GAS TANKERS
Advanced Course
INTRODUCTION
1
INTRODUCTION
1.1 ABOUT STCW International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW), 1978, as amended, sets qualification standards for masters, officers, and watch going personnel on seagoing merchant ships. STCW was adopted in 1978 by conference at the International Maritime Organization (IMO) in London, and entered into force in 1984. The Convention was significantly amended in 1995. The 133 current stateparties to the Convention represent approximately 98 percent of the world’s merchant vessel tonnage. 1.1.1 Limitations discovered Between 1984 and 1992, significant limitations to the 1978 Convention became apparent. Many people felt that the Convention included vague requirements that were left to the discretion of parties to the Convention. Others felt that there were growing problems with: (a) a lack of clear standards of competence, (b) no IMO oversight of compliance, (c) limited port state control, and (d) inadequacies that did not address modern shipboard functions. Meanwhile, the U.S. deferred ratification efforts and worked for almost a decade to effect necessary changes to our licensing regulations. 1.1.1 Amendments adopted in 1995 On July 7, 1995, a conference of parties to the Convention, meeting at IMO headquarters in London, adopted the package of amendments to STCW. The amendments entered force on February 1, 1997. 1.1.2 Effective dates The provisions of the Convention not tied to individual mariner certification became effective when the IFR (Interim Final Rule) was published. However, provision was made for certain new requirements to be introduced over a longer period. Full implementation is required by February 1, 2002. For issuance of licenses and documents, the effective dates of the new requirements will be according to transitional guidance published by the STW Subcommittee. Mariners already holding licenses have the option to renew those licenses in accordance with the old rules of the 1978 Convention during the period ending on February 1, 2002. Mariners entering training programs after August 1, 1998 are required to meet the competency standards of the new 1995 Amendments. For persons seeking original licenses, the Coast Guard anticipates that most new training requirements will be incorporated into courses approved by the Coast Guard, or by equivalent courses. To ensure that the competency objectives of the 1995 amendments are met, parties must implement quality assurance programs, with IMO reviewing each parties’ national program. Again, this represents a fundamental change in thinking for the international community. It will be mandatory that the "pulse" of the new system be checked on a recurring basis to ensure its "good health."
1.1.3 Familiarization training: Both the STCW Convention and the U.S. implementing regulations use the term familiarization training or similar terminology five different ways: a. Companies are required to ensure that seafarers who are newly assigned to a ship are familiarized with their specific duties and with all ship arrangements, installations, equipment, procedures and ship characteristics that are relevant to their routine or emergency duties. Written instructions are to be issued by the company to each ship to ensure this ship-specific familiarization takes place. b. All persons who are employed or engaged on a seagoing ship other than passengers are required to receive approved familiarization training in personal survival techniques or receive sufficient information and instruction to be able to take care of themselves and take proper action when an emergency condition develops. This includes locating and donning a lifejacket, knowing what to do if a person falls overboard, and closing watertight doors. c. Officers and ratings who are assigned specific duties and responsibilities related to cargo or cargo equipment on tankers must complete an approved tanker familiarization course if they have not had a minimum period of seagoing service on tankers. d. Masters, officers and other personnel who are assigned specific duties and responsibilities on board ro-ro passenger ships must complete familiarization training which covers subjects such as operational limitations of ro-ro ships, procedures for opening and closing hull openings, stability, and emergency procedures. e. Masters, officers and other personnel who are assigned specific duties and responsibilities on board passenger ships other than ro-ro passenger ships must complete familiarization training which covers operational limitations of passenger ships. 1.2
THE COURSE The International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW 78/95), which contains mandatory minimum requirements for training and qualifications of masters, officers and ratings of chemical tankers. This training is divided into two parts: Level 1: Chemical tanker familiarization - a basic safety training course for officers and ratings who are to have specific duties and responsibilities relating to cargo and cargo equipment
Level 1 training can also be covered through an appropriate period of supervised shipboard service where an approved shipboard training programme is conducted by qualified personnel Level 2: Advanced training programme on liquefied gas tanker operations. An advanced training programme for masters, officers and others who are to have immediate responsibilities for cargo handling and cargo equipment. In addition to level 2 training, such personnel must have completed level 1 and have relevant experience on liquefied gas tankers before signing on to these positions on board This course covers the requirements for level 1 and level 2 training required by STCW 95 Chapter V Regulation V/1 - 1.2, 2.2 and Section A-V/1 regulations 15 - 21
02.
Actual Gas Cargoes
2
ACTUAL GAS CARGOES
In the late 1920th transportation of liquefied gases in bulk started. In the very beginning it was transportation of propane and butane in fully pressurised tanks. When the steel quality became better and the knowledge about propane and butane was better they started to carry those liquefied gases under temperature control. From the mid-1960th we have carried fully refrigerated liquefied gases and now the biggest gas carriers are more than 125 000 m3. Liquefied gas is divided into different groups based on boiling point, chemical bindings, toxicity and flammability. The different groups of gases have led to different types of gas carriers and cargo containment system for gas carriers. • • • • • • •
IMO divides liquefied gases into the following groups: LPG - Liquefied Petroleum Gas LNG - Liquefied Natural Gas LEG - Liquefied Ethylene Gas NH3 - Ammonia Cl2 - Chlorine Chemical gases
The IMO gas carrier code define liquefied gases as gases with vapour pressure higher than 2,8 bar with temperature of 37,8oC. IMO gas code chapter 19 defines which products that are liquefied gases and have to be transported with gas carriers. Some products have vapour pressure less than 2,8 bar at 37,8oC, but are defined as liquefied gases and have to be transported according to chapter 19 in IMO gas code. Propylene oxide and ethylene oxides are defined as liquefied gases. Ethylene oxide has a vapour pressure at 37,8oC on 2,7 bar. To control temperature on ethylene oxide we must utilise indirect cargo cooling plants. Products not calculated as condensed gas, but still must be transported on gas carriers, are specified in IMO’s gas code and IMO’s chemical code. The reason for transportation of non-condensed gases on gas carriers is that the products must have temperature control during transport because reactions from too high temperature can occur. Condensed gases are transported on gas carriers either by atmospheric pressure (fully cooled) less than 0,7 bars, intermediate pressure (temperature controlled) 0,5 bars to 11 bars, or by full pressure (surrounding temperature) larger than 11 bars. It is the strength and construction of the cargo tank that is conclusive to what over pressure the gas can be transported.
Examples of some gas pressure at 37,8oC and boiling point at atmospheric pressure: Condensed gas
Methane CH4 Propane C3H8 n - Butane C4H10 Ammonia NH3 Vinyl Chloride C2H3Cl Butadiene C4H6 Ethylene oxide C2H4O 2.1
Boiling point Gas pressure at atmospheric pressure 37,8oC bars in oC absolute Gas - 161 12,9 - 43 3,6 - 0,5 14,7 - 33 5,7 - 14 4,0 -5 2,7 10,7
at
LPG
LPG - Liquefied Petroleum Gas is a definition of gases produced by wet gas or raw oil. The LPG gases are taken out of the raw oil during refining, or from natural gas separation. LPG gases are defined as propane, butane and a mixture of these. Large atmospheric pressure gas carriers carry most of the LPG transported at sea. However, some LPG is transported with intermediate pressure gas carriers. Fully pressurised gas carriers mainly handle coastal trade. LPG can be cooled with water, and most LPG carriers have direct cargo cooling plants that condenses the gas against water. The sea transport of LPG is mainly from The Persian Gulf to Japan and Korea. It is also from the north- west Europe to USA, and from the western Mediterranean to USA and Northwest Europe. LPG is utilised for energy purposes and in the petro-chemical industry 2.2
LNG
LNG - Liquefied Natural Gas is a gas that is naturally in the earth. Mainly LNG contains Methane, but also contains Ethane, Propane, Butane etc. About 95% of all LNG are transported in pipelines from the gas fields to shore, for example, gas pipes from the oil fields in the North Sea and down to Italy and Spain. Gas carriers transport the remaining 5%. When LNG is transported on gas carriers, the ROB and boil off from the cargo is utilised as fuel for propulsion of the vessel. Cargo cooling plants for large LNG carriers are very large and expensive, and they will use a lot of energy. Small LNG carriers have cargo-cooling plants, and can also be utilised for LPG transportation.
The sea transport of LNG is from the Persian Gulf and Indonesia to Japan, Korea and from the Mediterranean to Northwest Europe and the East Coast of USA and from Alaska to the Far East. LNG is used for energy purposes and in the petro-chemical industry. 2.3
NGL
NGL - Natural Gas Liquid or wet gas is dissolved gas that exists in raw oil. The gas separates by refining raw oil. The composition of wet gas varies from oil field to oil filed. The wet gas consists of Ethane, LPG, Pentane and heavier fractions of hydrocarbons or a mixture of these. Atmospheric pressure gas carriers and semipressurised gas carriers carry the most of the wet gas. Ethane can only be transported by semi-pressurised gas carriers, which have direct cascade cooling plants and are allowed to carry cargo down to –104oC. This is because Ethane has a boiling point at atmospheric pressure of –89oC. This will create too high condense pressure if using water as cooling medium. The cargo is condensed against Freon R22 or another cooling medium with boiling point at atmospheric pressure lower than –20oC. Wet gas is transported from the Persian Gulf to the East, Europe to USA and some within Europe. There is also some transport of wet gas in the Caribbean to South America. NGL is utilised for energy purposes and in the petro-chemical industry. 2.4
2.5
COMPOSITION OF NATURAL GAS
LEG
LEG - Liquefied Ethylene Gas. This gas is not a natural product, but is produced by cracked wet gas, such as, Ethane, Propane, and Butane or from Naphtha. Ethylene has a boiling point at atmospheric pressure of -103,8oC, and therefore has be transported in gas carriers equipped with cargo compartment that can bear such a
low temperature. Cascade plants are used to condense Ethylene. As critical temperature of Ethylene is 9,7oC one can not utilise water to condense Ethylene. The definition of Ethylene tankers is LPG/LEG carrier. Ethylene is very flammable and has a flammable limit from 2,5% to 34% by volume mixed with air. There are stringent demands regarding the oxygen content in Ethylene. The volume of ethylene must be less than 2% in the gas mixture to keep the mixture below the LEL “lower explosion limit”. Normally, there are demands for less than 0,2% oxygen in the gas mixture in order to prevent pollution of the cargo. Ethylene is utilised as raw material for plastic and synthetic fibres. Ethylene is transported from the Persian Gulf to the East, the Mediterranean to the East and Europe, the Caribbean to South America. There is also transport of Ethylene between the countries Malaysia, Indonesia and Korea 2.6
AMMONIA NH3
The next gas we will focus on is Ammonia, which is produced by combustion of hydrogen and nitrogen under large pressure. Ammonia is a poisonous and irritating gas, it has TLV of 25 ppm and the odour threshold is on 20 ppm. It responds to water and there are special rules for vessels that transport Ammonia. We can locate the rules in the IMO Gas Code, chapters 14, 17 and 19. When ammonia gas is mixed with water, a decreased pressure is formed by 1 volume part water absorbing 200 volume parts ammonia vapour. A decreased tank pressure will occur if there is water in the tank when commence loading ammonia and the tank hatch is closed. With an open hatch, we can replace the volume, originally taken up by the ammonia gas, with air. One must not mix ammonia with alloys: copper, aluminium, zinc, nor galvanised surfaces. Inert gas that contains carbon dioxide must not be used to purge ammonia, as these results in an carbamate formation with the ammonia. Ammonium carbamate is a powder and can blockage lines, valves and other equipment. The boiling point for ammonia at atmospheric pressure is –33oC, and must be transported at a temperature colder than –20oC. One can cool ammonia with all types of cargo cooling plants. Ammonia is transported with atmospheric pressure gas carriers or semi-pressurised gas carriers. Gas carriers carrying Ammonia must be constructed and certified in accordance with IMO’s IGC code for transportation of liquefied gases. The definition for ammonia tanker is LPG/NH, carrier. Ammonia is utilised as raw material for the fertiliser industry, plastic, explosives, colours and detergents. There is a lot of transportation from the Black Sea to USA, from USA to South Africa and from Venezuela to Chile. 2.7
CHLORINE CI2
Chlorine is a very toxic gas that can be produced by the dissolution of sodium chloride in electrolysis. Because of the toxicity of Chlorine it is therefore transported in small quantities, and must not be transported in a larger quantity than 1200m3. The gas carrier carrying chlorine must be type 1G with independent type C tanks. That means the cargo tank must, at the least, lie B/5 “Breadth/5” up to 11,5 meter
from the ships side. To transport Chlorine, the requirements of IMO IGC code, chapters 14, 17 and 19 must be fulfilled. Cooling of Chlorine requires indirect cargo cooling plants. The difference of Chlorine and other gases transported is that Chlorine is not flammable. Chlorine is utilised in producing chemicals and as bleaching agent in the cellulose industry. 2.8
CHEMICAL GASES
The chemical gases mentioned here are the gases produced chemically and are defined in IMO’s rules as condensed gases. Because of the gases’ boiling point at atmospheric pressure and special requirements for temperature control, these gases must be carried on gas carriers as specified by the IMO gas code. Condensed gases are liquids with a vapour pressure above 2,8 bars at 37,8oC. Chemical gases that are mostly transported are Ethylene, Propylene, butadiene and VCM. Chemical gases that have to be transported by gas carriers are those mentioned in chapter 19 in IMO IGC code. There are, at all times, stringent demands for low oxygen content in the cargo tank atmosphere, often below 0,2% by volume. This involves that we have to use nitrogen to purge out air from the cargo compartment before loading those products. In addition, even though the vapour pressure does not exceed 2,8 bars at 37,8oC such as, ethylene oxide and propylene oxide or a mixture of these, they are still in the IMO gas code as condensed gases. Gas carriers that are allowed to transport ethylene oxide or propylene oxide must be specially certified for this. Ethylene oxide and propylene oxide have a boiling point at atmospheric pressure of respectively 11oC and 34oC and are therefore difficult to transport on tankers without indirect cargo cooling plants. Ethylene oxide and propylene oxide can not be exposed to high temperature and can therefore not be compressed in a direct cargo cooling plant. Ethylene oxide must be transported on gas tanker type 1G. Chemical gases like propylene, butadiene and VCM are transported with mediumsized atmospheric pressure tankers from 12000 m3 to 56000 m3. Semi-pressurised gas carriers are also used in chemical gas trade and then in smaller quantity as from 2500 m3 to 15000 m3. Chemical gases are transported all over the world, and especially to the Far East where there is a large growth in the petro-chemical industry. Chemical gases are mainly utilised in the petro-chemical industry and rubber production.
2.9
LNG CONDSATION PLANT FLOW DIAGRAM
2.10 OIL/GAS FLOW DIAGRAM
2.11 PRODUCTION OF CHEMICAL GASES
2.12 CONNECTION TABLE
Table showing connection between cargo temperature and type of compartment and secondary barrier requirement Cargo temperature at atmospheric pressure Basic tank type Intergral Membrane Semi-membrane Independent Type A Type B Type C Internal insulation Type 1 Type 2
- 10oC and above
Below -10oC down to –55oC
No secondary barrier required
Hull may act as secondary barrier
Below -55oC
Separate secondary barrier where required Tank type not normally allowed Complete secondary barrier Complete secondary barrier Complete secondary barrier Partial secondary barrier No secondary barrier required Complete secondary barrier Complete secondary barrier is incorporated
03-
Cargo Compartment Systems
3 CARGO COMPARTMENT SYSTEMS Cargo compartment systems on gas carriers are divided into groups and types. The group division indicates how the cargo tanks transfer dynamic strength to the vessel hull. Cargo tanks that will be used on gas carriers must at all times have a documented strength and certification of welded joints and steel quality. The cargo tanks on gas carriers are rarely a direct part of the hull, but rather tanks installed into the hull and isolated from the hull. Gas carriers are built with two or more spaces where the cargo tanks are installed. The space where the cargo tank is installed is called hold space. How much hold space volume the cargo tank absorbs depends on the cargo tank’s shape. Cargo tanks isolated from the hull, for example, cylinder tanks, must be electrically grounded with a wire or steel strip to the hull.
Table showing connection between cargo temperature and type of compartment and secondary barrier requirement Cargo temperature at atmospheric pressure
- 10oC and above
Below -10oC down to –55oC
Below -55oC
Basic tank type
No secondary barrier required
Hull may act as secondary barrier
Separate secondary barrier where required
Integral Membrane Semi-membrane Independent
Tank type not normally allowed Complete secondary barrier Complete secondary barrier
Type A Type B Type C Internal insulation Type 1 Type 2
Complete secondary barrier Partial secondary barrier No secondary barrier required
Complete secondary barrier Complete secondary barrier is incorporated
Cargo tanks that are built for fully refrigerated gas carriers, and tanks with MARVS less than 0,7 bars, must at all times have full or partly secondary barrier. Secondary barrier is a tank or hull construction built outside the cargo tank itself, either in the insulation between cargo tank and hull, or in the hull around the cargo tank. If the hull around the cargo tank is used, it will be the ballast tank, ships side or cofferdams that is the secondary barrier. When utilising the hull around the cargo tank as the secondary barrier the vessel is limited as it will not have the capability to transport cargo colder than –55oC.
Secondary barrier will prevent cargo liquid from any possible leaks coming from the cargo tank cooling the environment around the cargo tank, for example the ship sides. The secondary barrier must have a construction that, at a minimum, keeps the cargo liquid away from the surroundings for at least 15 days and maintains its full function at static lurch of 30o. All cargo tanks on gas carriers are constructed to a given excess pressure and vacuum. The safety valve’s maximum allowed set point, called MARVS, is stated in accordance to specification and pressure test, stated by the manufacturer of the cargo tank. The tolerance of vacuum on the cargo tanks is stated in bars, kg/cm2 or percentage of vacuum. MARVS and vacuum for each cargo tank must be specified in the vessels “Certificate of Fitness”. US Coast Guard has more stringent rules for safety margins for pressure tanks than IMO, this indicates that cargo compartment on gas carriers have different MARVS pressures for IMO and USCG. In hold spaces and inter barrier spaces there are demands for an own bilge system that is independent from the vessel’s other bilge systems. This is arranged with independent ejectors or bilge pumps in the spaces and usually one in each side of the space. Inter barrier space is the space between the cargo tank and the secondary barrier. The bilge arrangement is meant to pump out the cargo if there has been a leakage from the cargo tank. The system can also be utilised to remove water from the hold space or inter barrier space if there is accumulation of condensed water. If we have to pump water we must be sure that all connections to the loading system is disconnected. On atmospheric pressure tankers, hold space and inter barrier space must at all times have a neutral atmosphere, either by dry inert or nitrogen when loaded with flammable cargo. Nitrogen or dry air must be utilised when the cargo content is Ammonia or nonflammable cargo. When the cargo is Ammonia one must under no circumstance utilise inert containing CO2 in the spaces, because Ammonia has a reaction on CO2 and form a material called Ammonium Carbamate IMO divides the cargo tanks into 4 main groups: Integrated tanks Membrane tanks Semi - Membrane tanks Independent tanks, type A, B, and C The characteristics of integrated, membrane and semi membrane tanks is that they all transfer static stress in the form of tank pressure to the hull around the cargo tank when this is loaded. Independent tanks only transfer the weight of the cargo tank and the cargo to the hull fundamentals, but does not transfer static pressure.
3.1
INTEGRATED TANKS
The first cargo compartment system we will look at is integrated cargo tanks. It is the same type of cargo compartment that we have on oil tankers, OBO carriers and product tankers. The cargo tank is an integrated part of the hull so the hull absorbs the weight and pressure from the cargo. This type of cargo compartment is less suited and rarely approved for gas transportation. If we transport cargo colder than –10oC, this type of cargo compartment is not approved. Then low temperature steel in the cargo compartment is required. International rules also require a minimum distance from the ship's side to the cargo tank of 760 mm for guiding of toxic or flammable cargo. This prevents pollution from collision or run grounding. Example of integral tank
3.2
MEMBRANE TANKS
Membrane compartment are divided into two groups, membrane tank system and semi- membrane tank system. Membrane tank system is built up with two equal membranes, while semi-membrane system have a membrane against the cargo and metal or veneer as secondary barrier. Common for all membrane tanks is that there is no centre bulkhead for reducing the free liquid surface, but is built up with a trunk for narrowing the tanks up against the top of the tank. 3.2.1 Membrane tank system Membrane tank is a cargo tank built of thin plate of invar steel, stainless steel or ferro nickel steel with a content of 36% nickel. Characteristic for these types of steel is a very small thermal expansion coefficient approximate equal 0. The tank shell and the secondary barrier are built in profiles formed as a membrane; this renders the material thickness small and no more than 10 mm thick. The membrane thickness is normally of 0,5 to 1,2 mm. There is insulation between the secondary membrane and the hull. The insulation is often perlite filled in plywood boxes, placed outside each other like building blocks, or polyurethane gradually sprayed directly on as the tank is built up. The hull takes up all weight from the cargo, and the membrane
takes up the thermal expansion. Normal excess pressure for such cargo tanks is 0,25 bars, and there are demands for secondary barrier. We can utilise the hull as secondary barrier for cargo temperatures down to –55oC, but we must utilise low temperature steel in the hull round the cargo tank. Frequently ballast tanks or cofferdams form the hull structure around the cargo tank. For cargo colder than –55oC a tank must be placed into the insulation as secondary barrier. French Gaz-Transport patent utilise two identical membranes outside each other as primary and secondary barriers, with 36% nickel steel or invar steel. The insulation in Gaz-Transport patent is perlite filled with plywood boxes. Technigaz membrane system utilises stainless steel in the main membrane and veneer in the secondary membrane. The main membrane is welded together of small plates by a special shaping so that the tank tolerates expansion, the plate thickness is about 1,2 mm. The first tanks from Technigaz utilized veneer plates, as secondary barrier and balsa as insulation. Polyester-coated aluminium foil is now utilised as secondary barrier, and polyurethane foam for insulation. These tank types are utilised on large LNG and LPG tankers. 3.2.2 Sketch on membrane tank
3.3
SEMI - MEMBRANE TANKS
These are tanks used on large LPG tankers. Semi-membrane tanks are built up with an inner tank, insulation, membrane and insulation against hull. It is the membrane that takes up the thermal expansion. The tanks are built of aluminium, ferro nickel steel with 36% nickel, or built of stainless steel. The insulation is mostly perlite, but can also be polyurethane or polystyrene. The hull absorbs all dynamic loads from the cargo tank when the tank is loaded. Normal excess pressure for such cargo tanks is 0,25 bars, and there is a demand for secondary barrier. One can use the hull as secondary barrier for cargo temperature down to –55oC, but one must utilise low temperature steel in the hull around the cargo tank. One can also place a tank into the insulation as secondary barrier. One cannot utilise the hull as secondary barrier for temperature colder than –55oC. A membrane inside is then
built in the insulation as secondary barrier. This tank type was designed for LPG transportation, but no LPG tankers are built with this tank type. In recent years, Japanese yards have started to utilise this tank type on large LPG tankers. 3.3.1 Example of semi-membrane tank
3.3.2 Cross-section of gas tanker with membrane tank
3.3
INDEPENDENT TANKS
Independent cargo compartment is cargo tanks that do not transfer the pressure loads to the hull when they are loaded. Therefore, only the tank weight is transferred to the cradles or the support points in the hull. The cargo tanks are built with support to prevent the tank from slipping forward, astern, to the side or floating up. Independent tanks are divided into three types: A, B and C. This division distinguishes between the pressure the tank must tolerate and the demands for secondary barrier. Independent tank Type A has the weakest strength of the independent tanks, and there are demands for full secondary barrier. Independent tank type B has greater strength than type A does, and only demands a partly
secondary barrier. Independent tank type C is a pressure tank with no demands for secondary barrier. 3.4.1 Independent tanks type A Independent tank type A could be a prismatic tank and built in 3,5% nickel steel, coal manganese steel or aluminium. The material is a recognised standard, steel quality approved by the class companies. This type of cargo tank is utilised for carrying LNG, LPG and ammonium. This type of tanks is built for excess pressure less than 0,7 bars. Normal operating pressure is 0,25 bars. The cargo tanks are mounted on building blocks so the tank can expand freely. On top of the tanks and in the ship side or up under deck, brackets are welded to prevent the tank from floating up. 3.4.2 Example of “anti float” brackets A full secondary barrier for this type of tank is required. On LPG tankers designed for minimum temperature of –48oC, the hull is generally used as secondary barrier as low temperature steel is used in the hull construction around the cargo tank. If the hull is not utilised as secondary barrier an extra tank around the main cargo tank are constructed. This is done by building a tank of veneer plates around the cargo tank with polyurethane foam as insulation in between. One can also use nitrogen or inert between the tanks as insulation. 3.4.3 Independent tanks type B Independent tank type B is a prismatic tank, spherical tank or membrane tank. These tanks are designed and model tested, and they have better quality than type A tanks. This tank type is used for large LPG and medium-sized tankers.
Prismatic tanks are produced in aluminium or 3% nickel steel in stiff plates. The tanks rest on reinforced plywood supports for free expansion. The tanks are normally provided with centreline bulkhead to reduce the free liquid surface. The tanks are insulated with polyurethane or perlite. Submerged pumps or deepwell pumps are utilised as discharging pumps.
Spherical tanks produced by Moss-Rosenberg patent are produced in aluminium or 9% nickel steel. The tanks are supported with cargo tank shirt at equator and down to the hull. Around the tank that is above deck there is a waterproof cover. The tanks are equipped with submerged pumps. Polyurethane is often utilised as foam on type B tanks as insulation; this is sprayed directly on the tank shell. Other types of insulation are polystyrene plates placed in layers, or perlite either filler around the tank or placed in small veneer cases. The insulation on spherical tanks is spinned on from the bottom and up. 3.4.4 Independent tanks type C Independent tanks, type C are either spherical tanks or cylinder tanks. The tanks are built in carbon manganese steel, 2 – 5 % nickel steel or acid-proof stainless steel. This type of tank has a large rate of security, and therefore does not need secondary barrier. This tank type is utilised for fully pressurised gas carriers and semipressurised gas carriers. Tanks type C utilised on gas carriers are built in sizes from 300 m3 to 2500 m3.
Either submerged or deepwell pumps are utilised as discharge pumps. The tanks are stored on cradles and welded to one of the cradles. The other cradle functions as a support for the tank to expand freely. Some patents keep the tanks down in the cradles by steel bands that are extended over the tank and fastened to the cradle. Another patent is to weld “anti float” brackets on top of the cargo tank and up under deck to prevent the tank from floating up. Tanks designed for cargo colder than –10oC must have insulation. Normally polyurethane or polystyrene is utilised as insulation. The insulation is either sprayed directly or placed on in blocks on the cargo tank. The thickness of the insulation is dependent of the quality of the insulation material and the temperature of the cargo. The thickness of the insulation on tanks that carry ethylene is about 200 mm.
3.5
TYPES OF GAS CARRIERS
Gas carriers are tankers constructed for transporting liquefied gases in bulk. IMO defines liquefied gases as products with a vapour pressure exceeding 2,8 bar absolute at a temperature of 37,8oC. Gas carriers are built according to IMO’s Gas Codes. There are three versions of gas codes; the first deals with existing gas carriers and passes for gas carriers delivered before 31st of December 1976. The next code passes for gas carriers delivered on or after 31st of December 1976, but before 1st of July 1986. The third gas code, IGC Code passes for gas carriers started or the keel set after the first of July 1986. The latest gas code is for gas carriers that keel is laid and 1% of the construction mass is used on 1st October 1994. The gas code has a content in demands for damage stability, gas tankers cargo handling equipment, cargo tanks, steel qualities in cargo tanks, pipe systems for cargo handling, personnel protection, safety valves, etc. Gas carriers are divided into three main groups and four types. The gas carrier owner decides which group and type the carrier should have, according to the freight the vessel will trade. The three main groups are: • • •
Fully pressurised carriers: designed for excess pressure in the cargo tank above11 bar. Semi-pressurised carriers: designed for excess pressure the cargo tank on 0,5 – 11 bars, the pressure is normally 3 – 5 bars. Fully refrigerated carriers: designed for excess pressure in the cargo tank below 0,7 bars, the pressure is normally 0,25 – 0,3 bars.
Each of the groups is again divided into ship types dependent on the cargo's hazardous properties (i.e.: toxicity, flammability, reactivity etc.). It is the ship owner’s specification of the gas carrier, the international rules determined by IMO, national rules and class companies rules that decide to which group and ship type the carrier belongs. All gas carriers classed according to IMO IGC Code for transportation of gases mentioned in chapter 19, is given one of the following description types: 1G, 2G, 2PG or 3G. Ship type 1G is the type that can carry all cargoes mentioned in chapter 19 of the IGC Code, and has the largest rate of security to avoid pollution of the environment. Ship type 1G is a gas carrier that can carry all products mentioned in chapter 19 in the IGC Code, and requires largest rate of security to prevent leakage from the product to the surroundings. Ship type 2G is a gas carrier that can carry the products marked in 2G, 2PG and 3G in chapter 19 in the IGC Code, and that requires defensible security to prevent leakage of the product. Ship type 2PG is a gas carrier of 150 meters or less that can carry the products marked 2PG or 3G in chapter 19 in the IGC Code, and that requires defensible security to prevent leakage of the product. Also, where the product is transported in independent tanks type C, which are designed for MARVS of at least 7 bars. Then,
the cargo tank system is calculated for temperatures of –55oC or warmer. Gas tankers of 150 meters or more, but with the same specification, as 2PG ships must be calculated as 2G ships. Ship type 3G is a gas carrier that can carry the products marked 3G in chapter 19 in the IGC Code, and that requires moderate security to prevent leakage of the product. The ship type is reported in column c in chapter 19 in the IGC Code. The type of gas carrier is specified in the vessels IMO Certificate of Fitness. On the certificate, there is also a product list of which products the vessel can carry. The type description of the gas carrier is given by the year when the keel was laid and the cargo tanks distance from ship side, damage stability, floating capability and of what material the cargo tank is made. As an example on ship type 1G, the cargo tank must lie at least B/5 parts up to 11,5 meters from the ship side. From the bottom plate and up to the tank no less than 2 meters or B/15 parts. B is equal to the vessel breadth. This type of carrier must tolerate any damage to the ship side along the whole ship’s length. All information of the demands made for the different ship types is located in IMO Gas Code, and all gas tankers must have this publication onboard.
3.6 FULLY PRESSURISED CARRIERS Fully pressurised gas carriers were the first generation of gas carriers that were built to transport liquefied gases in bulk. This type of gas carrier trades mostly where LPG is consumed as energy, such as house heating and cooking etc. The trade area is often limited to near coastal waters. This type of gas carrier is still built, but is built to be more modern with discharging pumps in cargo tanks and indirect cargo cooling plant for more flexible cargo handling. We divide this type of gas carrier into two, one for LPG trade and one for Chlorine trade. 3.6.1 Fully pressurised LPG carriers This type of gas carrier is the type that in proportion to displacement can carry the lowest weight of cargo, this because it is transported under pressure at the surrounding temperature, “ambient”. When the tank pressure increases the cargo’s temperature also increases and the density of the liquid will be lower. The cargo tank construction itself is heavy as these are built of common ship steel with a thick tank shell to endure high pressure. There are no requirements for insulation of the cargo tanks because these carriers are not allowed to transport cargoes with temperature colder than –10oC. These gas carriers are built in sizes up to about 3000 m3, and are built for an excess pressure corresponding to an ambient temperature of 45oC. Propane has a saturation pressure of 17,18 bars at 50oC. IMO has a requirement when building fully pressurised tanks that they must be able to bear ambient (surroundings temperature) cargo with a temperature on 45oC. The type of cargo determines the excess pressure for which the tanks must be built. Normally, fully pressurised carriers LPG have a relief valve setting at 18 bars, consequently, they can also carry propylene in tropical waters. This type of gas carrier is easy to operate, because the cargo does not need to be cooled down on the sea voyage. To prevent vapour into the atmosphere when loading, they can remove the excess vapour by having vapour return to shore. Fully pressurised gas carriers don’t need discharge pumps in the cargo tanks, because the excess tank pressure will discharge the liquid to shore. Hot gas from shore can be used to hold the excess pressure in the cargo tank. If there is no utilisation of the discharging pumps while discharging, the cargo tank’s excess pressure must at all time be higher than the shore backpressure. Some fully pressurised gas carriers are equipped with booster pump(s) (auxiliary pump) on deck. This pump is used to discharge against a higher pressure than the excess pressure in the cargo tanks. Booster pump is a one-stage centrifugal pump installed on deck close to the ship manifold. Normally a booster pump manages to increase the pressure up 9 bars. If the cargo tank’s pressure is 7 bars, then we can manage 16 bars on the discharge line with the booster pump. We must bear in mind that when running the booster pump against maximum pressure, the flow through the pump is very low. We must always prime the booster pump before starting it, generally by draining the discharge line to the ventilation mast. It is the pressure in the shorelines that determines the manifold’s pressure and whether we should use the booster pump or not.
Fully pressurised gas carriers are equipped with a heat exchanger (cargo heater) connected to the loading lines with vales and spool piece (adapter). When the heat exchanger is not in use it is segregated from the liquid line. The heat exchanger is used when we are loading a cargo with temperature below 0oC, for example, propane at atmospheric pressure corresponding to –42,8oC directly into the vessels cargo tanks. Then the cargo has to be heated to above –10oC before we load it down to the cargo tank. Fully pressurised gas carriers have a small cargo compressor to produce excess pressure in the cargo tanks or remove over pressure from the cargo tanks. Vapour is sucked from the cargo tanks to the compressor, and hot vapour is returned back to the cargo tanks. These compressors are in general small, and are utilised only for holding the temperature on the cargo. Fully pressurised gas carriers are constructed with independent tank type C, cylindrical or spherical tanks. These are tanks installed on “cradle-like” supports down in the hold space (the space around the cargo tank), and the ship hull doesn’t recover dynamic loads from the cargo tanks. Actual cargo for fully pressurised carriers is LPG and some chemical gases. The kind of cargo each vessel can carry is stated in the vessel’s IMO Certificate of Fitness. Fully pressurised gas carriers are most utilised for carrying of ambient LPG and some chemical gases as propylene, mainly in the Far East, South America, the Caribbean and The Mediterranean. Advantages: • • • • • •
Easy to operate because all discharging takes place without pumps. Low costs in building because common steel is utilised in the cargo tanks. Low costs for maintenance, because there is little mechanical utility equipment for cargo handling. Simple discharging/loading equipment on deck. No insulation of tanks or liner, no need in maintenance of the insulation, and one can easily inspect the cargo tanks and the lines from the outside. Transporting the cargo by surrounding temperature (ambient), no cooling of the cargo gives low energy consumption.
Disadvantages: • • • • •
Small amount of cargo in proportion to displacement as the cargo is transported ambient. Limited trade area because of dependence of discharging to pressure tanks on shore. Limited cargo volume because the tankers are not built large than 3000m3. Unable to have cold cargo in the tanks because of the steel quality. Heavy cargo construction because of toleration of the pressure.
3.6.2 Fully pressurised chlorine Cl2 carrier These tankers are built as fully pressurised tankers LPG, but because of the toxicity of chlorine, special requirements are set on this type of gas carrier. The requirements are stated in the IGC code chapter 14, 17 and19. This type of ship must not have cargo tanks larger than 600 m3, and total capacity must not exceed 1200 m3. Consequently, these gas carriers are smaller than the common fully pressurised gas carriers LPG. The cargo tanks must be built for an excess pressure not lower than 13,5 bars, which is saturation pressure for Chlorine at 45oC. Tanks and lines must be built in steel quality that tolerates a temperature down to –40oC. Cargo lines must at maximum have an inner diameter of 100 mm. Tanks and lines must be insulated. Polyurethane or polystyrene is utilised as insulation. This information is at all times specified in IMO Certificate of Fitness. There is also a summary in the certificate of fitness as to what type of cargoes the actual tanker is allowed to carry. This type of gas carrier often has an indirect cargo cooling plant with coils welded to the outside of the tank shell. In general ethanol is used as cooling medium against Freon (R22) in a small freon cooling plant. Other indirect cargo cooling plants utilise freon as the cooling medium by directly pumping freon in and around the coils. It is prohibited to use any kind of direct cargo cooling plant on chlorine. To discharge these type of gas carriers the cargo tanks excess pressure is used. Either the pressure established by dry nitrogen or only the tank pressure is used. Chlorine vapour obtained from shore via the ship’s vapour lines can also be used for discharging. Some chlorine carriers are also equipped with submerged pumps in the cargo tanks. This type of gas carrier mostly stays in the chlorine trade, because of the toxicity of the cargo. There are few cargo owners that accept to load other products after Chlorine. Chlorine carriers can, if they are accepted, also carry LPG and some chemical gases depending on the relief valve’s set point. Because of the toxicity of chlorine it is necessary that the chlorine carriers are equipped with a chlorine absorption plant connected to cargo tanks and cargo lines. The absorption plant must neutralise a minimum 2% of total cargo capacity. The gas detector onboard must measure 1 ppm chlorine and alarm setting at 5 ppm. The gas detector must scan the bottom of hold space, line from safety valve, the outlet from chlorine absorption plant, into ventilation for accommodations and all of the gas area on deck. Advantages: They are easy to operate. Simple cargo handling equipment on deck. Tanks and lines are insulated. They have an indirect cooling plant, and are thereby capable to cool cargo.
Disadvantages: They are small tankers, and have thereby low loading capacity. Expensive to build in proportion to the cargo amount they can transport. The tankers are mainly designed for Chlorine.
3.7
SEMI PRESSURISED GAS CARRIERS
Semi-Pressurised gas carriers are a development from fully pressurised carriers. Semi-pressurised carriers are equipped with discharging pumps in the cargo tank, cargo cooling plant, heat exchanger (cargo heater) and booster pumps. In addition, the tanks and lines are insulated, normally with polyurethane or polystyrene. This renders the ship type with more flexibility than other gas carrier types. Semipressurised tankers are divided in two types - Semi-pressurised carrier LPG/LEG and Semi-pressurised tanker combined gas and chemicals. 3.7.1 Semi-pressurised LPG/ LEG carriers Semi-pressurised carriers are more complex than fully pressurised carriers due to their extended cargo handling equipment. Semi pressurised tankers are equipped either with direct cargo cooling plant or cascade cargo cooling plant. Which type of cargo cooling system the gas carrier is equipped with depends on the type of cargo it is meant to carry. If the tanker is carrying LPG or Ammonia with a boiling point at atmospheric pressure warmer than –48oC, the choice is generally direct cargo-cooling plant. If the vessel will transport cargo with a boiling point at atmospheric pressure colder than –48oC, the vessel must be equipped with cascade cooling plant. Before loading cold cargo, the cargo tank steel must be cooled down to approximate 10oC above cargo temperature. It is common that the first 30oC can be cooled the first hour. Thereby we can cool down the shell by 10oC an hour until it is about 10oC above the cargo temperature. The cooling of the tank steel must be done to prevent thermal expansion and crack in the tank shell. A tank of 1000 m3 that is cooled from 20oC to –103oC shrinks about 5 m3. In addition, when the shell is cooled down, the time for loading will be reduced and thereby reduces the time ashore. That will save harbour expenses for the ship owners or the charterer. It is specified in the operating manual for each vessel how to cool down the cargo tank shell. We must be attentive to this, because uneven thermal shrinkage of the cargo tank can lead to damage to the cargo tank. Semi-pressurised gas carriers are normally built in sizes from 2000 m3 to 15000 m3. They are designed to carry cargo with temperatures down to –48oC for LPG and Ammonia, and –104oC for LEG.
Semi-pressurised gas carriers are utilised for transportation of petrochemical gases, such as, Propylene, Butadiene, Ethylene and Ammonia, but also for gases, such as,
Propane, Butane and Ethane. There have been plans to build semi-pressurised tankers up to 36000 m3, but they are still not built. Semi-pressurised gas carriers have independent tanks type C either as cylinder or spherical tank designed for tank pressure between 0,5 – 11 bars. Either nickel steel or coal-manganese steel is used in the cargo tanks. Semi-pressurised carriers with spherical tanks utilise the same steel quality as in cylinder tanks. The cylinder tanks are often a combination of twin tanks that are situated longitudinally of the ship, and a single situated abeam. The tanks are placed below deck, but some vessels also have cargo tanks on deck. This information is, at all times, specified in IMO Certificate of Fitness. In the IMO Certificate of Fitness, there is also a summary of cargo the vessel can carry. The tanks are placed in “cradle-like” constructions and are welded to one of the cradles; the other cradle then functions as cargo tank support by expansion of the tank. The tanks are either strapped down with steel bands or the brackets are welded on to prevent the tanks from floating up. Between the cradle and the tank shell there is a layer of hard wood that acts as a fender to prevent damage to the cargo tank against the cradle, and acts as insulation against the steel in the cradle. On some vessels, the cargo tanks are attached to one of the cradles, and free in the other cradle for free expansion of the cargo tank. The spherical tanks are also installed in a “cradle-like” construction, and brackets (anti float) are welded on top of the cargo tank to prevent the cargo tanks from floating up. The support goes towards a bracket in the hull of the tanker, either up under deck or in the ship side. Actual cargoes for Semi-pressurised gas carriers are LPG, LEG, Ammonia, Ethylene and some chemical gases. Semi-pressurised gas carriers are the type of gas carriers that is most flexible for change of cargo and cargo handling. Advantages: • • • • •
Very flexible, can load and unload temperate cargo. Can heat the cargo while at sea and while discharging. Can transport fully cooled cargo, and thereby handle heavier cargo, lower temperature, and larger density. (Notice the safety valves set point). Easier tank construction than fully pressurised tankers. Can cool the cargo on route, no dependence at loading to remove excess pressure.
Disadvantages: • • • •
Expensive to build, costly cargo handling equipment. Complicated to operate because of the cargo handling equipment. Uses more energy than fully pressurised tankers. Limited cargo amount (maximum approximate15000 m3).
Semi-pressurised tankers with deck cargo tank or some transverse cargo tanks can have stability problems in loading/discharging. This is specified in the operating manual and the stability book for the tanker, and the operators onboard must consider this. 3.7.2 Semi pressurised tankers (Combined gas/chemical) These gas carriers are constructed like other Semi pressurised tankers, but they are classified both according to IMO gas and chemical codes. This involves separate liquid and vapour lines from each tank to the manifold, in order to segregate all cargo tanks from each other. This means that this type of gas carriers can load equally as much different cargo as they have cargo tanks. The cargo tanks on this type of gas carrier are the independent type C cylinder, generally single transverse or small alongside twin tanks. Cargo tanks, lines, and valves are constructed in stainless steel, and these gas carriers are equipped with indirect cargo cooling plant in addition to cascade cargo cooling plant. They are constructed to transport cargo from –104oC to 60oC. The indirect cargo cooling plant is often equipped with a coil welded outside the tank shell, where Ethanol is used either to cool or heat the tank steel. When cooling the tank steel, the Ethanol is cooled with the help of freon (R22) cooling plant. The Ethanol is also utilised to heat the tank steel; it is then heated with the tanker steam in a heat exchanger and pumped in and around the coils. These gas carriers are normally designed for 3 - 4 bars excess pressure and are built in sizes from 4000 m3 up to 15000 m3. Actual cargo LPG / NH3 / LEG / chemical gases and chemicals. Advantages: • • • • •
The tankers are very flexible, can transport both chemicals and gas. Tanks and lines are stainless steel. Direct and indirect cooling/heating. Can load and discharge tempered cargo and fully cooled cargo down to -104oC. Access to many smaller ports/harbours because of relatively little draught.
Disadvantages: • • • •
Expensive to build. Demanding to operate because of complicated cargo handling equipment. Limited cargo volume because of the tanker’s size. The stability is a problem when loading/unloading when there are many transverse cargo tanks or deck cargo tanks. This is specified in the operational manual and the stability book.
3.8
FULLY REFRIGERATED CARRIERS
Following semi-pressurised gas carriers, the first atmospheric pressure gas carrier was delivered at the end of the 1950s. These gas carriers are built in sizes from 15000 m3 to 120000 m3, and are designed for excess tank pressure less than 0,7 bars. These gas carriers are built either with independent tank type A or type B as prismatic or spherical tanks, or with membrane tanks. With prismatic or membrane tanks the volume of the hull is utilised, and tank construction is below deck. With spherical tanks, about half of the cargo tank is above deck because the vessel’s hull is lower than what you find with prismatic or membrane tanks.
3.8.1 Fully refrigerated LPG carriers The cargo tanks on fully refrigerated LPG carriers are normally built of low temperature carbon-manganese steel. The cargo tanks are designed for LPG, Ammonia and some chemical gases with minimum temperature of –48oC. The cargo tanks are normally insulated either with Polyurethane or Polystyrene. Some of the older fully refrigerated gas carriers have Perlite as tank insulation. Fully refrigerated gas carriers are normally equipped with independent type A or B prismatic cargo tank or membrane tanks. Fully refrigerated carriers with independent tank type A must have a full secondary barrier. This is achieved by using low temperature steel in the hull structure around the cargo tank. If independent tank type B is utilised either prismatic or spherical tanks, only a partly secondary barrier is demanded. This is achieved by utilising low temperature steel in the hull under the cargo tank. Independent prismatic cargo tanks are normally divided into two in longitudinal direction with a centre bulkhead that runs to the top of the tank dome. The centre
bulkhead is built to improve the stability on the carriers by reducing the effect of the free liquid surface when the tanks are loaded. There are normally one or more valves in the centre bulkhead that is called intermediate valves. These intermediate valves are installed down in the pump sump for the liquid to flow from one side to the other. It is important that the intermediate valves are closed when there is no loading or discharging of cargo. Normally there are two pumps in each cargo tank. With the intermediate valves open, one can discharge the entire cargo tank with one pump.
Fully refrigerated carriers with membrane tanks are without a centre bulkhead. Such gas carriers are built with a trunk on deck that the membrane tank is formed out of, and thereby reduces the effect of the free liquid surface. Fully refrigerated carriers are generally equipped with the same cargo handling equipment as Semi-pressurised carriers. Some carriers also have coils in the pump sump that is used for liquid free the tank, hot gas is blown through the coils. Some carriers are also equipped with strip lines in the tank that either are connected to ejectors or transportable membrane pumps, this is utilised when loading naphtha etc. Some atmospheric pressure tankers do not have booster pumps or heat exchangers (cargo heaters). Actual cargo for this type of gas carrier is LPG, Ammonia, Naphtha, and some chemical gases, such as, Propylene, Butadiene and VCM. Information of the type of cargo the tanker transports is located in IMO Certificate of Fitness. When atmospheric pressure gas carrier are carrying flammable products, the hold space or the inter-barrier space must have a content of neutral atmosphere with either dry inert gas or dry nitrogen. When carrying non-flammable products, one utilises dry air or dry nitrogen on the hold space. This gas carrier type carries a lot of LPG from the Persian Gulf to the Far East and USA. Ammonia is transported from The Black Sea to USA and the Far East. Advantages: • • • • •
Transports large weight in proportion to volume because the cargo is at all times loaded and transported at atmospheric pressure. Easier cargo tank construction than Semi pressurised tanker Tanks and lines are insulated. Have large cargo cooling plant. Large tankers are more efficient (cargo weight).
Disadvantages: • • • •
Not so flexible for cargo change as Semi pressurised tankers. Pressure limitation, not possible to heat up cargo on route. Carrier without heat exchanger (cargo heater) can only unload at atmospheric pressure (fully cooled). Limited access on terminals and ports with limitations to draught.
3.8.2 Fully refrigerated LNG carriers These gas carriers are special as they are designed for loading gas at atmospheric pressure with a temperature down to –163oC. Fully refrigerated LNG carriers are either built with independent tanks type B Moss-Rosenberg patent with spherical tanks or French patents that utilises membrane tanks. Spherical tanks of Moss Rosenberg patent are built in aluminium. French patents with membrane tanks are built either in stainless steel, 9% nickel steel or ferro nickel steel that have a 36% nickel content. Common for all these steel types is that they have a thermal expansion coefficient close to 0.These gas carriers are built from 20000 m3 to 125000 m3. The largest LNG carriers are, at all times, contracted on basis of long cargo contracts over about 25 years. This is because these tankers are very expensive to build, and are designed for LNG trade. The LNG tankers compete with gas transportation in pipelines on shore, and the sea transport amount to about 5% of the total LNG transport. These tankers are special in that the vapour boil off from the cargo is utilised as fuel to the vessels propulsion. For the large LNG tanker, the vapour boil off is between 0,18% to 0,25% of the cargo capacity per 24 hours. It is possible to produce cargocooling plants for the large LNG tankers, but to cool 125000 m3 LNG about 6000 kW/h is required. This indicates that this is too expensive, and it is more appropriate to utilise the vapour boil off for propulsion. The smaller LNG tankers on the other hand have a cargo cooling plant, and they transport some in LPG/LNG/LEG trade.
LNG carriers have a special procedure for cooling the cargo tanks before loading, which is specified in the tanker’s operation manuals and certificates. The tankers are equipped with a spray plant where Methane is pumped into the tank’s spray line (perforated lines), which is installed inside the cargo tank. Understandably, one must cool the cargo tanks a considerable amount of degrees to be ready to load. One must never begin to load a cargo tank before there is –136oC in the middle of the tank, or by the tank’s equator.
04-
FREIGHTING
FREIGHTING The right to charter has an international colouring. This is naturally because the activity over the world’s oceans is linking countries, nations and traditions closer together and creates a need of uniformity. That is why common regulations concerning transporting products is so important, but still there is a long way to go. Ship trading is risky, and in extreme cases this fact was recognised in the days of sail-ships. In these days the owners of the ships had a limited responsibility due to miscalculations, and where held responsible the limitations to the area accounted for was the value of the ship and the freight. The rules concerning the responsibility during transport gives the answer to how far, and to which extend, the owner is responsible to the economical loss the cargo owner suffers by loss or damage on goods or by delay during the voyage while it still was in the ship’s owner custody. 4.1
HISTORY
In the last century the ship owners signed off any responsibility due to damages during the voyage. Clauses were made to liberate the owners from almost everything. American cargo owners had agreements in the American court of law saying that many of these clauses had no value, in other words: the clauses liberating the ship owners responsibility due to miscalculations and negligence was not valid. The motion was against the hip owners both in USA and Europe. The ship owners themselves decided that something had to be done, and in 1924 as a result of several maritime court conferences, “Bill of Lading” convention was established, called The Haag-Regulations. The Haag regulations was ratified of most maritime trading nations, and this resulted in an almost united regulation of the most important conflicts concerning transporting of goods, as well as the understanding of what a bill of landing shall contain and the responsibility connected to the information about the product. Norway acknowledged the Haag regulations in 1938. Still the need to improve existed and after the conference in Stockholm in 1963, another protocol was developed with some proposals to improvements due to the Haag regulations. The convention changed in constitution and now called the Haag-Visby regulations. The Haag-Visby regulations represent no longer fully and updated international accepted regulations, for instance to general cargo transporting. A new conference was held in Hamburg and the purpose is that the Hamburg rules shall take over for the Haag-Visby rules when they are ratified by a satisfactory number of marine trading nations. 4.1.1 The Conventions decisions Usually the parts in negotiations can request their desires concerning transport of a product or an entire ship cargo, by the commitment in a freight contract. But the participants can not make the deal totally as they want. The regulations in the “Bill of
Lading” has to be followed, and the amount the ship owner have to be responsible to, must be, according to the responsibility rules and not a smaller amount. On the other hand, the opportunity to commit the shipping company to be responsible to a higher amount is available. So, there is no way to avoid the regulations due to the bill of lading and the transport responsibility according to the conclusions in the international Haag-Visby regulations. In addition to the decisions by the court of law about the partner’s deal, the customary practice will play an important role when it comes to how a freight contract shall be understood. When the partners implicated have come to an agreement, a contract is established and ready to sign. This deal is called A «charter party». Such a deal was in the past considered to be a quit simple document. This is not the case any more. A number of the larger charterer have today their own charter party formula. It exists a number of different charter party formulas due to load and trade. It is quit obvious that a charter party for a load of cattle will have to consist of a number of regulations totally different from a ship carrying a cargo of gas or a ship carrying crude oil. In freighting, a number of expressions which is important to be fully aware the meaning of, is used, therefore look at the enclosure list with commonly used expressions in the end of this part of the compendium. As mentioned before a «charter party» is a contract about a transport mission. When the product is taken on board a document shall be issued called a “Bill of Lading”. A Bill of Lading is a document confirming the acceptance of the cargo (product) to carry from one determination place to the other. In bill of lading the ship owner is described as the freighter of the cargo, and the one who go cargo needed to be transported is called the charter. 4.1.2 Freighting in General We shall now go through a little extra about the different ways of freighting, but first let us briefly say a little of what normally happens before the negotiations start. Thereafter we will talk some about the partners involved, the mechanisms in the market, and the shipping language with the expression habits, the rate system, the freight market, and a little about technicalities during negotiations. Two of the partners often mentioned in freighting are the charter one having the cargo and the freighter whom is the ship owner. In a simple way, for example an oil company has a cargo and wishes this cargo to be shipped from the Arabian Gulf to SE Asia. So the company then contacts their broker in London delivering a brief description of the cargo. For example, 4500 metric tons of propylene, which is to be loaded in Al Jubail in the time 17-20th of May. The London broker will then send off a telex with this cargo information and send this telex to his broker associated in various countries, based on this brokers knowledge and considerations due to where hip owners with suitable tonnage is established around the world. These brokers have a wide information of ship types and companies and when the ships are available which is extremely useful and necessary. When it concerns shipping the free marketing mechanism is ruling. Offer and demand control the whole scenario. When little activity in the market, it referred to
as quit and slow. When the market is active, it is referred to as a lively demand. The freighting level will rarely be constant. In a n active market with lively demand the freight rates increase. This due to the fact that both ship owner and the broker instantly read which way the demand is going. Therefore the freight rate gets higher, and because there is shortage of tonnage, the oil company has no other alternate than to pay. Similarly a quit market will lead the oil companies to acknowledge that there is no competition in market, and that ships want contracts, and since several is competing, the oil company can press the market down and have a ship with the lowest freight rate signed up. How far down the rate is pressed is dependent of the conditions. A ship owner will probably not choose to lay up the ship until the freight market is so low that the transporting income does not cover the running costs. In a short period the owners may choose to sail with loss, reading the market and consider this as a temporary down period. It is really extremely expensive to lie up the ships and then break open again. Further on the low rate over a longer period will lead to less contracting. When the market is strong and active, will on the other hand the contracting increase. The same is to be said about buying and selling «second hand tonnage». Later on we will, when viewing the different types of transport forms, discuss the different shipping expressions. The shipping language itself is English, but what make it difficult to understand is all the abbreviations and special expressions. Some of these special expressions will be found in the loading instructions, so we will try to get the most important ones with us. Since the wars ended oil tankers have developed different rating systems, and the one valid these days is called «World scale». In world scale (World-wide Tanker Normal Freight Scale) is abbreviated to WS or W, and we find a number of basis rates for oil cargo voyages between typical disembarkation ports and receiving ports throughout the world. 4.1.3 Basic Rate The basic rate is calculated and intended a standard oil tanker with a loading capacity of 75000 metric ton, and an average speed of 14,5 knots, and a day and night consumption estimated to 55 metric tons HVF (heavy fuel) and 100 metric tons to other purposes, plus 5 metric tons in each port. When the partners have agreed, the contract is written. The brokers then draw the contract, usually on a standard formula called a Charter Party. Usually the charterer’s broker finishes the charter party and often signs the contract on behalf of the charterer. The owner’s broker usually signs the charter party according to received power of attorney after the owner have controlled a «working copy» of the charter party Common procedure is that the charter himself sign the document. Often the owner or his broker who is signing the charter party on the ship owners behalf. The broker as a connecting link is extremely important and must be accurate and see that no mistakes are being made which can cause conflicts leading to claims for compensation. He will often as the negotiations is ending, send a rundown with all the conditions agreed to, so both parts can read through it and confirm the conditions agreed on.
4.2
THE NEGOTIATION
The parts in the negotiations 1. The ship owner is the one offering the ship services due to transport of cargo 2. The charterer is the part who has cargo to transport and need tonnage. 3. The broker is the part negotiating a final contract between owner and charterer.
The parts have start the negotiations, the picture above symbolise only the negotiation, not the negotiation form. Now a few words about the negotiations itself. Naturally no deepen details is mentioned. Let us go back to the oil company which had a cargo of 4500 metric tons from Al Jubail going to SE Asia: The procedure in the negotiations can vary, but in this case the English broker sent the order to several of his broker connections in countries which he meant had suitable tonnage in hand, among others several Norwegian brokers. These brokers passes on the request from the oil company to the different owner connections.(many owners have their own charterer sections). Only few minutes after the Oil Company informed the market about the cargo, the order came ticking into the owners office around the world. The owner «Transporter Gas» has just the oil tanker available, LPG «Seagull» which can load about 4500 metric tons of propylene and is found in the right position to be ready for loading at the requested time. The owner is Norwegian and it is morning, and no especial time difference between London and Oslo, so the owner is ready to set up LPG «Seagull». The owner work out the conditions and makes an offer which the Oslo broker send via London to the oil company. This offer stand on hold for
about half an hour in our example, «subject reply within 1230 hours», this is due to the fact that a new opportunity is likely to occur sudden in the market, and rapid replays is necessary. Such a primer offer is usually very short , there is no negotiations yet, but it is a way to check the interest in the market for business, «subj. Details and C/P conditions». Ref. your order Ras Tanura - Europe pleased to offer you: Subj. Replay here 1230 hrs. Oslo time. LPG «Seagull». 8250 metric tonn dwt. on 8,2 mtrs. Summer draft. (followed by a short description of the ship). Loa/beam: Blt: Tank cap: Last 3 cargoes: Cargo: Segregation. Load: Disch. Laycan: Rate: Demurrage: Haag-Visby: GA/ARB: WSHT: TTL(total) CP form:
126,1m/17,8m. 1982/class GL/flag Norwegian. 8073 cbm. (98%). Propylene. Min. 4500 metric ton up to full cargo. Max. 2 grades within vessel’s natural segr. One safe berth Al Jubail. One safe port SE Asia. May 17/20 1994. 68 USD/mt. USD 14000 pdpr (per day pro rata) Haag-Visby rules. (general average, arbitration) London, English law to apply. (worldscale`s hours terms)-72hrs. Laytime. 2,5% commission on FRT/DEM. ASBATANK (Tanker Voyage Charter Party )
Sub further terms details. The Owner (company) will now wait in excitement for replay, so that a “counter” is received, meaning a counteroffer. May be nothing happens. Other shipping companies may have reacted even faster with a replay, a ship with better equipment and position or a better previous cargo history, that means what cargo the ship has been carrying on previous voyages. It can also happen that another owner offers a lower freight rate. But any way, in our example, this ship is favourable to the charterer. Mostly of our offer is accepted and we receive the following telex: Subj. Reply London 1245hrs. Accept discharge range. Rate: 62 USD/mt. DEM: USD 9000 PDRP
Naturally, in the cards played one has to compromise, and after negotiating to and from one agrees that the owner and the charterer commits to a rate on 62 USD/mt and a DEM 9000. When the negotiations are completed, the charterer send a rundown, where the total contract is confirmed. The charter party is written later. The parts involved have committed themselves on the basis of negotiations and the rundown which have been read and accepted, but there is always written a charter party , and this is functioning like a contract between the owner and charterer. Earlier this was a relatively simple document written in a few lines describing the ship, cargo, freight and the voyage. Today a charter party is a extensive document with standard clauses which are supplied with several additional clauses (riders) which are fastened to the charter party. These additional clauses is regulating the charterer special needs or other more practical conditions connected to the transport. Further on it can be clauses concerning responsibilities damaging the cargo, shut downs, clauses of war, dividing expenses and other information about the ship, which will be of interest to the charterer. 4.3
BILL OF LADING
Choose to enlighten the document Bill Of Lading before going further into the other variants of charter parties. A bill of lading is defined as a document consisting of: A confirmation from the owner on the acceptance of specific goods, cargo to transport (carry). A promise to transport the goods to a certain destination. A promise to deliver goods at a determinate place in return of the document “Bill Of Lading”. After receiving the cargo, the ship captain or the vessel’s agent issues a Bill of Lading. This will be handed over to the shipper so that the cargo can be delivered to the one holding the original Bill of Lading (the shipper or the buyer). This third party is the one who owns the cargo and is the receiver. We have different types of Bill of Ladings, but the regulations are the same. When the cargo is transported by an oil tanker, the cargo owners, after checking when the ship is ready to embark, deliver the cargo to the tank company’s “Warehouse”. The tank company then issues a “booking note” referring to the company’s Bill of Lading conditions and this ensures that the Bill of Lading is issued when the cargo has been loaded. From time to time, this “booking note” is skipped over. The cargo is then simply received with a confirmation of the Bill of Lading, which confirms that the cargo is received for transport. If the cargo is loaded on board a known and named ship, the Bill of Lading will be an “Onboard” Bill of Lading. A Bill of Lading can also state that the transportation may be executed with more than just the owner’s ship. This Bill of Lading is called a “Through” Bill of Lading. A document for the whole transport can be issued, when transport is executed by car, ship, or railroad. This is called Combined Transports and the document is called a “Combined Transport Document.” Along the Norwegian coastline no Bill of Lading is used, but a Freight-bill is issued instead. The strict rules for Bill of Lading responsibilities do not apply to the Freightbill. In special areas, like the Northern Sea and in North Atlantic trade, a Bill of
Lading is not always used. However, a document referred to as “Waybill” is used and this document is not enforced as a Bill of Lading. The Bill of Lading shall state the day and place where issued along with the shipper’s name. In addition, a recipient Bill of Lading will state the destination for cargo delivery. An Onboard Bill of Lading will provide the ship’s name, nationality and location where the cargo was loaded. Further, if the shipper demands, the following will be noted: The type of cargo and either the cargo weight measure or quantity of goods, based on the shipper’s written task. The shipper will deliver the necessary identification marks in writing, before the loading starts, provided that the marks are clearly indicated on the cargo. The visible condition of the cargo. On the receiver Bill of Lading, the day receiving the cargo. On the Onboard Bill of Lading, the day when the loading ended. Where to and to whom the cargo will be delivered. The size of the freight and the other terms related to the transportation and the cargo delivery. The owner must be critical and should, of course, in no way avoid controlling the shipper’s information, if there is any reason to suspect they are incorrect. If this is the case, he must control the information himself to ensure accuracy. The shipper is accountable to the owner for the accuracy of the cargo information regarding his task or as requested in the Bill of Lading. As receiver, the shipper can in the Bill of Lading suggest a special person, order or holder. The person standing as a receiver can give this right to another who can demand the cargo be delivered to him. A Bill of Lading is issued in as many copies as the shipper demands, but the number should be referred to in text and the wording should be similar. The regulations concerning the Bill of Lading responsibility are very strict. This is because a Bill of Lading is a negotiable document - a document which represents the cargo (the cargo’s ID card) and which can be negotiable in form of buying and selling several times during the ships voyage. Therefore, the information in the Bill of Lading must be correct. The buyer of cargo, which is under transportation or scheduled for transport, has paid for the cargo and in return was granted the Bill of Lading. Upon arrival the buyer receives the cargo upon presenting the Bill of Lading. A Bill of Lading functions as a certificate that the cargo is as described in the document. Protests from the owner to the shipper cannot be set in force if the Bill of Lading has been acquired from a credulous third person. When the person at the receiver location shows a Bill of Lading, he is regarded as the legitimate receiver of the cargo. It is enough that one Bill of Lading is presented where several are issued, if the others are delivered to their right owners. (This will be viewed on the next page, “Indemnity Clause”). If more receivers appear and can legitimate themselves by presenting examples of the Bill of Lading, the cargo will be held back until the correct receiver is found.
We already have mentioned that the Bill of Lading represents the cargo, and that this is valid as evidence -confirming- that the owner has accepted and loaded the cargo when no other information is available. Counter evidence cannot be produced if the Bill of Lading has been acquired in good faith from a third person. If an owner knew, or ought to have known that information about the cargo was incorrect, there is no way to be free of responsibility. The Bill of Lading must in such a case show that this information proved otherwise. The owner is also responsible to the third person if cargo damage or cargo lack has been kept hidden without being noted in the Bill of Lading, even when the owner should have discovered this information. So, if the third party involved suffers loss when cashing in the Bill of Lading, trusting that the information stated is correct, the owner will be responsible. This is true when he has or should have realised that the information in this document was incorrect, and could have caused misunderstandings for a third party. Law in its execution forces Haag-Visby regulations. There is no way to avoid any responsibility for the conditions. The conditions in the Convention concerning responsibility for loss and damages is not an impediment for changes in the preserving of the cargo and the handling before loading and after discharging. The owner, or someone he answers for, is responsible for losses regarding his obligation to keep the ship in seaworthy condition at the start of the voyage. The Haag-Visby regulation’s Bill of Lading paragraph does not apply to freight contracts. However, if a Bill of Lading is issued under a charter party, it must satisfy the conditions set in the Convention. The situation will then be to judge, based on the Haag-Visby regulations for all Bill of Lading issued for transport of a cargo, between two flag states when: The Bill of Lading is issued in a country which has ratified the Convention, or: The transport is from one port in a country using the Convention already, or: The contract contains information which confirms that the country, according to its Convention rules or laws, have agreed to use the Convention in the transport situation. When a ship is time chartered, the charterer will request that a Bill of Lading is issued. The same regulations concerning responsibility is valid as long as a document is issued. Also, the captain must make sure that no Bill of Lading is signed which could lead to claims of responsibility against the owner which was caused by incorrect description of the cargo. Here the same sort of difficulties as mentioned earlier can occur, therefore any remark concerning the cargo and its condition etc., must be noted in the document before signing it. A charterer will press a captain to issue a clean Bill of Lading regardless. Here the time charterer is important. A large reputable company can accept a “Back Letter” or “Letter of Indemnity” as satisfactory, but be careful and leave the decision to the owner. Be aware, that P/I companies do not cover the owner’s loss if something goes wrong. The owner’s would have to face claims of responsibility as a result of delivering cargo without having accepted the necessary Bill of Lading from the receiver, as required by law. When delivering cargo, the captain must ensure and control that the cargo is exclusively delivered to the one presenting the original Bill of Lading. If another receiver than the one presenting in the Bill of Lading appears, it necessary to
examine if the person involved is right according to the transport declarations (notes added on the back side of the Bill of Lading, will confirm this). Besides, the ship’s captain must respond similarly due to the Bill of Lading, both when on time charter party or travel charter party and in the owners regular trading. 4.3.1 Indemnity Clause This can cause some bother. If the original Bill of Lading cannot be produced, then the owner has a duty to deliver the cargo in exchange for a “Letter of Indemnity” (a written guaranty from the charterer). This is nothing like a bank guaranty, and cannot always be considered satisfactory security for the cargo delivery. Some oil companies use this, especially in short voyages. The owner’s instruction must, in these cases, be followed by the captain in charge. The ship’s captain has to show the utmost of caution when treating the loading contracts, especially the Bill of Lading. Any doubt whatsoever, remember to follow the owner’s instructions, prospectively contact the owner’s charterer office for further information. 4.4
FREIGHTING FORMS
A charter party is a document, which is a written confirmation between the owner and a charterer about a commissioned transport. Several standard charter party outlines exist concerning different types of freighting and their single type of cargo. Large charterer usually have their own formulas with specific conditions added. Be aware that the charter party outline also can be divided in sub groups, however this will not be viewed her. The central issue in any charter party is who the owner is and who is controlling the ship, the owner or the charterer and in some situations this can be hard to determine. 4.4.1 Voyage Charter Party Voyage chartering is probably the most common type of freighting. A number of charter party formulas exist that are used. There will, in some cases, be clauses in the document, which almost completely favour of the charterer. Here the shipping company (owner) watches out for this and makes an effort to improve the terms. Whether this turns out to be a success is no guaranty, frequently there is a situation described as “take it or leave it”. But in an active good market, with a shortage of ships available in the right location, then better conditions can possibly be negotiated with success. If the market activity is slow, the solution will be to accept the terms and be happy that the ship is in trading. To get a more neutral charter party formula and better adjusted to the interests of both parties, two organisations have been especially working hard to accomplish standard charter party outlines. That is The Baltic and the International Maritime Conference (BIMCO) in Copenhagen and the English shipping organisation Chamber of Shipping of United Kingdom in London. The best-known voyage charter party within the tank trade is “Tanker Voyage Charter Party”. Most of the oil companies have their own charter party formulas which, broadly speaking, follow each other. In this freighting form (voyage charter party), the owner is responsible for the operating expenses including bunker costs, as well as, the load/discharge port
expenses which are specified in the charter party, unless other terms are agreed to. Read the charter party. In voyage freighting, the charterer makes an agreement on the freight before each single voyage. The ship is paid based upon the transported quantity. We will view the most important facts in the charter party negotiations. We will look at a typical voyage charter party and go through the most important content. As you will notice, this is divided in two, Part I and Part II. The conclusions included in Part I have higher priority than the conditions in Part II. According to common interpretations practice, the written word is preferred over the printed, if these should come in conflict to each other. In the end of this chapter you will find a copy of a voyage charter party “ASBATANKVOY” which we use as the starting point here. Preamble: Here is the information about the partners committed in this deal, as described in Part I and Part II, and which ship this concerns. PART I A: Description and position of vessel: The ship’s loading capacity for the cargo to be transported (on oil tankers it is smart to note the pumping capacity). The loading capacity can be noted in i.e. m3 or in metric tonnes. Here we should remember the expression: % MOLOO or % MOLCO. % MOLOO means “more or less in owner’s option”, in other words, the ship’s captain can adjust the amount of cargo as specified by a % up or down. % MOLCO means “charter option”. The position of the ship and when it can be ready in loading port. B: Lay days/Cancelling (lay/can May 17-20 1994): Lay/can estimates the time frame (window) when the ship can arrive in loading port and be ready to load. If the ship appears ahead of time, it cannot demand to start loading before the lay days start, in this example 17th of May. If the ship arrives later than the cancelling date, in this example the 20th of May, the charterer may cancel the contract or renegotiate. C: Loading port(s) Range: It is important to agree on the load and discharge place, but this can be described in different ways: A determined dock: for example berth 1, Mongstad. A determined port: for example one safe port. A determined area: for example one safe port, Europe. An ordered port: for example Gibraltar for order. Several ports: for example Rotterdam, Mongstad. It is then the owner’s duty to bring the ship to the determined port. In one part of the charter party, we have a so-called near clause which protects the owner from impediments which may arise after the charter party has been agreed to “or so near there to as she may safely get and lie always afloat”. D. Discharging port(s) range: The discharging ports can be agreed to as referred to in point “D”. E. Cargo description: It describes here how much and what kind of cargo the ship should load. MOLOO & MOLCO are possibly used. If several grades are to be loaded
the expression “Within vessel’s natural segregation”, abbreviated to (WVNS), can appear. F. Freight rate: Here the freight rate is given in World Scale or prize pr unit either metric ton or cubic meter. World Scale is viewed earlier in this part of the compendium. The freight incomes in our example should be based on the agreement in the charter party, as follows: (185000 x 62 x 16,50): 100 = US dollar 1,892,550. For cargoes based on price pr. metric ton as in this example 65 USD/mt. We then multiply rate with estimate cargo as e.g. 4500 mt x 65 USD/mt = 292 500 USD. G. Freight payable to: Noted here is the name and address of the receiver of the freight income. Paying currency is also noted. H. Lay time: Normally agreed lay time in loading/discharging port is 72 hours (agreed in World Scale). This time is also dependent on the capacity of the discharging port, not only the ships. We will look closer at this when some of the parts in Part II of the charter party is viewed. I. Demurrage: This is the compensation the owner can claim from the charterer if the charterer use a longer time for loading and discharging than estimated. Demurrage is agreed to when the charter party is negotiated and is likely to be set to a fixed cost per 24 hours. In our example we negotiated a demurrage of US dollars 11500. - per 24 hours.
We here show a demurrage calculation based on the information from a completed voyage charter party: Total laytime according to CP 72 Timer Agreed demurrage according CP 11 500 USD pdpr Loading port Moored at loading port 17.05.94 06:00 NOR delivered 17.05.94 06:00 NOR accepted 17.05.94 06:00 Commence loading 17.05.94 07:00 Loading complete arm disconnected 18.05.94 22:00 Total hours used on loading 39 Timer Total hours NOR accepted to loading 40 Timer complete Discharge port Arrival anchorage discharge port 06.06.94 03:30
Moored discharge port
06.06.94 16:00 NOR delivered 06.06.94 03:30 NOR accepted 06.06.94 09:30 Commence discharging 06.06.94 17:00 Completed discharging arm disconnected 08.06.94 08:00 Total hours used on discharging 39 Total hours NOR accepted to discharging 46,5 comp. Total hours on NOR in both ports 85,5 Total laytime according to CP 72 Demurrage base 13,5 Demurrage claim is then (11500 / 24 x 6468,75 13,5)
Timer Timer Timer Timer Timer USD
Based on these facts, we now know what the charterer has to pay the owner if all delays are to be claimed on the terminals. J.
Commission of: Here it is written the percent owner has to pay e.g. 2,5%.
K.
The place of General average and arbitration: Here it is stated if London or New York rules have to be used to settle claims.
L.
TOVALOP: It is now cancelled
M.
Special provisions: The amount of numbered additional clauses is given here.
Signatures: Here the owner and the charterer’s broker sign the charter party. This was Part I in the charter party. We will take a further look at the clauses in Part II of the charter party. Before looking at Part II, let us be enlightened on the issue “Subjects”!. Often the charterer, before the affair is completed, takes into consideration certain circumstances, i.e. the cargo can be delivered to the loading place (subject stem), that the shipper accepts the ship, that the confirmation is approved be the company’s management or that the cargo is being sold. Usually the agreement is a fact when the agreement is abandoned. Until this is a fact, the owner is bound. If the cargo is abandoned both parties are free.
PART II. Now we will look at some of the clauses written in Part II of “ASBATANKVOY”, you must read the charter party very closely, this is IMPORTANT. Warranty – Voyage – Cargo: (Clause 1) The vessel must have all certificates valid that is required according to PART 1. Dead freight: (clause 3) This clauses puts a claim on the charterer to come up with full cargo for the ship, but if he does not, the owner will suffer a freight loss. Full cargo is determined by what stands in the charter party Part I, A, iii. Therefore, if the charterer does not deliver full cargo the charterer must pay dead freight costs. Dead freight is the freight which compensates for the difference between the cargo the ship could have loaded if the charterer had supplied full cargo versus the real cargo the ship actually received. If the owner has had advantages like less load/discharge expenses and shorter time in loading and discharging, then this will be considered when calculating the dead freight. The owner should not be better off than he would have been with full load. If the charterer does not deliver cargo at all, the freight received is called fault freight. For example, if our cargo is described at 4500 metric tonnes 5% MOLOO, the maximum cargo the captain can request (4500 : 100 ) x 105 = 4725 metric tonnes. The charterer will not give the vessel more cargo than 4725 mt even if the captain say he can take e.g. 4925 metric ton. When not receiving cargo the ship is booked for, the captain has to deliver a written protest in the load port, and make the shipper aware of the ship’s capacity to load more, and at the same time calculate how much cargo is lacking. In our example, we will find the dead freight claim based on the charter party. Received cargo is 4650 metric tonnes. The ship can load 4725 metric tonnes based the charter party. The dead freight base will be: (4725 - 4650) = 75 metric tonnes. Naming loading and discharging port: (clause 4) Charterer have to name the load port 24 hours before the vessel readiness to sail from previous discharge port, bunkering port or when the charter party is signed. Laydays: (clause 5) Commence of laytime does not start before the date and time stated in Part 1. If the vessel has not delivered NOR before 04:00 PM the cancelling date the charterer have option to cancel the chart. Notice of readiness/Running time: (clause 6) This is the message, which is given when ready to load/discharge. NOR is not provided until the lay day (agreed to in Part I) starts. NOR is given when the ship arrives and is shown the waiting place. The lay time starts to run 6 hours after NOR is sent. If the ship goes straight to port and starts loading/discharging, the lay time starts to run even before the end of the first mentioned 6 hours.
This will be specially noted in the charter party. It is important that all hours, concerning sending and receiving of NOR, is correct. Running lay time is cancelled in the following situations: • • •
From waiting place to load/discharge place. Ballast handling (when this prevents load/discharge). Time for stops which owner and ship causes.
Time for tugboat, pilot, strike, etc. Lay time: (clause 7). The determined number of hours for loading/discharging is written in this clause. Note specially here “and all other Charter’s purpose whatsoever”. The lay time only counts as long as the ship’s loading/discharging capacity is fully used. If an impediment is caused due to the charterer’s responsibilities, the time is counted. Demurrage (clause 8): This is discussed on page 3 in this part. Safe berthing/ Shifting: (clause 9) The charterer can rightfully shift the ship within the limit of load/discharging port. The freight does not cover this, but the lay time continues to run. The charterer must cover running expenses. Pumping in and out: (clause 10). Cargo is loaded on the charterer account, when the cargo is received onboard it is on the owners account. Ice: (clause 14). The ship should not trade in ice or be forced to follow an icebreaker. The important point here is that the ship's captain follow the nominated ports at all times. Read this text thoroughly. Quarantine: (clause 17). Delay in time caused by quarantine is counted as lay time, if the quarantine was in force at the time the charterer nominated the port. Agency: (clause 22). The company in the nominated load/discharging ports must use the charter agent. These agents should be considered to be the owner’s agent and is paid by the owner. Oil pollution prevention: (clause 26) The owner shall ensure that the ship captain is performing the following: 1. Everything based on MARPOL 73/78, particularly Regulation9/Chapter 2 of the International Convention for the Prevention of Pollution from ships 1973. 2. Oil and oily water should be collected in the ships slop tank while cleaning tanks, and after a maximum settling time the separated water is pumped over board as stipulated in MARPOL 73/78. 3. There after the charterer should be informed of the amount of oil and water which is left on board in addition to details concerning slop left over from
earlier voyages, called collected washings. The charterer has the right to decide whether this slop will be delivered ashore or be kept on board to eventually be loaded on top of this (LOT.) Bills of Lading: This clause is extremely important and comprehensive and has to be studied carefully. See the section of Bill of Lading chapter 3 in part 4 This was some of the printed clauses in Charter party ASBATANKVOY. The type of charter party vary from company to company. In addition to the printed clauses we have option to write specified clauses.
4.5 TIME CHARTER PARTY Besides voyage freighting, the time charter party is the most important form of chartering. Here the charterer hires a ship for a certain period of time, for example 1month, 6months, 2years or 15years. The time period can vary, but the principle will remain the same. In the chartered period, the time charterer can freely dispose the ship for his purposes within the frames which the charter party contract is drawn. He supplies cargo, instructs the ship, pays the variable expenses such as bunkers, canaland harbour costs in load/discharging port. The owner is paid for the time the charterer uses the ship, either at a certain freight rate per tonnes dead weight per month, or as regular rate per 24 hours if it is a short termed time charter, or as a rolled-out sum per month or per fortnight. One can say that time charter is not quite as exciting as voyage chartering. If the ship is at sea, running satisfactory, the owner will have secure income, but the opportunity to make large profits if the activity in the market suddenly explodes, is shut off. In association with an expensive new building, an owner is often interested in getting a long-term charter party for the ship. That will ease the business financing and offer security, especially in the first years of a ships active time. In long-term charter parties it is common practice to have a clause protecting the owner completely or partly of rising running costs (Escalation Clause). There are several reasons for using the time charter. The regular shipment of large quantities of goods, for a special oil company, can be one reason. Others can be covered by using own tonnage, some will possibly be covered by time chartering available ships and the marginal need will be covered by voyage freight. An owner may also have faith in a rise in freight rates and charter a ship for i.e. 6 months. When it is a success you earn money, when you fail you lose money because you do not get covered for the time freight or the variable expenses. Time chartering is a way to take advantage of the market, if the freight income is higher than the time charter rent you are paying. Just as in the voyage charter party, the time charter party has many different formulas. During a time charter party, it is important that the description of the ship is accurate. The characteristics of the ship are of particular value for the time charterer, and the time charter (freight) he will pay. If it is discovered that the owner has given incorrect information concerning the ship, or the ship does not fulfil the description, the charterer can demand a reduction or compensation for the damage he proves to suffer. In situations where the error is essential, the charterer can cancel the contract and demand compensation. With a time charter, a discussion most likely to appear, concerns the ships ability to load, load/discharge equipment, speed and the consumption of bunkers. A charterer will usually watch closely and make sure the ship redeems the claims in the charter party. Usually the time charter will be determined due to the dead weight of the ship indicated by the summer marking, and especially when it comes to talking about heavy cargo like oil. The time charterer pays freight for the time disposal of the ship. So the ships speed and consumption is of great importance. The time charterer operates the ship and pays the bunkers expenses. Over the years, the discussion has been endless concerning speed and consumption. This is likely to continue as long as time charters are involved. The owner is seeking
the business and has, therefore, described the ship as positively as possible, but since the charterer is paying he will watch the consumption from month to month.
A time-chartered ship is rented fully crewed and equipped, and the time charterer shall operate the ship. The captain’s position on board can also appear difficult in this connection. The owner employs him for the responsibility of the ship handling, navigation and security. He also will take orders from the charterer concerning loading, discharging and sailing. The charterer will of course be interested in a quick load, quick sailing, and quick discharging. This can be a heavy load and interfere both with the safety and the conditions of the ship which may not always be in the interest of the owner. It is of extreme importance that the ships captain is not influenced by the charterer on behalf of the owner, but still within reasonable limits try to maintain a healthy relationship with the charterer. This relationship is very important. In voyage freighting the owner pays all expenses connected to the voyage. In a time charter it is different, and therefore it is important to know whether the expense is put on the charterer or the owner’s bill. The owner company is obligated to have crew and a seaworthy ship at the charterer’s disposal. The owner will be responsible for the following expenses: • • • • • • • •
Salary and other crew expenses Food Ships insurance Deck-and machine accessories Lubricating oil Repair expenses/dock setting Classification expenses Interests, part payments and administration
The following expenses will be paid by the charterer: • • • •
Bunkers Port expenses Load/discharge expenses Pilot and canal expenses
The ship is delivered when the owner can present the ship to the charterer, in a port or out at sea at a determined position and on a determined date. The time charter is payable from the moment the ship is delivered. If the delivery is executed in a port the owner will, if nothing else is agreed to, pay the incoming expenses while the charterer is paying the outgoing expenses. If the ship arrives at delivery port in ballast, the usual procedure is to charge the charterer all the arrival expenses from the time the ship embarked the pilot on board, if nothing else is agreed to. Upon delivery time, it is important to check that all bunkers are surveyed on board. This is
the owner’s property and must be paid for by the charterer. Usually both parties sign a delivery certificate. 4.5.1 Delivery Certificate During the charter party’s valid time period, the charterer may use the ship as he wishes and may plan voyages wherever he wants. This can be done within the limits set by the charter party. The charter can be limited by geographical limitations, ice conditions and wars, which affects the use of the ship. A ship can be used for worldwide trade, but with the enclosure “within institute warranties limits” (I.W.L.). When ordering a ship to areas where additional insurance are to be paid, the owner should preserve these expenses to be charged by the charterer. The time charter has a set duration and is then returned to the owner. Some difficulties can arise, because it is hard to schedule a ship to be delivered on date, so a little room has to be allowed. For example, the charter party can add a clause saying “14days more or less in charterer’s option” or something similar to this. The market is of vital importance regarding the choice, made by the charterer, to return as soon as possible or as late as possible. The charterer should use the ship for its missions in the determined period. If the charterer cannot use the ship, because of breakdown or other delays the owner is responsible. The ship will appear in “off hire” and then the charterer does not pay freight until the ship is operating again. Bunkers, which are consumed in an “off hire” situation, have to be paid by the owner. The off hire period is not added in the charter party unless expressively agreed. Usually the time of return will be noted. The owner will be very interested in this in order to plan further activities for the ship. At return a Redelivery Certificate is signed which will note the time, date and remaining bunkers at delivery. When the charterer has kept the ship beyond the reasonable agreed time, it is called “overlap”. When overlap occurs the owner can claim market freight for the time lost, if the market freight is higher than the time freight. In extremely late re-deliveries, compensation can be claimed, if the charterer causes cancelled business, which would have earned profit above the market level. If not specially determined, the time freight is due to be paid in advance, as soon as the ship is delivered on time in the right place, and fully equipped according to the charter party. The payment is usually in advance, monthly or in 14 days termin. If the captain is instructed not to sign the Bill of Lading, this is a successful way to pressure an unwillingly charterer. Therefore it is especially important to examine the time charterer’s financial capacity in long-term charter parties. A middle sized or middle solid charterer can be influenced to get a bank guarantee, i.e. for 4 months freight in advance. Upon re-delivery, the ship should be in the same condition as at the delivering time, except from the usual, normal fair wear and tear. With this I hope you have got some understanding of a time charter party.
4.6 CONTRACT OF AFREIGHTMENT (COA) The third charter form is the freight contract (transport contract) or the quantity contract which is almost similar to the voyage charter party. This is a contract for the transport of large quantities of oil, ore, grain, etc. over an agreed period of time. During such a transport contract the owner company is not obliged to use a special ship, but the owner accepts to make a suitable ship available, within the limits of responsibility drawn in the contract. The charterer is obligated to deliver the cargo. The afreightment contract is usually an agreement between owners of big quantity cargo and transport companies as big ship owners. The advantage for a charterer is the covering of the shipping program with only one company or group, and in this way he avoids having to find a charter for every shipment. He knows that in the contracting period, when selling the cargo, what the freighting element will be. Furthermore, the charterer can, in some situations, negotiate a freight contract in a time when he expects the future rate to rise. The ship owner however may believe that the activity in the market is sinking, and can in this way secure himself against down periods. He can go in the market and include tonnage in time charters or voyage charters at a lower freight than he himself receives and be compensated by the difference in income, if he is short of tonnage. The afreighting contracts are often valid for a rather long period of time and large quantities. An example, 1.000.000.- tonnes oil pr year in a period of 2 years. It is easy to realise that in a long-term transport contract many unforeseen situations may occur and the common procedure is, therefore, to make clauses in the contract to protect against these unforeseen situations. So a freight contract stipulates no special ship and the owner can freely use any suitable ship to execute the transport mission. Usually the loading dates are determined within certain marginal (windows) and then it is important to follow these and the determined quantity accurately. In addition, it is especially important to obey the reporting regulations at all times. Every single voyage, under a COA (Contract of Afreightment), is provided with the regulations commonly found in a voyage charter party. Very often the shipments are carried out regularly through the time period (fairly spread out), but occasionally the activity in shipments vary with the season, etc.
4.7
DEMISE CHARTER
The fourth charter form is demise charter. This form of charter lets the charterer rent the whole ship from the company. Demise charters is, for example, where the renting contract says the charterer shall pay all expenses concerning, crew payments, maintenance and all running expenses. Just as in the time charter, the freight is pre-payable per month in a demise charter. The charterer uses the ship in the chartered period as if he is the owner and pays all expenses and maintenance in the period including the crew expenses. The crew is hired by charterer and employed by charterer and not by the owner. In juridical sense a demise charter is an owner.
Occasionally, the company reserves the right to insure the ship or at least the charterer. If the charterer will arrange this before hand, then the terms must be accepted regarding the insurance amount and the conditions. The company has to make sure that the demise charter insures the ship responsibly and with a first class insurance company. Usually only the owner will pay the capital expenditures relating to the ship, but other arrangements can be made. Some times the charterer reserves the right to have some of their own senior officers on board the ship. The expenses connected to this are agreed to before hand. The demise charter is usually a long-term contract, and often in combination with special finance and sales contracts. Often the charterer has an option to buy the ship at the end of the charter party period. In other cases, it can be agreed that at the end of the charter party period, the ownership passes to the demise charter. With the demise charter, the demise charterer has bought the ship, paying the freight in monthly terms. This has two elements - a freight element and a buying element, which is already included in the monthly hire. Now the charter party forms are viewed, and here we choose to enlighten you on the word “Seaworthiness” as a conclusive and very important part of negotiated contracts.
4.7.1 Seaworthiness The owner is obligated to make a completely seaworthy ship available, if nothing else is noted. Seaworthiness cover many areas we do not have in mind daily. The following will illustrate “seaworthiness”: · (Cargo-worthiness) the ship is able to receive cargo at determined date and time · Sufficient amount of bunkers, and quality · Load/discharge equipment should be operative · Authority Regulations and Classification companies must be satisfied · Complete and fully qualified crew · No heating coil leakage, or leakage between cargo tanks · Seaworthy at the departure load/discharging ports If the charterer discovers lack of seaworthiness before a voyage he can demand this to be improved within reasonable time, or in the worst case, cancel the contract. When the voyage is started and lack of seaworthiness is discovered, the charterer can not cancel the contract, but will then claim compensation for losses due to the lack of seaworthiness. Still the assumption of delivering cargo from the charterer is that the ship is seaworthy.
4.7.2 Cargo samples Make sure that samples are taken of the cargo. This is important, in case of requests later on concerning the quality of the loaded oil. Keep samples as evidence, and be prepared in case claims from the receiver should eventually come. The samples should be kept on board at least 12 months. Usually, samples of the cargo are delivered on board and then to be handed over to the receiver of the cargo. Be quite sure that the samples are representative of the actual cargo, before you sign any receipt approving the samples. (For receipt only). 4.7.3 Frozen in If the ship is icebound because of the charter’s orders to lay in this specific port, the charterer pays the time loss. 4.7.4 Maintenance The owner of the vessel pays for classification, etc., all maintenance like docking period and if the ship goes off-hire.
05- Chemistry And Physics
BASIC ORGANIC CHEMISTRY Organic chemistry mostly deals with chemical compounds containing carbon. 5.1
THE PERIODIC SYSTEM
The periodic system is built on the principle that the electrons in the outer shell determine the chemical properties of a material. An atom consists of protons, neutrons and electrons. Protons and electrons form the atomic nucleus. The electrons move with high velocity around the nucleus, at different levels and orbital. The levels are numbered from K to Q and called electron shells. At maximum, there can be 8 electrons in the outer shell. There are equal numbers of protons as electrons in an atom, meanwhile the number of neutrons may vary.
The periodic table arranges the 106 elements in increasing number of electron shells. Each vertical column is one of the periodic table’s main groups. The number of electrons in the outer shell is always equal to the atom’s main group number. Two of the main elements in the periodic system are Hydrogen and Helium, and fall under group IA and VIIA. The atomic models are illustrated as follows: Carbon falls under group IVA and has 4 electrons in the outer shell.
The elements in the group IA have only one electron in the outer shell.
It is therefore easy to emit one electron to elements within group VIIA, which has seven electrons in the outer shell and is “short of” one electron to fill up the outer shell. Such mutual sharing of an electron is called ion bonding. An example for such a bonding is when Sodium (Na) and Chlorine (Cl) bond with one another and form Sodium chloride or cooking salt. Na + Cl
Na+ + Cl-
NaCl
Sodium “emits” the only electron to Chlorine, and is thereby positively charged. Chlorine “receives” the electron and is thereby negatively charged. We call this mutual sharing of electrons, covalent bonding. Covalent bonding is common in both organic and inorganic chemical reactions. When two or more atoms bond together, they form a molecule. There are 8 side groups between the main groups IA and IIA. All the elements in the side groups are metals, and they easily form alloys with one another. The rows in the periodic chart indicate the periods. The 7 periods indicate the number of electron shells. Sulphur is located in row 3 (period number 3) and has thereby 3 shells. We also look at the electron shells as the electrons’ energy level. The elements in group VIIA are named noble gases. Noble gases occur only in atomic form.
Most inorganic elements are metals. The metals form metal bonding where the atomic are organised close together. The individual element has numbers from 1 to 106. The periodic system’s number is the element’s atomic number. The atomic number also indicates the total number of electrons in the atom.
5.1.1 Carbon You find the element Carbon in the main group IVA/period number 2, which has four electrons in the outer shell number 2. The atomic number for carbon is 6, which means there are totally 6 electrons divided between two electron shells with 4 electrons in the outer shell, and 2 electrons in the innermost. There are many isotopes of carbon. Isotopes have the same number of protons, but different number of neutron in the atomic nucleus. There are two natural forms of Carbon, graphite and diamond. Carbon is not particular reactive in room temperature. When heated, it will easily react with for example, Oxygen. We say that carbon is combustible. The different products of the combustion are dependent of accent to oxygen. C + O2 = CO2 + 393 kJ (at complete combustion) (at incomplete combustion) C + 0,5 O2 = CO + 113 kJ Both reactions are exothermic, that means heat is produced in the chemical reaction. Both reaction products are also gases. Carbon dioxide, CO2, is the product of complete combustion of carbon and carbon monoxide, CO, which is the product of incomplete combustion of carbon. A partly incomplete combustion produces both less heat and more formation of carbon monoxide than a complete combustion. Carbon monoxide is odorless and a very poisonous gas that always is present in a real combustion process. Inert gas produced in an inert gas generator or flue gas plant onboard will always contain carbon monoxide due to incomplete combustion, especially when the air excess is reduced. Poisoning of carbon monoxide occurs because the hemoglobin in the blood reacts much easier with CO than with oxygen. When you breathe a mixture of these two gases, CO is thereby first absorbed in the blood and seizes the absorption of oxygen. The result of this poisoning is a sort of suffocation at very low concentrations. These relations are very important to notice. You must always check the cargo tank atmosphere for carbon monoxide before personnel are allowed to enter the tank. 5.2
HYDROCARBON GROUPS
Hydrocarbon is a common expression for all chemical compounds that includes carbon and hydrogen. You find the element carbon in only two different natural conditions, as graphite and as diamond. Carbon is the element that naturally forms most natural chemical compounds. It is not reactive in room temperature, but it will when heated up react more easily with, for example, the oxygen in air. We say that the carbon is combustible. The combustion is exothermic, which is a reaction that produces heat. Hydrogen is the smallest main element. The gas (H2), is light and is flammable in air. There are small quantities of hydrogen in free natural form on earth. Hydrogen is strongly widespread, first of all in form of water and naturally compounds together with carbon. Crude oil and natural gas consist mainly of a mixture with various unequal hydrocarbon compounds. Following sketch indicates an example of a natural gas’ composition:
Carbon has four electrons in the outer electron shell that can be divided with others. You may look at the four electrons as four “arms” that can connected to the hydrogen atom’s single “arm”, and creates hydrocarbon compounds.
Some of the hydrocarbon compounds are naturally created; other are only created in chemical controlled processes. To simplify the overview of these natural components, and all new hydrocarbon compounds that is created in the petrochemical industry, the different hydrocarbon compounds are grouped dependent of how the “arms” or the chemical bonding are between the two atoms. The most important hydrocarbon groups are: · Alkanes, also called Paraffin’s · Alkyls · Alkenes, also called Olefins · Alkynes, also called Acetylides · Alkadienes, also called Di-olefins · Cyclo-alkanes · Arenes · Alcohol · Aldehydes · Ketones
In addition to above listed hydrocarbon groups there are others like Carboxylic acid, Esters, Ethers etc.
5.2.1 Alkanes Alkanes are the simplest hydrocarbon compounds and is the major part of crude oil and natural gas. The carbon atom’s four arms are united to the hydrogen atoms’ single arm and has this general molecule-formula:
CnH2n + 2
where “n” is a positive integer.
All alkane compounds have the ending “-ane”. The gas methane is the smallest molecule, and is the main component in natural gas. A methane molecule consists of one carbon atom and four hydrogen atoms.
By adding one carbon atom and two hydrogen atoms to methane, we get ethane, which is the next component in this group. By adding carbon atoms and hydrogen atoms, and at the same time maintain the same simple form of binding, new alkanes are formed. The third component in the alkane group is propane, C3H8.
When the number of carbon atoms increase, the number of possible bonding between the atoms increase. You can arrange 20 carbon atoms and 42 hydrogen atoms in 366319 different ways.
Many materials may have the same molecule formula, but the properties (boiling point, density, etc.) are different because the atom structure is different. Such bonding is called isometric bonding. Normal-butane and iso-butane are examples of isomers where both have the same molecule formula, but different properties.
n-Butane, C4H10
iso-Butane, C4H10 Chemical formulas and names are many times derived from each other. Pentane is derived from the Greek word “pent”. That means “five”, it refers to the number of carbon atoms in the material. Other names like methane and ethane are not following this system. These names are called trivial names.
In the following list, some of the most common alkanes are listed with melting- and boiling point at atmospheric pressure. Note that melting point and boiling point increase by the length of chain for the straight-chained hydrocarbons.
CH4
Melting point o C -182,5
Boiling point o C -161,6
Number of isomeric compounds 0
Ethane
C2H6
-183,2
-88,6
0
Propane
C3H8
-189,9
-42,5
0
n-Butane
C4H10
-135
-0,5
2
iso-Butane
C4H10
-145
-11,7
n-Pentane
C5H12
-130
36
3
n-Hexane
C6H14
-95
69
5
n-Heptane
C7H16
-91
98
9
n-Octane
C8H18
-57
126
18
n-Nonane
C9H20
-54
151
35
n-Decane
C10H22
-30
174
75
Name:
Formula
Methane
5.2.2 Alkyls If one hydrogen atom is removed from an alkane molecule, an alkyl molecule is created. The different compounds are named by the alkane, but with the ending “-yl” instead of “-ane”. The general molecule formula for alkyl groups are: CnH2n + 1 The compounds in this group are chemical products where the CH-group is attached to various alcohol and chloride compounds. 5.2.3 Alkenes You do not find alkenes in the natural forms. These compounds are produced in a cracking process within the petrochemical industry. Alkenes are hydrocarbons with a double bonding between two of the carbon atoms. The general molecule formula for alkanes is:
CnH2n The simplest alkene is ethylene, C2H4, that is produced by cracking of for example propane, ethane, butane or naphtha.
The next alkene is propylene, C3H6, which is produced by cracking other hydrocarbons or naphtha. The alkenes are so-called unsaturated hydrocarbons. The double bonding may easily loosen up, “arms” that are attached to several hydrogen atoms released, and the alkenes may change back to (chemical reaction) alkanes.
Name:
Formula
Ethylene (ethene) Propylene (propene)
C2H4
Melting point o C -169
Boiling point o C -103,7
Number of isomeric compounds 0
C3H6
-185,2
-47,7
0
1-Butene cis-2-Butene
C4H8 C4H8
-185,4 -138,9
-6,3 3,7
4 4
trans-2-Butene iso-Butene
C4H8 C4H8
1-Pentene
C5H10
-105,6 -140,4
0,9 -6,9
4 4 6
The number of isomeric compounds increase by the number of carbon atoms. Double bonding also gives additional possibilities for combination because the double bonding may be located on several different places inside the molecule. The following molecules have the same molecule formula, but different structure and thereby different properties. Notice the difference between a cis-bonding and a trans-bonding. 1-Butene
Cis-2-Butene
Trans-2-Butene
5.2.4 Alkadienes Alkadienes are hydrocarbons with two doubles bonding in the molecule. The general molecule formula for alkadienes is:
CnH2n - 2
Propadiene
1,3 Butadiene Name:
Formula
Propadiene C3H4 1,3-Butadiene C4H6
Melting point o C -136,5 -108,9
Boiling point o C -34,5 -4,4
Number of isomeric compounds 0
5.2.5 Alkynes Alkynes are hydrocarbons with a triple bonding between two carbon molecules. The alkynes have the same general formula as for the alkadienes:
CnH2n - 2
Alkynes are unsaturated hydrocarbons, and form a homologous serial. The simplest compound within this group is etyne, C2H2.
Name:
Formula
Etyne (Acetylene) Propyne (Allyene)
C2H2
Melting point o C -82
C3H4
-102
Boiling point o C -84
Number of isomeric compounds 0
-23
0
5.2.6 Cyclo alkanes Cyclo alkanes are hydrocarbons with single bonding between the carbon atoms, but the molecules form a circular structure. The compounds are saturated, and form a homologous serial. The general molecule formula for the cyclo alkanes is;
CnH2n The circular structure of the cyclo propane:
Name:
Formula
Cyclopropane Cyclobutane Cyclopentane Cycloheptane
C3H6 C4H8 C5H10 C6H12
Melting point o C -126 -50 -93 6
Boiling point o C -34 13 50 81
Number of isomeric compounds
5.2.7 Arenes Arenes are cyclic, but unsaturated hydrocarbons because of its double bonding. The compounds are aromatic. Benzene, which is very stabile and frequently used together with other products in the petrochemical industry, is a well-known product within this group.
Name:
Formula
Benzene
C6H6
Melting point o C 5,5
Boiling point o C 80,1
Number of isomeric compounds
5.2.8 Alcohol’s Alcohol are organic compounds where the functional group is the hydroxyl-group – OH. All alcohol ends with “-ol”. The different alcohol’s are divided in subgroups, dependent of the form of bonding. Name:
Formula
Methanol Ethanol
CH3OH C2H5OH
Melting point o C -97,8 -117,3
Boiling point o C 64,5 78,3
Number of isomeric compound
5.2.9 Aldehydes Aldehydes have one functional group –CHO. Name:
Formula
Formaldehyde HCHO Acetaldehyde CH3COH
Melting point o C -118 -123,5
Boiling point o C -19 20,2
Number of isomeric compound
Ketones 5.2.10 Ketones are compounds where the functional group is the carbon-group. Name:
Formula
Acetone
CH3COCH3
Melting point o C -94,3
Boiling point o C 56,2
Number of isomeric compound
5.3
CHEMICAL REACTIONS
New products are continuously made in the petrochemical industry by allowing hydrocarbon compounds participate in chemical processes and reactions. 5.3.1 Unsaturated chemicals Unsaturated chemical compounds contain one or several double or triple bonding between the carbon atoms. They can easily saturate the vacant valences in a chemical reaction. A chemical reaction may take place: · ·
by mixing unsaturated compounds with another product. by increasing the temperature and pressure in the chemical compound, alone or together with other compounds.
To visualize an unsaturated compound, a solvent bromine and water can be used. If you mix bromine (Br) with a saturated oil, the bromine-coloured water will disappear, because the double bonding is opened and bromine appear in every vacant valence. A chemical reaction has appeared between two compounds, and a new compound is created. If you combine ethylene and bromine, this chemical reaction will take place:
When unsaturated chemical compounds are heated under pressure, the molecules react with each other and form large molecules, so-called macromolecules. This is called polymerisation. To start the reactions or to increase the velocity of reaction, a catalyst is often used. A catalyst is a material that increases the velocity of reaction in a chemical process without changing its own state. Linear polyethylene is a plastic raw material, which is a polymer of ethylene produced by polymerising ethylene with a peroxide catalyst. Benzol peroxide is an example of peroxide used as a catalyst for production of polyethylene. Other types of polymers are made of ethylene or together with other hydrocarbon. The properties are different, and the plastic raw materials are used alone or together with others when producing plastic products. Most plastic raw materials are produced like this. Molecules or mixture of molecules which is capable of polymerise, are called monomers. The number of monomers taking part of a polymerisation may be many thousand. A linear polyethylene has a molecular weight of more than 6000, others are considerably smaller. The molecular weight is controlled by temperature, concentration of catalyst or amount of ethylene.
It is not only the unsaturated hydrocarbon compound that may polymerise. In 1907, Baekeland managed to control three-step polymerising with phenol and formaldehyde. The product “Bakelite” was the first synthetic polymer that was produced, and has great significance even today. The following list demonstrates some of the most common plastic materials today, and how they are produced: Polymer Polyethylene Polytetrafluorethylene (PTFE) Polyvinylcloride (PVC) Polypropylene Polystyrene
Monomer: (CH2)n (C2F4)n
Polymerisation: Ethylene Tetrafluorethylene
(H2CCHF)x (C3H5)n (C6H5CHCH2)n
Vinylcloride Propylene Styrene monomer
5.3.2 Peroxides and inhibitors Peroxides are highly explosive, and can form into unsaturated compounds, as for example butadiene and VCM if oxygen is present. They can appear as powder in pipes and tanks and are very unstable and can easily explode. The formation of peroxides in butadiene can entail polymerising with powerful heat generation. To avoid such a chemical reaction, the content of oxygen in the tank atmosphere is kept as low as possible. To assure that all oxygen is removed, an inhibitor is added to the individual cargo. An inhibitor is a material that itself, in low concentrations, reacts with the oxygen. Some types of inhibitors have the capability to react with radicals so that the velocity of reaction reduces or to cease up. Most inhibitors are very dangerous to our health, and must therefore, be handled with the utmost care. 1,3 Butadiene and VCM are examples of cargo that are added inhibitors. Approximately 5 ppm hydroquinon is added to VCM to prevent polymerisation. US Coast Guard requires that one add 100 ppm TBC (Tertiary Butyl Catechol) to 1,3 butadiene to prevent a polymerisation with strong heat generation. Humidity and water will reduce the effect of inhibitors, in some cases water will accelerate a chemical reaction. Cargo that is inhibited must have a certificate with: • name and amount • inhibitor date and for how long the inhibitor is efficient • precautions, if the voyage lasts longer than the effect of the inhibitor • eventual temperature limitation The above mentioned inhibitors are only present in the liquid phase. In all probability, dangerous peroxides will be formed inside the lines of the cooling plant’s “condensate” system. It is recommended that these parts of the system are checked regularly, when the inhibited cargo is cooled. Further, it is recommended to circulate some inhibited liquid through the part of the system where “condensates” remains without the inhibitor.
Introductorily, we have said that polymerising can occur if the temperature is high enough. The following restriction of maximum outlet temperatures from the compressor is required: · maximum 60 oC for butadiene · maximum 90 oC for VCM 5.3.3 Reaction with other cargo and materials Some cargo can react strongly with other cargo. This makes great demands for cleaning, before loading and full segregation against other cargo. Whenever cargo segregation is required, spool pieces must be used. It is important that all materials are compatible with which the cargo can come in contact. The material must, for example, in all gaskets that can be in contact with propylene oxide be of PTFE or a similar approved material type. 5.4
TEMPERATURE, HEAT, ENERGY AND PRESSURE
When you mix cold and warm water, the temperature will eventually be a specific average temperature. If multiple objects, with different temperature, are placed in a room, the temperature of the objects and the temperature in the room will eventually be the same. This is the heat theory’s O. law that forms the basis for measuring the temperature with a thermometer. Empirically, the heat will, at all times, move from the warmer material to a colder material. One can prevent heat transmission by a heat-insulating barrier. If the barrier is totally heat insulated, it is called adiabatic. If a material, or a collection of materials (a system), is totally surrounded by adiabatic barriers, no heat exchanging can occur with the surroundings. The temperature characterises a fixed stated condition of a material, and can only be measured indirectly by measuring another directly measurable size, that changes with the temperature. With a mercury thermometer or an alcoholic thermometer, the temperature can be determined from how the liquid changes the volume. Because of this, the individual materials change the volume differently at varying temperatures. Graduation on a mercury thermometer must necessarily be different from the graduation of an alcohol thermometer. One calibrates thermometers by measuring the temperature at one or several fixture points. Boiling water at 760 mm HG is one example of such a fixture point. Measuring the temperature based on other material’s characteristics has its obvious weaknesses. Different temperature scales make its impossible to establish a uniform method of many thermal calculations. William Thompson, later ennobled as Lord Kelvin, was one of many physicists that worked to find an absolute temperature scale independent of another material’s properties. He defined a theoretical temperature scale, which later acquired the name “Kelvin” (or K). The Kelvin scale, adopted by the SI-system, begins at the absolute zero point, which is defined to –273,15oC, but has the same graduation as the Celsius-scale. In most thermal calculations, it is sufficient to estimate the zero point of the Kelvin scale at – 273oC, which will be used as conversion factor in this compendium.
A temperature difference has the same measured value in Kelvin as in the Celsiusscale. Converting from Celsius to Kelvin is: Temperature in Kelvin ⇒ 273 + temperature in Celsius degrees - 50oC
= (273 + (-50))K
= 223 K
0oC
= (273 + 0)K
= 273 K
+50oC
= (273 + 50)K
= 323 K
When a car decelerates, the velocity of the car delerates, the kinetic energy is reduced. The temperature on the brakes rises simultaneously. An electric driven compressor, which compresses the gas, increases the temperature of the gas, but consumes electrical power simultaneously. If we switch on the hotplate, the temperature rises, simultaneously the hotplate consumes electrical energy. When oil burns, chemical combined energy falls as the temperature rises. Mutually, for all these phenomena is that a temperature increase is in progress simultaneously as we copy energy in one or another form. Mechanical energy, electrical energy, chemical energy, or heating energy, are all expressions for energy that can be summarised as: “Energy is the capability to perform work” We will mainly go into two forms of energy in this compendium, potential (position) energy and kinetic energy. A temperature increase can take place by transfer of heat from one material to another at lower temperature. It is natural to conclude that heat transfer is also a form of energy transfer. Heat is often defined as the energy that is transferred from one system to another, because of difference in temperature. We measure heat with the same unit as all other forms of energy. The heat (quantity) required to heat up one gram of water one degree Celsius at a temperature of 14,5oC, is defined as one calorie (cal). The SI-system uses the unit “Joule” for heat. If 41855 Joule is supplied to 1 kg water at a temperature of 14,5oC, the temperature will rise 1K. Conversion between the units calorie (cal) and Joule (J) is therefore; 1 cal = 4,1855 J
or
1 kcal = 4185,5 J
One can transfer heat from one place to another in different methods. If an iron bar is heated at one end, the whole bar will gradually be warm. The heat will spread through the material and we say that the heat occurs as stationary thermalconductance.
At thermal conducting, the heat is, at all times, deducting from a location with higher temperature to a location with lower temperature. The speed of the thermal conductance is a temporary dependent of the material. Metals are good heat conductors, gases the inferior. Stationary thermal conductance or heat flow through one level is defined as: F = l/d x A x (T1 - T2) where: l = specific thermal conducting ability or thermal conductivity with unit W/mK δ = the thickness of the material in m A = the area of the material in m2 T1 = temperature on the warmest side in Kelvin T2 = temperature of the coldest side in Kelvin General expressions for heat flow, heat quantity, work or energy is always with the SI-unit J (Joule). When above mentioned energy forms are measured or calculated during a stated period, the measuring unit is always in J/s, which is the same as Watt (W). In liquid and gases, the molecules move by heat conductance. The heat alters the density, and different density provokes flow that takes the heat ahead. When heat conductance takes place by movement in the material, we say that the heat is transferred by convection. Heat can also be transferred by radiation. In the vicinity of a heated material in cold surroundings, one can feel the heat at a long range. We say that the material radiates heat or emits heat rays. You can not lead the heat you feel through the air, because as you hold a shield against the heat source, the heat is gone. To produce heat from radiation, you have to stop it with a material. The radiate is slightly enervated in air. The faster and darker the material is, the easier the radiate absorbs and converts to heat. The influence of heat radiation is often underestimated. Understanding this type of heat transfer is important in the work of reducing heat transfer. Emission or emanation is also a form of radiation. A practical example is in a thermos. Double walls with vacuum alone does not prevent transfer of products, in spite of fact that thermal conductance in vacuum is zero. Some heat is transferred by radiation between walls. To eliminate this, the sides that face into vacuum are silvered. This reduces the heat radiation to a few percent. You can express the relation between absorption and emission as the absorption capability of two materials that has to conduct to each other as emission capabilities. One measures and describes pressure in different ways independent of what is measured, who measures it and how it is measured. The engineer reads the pressure in a system on a manometer. The mate reads the atmospheric pressure on a barometer or a mercury column. In thermal technical charts and diagrams, the expression absolute pressure is used.
One can easily describe the different “pressure” by help of a diagram. The lowest possible pressure that can exist is vacuum. Therefore one estimate absolute pressure from this starting point. The pressure of the manometer is pressure above the atmospheric pressure. As the atmospheric pressure will vary, the logical choice is to show pressure dependent values as a function of absolute pressure, like for example in thermal technical charts and diagrams. On can use a mercury column or a water column, both to indicate atmospheric pressure and excess pressure. A normal atmospheric pressure is defined as 760 mm Hg.
The gravity of or the pressure that a liquid column of 760 mm Hg amounts to can be calculated in this way: p=r xgxH where: p= pressure in N/m2 (Newton per square meter) r= the density of liquid (The density of mercury 13595,1 kg/m3 g= the gravity of shaft ration (9,81 m/s2) H= height of the liquid column in meter (760 mm = 0,76 meter) p = (13595,1 x 9,81 x 0,76)N/m2 = 101359 N/m2 1 normal atmospheric pressure define as = 1013 mill bars 1 atmosphere = 105 N/m2 = 100 000 Pa = 100 kPa = 1 bar
5.5
OIL PHYSICS – AGGREGATE CONDITIONS
History Back in the antiquity, about 2500 years ago, oil was mentioned in scripts from Asia. The scriptures describe hot springs and that oil lamps were known and in use. However, centuries went by before the oil was in common use. As a matter of fact danger of fire was one reason which prevented utilisation of oil. In USA, which today is rich in oil sources, did not take the oils in common use until the midst of the last century. At first the oils were used in medical treatment then to heal such as rheumatism and pneumonia. The first well was drilled in USA in 1859 and supplied 1500 litres oil every 24 hours. The oil consumption and development increased dramatically from this time on. John D. Rockefeller founded the «Standard Oil Company» in 1870. The Dutch « Royal Dutch Oil Company» was founded in 1890. In 1909, the English founded «AngloIranian Oil Company». This development has continued up to this day where these companies operate around the world. The need for oil increased at the same speed as the oil discoveries increased. At the same time, knowledge about oil’s nature, physics and chemistry improved. 5.5.1 The Oil Transportation As a start, the oil was transported in barrels by ordinary liners. The oil tankers today were first used at the end of the last century. These tankers have since then changed dramatically through a radical process up to today’s technically advanced tankers. Due to the world’s oil demand, tank tonnage has increased enormously, along with the average size of ships. The personnel (crew) operating the ships provide a wide range of knowledge regarding ship operations and its specific cargo. To be an educated and qualified “Ship Officer”, it is necessary to have a basic theoretical knowledge and a lot of practical experience on board the ships. In this part of the compendium, the oil’s physical properties are reviewed. Physics is the learning of different substances and property forces and their energy form. Chemistry is the learning of the substance’s composition and the substantial or permanent changes these substances may under go. The theory about atoms and molecules understands that substances are able to divide into smaller parts, atoms and molecules. 5.5.2 Aggregate States Solid, fluid and gas form conditions must be seen in connection with the understanding of molecule forces. An example: When splitting a piece of wood the molecules separate along the split area. The force being used corresponds with the force binding the molecules together. If you now wish to force the two pieces together again, a certain power has to be used. The first power in use will be called the force of cohesion. The second power in use will be called the force of expansion. Cohesion and expansion summed, is called intermolecular force.
The cohesion is due to the fact that all substances (elements), including the smallest parts in a substance, execute a mutual back sweep on each other. We therefore have the same nature as the force of gravity. It decreases rapidly when the distance between the smallest parts increases. The expansion is due to the fact that the molecules in all substances, both solid, fluid and gas forms execute movements or vibrations and therefore fill-up an entire room. 5.5.3 Solid Substances A solid substance has a fixed form and fixed volume. An iron bar is resists being lengthened or pressed together. The molecules in the iron bar will try hard to keep a certain mutual distance to each other. The iron bar is resistant to rubbing and bending. In solid substances the molecules have fixed places according to each other, and the same applies to the substance’s volume and form. This is because the cohesion and expansive force is very large. 5.5.4 Fluids Fluids have a fixed volume, but do not have any fixed form. To squeeze water in a cylinder with tight-fitting piston is hard without using large power. Similar to the molecules in solid substances, the molecules in a liquid substance have a certain mutual distance between each other. The liquid molecules on the other hand have no fixed positions according to each other. Liquid will always be shaped based on where the liquid is stored. The cohesion force in liquids is not powerful enough to prevent the molecules from moving freely according to each other. However, the force is still strong enough to maintain the distance between each of the fixed molecules. The expansion force is equal as in solid substances. 5.5.5 Gases Gases have no fixed volume or fixed form. A gas will always try to fill as great a volume as possible, and will therefore fill the room, the tank and so on, where the gas is stored. The cohesion force in gas is too small to prevent the molecules from changing both the distance and the position in accordance with each other. The expansion force gets free scope and the gas expansion is total and unlimited. By exposing gas to forces greater than the expansive force itself, the gas will be compressed.
5.5.6 Phase changes
Any substance can be transformed from one condition to another, by means of temperature changes or varying temperatures and pressure. Ice, water and water vapour are the same substance in different forms. The transformation between cohesion and expansion with water molecules goes through these three phases solid substances, liquids and gases. 5.5.7 Melting When a solid, pure crystal substance is continuously supplied with heat, the substance will melt. For example:
1 kg of ice with a temperature of -20oC exposed to heat (the pressure is 1 atm). A thermometer placed in the ice will show a rise in temperature up to 0oC, which is melting point of the ice. The heat supplied after the melting point is achieved will have no effect to any temperature rising, as long as the ice is present. During the melting, the temperature is invariable, and the heat supplied during the melting process is consumed in melting the ice. When all the ice is melted, the temperature in the water will rise. So, the amount of heat supplied to 1 kg of the
solid substance, in order to reach the melting point where the change from solid to liquid form occurs, is called the “melting heat”. The heat needed to transform a solid substance at a given temperature, into a liquid substance with the same temperature, is called the “specific melting heat”. The unit for specific melting heat is Joule/kg. The heat necessary to evaporate one kilo of a certain liquid substance is called “specific melting heat”, abbreviated “r”. The unit for specific evaporation heat is J/k. 5.5.8 Enthalpy A substance’s total energy consists of the external energy (work) plus the internal energy. Enthalpy is an expression for a substance’s internal energy abbreviated “h”. This enthalpy is an expression of how much energy is tied up in one kilo of the substance. The unit for enthalpy is Joule/kg. The comparison of enthalpy to temperature change of gradients shows how much energy is needed to be supplied to bring ice through the three different stages. 5.5.9 Evaporation A liquid change to gas is called evaporation. This may happen by evaporation or boiling. To achieve evaporation, heat of evaporation is needed. Some liquids evaporate very quickly, such as gasoline and ether. Other liquid substances evaporate very slowly, such as in crude oil. Evaporation is vapour formed out of the liquid surface and occurs at all temperatures. This is explained by some of the liquid’s surface molecules being sent into the air, which is strongest at high temperatures, dry air and fresh wind. The specific temperature calls the amount of heat needed for one kilo of liquid with fixed temperature to form into one kilo of steam with the same temperature”. The heat from evaporation is set free when the steam forms to liquid again, or condenses. The heat necessary to evaporate one kilo of a certain liquid is called “specific heat of evaporation”, abbreviated as (r). The unit for specific heat of evaporation is J/kg.
5.5.10 Boiling Boiling is steam formed internally in the liquid. The boiling occurs at a certain temperature, called “the boiling point”. Water is heated in normal atmospheric pressure (1 atm), in an open container. In common, some parts of air are always
dissolved. The rise in temperature is read from a thermometer placed in the liquid’s surface. When the temperature has reached 100oC, steam bubbles will form inside the liquid substance, especially in the bottom of the container. With continuous heat supply, the bubbling will rise like a stream towards the surface and further up into the air. The water is boiling. The formation of bubbling steam can be explained as follows: During the heating, the water molecule’s kinetic energy increases, consequently the molecules demand more space. During the boiling, as long as there is water in the container, the temperature will be 100oC. The boiling point is dependent upon the pressure. If the steam or the atmospheric pressure increases above liquid substance, the boiling point will also rise. If the surface temperature is just below the boiling temperature, then the water steam will evaporate on the surface. The evaporation point and the boiling point will be the same accordingly. The pressure from the surrounding liquid is the total amount of pressure above the liquid, Pa, plus the static liquid pressure. P = Pa + (ρ x g x h ) P = pressure in Pascal (100 000 Pa + 1 bar) Pa = barometer pressure ρ=
the liquid density in kg/m3
g = force of gravity acceleration (9,81m/s2) h = liquid column in meter.
When reducing the pressure above the liquid, the boiling point will also be reduced. A practical use of this characteristic is the production of fresh water on board (fresh water generator).
5.5.11 Condensation Condensation is the opposite of evaporation. If a gas is to be changed to liquid at the same temperature, we must remove the heat of evaporation from the gas. A gas can be condensed at all temperatures below the critical temperature. By cooling a gas, the molecule speed decreases hence the kinetic speed. The internal energy decreases, as well as, the molecule units and liquid forms. 5.5.12 Distillation Distillation is a transferring of liquid to vapour, hence the following condensing of vapour to liquid. Substances, which were dissolved in the liquid, will remain as solid substance. With distillation it is possible to separate what has been dissolved from the substance which was being dissolved. When a mixture of two liquids with different boiling point is heated, will the most volatile liquid evaporate first while the remaining becomes richer on the less volatile? On board, for instance, seawater is distillated by use of an evaporator.
5.5.13 Saturated, Unsaturated or Superheated Steam Let us imagine boiling water, releasing vapour from a container, leading the steam into a cylinder that is equipped with a tightening piston, a manometer and two valves. The steam flows through the cylinder and passes the valves, whereon the valves are closing. There now is a limited and fixed volume of steam in the cylinder. Around this cylinder a heating element is fitted. Vapour from the container is constantly sent through this heating element to ensure that the temperature is maintained constant. The piston is pressed inwards, and now the manometer should show a rise in pressure. But, the manometer shows an unchanged pressure regardless how much the volume is reduced. What’s happening is, the further the piston is pressed inwards, some parts of the steam is condensed more using less volume. The vapour from the heating element removes the condensed heat, which is liberated during the condensation process. We find that the amount of steam, which is possible to contain per volume unit, remains constant when the steam’s temperature is equal to the condensation point at the set pressure. The room cannot absorb more vapour, it is saturated with steam and called “saturated”. If the piston is pressed outwards, the pressure will still show constant. The conclusion is: · With temperature equal to the condensation point by set pressure, steam is saturated. · Steam above boiling water is saturated. · Saturated steam with a set temperature has a set pressure. This is called saturation pressure. · With constant temperature saturated steam cannot be compressed.
This also concerns vapour as saturated steam of other gases. Using the same cylinder arrangement as before. The cylinder contains saturated steam, no water. The piston is drawn outward. When no water exists over the piston no new steam will be supplied underneath. The manometer will now show reduced (falling) pressure as the steam expands. When saturated steam expands without supplying new steam, it is called unsaturated steam. The room has capacity to collect more steam. 5.5.14 Unsaturated steam contains lower pressure than saturated steam at the same temperature. The unsaturated steam in the cylinder can be made saturated again in two ways. Either by pushing the piston inward to the originated position, or let the unsaturated steam be sufficiently cooled down. When the temperature is reduced, the saturation pressure will reduce. Unsaturated steam will, in other words, have a too high temperature to be saturated with the temperature it originally had. Therefore, this often is referred to as superheated steam.
5.6
THE GAS LAWS
The gas laws are laws that describe the basic facts for ideal gases. Many actual gases under pressure and temperature that we normally get in touch with can not observe as ideal gases. Calculations based one-sided of the gas laws, will therefore necessarily often depart from reality. The gas laws are meanwhile important by that the laws establish simple and clear connections by the condition changes of the gases. 5.6.1 The Boyles law Boyles law, of Mariottes law establish that when the gas quantity is confined and the volume varies under constant pressure, the pressure will vary so that the product of pressure and volume is constant. The law can also express as: p x V = constant One illustrate the law a by thinking a cylinder filled with gas. A well-adjusted piston closes the gas inside the cylinder. The pressure in the gas is p, by a volume V, before changing. If the piston is removed so that the volume alters to V, the pressure p after volume change is: p1 x V1= p2 x V2 p2 = (p1 x V1) / V2
A change of state in the gas where the temperature is constant is called an isotherm alteration. The Boyles law agrees to good approach for air and hydrogen up to about 100 pressures of the atmosphere. For other gases as carbon dioxide, the law is only for lower pressure. If the pressure is 1 bar and the volume 1 litre before alteration, after reducing the volume to half, the pressure will be: p2 = (p1 x V1) / V2= (1 x 1) / 0,5 = 2 bar
5.6.2 Gay-Lussacs laws Gay-Lussacs 1.law establish that the gas volume varies proportionally in condition to the absolute temperature of the gas when the pressure is constant. The law can also express as: V1 / T1 = constant The law can illustrate by thinking a cylinder filled with gas. A good adjusted piston that moves free shuts the gas inside the cylinder. The pressure in the gas is constant and determined by the weight of the piston. If you heat the gas so that the temperature alters from T1 to T2, the volume alters from V1 to V2. The new volume is: V1 /T1 = V2 /T2 V2 = (V1 x T2) / T1
An alteration of state in the gas under constant pressure is called an isobar change. Gay-Lussacs 2.law establish that the pressure of a gas quantity is proportional to the absolute temperature of the gas when the volume is constant. The law can also express as: p1 / T1 = constant One can illustrate the law by thinking a cylinder filled with water. The piston is locked so that the volume stays the same. If you heat the gas so that the temperature is altered from T1 to T2, the pressure will alter from p1 to p2. The new pressure after heating will be: p1 /T1 = p2 /T2 p2 = (p1 x T2) / T1 A state of proportion in the gas with unaltered volume is called an isochor alteration. 5.6.3 The absolute zero point. Gay-Lussacs experiment is used to decide the absolute zero point. If you heat a small glass tube in water where a small mercury droplet fences an air column, the state between the air volume and the temperature is plotted in a diagram when the temperature changes. When the temperature rises, the volume increases. The read off values for temperature and volume is close to a straight line. The differences are so small that they are inside the accuracy. The line that emerges shows how the volume varies with the temperature under constant pressure. The pressure will at all times during the experiment be the total amount of the atmospheric pressure and the weight of the mercury droplet.
The Avogadros law The Avogadros law says that equal volumes of two gases with the same pressure and temperature contains the same amount of molecules. A conclusion of this statement is that the state between two gases density (p) at the same pressure and temperature, has to be equal to the state between the masses of the individual molecules in the gases or the state between the relative molecule masses (M). r1/r2 = M1/M2 The Dalton Law The Dalton law say that the total pressure in a gas mixture is equal to the total amount of the partial pressures (part-pressure), that each of the gases will alone in a room with the same temperature as the mixture. The law expresses as: ptotal = p1 + p2 + ………….pn The Dalton law is logical. Every gas fills all the volume, independent of other gas molecules that are present. The molecules itself obtains itself an utmost small part of the volume. Therefore every gas will have a pressure that responds to this. One can also see the restriction of the law from this explanation. It has no longer any existence when the pressure is so large that the molecules occupy a perceptible part of the volume. It has also no accuracy when the gas molecules has influence on one another, and also not if the gases has a chemical reaction against one another. The Joules law The law of Joules say that the inner energy in a precise amount of ideal gas only depend of its temperature and is independent of the volume. If pressure and volume is changed in a process, the inner energy will remain constant if the temperature is constant. According to the kinetic gas theory, the inner energy in an ideal gas is equal the complete kinetic energy that the molecules have because of its disordered movement. This can express, as the inner energy in a precise amount of ideal gas is proportional with the absolute temperature. A conclusion of this statement is if an ideal gas expand (gets a larger room), the temperature and with that the inner energy will remain unchanged after expansion. Indirectly, the law is demonstrated by experiments with actual gases. These experiments indicate that the inner energy of an actual gas is dependent of the gas volume, but this dependence decreases the more the gas approach to become an ideal gas. In an actual gas the force of attraction works between the molecules. The force of attraction between the molecules by usual pressure is small, but is not equal zero. It is therefore necessary to perform work to increase the distance between the molecules and expand the gas volume. If the expansion is adiabatic, that is without heat exchange between the surroundings. This work can only be because in expense of the molecule kinetic energy and the temperature of the gas sink.
Joules-Thompson effect The Joules-Thompson effect describes the divergence from the Joules law of an actual gas. According to Joules law, the temperature will not change if a gas expand freely without working. Practical the temperature will fall freely for most gases of hydrogen and helium that is heated during expansion. When air expand from about 50 bar to the atmospheric pressure, this is cooled with about 13K. It is the result of this effect one can observe or feel when air or another gas is let out from an air bottle and the delivery valve (expansion valve) is noticeable colder. Cooling plants that are used on board expand the vaporisation of the gas. The Joules-Thompson effect in such plants is insignificance and therefor not calculated with. Diffusion Bromic gas that has a brown colour is well suited to demonstrate diffusion between gases. If you fill a glass with bromic gas and a glass filled only with air on top, one can after a while se that the content in both glasses is gradually brown-coloured. Diffusion has taken place. In despite of that bromic gas has five times as large density as air diffuses that gas up in the top glass with air. All gases can mix at diffusion. As the molecules in the gases are accidental and unorganised, a precise gas molecule will over time come any where in the room that is available (according to the kinetic gas theory). From the kinetic gas theory it is natural to draw the conclusion that the diffusion velocity is faster the larger velocity the molecules have. At experimental experiments the Englishman Graham reached following connections: The diffusion velocity for a gas is converted with the square root of the density of the gas and directly proportional to the square root of the absolute temperature. These can mathematical express as: v1/v2 = √r2/√r1 As equal volume of two gases contain, at the same pressure and temperature, that same amount of molecules (Avogadros law), the state between the density of the gases (p) and the masses of the individual molecules and the relative molecule masses (M) be: r1/r2 = M1/M2 From above mentioned two expressions, gases diffusion velocity can express as: v1/v2 = √M2/√M1 This formula can be used to find how fast gases diffuse in proportion to one another. When the molecule mass to nitrogen is 28 and the molecule mass to hydrogen er 2, we find the relative diffusion velocity for nitrogen to: v1/v2 = √M2/√M1 = √32/√2 = 4 that shows that hydrogen diffuse 4 times faster than nitrogen.
BASIC REFRIGERATION There is seven different principals for cooling, but we are to concentrate about the process that has been known the longest and that has the largest distribution. The process of evaporation is the process that is used at the most in modern cooling technique. It is here the cooling medium evaporation heat that is utilised to transfer heat from one place to another. For a liquid to evaporate one must supply heat to the liquid. The heat is taken from the surroundings that thereby are cooled. In an air condition plant on board, Freon liquid is lead into an evaporator, the heat from the air is transferred to the liquid that evaporate, and the air is cooled. One must here emphasise that there is always in speak of transport of heat from a warmer media to a relative colder media. Equal that water flows from a higher level to a lower because of that the gravity will heat from a higher temperature to a lower. How can then heat transfer from the relative cold Freon gas that is sucked back to the compressor and transfer to the relative much warmer seawater? To elevate water from a lower to a higher level, work has to be done (by help of a pump) of the water. To transport heat from a lower temperature to a higher or likewise.
The thermodynamics 2nd main sentence say: “Heat can only be transported from a body with low temperature to a body with higher temperature by converting of mechanical work.” It is this law that is utilised in any cooling plants or condensation plant for cargo on gas ships. Heat is transferred from the relative cold cargo gas to the relative much warmer seawater. For this to be possible one must perform a work on the gas by the compressor compressing the gas to a higher pressure and temperature than the seawater; and heat can thereby transfer from the gas to the seawater in a heat exchanger. Since the compressor secures a continuous high pressure and temperature in the heat exchanger, both the supplied heat quantity under compression and the evaporation heat transfer to the seawater. The gas condenses and is allowed back by a regulation valve and back into the tank. The job of the regulation valve is to secure a liquid lock for thereby to maintain a high pressure in the condenser, and to distinguish between the low-pressure side and the high-pressure side in a condensation plant. Without this valve it is impossible to maintain the condensation pressure and keep the cooling process up. In many cooling plants thermostatic expansion valves are used at this purpose. A thermostatic expansion valve is not regulated by the liquid level in a liquid collector before the valve, but by the overheating temperature inside the evaporator. Regardless of the valve is a regulation valve or a thermostatic expansion valve, the job is the same and the valve has no “cooling technical” qualifications in itself. (ref. Joules-Thompson effect). On ships that transport condensed gases in bulk, the cargo in the cargo tanks will at all times be in its boiling point. As the temperature difference between the cargo and
the surroundings are partly very large, heat will transfer from the surroundings to the cargo. Isolation will never prevent heat-transfer, only reducing this. The heat to the cargo will lead to temperature increase with thereby following pressure increase. This process will unprevented be in progress until the temperature of the cargo is equal as the surrounding temperature. If the cargo is propylene and the surroundings ambient temperature is 27oC, the pressure in the cargo tank will gradually build up to about 11 bar. With exception of fully pressurised gas carriers, there is no gas ship with cargo tanks that is constructed to resist such a pressure. To maintain the tank pressure less than the pressure the cargo tanks is designed for (MARVS), it is necessary to remove the supplied heat. This can be done in three different ways. We can condensate the vapour back to the cargo tank, we can use the vapour as fuel or we can blow the vapour out in the atmosphere. LNG ships use the vapour in the propulsion machinery and that is a part of the chart. How the cooling plant is constructed depends of the size of the ship and what kinds of cargo the ship is built to carry. Roughly the cargo cooling plant is divided into in three main types: •
Direct cooling plants; the cargo condensing directly against seawater.
•
Cascade plants; the cargo gas is condensing at a cooling media as for example R22
•
Indirectly cooling plants; the cargo is cooled or condensed against a cooling media or a without compression of the cargo gas.
As mentioned earlier, there is unqualified necessary to cool a compressed gas under its critical temperature to condensing this. As the condensing temperature normally is no lower than 5oC above the seawater temperature, we can set a cool technical limit for what can be condensed against seawater. The diagram shows a saturation curve and the critical point for the most actual gas cargo. It is here drawn a reflected boundary line at a condensing temperature of 37oC. We then suppose that the seawater temperature in the area the ship is trading will be maximum 32oC and that the highest condensing temperature thereby is 37oC.
06-
Cargo Handling Equipment
6
CARGO HANDLING EQUIPMENT
Centrifugal pumps are utilised as main unloading pumps on gas tankers. The unloading pumps are located down in the cargo tank’s swamp or as close to the tank bottom as possible. This is because the centrifugal pumps do not suck, and are thereby dependent upon good drainage. The pumps are either the deepwell pump type, submerged type or booster pump. Normally, the number of revolutions on deepwell and submerged pumps lie on 1300 – 1800 RPM. Pumps driven with hydraulics have the advantage that the number of revolutions can be adjusted. Electrically driven pumps normally have a stated number of revolutions, but lately they are delivered with a variable number of revolutions, for example 1370/800 RPM. Booster pumps normally have revolutions from 3500 – 4000 RPM. It is very important to follow the user manual supplied by the pump manufacturer to ensure what to do before we start a pump, and what routines to follow at overhaul and inspection of the pumps. 6.1 DEEPWELL PUMP Deepwell pump is the pump type that is often used on gas tankers. Deepwell pumps are pumps with a long shaft between the driving motor and the pump. The shaft goes inside the tank’s discharge pipe from the pump up to the tank dome. The discharge pipe is a solid pipe that goes up through the tank and out to the flange on the tank dome to the liquid line. The discharge pipe is constructed with several lengths with pipes, and there is a shaft bearing on each flange. The bearings are lubricated and cooled down by the liquid that is pumped from the tank. It is very important not to run the pump without liquid. This may result in damage of bearings and then the shaft. The motor that drives the pump is either electric or hydraulic. There is a mechanical sealing device between the motor and the discharge pipe in the cargo tank. When using the pump, we must have at least one bar higher pressure on top of the mechanical seal than we have in the tank. It is important to closely read the pump’s user manual about the routines before discharging, because the routines vary some from different manufacturer.
6.1.1 Submerged pump Submerged pumps are multistage centrifugal pumps that are often used as discharge pumps on large LNG and LPG tankers. The motor and pump are submerged down in the tank sump or as close to the tank bottom as possible. The motor is connected directly to the pump with a short shaft on this type of pump. The liquid that is pumped lubricates and cools the pump’s bearings. It is therefore essential that the pump is used only when there is liquid in the tank. The liquid is pumped up through the tank’s discharge pipe and up to the liquid line. This type of pump is equipped with electrical motor. The cables to the electric motor are either made of copper or stainless steel. If copper is used in the cable, the cables must be sheathed with stainless steel to prevent damage on the cable from corrosive cargoes. When transporting Ammonia, the cable and engine must be sheathed with a thin layer of stainless steel. It is important that the stainless steel sheathing is kept unbroken, and we must avoid a sharp bend on the cable to protect the stainless steel sheath. One must at all times check the resistance of the cable insulation before starting the pump. Submerged pumps are also installed as portable pumps. The discharge pipe is then the steering pipe for the pump. At the bottom of the discharge pipe it is a non-return valve that opens when pump is lowered and shut when the pump is taken up. Before opening the discharge pipe it must be gas freed, this is done either with inert gas or Nitrogen.
6.1.2 Booster pumps Booster pumps mentioned here are auxiliary pumps for cargo handling. The pump is one-staged centrifugal pump and is often installed on deck near the pipe manifold. The booster pumps on gas tankers are used either as a main discharge pump, auxiliary discharge pump, deck tank supply pump or heater feed pump. The booster pumps are driven with electric or hydraulic motor. The engine and the pump are connected together with a short shaft with coupling in between. It is very important that the motor and the pump are aligned according to the manufacturer manual, and the clearances specified inside are followed. Booster pumps that are regularly utilised should, as a good rule, be turned by hand once a week to prevent destruction of the motor and pump bearings. It is important that the booster pumps are blended off on LPG/LEG tankers when carrying cargo with lower temperature than –50oC. Booster pumps are rarely designed for temperature lower than –50oC.
6.1.3 6.1.3 Hold spaces and inter barrier spaces In hold space and inter barrier space there is requirement of drainage system separated from the machinery drain system. The drain system could be submerged pumps, deepwell pumps or ejectors. These pumps can be used to drain water or cargo spill from the bilge. Generally, there are spool pieces (short pipe pieces) that are produced especially for each hold space and on each side and fit both to the cargo system and the seawater system. It is important that the spool pieces are disconnected, and the flanges are blinded off when the bilge system is not in use.
Example of ejector in hold space
6.2 LOADING LINES, PIPES AND VALVES 6.2.1 Loading lines and pipes The loading lines and pipes mentioned here refer to gas carrier’s cargo handling system. This involves liquid lines, vapour lines, condensate return lines, lines to vent mast, pipes inside the cargo tank and seawater pipes to the cargo cooling plant. All loading lines on gas carrier: liquid lines, gas lines and lines to vent mast have the same requirements as pressure vessels regarding of temperature and pressure they are meant to handle. All welding on pipes exceeding 75 mm in diameter and 10 mm wall thickness or more must be X-rayed and classed by the class company. The same regulation do we have on flanges and spool pieces also. All loading lines outside the cargo tank must be produced by material with melting point no less than 925oC. The loading lines on gas carriers are mostly produced of stainless steel, but low temperature nickel steel is also in use. All loading lines with an outside diameter of 25 mm or more must be flanged or welded. Otherwise, lines with an outside diameter less than 25 mm can be connected with treads. Loading lines designed for cargo with low temperature, less than –10oC must be insulated from the ship hull. This to prevent the ship hull to be cooled down to below design temperature. The hull has to be protected against cold cargo spill under spool pieces and valves on all liquid lines. This is done with wood planks or plywood. To prevent cold cargo spill on the hull plates, a drip tray must be placed under the manifold flanges. All lines that are thermally insulated from the hull must be electrically bonded to the hull with steel wire or steel bands. On each flange on lines and pipes where gaskets is used, there must be electrical bonding with steel wire or steel band from flange to flange. On all cargo lines where it can be liquid it is required with safety valve. Vapour from the safety valve outlet must go back to the cargo tank or to the vent mast. If the return goes to vent mast the pipe must be equipped with a liquid collector to prevent liquid to the vent mast. The safety valve’s set point is dependent upon the pressure for which the line is designed. The safety valves must be tested and sealed by the ship Class Company.
6.2.2 Valves The most common valves used on the cargo handling equipment on gas carriers are ball valves, butterfly valves and seat valves. All valves used on cargo lines have to be installed with flanges, and the valves must be electrically bonded to the line either with steel wire or steel bands. 6.2.3 Ball valves On semi and fully refrigerated gas carrier’s ball valves are often used on the cargo lines and cargo cooling plant. The ball valves tolerate high pressure and large thermal variations, and they are also approved for chemicals. The valve seats and sealing devices are produced in Teflon, the ball and spindle is produced in stainless steel. The ball valve principle function is the pressure on one side of the ball forces the ball against the seat and the valve is closed. If the pressure is equal on both sides of the valve, leakage may occur. On some types of ball valves the ball is fastened to the spindle, other types of ball valves have floating ball. With a floating ball the pressure is equal all around the ball, and the ball is pressed even toward the seat. With the ball fasten to the spindle it is pressed aslant towards the seat and the valve seat can be damaged and the valve will leak. Frequently, particles are left between the valve ball and the valve house, and these particles can easily cause damage to the valve seat and the ball. The valves must from time to time be opened and the ball and seat have to be cleaned especially the manifold valves. There is a drain hole on the ball itself. It is of importance to ensure that when the valve is closed, the drain hole pointing where it is least natural pressure, then the liquid inside the ball can be drained or boiled off. This prevents large pressure inside the ball, liquid expansion and wreckage of the sealing devices around the spherical occurs.
Sketch of operation of drainage hull:
Advantages: Ball valves tolerate large pressure and thermal variations due to the shape of the ball. Tolerates both gases and chemicals. Easy to maintain and overhaul. Disadvantages: The valves are expensive, and have costly spare parts. They can be difficult to shut at temperatures down to –90oC and colder (this can be relieved by adding a thin packer between the to parts of the valve house). Ball valves are unfavourable as regulation valves, as it is difficult to adjust to low flow through the valve. 6.2.4 Butterfly valves Butterfly valves are often used on the seawater line on gas carriers, such as water to heat exchanger (cargo heater), seawater condenser, oil cooler, the compressors etc. Butterfly valves are also often used on lines with large diameter as cargo lines, where there is not such a large pressure or thermal difference. Butterfly valves should be moved at regular intervals to prevent the seat from fastening and be damaged and cause leakage valve. Advantages: This type of valves has more reasonable price than ball valves. They have lower weight than ball valves to corresponding pipe diameters. They are better than ball valves for regulation of flow. Disadvantages: They are exposed to cavitation damage on the valve seat and flap when too high liquid flow through the valve. They are less suitable at low temperatures than ball valves. Seat valves Seat valves are frequently used as one-way valves (check valves) on loading lines, as the pressure valve on the discharging pump, on condensate return lines back to the
cargo tank and on the inert gas lines. Seat valves are opening by turn the spindle anti clockwise and the valve seat can wander freely on the spindle. When the pressure increases in the line under the valve seat, the seat is lifted up and the valve is open. When the pressure ceases under the valve seat or the pressure increases above the valve seat, the valve seat will drop down and shut the valve. Opening or choking the valve regulates the amount of flow through the valve. Example on seat valves:
Sketch on spring-loaded seal valve:
Seat valves that are used as check valves, must be overhauled at regular intervals, and especially the seat and contact faces must be polished/grounded as they are expelled for mark and wear and tear when the valve operates often. The seat valves must also be moved regularly when they are not in use for a long period of time.
Advantages: The seal valves are reliable and simple to operate. Have large range of utilisation. Have few wearing parts. Reasonable to maintain. Disadvantages: Require strict inspection. Start leaking if wrongly operated. Needle valves Needle valves are used for regulation of cargo cooling plants, both air regulation and for regulation of Freon in cascade cooling plants. The needle valve is the valve type that empirically is best suited for regulation of low flow volume. HEAT EXCHANGER Heat exchangers are utilised in several different parts of cargo handling on gas carriers, as heat exchangers (cargo heater), condensers for cargo cooling plant, vapour risers, super heaters and oil coolers for compressors. In most of the heat exchangers seawater is used as the medium on gas carriers, which the products are cooled or heated against.
The heat exchangers that are used for cargo handling must be designed and tested to tolerate the products the gas carrier is certified for. Heat exchangers that are used for cargo handling are considered as pressure vessels, and IMO requires one safety valve if the pressure vessel is less than 20 m3 and two safety valves if it is above 20 m3. All heat exchangers that are used for cargo handling must be pressure tested and certified by the gas carriers Class Company. Heat exchangers where water is used as the medium and are utilised for heating have little or no effect with water temperature less than 10oC. Seawater became ice at about 0oC and starts to free out salt at about 50oC. So with operating
temperatures with a larger variation than from 10oC to 45oC, one ought to use another cooling medium than seawater. Some terminals do not accept water as medium in heat exchangers, therefore one must either heat the cargo on route at sea or the gas carrier must have heat exchangers that do not use water as medium. It is of importance to ensure that the water out of a heat exchanger is never below 5oC. These prevent the water in the heat exchanger from freezing and eventually damage the heat exchanger. Tube heat exchangers Tube heat exchangers are produced with tube bundles either as straightened pipes or u-formed pipes placed into a chamber. The pipes in the tube bundle have an inside diameter on 10 to 20 millimetres. There is a cover installed on each end of the chamber to clean the pipes more easily and maintain these. It is, at all times, important to ensure that the velocity of the liquid that is being pumped through the heat exchanger is not too high, to prevent cavity damage in the tube bundle or the end covers.
The tube bundle is made of stainless steel, carbon steel, copper-nickel alloy, aluminium-brass alloy or titan. Which choice of material one decides to choose, depends on the product one will operate and the costs associated with the investment and maintenance. In tube heat exchangers, where seawater is used as medium, the product to be heated goes in the tube bundle. This prevents remaining seawater from freezing or prevents remnants of salt deposits inside the tubes. Tube heat exchangers must at regular intervals be cleaned to prevent particles from settling inside the tubes in the tube bundle or in the end covers. One must closely check for cavity damage when cleaning the heat exchanger. Ensure that the gasket is produced in a quality that tolerates the products and temperature one operates it with. Also, ensure that the gasket is correctly placed.
Plate heat exchangers Plate heat exchangers are more utilised in cold storage plants on shore, for example in the fish industry and the meat industry. Plate heat exchangers are built with thin plates with double liquid channels. The plates are installed with the flat side toward each other. The cooling medium and product are pumped each way in the channels to achieve the best possible cooling or heating. Water or oil is used as the cooling medium and is dependent upon the temperature of the product that is to be cooled or heated. Plate heat exchangers are also used as condensers on newer cargo cooling plants aboard gas tankers.
Plate heat exchangers must be cleaned at regular intervals to prevent the channels from clogging with salt deposits or particles from the medium or the product. One must ensure, after cleaning, that the gaskets are properly placed, and that one uses gaskets that tolerate the medium and temperatures one operates within the heat exchanger Different heat exchangers utilised onboard gas carriers for cargo handling Cargo heater: A cargo heater is used to heat the cargo when discharging to an ambient shore tank. A cargo heater is also used when loading a fully pressurised gas carrier with cargo with temperature less than –10oC. Seawater or oil is used to heat the cargo in the cargo heater. It is of importance to remember that the cargo heater is full of water and have good flow out with water before letting cold cargo into the heater. Fully pressurised gas carriers are carriers that are designed to transport condensed gases at ambient temperature, and they normally don’t have cargo cooling plant.
CARGO CONDENSER: Cargo condensers in a direct cargo cooling plant condensate the vapour against sea water, Freon or other medium as propylene after it is compressed in the cargo compressor. Cargo condensers in a direct cargo cooling plant can on some gas carriers also be used as cargo heaters and are designed in low temperature steel that tolerates a minimum of –50oC. Intermediate cooler An intermediate cooler is used in a 2-stage direct cargo cooling plant and cascade cooling plant. Vapour from the first stage on the cargo compressor is pressed down on the bottom of the intermediate cooler and is condensed against the cargo liquid in the bottom. The cargo compressor’s 2nd stage sucks simultaneously from the top of the intermediate cooler to keep the pressure down. Floaters or D/P-cells regulate the liquid level in the intermediate cooler. The condensate inside the coil came from the cargo condenser and is under cooled by the liquid in the intermediate cooler before it is pressured further back to the cargo tank. Sketch of intermediate cooler
Freon condenser: Water is used to condense Freon in the Freon condenser in a cascade cooling plant. The liquefied Freon is used to condensate the cargo in the cascade cooling plant’s cargo condenser. Liquefied Freon is also used in indirect cargo cooling plants. The condensate is then pumped in pipe coils, and cools either directly on the tank steel or as a cooling medium for ethanol or other mediums. Vapour riser: A vapour riser is used to produce vapour from the cargo liquid. Steam or heated oil is used to heat up and vapour rise the liquid. The liquid is pumped from one of the cargo tank, deck storage vessel or from a shore tank and into the vapour riser. The vapour is used to gas up or maintains the pressure in one or several cargo tanks. Oil coolers: The cargo cooling plants oil coolers use water as a cooling media. The oil coolers must hold the oil temperature on the different compressors within the specifications determined by the manufacturer of the cargo cooling plant.
CARGO COOLING PLANT 6.4.1 Compressors Compressors are used as vapour pumps in all modern cargo cooling plants, either to compress or pump cargo vapour. Compressors are also used to compress or pump cooling medium as Freon vapour on indirect cargo cooling plant and cascade plant. The compressors in the cargo cooling plants are produced either as piston, screw or centrifugal type. We will now look at the different types of compressors and starting with piston compressors. 6.4.2 Piston compressor Piston compressors used directly against cargo are of oil free type. Oil free compressors are used to prevent pollution of oil into the cargo, and thereby contamination of the cargo. All cargoes we are cooling demand a high rate of purity. Consequently, it cannot be mixed with oil or be polluted by other products. With an oil free piston compressor, we mean that the cylinder liners are not lubricated or cooled with oil. Piston compressors that are used against Freon normally have oil lubrication of cylinder liners. Piston compressors are either built with cylinders in line, v-form or wform. Compressors with cylinders in line are built with two or three cylinders either single-acting or double-acting. V-form compressors are built with two, four, six, eight or twelve cylinders and are single acting. 6.4.3 Double-acting compressors Double-acting compressors are normally oil free and compress the vapour above and under the piston. The vapour is compressed on top of the piston when the piston goes up and vapour is sucked into the cylinder below the piston. The vapour is compressed below the piston when the piston goes down and is sucked into the cylinder above the piston. This indicates that each cylinder has two suction valves and two pressure valves. The pistons are equipped with compression grooves and are not equipped with piston rings. There is no oil lubrication of the piston itself, but there is oil in the crankcase on the compressor. It is of importance that the sealing device between the cylinder liner and crankcase is intact. In the first stage, the oil pressure in the crank is checked and compared to the suction pressure and the cargo tank pressure. Check the user manual for the cargo compressors and the marginal values for the pressure difference with oil and suction. This type of compressor is used as cargo compressor onboard gas carriers. It is important to change the oil in the crank when changing cargo. This to prevent pollution to the next cargo from the previous cargo. Small amounts of leakage between the cylinder and crank will at all times occur, so the oil in the crank contains some of the product that is cooled.
6.4.4 Single-acting compressors Single-acting compressors compress and suck the gas on one side of the piston and then normally above the piston. A suction valve and pressure valve is then installed in the top of the cylinder. The cylinder top is spring-loaded as a safety precaution against liquid “knock”. The compressors are built with the cylinders in pairs: two, four, six, eight and twelve, then often as v-form or w-form. Single-acting compressors are used both as Freon and cargo compressors on gas tankers. Piston compressors are operated by electric motor with direct transmission or strap transmission with a constant number of revolutions. The number of revolutions is between 750 to 1750 rpm. Unloading of the compressor occurs by hydraulic lifting of the suction valves. The drawback of piston compressors is that they are vulnerable when the cylinder liner is filled with liquid and they also have relatively low capacity for cooling. Onboard many gas tankers, there is a liquid receiver on the vapour line between the cargo tank and the cargo compressor, which prevents the liquid from being carried with into the compressor. The liquid receiver is equipped with a level alarm to control the liquid level.
6.4.5 Screw compressor Screw compressors are either oil free or oil lubricated. The type used on the cargo side must be of oil free type for the same reason as the piston compressors. The principle for screw compressors are two rotating screws, the screw that operates has convex threads and the operated screw has concave threads which rotates them in different directions. Vapour is screwed through the threads and with rotation on the screws, the confined gas volume decreases successively resulting in compression. Please also refer to “cargo cooling process” for more information.
The advantage with screw compressors is that they wear few parts and have low weight in proportion to cooling capacity. Oil free screw compressors are operated by electric motors with a constant number of revolutions and have a gear transmission for the compressor, which has approx. 12000 rpm. The high speed prevents leakage between the pressure and suction side. Screw compressors with oil injection in the rotor house have a lower number of revolutions, about 3500 rpm. One can also use electric motors with direct shaft transmission.
Oil free screw compressors are used on the cargo side. On the Freon side, compressors with oil injection are used. The oil causes a film on the outside of the rotors that prevents leakage between the pressure and suction side. This compensates for the temperature difference inside the compressor. The capacity of screw compressors is adjusted by a slide, which is inside the compressor. However, when we reduce the capacity the excess gas flows back to the suction side. Screw compressors are not destroyed if they suck liquid, as we find with piston compressors.
Cargo compressors with motors that are installed inside a deckhouse have two parts, one room for the compressors and one room for the motors. The room where the motors are installed is gas safe with a constant excess pressure of air preventing flammable gas from flowing in. If the excess pressure is too low, the power to the electric motor room will be shut off and the cargo cooling plant stops. The shaft from the electric motor room to the compressor room is rendered gas-tight. A mechanical seal device with automatic oil lubrication is normally used. To prevent bearing breakdown, it is important that electric motors and compressors are aligned according to specifications from the manufacturer of the compressor and motor. 6.4.6 Centrifugal compressors On gas tankers, centrifugal compressors are used to deliver vapour to shore or to supply the cargo compressors with vapour from the cargo tanks. Centrifugal compressors are operated by electric motor, hydraulic motor or with steam, and have a gear transmission. The compressor has a number of revolutions from about 20000 rpm to over 35000 rpm. This high number of revolutions sets large demands on accuracy and tolerances at aligning motor and compressor. The centrifugal compressor is built on the same principle as a centrifugal pump. When a centrifugal compressor is used to feed the cargo compressor, it creates a higher suction pressure on the cargo compressor, and thereby gives better cooling capacity. Another area of operation for centrifugal compressors is pumping vapour back to shore tank while loading. The centrifugal compressor can also be used when changing cargo. Either to blows hot vapour or to be used as ventilation fan. The use of centrifugal compressors depends on how flexible the piping system to the compressor is. On gas tankers, the centrifugal compressor is mounted on deck close
to the cargo manifold. The capacity of the centrifugal compressor is from approximate 2000 m3 and upwards.
6.4.7 Indirect cargo cooling plant Indirect cargo cooling plants are used on cargoes that not can be compressed or exposed to high temperatures, as they either polymerise or start chemical reactions. Typical cargo that uses indirect cooling is propylene oxide, ethylene oxide, mixed propylene oxide and ethylene oxide and chlorine. There are some different methods for indirect cargo cooling. One type of indirect cargo cooling plant use the discharge pumps and pumps the cargo liquid through a Freon heat exchanger and back to the cargo tank. This method is energy demanding as we have to use discharging pump, Freon cooling plant and seawater pump to control the cargo temperature. On this type of cooling plant the discharge pump should be of submerged type, deepwell pumps can also be used but we must try to avoid running those while at sea. Deepwell pumps with revolution regulation can be used if the ship is not rolling or pitching to mush. 6.4.8 Indirect cargo cooling plant with utilisation of discharge pump Another indirect cargo cooling plant resembles the first a lot, but the discharge pumps are not used. Instead the cargo vapour is condensed in a Freon heat exchanger and the condensate is pumped back to the cargo tank with a small pump. This indirect cooling requires less energy than if one also uses discharge pumps. A third indirect cargo cooling plant also uses a Freon cooling plant where cold Freon liquid is pumped to a coil installed inside the top of the cargo tank or is welded around the outside of the cargo tank. The Freon compressor sucks Freon vapour from the Freon liquid collector then presses the vapour to the condenser where it is condensed against seawater. One can also use ethanol in this cooling system; ethanol is then pumped round in the coils and Freon is used to cool down the ethanol.
6.4.9 Indirect cargo cooling plant with utilisation of Ethanol in coil round the cargo tank
6.4.9 Direct cargo-cooling plant A direct cooling plant is used to control temperature on cargoes as LPG, Isobutane, Ammonia and some chemical gases like VCM, Propylene and Butadiene. Common for all direct cargo cooling plants is that the cargo vapour is compressed directly in the compressor. It is the seawater temperature and the type of cargo that decides which condensation pressure is achieved provided that the cargo is pure. If one for example has loaded Propylene and the seawater temperature is 20oC, the condensation pressure will be approximate 9 bars. The pressure needed in proportion to the temperature is located in the density table for the actual cargo.
SKETCH OF TWO-STAGE DIRECT COOLING PLANT
Direct cargo cooling plants are operated as one or multistage, dependent upon the type of compressor, the cargo and the temperature on the seawater. Most gas carriers that are designed for LPG have direct cargo cooling plants that can be operated as a one-stage or multistage operation. With one-stage direct cooling, vapour is sucked by the cargo compressor from the cargo tanks. The vapour is then pressed to the condenser and assembles in the liquid collector. The liquid level in the collector is regulated either by two floaters or the differential pressure above the liquid level in the liquid collector. The condensate is pressured back to the cargo tank from the liquid collector via a regulation valve and in the condensate return line. To use one-stage cooling, the pressure difference between tank pressure and condensate pressure must be less than 6 bars. With 2nd stage direct cooling without an intermediate cooler the cargo compressor sucks from the cargo tank with the 1st stage cylinder. The vapour is thereby pressed to the compressors 2nd stage suction side and then to the cargo condenser where the vapour is condensed against seawater and collected in the liquid collector. The liquid is pressured back to the cargo tank via a regulation valve and the condensate return line from the liquid collector. The pressure in the liquid collector is equal to the pressure in the cargo condenser, and is at all times higher than the cargo tank pressure. 2nd stage direct cargo cooling plant is delivered with or without an intermediate cooler. Some direct cargo cooling plants are delivered with intermediate cooler (inter cooler), this achieves lower temperature and pressure on the 2nd stage suction side. These cargo cooling plants are used on semi-pressurised LPG carriers and atmospheric pressure LPG/ NH3 carriers.
6.4.11 Cascade cooling plant / direct cooling Cascade cooling plant is basically a direct cargo cooling plant where the cargo is condensed against Freon and Freon is condensed against water. Sketch of cascade plant
In a cascade cooling plant there is a Freon cooling plant in supplement to a direct cargo cooling plant. The Freon cooling plant contain of a compressor, Freon liquid collector, oil collector, Freon separator, regulation valve and pump. Freon are condensed against water, and the Freon condensate is pumped from the separator to the cargo condenser where the cargo is condensed. The Freon plant have to be started first, so the condensation and circulation of Freon in the cargo condenser is normal. There must be accuracy in the start-up to prevent oil from leaking with Freon and removing the condensation effect. When the Freon plant operates normally, one can start the cargo compressor. On the cargo side in a cascade cooling plant there is mostly 2nd stage direct cargo cooling plant equipped with compressor, intermediate cooler, cargo condenser, liquid collector and regulation valve. The vapour is sucked from the cargo tank and into the cargo compressor’s 1st stage. The vapour is then pressured to the intermediate cooler where it is condensed against the liquid in the bottom of the intermediate cooler. The cargo compressor sucks vapour with the 2nd stage from the top of the intermediate cooler and press the vapour to the cargo condenser where the gas is condensed against Freon. The condensate is then pressured against a coil in the intermediate cooler and further through a regulation valve to the condensate line, and back to the cargo tank. This type of cargo cooling plant is used on semi-pressurised LPG and LEG carriers, and on large atmospheric pressure LPG carriers. A cascade cooling plant must be used for condensation of Ethane and Ethylene, but can also be used for Propane, Ammonia and Propylene. Some cascade cooling plants are constructed for use as a
two or one-stage direct cargo cooling plant. Generally when cooling Butane, Butadiene or VCM one can also condensate Propane and Propylene directly if the seawater temperature is low enough. This type of cargo cooling plant has a lower dependency of the seawater temperature than a direct cooling plant. The larger volume of Freon, seawater temperature has less influence on the plant. It is difficult to cool regardless of the kind of cooling plant’s if the surrounding temperature e.g. seawater temperature is higher than 35oC. 6.5 INERT GAS PLANT On gas carriers inert gas is used for different purposes, some are requirements other is to maintain the ships hull and spaces:
·
Have neutral atmosphere in hold and inter barrier spaces
·
Elimination of cargo vapour from the cargo tank when gas freeing
·
Eliminating oxygen from the cargo tank before loading
· Drying up hold spaces or inter barrier spaces to achieve a neutral atmosphere and to prevent corrosion in the spaces ·
Placing a neutral vapour above the cargo if required
When carrying flammable cargo on fully refrigerated gas carriers there is a requirement to have a neutral atmosphere in the hold space or inter barrier space either with dry inert gas or nitrogen. If the gas carrier does not have an inert gas plant or nitrogen plant, it must have a storage vessel with inert gas or nitrogen with capacity of 30 days and nights consumption. The definition of consumption here is the leakage in the vents and manhole. If the cargo is not flammable we can have dry air, inert gas or nitrogen in the spaces.
If the cargo is Ammonia, one must not use inert gas that contains carbon dioxide, only dry air or nitrogen, because carbon dioxide reacts chemically with Ammonia. It is always beneficial to keep spaces around the cargo tanks dry.
The inert gas generator is built up with a combustion chamber, scrubber, O2 analyser, dryer and heater. The fuel oil is injected into the combustion chamber, mixed with air, combusts and flue gas or inert gas is formed. The inert gas is blown through the scrubber where carbon particles and sulphur is washed out with the water. The O2 analyser measures the O2 content against the stated limits before the inert gas is blown further into the dryer. There is two types of dryers for inert gas plants either Freon dryer or absorption dryer. The most common is the Freon dryer.
After the scrubber the inert gas is cooled in a dryer to reduce the dew point. With use of Freon dryer the dew point will be minimum 5oC. Water is condensed out while the dew point is reduced and we have to avoid temperature to sink below 0oC so we do not clog the inert channel with ice. In an absorption dryer the inert gas is compressed with a compressor and pumped through a material that absorbs water and the dew point sinks to minimum –80oC. Strict demands are made regarding inert gas plants on gas carriers. IMO makes demands for maximum content of 5% O2 by volume. Inert gas is produced on gas tankers by their own inert gas generator. Inert gas produces by consuming gas oil, diesel oil or light fuel oil. The O2 content in the inert gas adjusts by quantity of air added to the oil that is injected into the combustion chamber. To achieve as pure inert gas as possible, very good combustion is required. A rich oil/air mixture gives a lot of carbon, high content with Carbon monoxide and low O2 content. A lean mixture (more air) gives higher O2%, less carbon and less carbon monoxide. The air/oil mixture is produced manually or automatically on the control board. 6.5.1 Sketch of inert gas plant
6.5.2 O2 Control O2 analyser is connected to a two-way valve where the inert gas either can be sent to a ventilation mast or to a consumption unit (dryer, tanker). The limit value is set manually between 5% by volume O2 and the minimum value for the inert gas generator for example 0,3% by volume O2. The inert gas then automatically goes to the vent mast if the O2 content reaches more than 5% by volume or below 0,3% by volume. O2 content is set to the required O2 volume, for example 1% by volume. The inert will then go to the dryer and is consumed when the O2 content is between 1% and 0,3%.
6.5.3 Drying The inert gas is saturated with water when it comes out of the scrubber, that means 100% humidity. The temperature on the inert gas after the scrubber is about 5oC above the seawater temperature. The inert gas therefore must be dried before it is sent to the cargo tanks, hold space or inter barrier space to prevent condensing of water into the tanks or spaces. The inert gas temperature should be higher than the atmosphere that one will inert. Inert gas dryer is a Freon heat exchanger, absorption dryer or a combination of both. 6.5.4 Freon dryer Freon dryer are frequently used and require less space in proportion to an absorption dryer. The principle with the Freon dryer is that Freon flows through small pipes in the inert gas channel. The inert gas is cooled down and thereby condensate the water from the inert gas when it passes the Freon pipes. The Freon is condensed in its own cooling plant. The temperature of the inert gas after the Freon heat exchanger must not be less than 5oC. The inert gas that comes out of the Freon dryer has a dew point of about 5oC and a water content of 6,75 gram per m3 inert gas. 6.5.5 Absorption dryer With use of absorption drier the inert gas is then pressed through a medium that absorbs water, for example silica gel or Aluminium Oxide. The inert gas has a temperature a bit above seawater temperature when entering into the dryer blower. The temperature of the inert gas is higher when it emerges from the dryer, from 30oC to 60oC, depending on the required dew point. The result of temperature increase is that the compressor compresses the inert gas. One can have an inert gas dew point down to –80oC with an absorption dryer, but the inert gas volume that is delivered for consumption decreases. The inert gas contains 0,0013 gram water pr m3 at a dew point of –60oC. Inert gas with temperature of 40oC and dew point at – 60oC has a relative humidity of 0,025%.
6.5.6 Inert gas heater An inert gas heater is a heat exchanger where steam or an electrical coil is used for heating the inert gas. The dried inert gas can absorb more humidity when it is heated. Heating reduces the relative humidity on the inert gas. The relative humidity is 28,72% at a temperature of 25oC with Freon dryer and an inert gas dew point of 5oC. If the inert gas is heated to 50oC, the relative humidity will sink down to 8,13%. It is of importance that one first removes humidity, and thereby heats the inert gas so it can absorb more humidity. The inert gas dryer and heater can also be used in connection with venting tanks and spaces with air. To maintain the function of the inert gas generator to specification, one must run it regularly, generally once a week and preferably several hours each time. This is a good opportunity to refill spaces and lines, which are not used.
6.5.7 Sketch of thermal drier
A B C D E F S
Drying tower Tower that is dried Heater Cooler Fan Water separator Solenoid valves
6.5.8 Sketch of pressure swing drier
6.5.9 Composition of inert gas and dew point
AFETY VALVES Gas carriers must have safety valves on all cargo tanks, spaces and cargo lines where cargo liquid residue may remain. Cargo tank’s safety valves are either pilot (pressure loaded) or spring loaded valves. Spring-loaded valves are normally used on
fully pressurised tankers and semi pressurised tankers with a tank pressure above 0,7 bars and on cargo lines. The pressure loaded valves are normally used on atmospheric pressure tankers and semi pressurised tankers. There must be two safety valves on all kinds of pressure vessels on more than 20 m3. There are also demands that there is a safety valve on all kinds of pressure vessels below 20 m3. The maximum set pressure on a cargo tank’s safety valves depends on the cargo tank MARVS. MARVS is maximum allowed safety valve set point. The pressure required by MARVS is located in the gas carriers IMO Certificate of Fitness. The cargo tank safety valve must be located on the tank’s highest point above deck. Each safety valve must be connected to vent mast without impediment or valves. The vent mast’s outlet must be at least B/3 or 6 meter above weather deck or gangway, B is the ships breadth. The distance should at least B or 25 meters from the nearest air inlet or opening in the accommodation. This distance can be shortened for gas carriers of less than 90 meters in length, but the flag state authorities, for example Norwegian Maritime Directorate, must approve it. All safety valves on cargo tanks must be prototype tested and approved by IMO and the gas carriers class company. The cargo tank’s safety valves must be tested within the IMO limits +-10% for 0 to 1,5 bars, +-6% for 1,5 to 3 bars and +-3% for 3 bars and higher pressure. The tanker’s class company has to seal the safety valves after authorised personnel have tested and calibrated the safety valves.
6.6.1 6.6.1 Cargo tank safety valve’s function Safety valves used on cargo tanks have one or more pilots to hold the valve closed. The safety valve contains of an adjusting spring, three membranes, two valve seats, an exhaust pipe and an equalising pipe. The pilot is adjusted by a pilot spring in order to get the needed pressure, for example 0,5 bars. The pilot valve’s seat is attached to two membranes and the pilot spring. The pilot main valve seat is attached to the main valve membrane. The pilot valve is connected to a pipe on the highest point on the cargo tank. There is the same pressure below and above the main valve seat and on the below the boost membrane when the pilot valve is shut. When the pressure in the cargo tank is higher than the pilot valve’s setting, the boost membrane will lift, pull the pilot seat up and the pressure above the main valve membrane is ventilated to the atmosphere. The pressure will now be higher above the main valve seat than below and the valve is open and vapour is ventilated to the vent mast. When the cargo tank pressure sinks again, the boost membrane will sink and the pilot seat will go to the shut position. The pressure above the main valve membrane
increases to the same pressure as in the cargo tank. The main valve seat will then be closed and the valve shut. 6.6.2 6.6.2 Example of a tank safety valve There are extra setters that are installed on the pilot valve to achieve the right set point on fully pressurised tankers and semi-pressurised tankers. The setter consists of an adjusting spring with spring tension equal to the pressure, for example 2,3 bars. When the setter is screwed down on the pilot, the set point will be at 2,8 bars. The cargo tank safety valves on atmospheric pressure tankers are normally the membrane type. The principle is the same as with seat valves. When the valve is shut there is equal pressure under and over the main membrane and under the boost membrane. When the pressure is higher than the pilot setting, the boost membrane in the pilot will press the pilot seat up and the valve start to open. When the pressure sinks, the pilot seat is pressed back and shut. The valve opens when the tank pressure exceeds the spring tension. When the tank pressure sinks below the spring tension, the valve shuts again. An adjustment screw is attached on top of the valve that is used for calibrating the spring tension.
On fully refrigerated gas carriers there is often options to mount extra weights during loading or change of cargo. The extra weights are mounted on top of the pilot and increases the set point with approximates 100 to 150 grams.
THE EXTRA SETTER IS NOT ALLOWED TO HAVE ON THE PILOT WHILE THE VESSEL IS AT SEA. 6.6.3 6.6.3 Safety valves on cargo lines/ pipes Seat valves are mainly used as safety valves on lines. These safety valves are springloaded and must be according to the certified line pressure. The set point and the number of the different safety valves can be found in the gas carrier valve list. The safety valves must be overhauled, pressure tested and calibrated by authorised personnel. Then sealed and by the ships class company. Example on safety valves on cargo lines/pipes
07-
Monitoring and control
7
MONITORING AND CONTROL
All gas carriers must have a minimum instrumentation for control of cargo and cargo equipment. The instrumentation varies from local instruments to remote instruments. The cargo control room or instrument room is located in a gas safe area, normally in connection with the deckhouse. 7.1 LEVEL GAUGES A level gauge for cargo tanks is normally of float type or radar type. The float type can be controlled by a guide-wire or placed inside a perforated pipe. The float’s movements are transferred to a counter through a steel band. The counter are normally local, but on new ships it is also remote reading. It is of importance to check that the counter is at the correct level. On the counter, there are marked values indicating the reading when the float is at the bottom and on the top. Control and necessary adjustments are made using these values. To avoid damage to the float and band, it is of importance that the float is hoisted and locked when the ship is sailing. Other types of floats are connected to an arm with a switch, which sets off or on an alarm circuit when the liquid reaches a given level. This type is used as level alarms in cargo tanks and in small tanks as liquid receivers. The liquid separator on the suction side of the compressor is an example of a tank equipped with this type of alarm. If liquid is carried away with the vapour to the compressor, the liquid will assemble in the liquid separator on the vapour line. The float in the liquid separator will at a stated level float up and actuate an alarm and then shutdown the compressor and liquid is prevented to came into the compressor.
7.1.1 High-level alarm To prevent over loading of cargo tanks high level alarms are required. There are two separate floats that give the alarms, one is high-level alarm the other is high-highlevel alarm. The high-level alarm is activated when the cargo tank is nearly full, normally at 95% filling. The high-high-level alarm is actuated at 98% or 99% filling. Valves on the actual tank are closed and discharge pumps are automatically stopped when liquid reaches the actual level. To open the valves and start the pump again the 98%/ 99% circuit has to be reset and switched off.
7.1.2 Emergency Shut-Down System (ESD) All gas carriers are equipped with an emergency shutdown system that is manually activated from at least two locations onboard. It closes the liquid valves in the cargo pipe-system. At least one of the locations must be located outside the cargo area or in a cargo control room. The system is normally a part of the cargo valve’s control system and both pneumatic and hydraulic system is used. The emergency shutdown system must be “fail safe”, that is by loss of pressure or voltage, the valves are shut. A spring or lack of pressure closes most of the actuators used in the ESD system. The liquid valves should fully close under all service conditions within 30 seconds. Thermal fuses (fusing plugs) that activate the emergency shutdown system are located on tank-domes and manifolds. On some gas carriers the fuses also activate the water spray system. If fire ignites or temperature rise of other reason, the fuses have to melt at temperature between 98oC and 104oC. When the emergency shutdown system is activated, the power circuit to the cargo plant is broken and compressors and discharge pumps will stop. Some terminals require that the ESD system onboard must be activated also from the jetty when handling cargo. Most gas carriers therefore have the possibility to place an extra ESD release button on jetty. For gas carriers with MARVS above 0,7 bars it is required that all pipes connected to the cargo tanks, with exception from pipes to the safety valves and instrument pipes, are equipped with remote-operated valves. Gas carriers with MARVS below 0,7 bars only require such valves on the manifold. A pneumatic or hydraulic actuator placed on the valve operates the valve in the pipe system and can also be remote controlled. The connected actuators for valves in the pipe-system have an adjustable closing time. Adjustable closing time is a requirement from IMO to avoid over loaded cargo tanks or pressure surge on the terminal pipes. The valve’s closing time can be regulated by adjustment of a needle valve on the actuator.
7.1.3 Pneumatic system In cargo control systems pneumatic is used to control cooling plants and valves. The emergency shutdown system can be complete or partly pneumatic. Pneumatics is also used to control the water spray-system and regulate the cargo cooling plant. For a satisfactory function of the control air systems it is of importance that the air is as dry as possible. Poor air-treatment and humidity is the major cause of interruption in such systems. Regular control and maintenance of the air dryer, normally by the “heatless dryer” type, is also of importance. 7.1.4 Example of pneumatic ESD system
7.1.5 Fixed gas detector system It is required that all gas carriers are equipped with a fixed gas detector that continuously measures the concentration of hydrocarbons in the atmosphere, and activates an alarm if the concentration exceeds 30% LEL. The fixed gas detector is normally located on the bridge or in the cargo control room. The detector pump sucks the atmosphere from sample points to the sensor from selected areas around the vessel. The following areas are specified in the IMO’s Gas code and must be monitored by this type of equipment: • • • • • •
· Compressor room · Electric motor room · Cargo control room unless designated as gas-safe · Airlocks · Hold spaces for all cargo tanks except for independent tanks type C · Other areas in the cargo area where flammable mixtures may accumulate • · Sample points at ventilation inlets to the accommodation, in the engine room and in the forecastle.
It is required that the detector must measure all sample points consecutively and not exceeding 30 minutes intervals on the same sample point. Both audio and visual alarms should be activated. The next page illustrates a flow sheet for one type of gas detector. The pump sucks continuously from all points, but only one point is measured at a time. When a point is measured, a precise time purges the instrument before measuring the next point. If the concentration at a measure-point exceeds 30% LEL, the alarm is released. An indicator panel on the instrument indicates, at all times, what point is measured and from where the alarm is released. If restrictions or similar in the suction-line take place, a “flow-failure” is released. The gas detector has a fixed connection for sample gas for calibration. Calibrating and testing the equipment must be done regularly. This is normally a routine that is executed once a week. It is of importance that calibrating and testing is logged. The instrument should regularly be calibrated for the cargo carried onboard. This is used to adjust the difference between the span gas and the cargo onboard.
7.1.6 Fixed gas detector
7.2 LNG CARRIERS AND VAPOR BOIL-OFF A re-condensation plant for LNG is both complicated and expensive. It is therefore more moderate to utilise the boil-off from the cargo as fuel. The amount of boil-off from a LNG ship lies at about 0,25% per day and night. For an 85 000 m3 tanker this covers about 60% of the fuel need. For a large LNG carrier the boil off covers a larger share of the fuel needed. The boil-off can be utilised as fuel in boilers, dual-fuel engines and gas turbines. Strict regulations are made for control and security when we use LNG as fuel, especially regarding ventilation. Emergency shutdown and double shut-off valves are essential on the vapour line. With a stop of the plant, the vapour line is flushed with nitrogen. Boilers and engines are both made for oil and gas. It is required that at all times we must use about 5% fuel oil on the pilot burner. This secures that all vapour in the pipe to the boiler is continuously set on fire. Example 1: A loaded LNG carrier of 87 000 m3 has a heat transfer to the cargo tanks of 700kW. The cargo temperature is -160 0C. How much vapour boils off per day at unaltered cargo temperature? The size of the heat transfer to the cargo tanks determines how much methane that is boiled off. We find specific heat of vaporisation for methane in the heat technical table to be 506,2 kJ/kg at -160 0C. This indicates that 506,2 kJ must be supplied to evaporate one kg methane. The total amount that evaporates at a heat transfer of 700 kW is then: (heat transfer x time ) / specific vaporisation 700 kW x (24 x 3600)s / 506,2 kJ/kg = 119.500 kg/day = 119,5 MT/day The ship can during the existing condition consume 119,5 MT per day and night for propulsion. This corresponds to 0,33% boil-off per day. The ballast journey to the next load port must be planned thoroughly when we use the boil-off as fuel on the boilers and machinery. The quantity needed of cargo for propulsion and cooling of cargo tanks must be calculated before commence discharging. The amount of boil-off from the cargo tanks is regulated with the spray pumps.
7.3 ELECTRICAL EQUIPMENT IN GAS HAZARDIOUS AREAS Electrical equipment should be of a type that eliminates the chance of fire or explosion when carrying flammable cargo. Precautions must be made to exclude spark sources from areas where flammable gases may appear. On shore one operates with different zones of explosive areas, as you found in the following table: 7.3.1 Zone 0
Explosive gases are present continuously or in long periods.
Zone 1
Explosive gases are present in periods during normal operating conditions.
Zone 2
Explosive gases are rarely present and occur only in short periods.
If applying these criteria to ships, zone 0 includes cargo tanks with cargo handling equipment, zone 1 includes the remaining part of a gas hazardous area and zone 2 includes the remaining part of the cargo area. Rooms containing cargo-handling equipment, for example, cargo compressor rooms must also be treated as zone 1.
7.3.2 Gas hazardous areas Gas hazardous areas are defined onboard as the areas in the cargo tank area that have no approved arrangement ensuring that the atmosphere has, at all times, no content of flammable mixtures. The areas are clearly specified in the regulations. Areas outside the specified gas hazardous areas are considered as gas safe. The electric motor room or the control room is an example of a gas safe area. These rooms are considered as safe in all circumstances provided that the ventilation system prevents flammable mixtures penetrating into the rooms. Normally this is arranged with the suction pipe for the ventilation to the rooms are located outside defined flammable areas and the room has always an overpressure. Access to the room is through a airlock, and the room is made “dead” in case of a failure in the ventilation system. GAS HAZARDOUS AREAS ARE SPECIFIED IN THE REGULATION ARE, AS FOLLOWS: • · All cargo tanks and rooms containing cargo tanks or pipes. • · Pump room, compressor room and other rooms where gas appears. • · A radius of 4,5 meters around ventilation outlet from the compressor room and pump room.
• · A radius around the ventilation outlet from cargo tanks at 9 meters. • · A zone on open deck in the cargo tanks area up to 2,4 meters above deck and 3 meters ahead and aft of the cargo tank area. • · A radius of 3 meters around pipe lines for cargo. All vessels are issued with a drawing that indicates these areas. The drawing normally named, “Hazardous Areas”, is posted around the vessel for the crew to familiarise themselves with these hazardous areas.
The conditions onboard and around operation of gas carriers are special, and the rules are more restrictive than ashore. The equipment onboard is exposed to hard strains, the maintenance is often more complicated, and none the less, the consequences of fire or explosion are very seriously. Ashore there are individual national rules. For gas carriers and other ships the Flag State, Class Society and IMO rules apply. Terminals and charterer also have individual rules that have to be followed. Special rules exist for pump room, compressor room, pipe tunnels, control room in the cargo tank area, electric motor room and the area on the forecastle. Access to electric motor room and cargo control room occurs through an airlock. These rooms must have a mechanical positive pressure system. The doors must be equipped with alarms and if both doors are left open too long and the pressure drop, the rooms automatically render “dead”. This is normally arranged by a DP-cell (differential pressure cell) that measures the pressure difference inside the room and the surroundings. Compressor room, pipe tunnels and pump rooms must be equipped with explosionproof lights. Cargo control, monitoring and automation equipment must be independently secure in a flameproof enclosure. The light arrangement disperses over at least two switchers, and all switches and protection equipment must be placed in a gas-safe area. Electrical cables are not allowed to be open exposed throughout the room. Explosion-proof and independently secured equipment are approved to be used in gas hazardous areas on deck. Automation and communication equipment must on the other hand be of independently secure enclosure.
THE FOLLOWING LIST GIVES A SUMMARY OF APPROVED ENCLOSURES WITHIN THE SPECIFIED FLAMMABLE AREAS:
Area:
Approved enclosure:
Pump room and compressor Electric motor and lighting fixture must room be Ex d. Alarm , automations and communication equipment must be Ex ia Cargo tanks
General demand for Ex ia
Open deck area
Ex d, Ex e, Ex ia and Ex ib are all approved.
The above-mentioned different types of electrical equipment characterise “explosion proof equipment”. This equipment is divided into three groups; each of them approved for its stated area. One must not fail to believe that if the equipment is “explosion proof”, it can be used in all flammable areas onboard. 7.3.3 Flameproof proof enclosure English: Flameproof enclosure American: Explosion proof German: Druckfeste kapslung CLC symbol: d Flameproof equipment does not secure against penetration of vapour. The enclosure must be mechanical and strong enough to resist the inner pressure that can arise because of an explosion. Eventual spurt of flame and warm gases will be sufficiently chilled to prevent ignition of the same gas mixture outside the equipment. Manufacturing longer openings with the help of threads and fissures constructively solves this. As we can see, the safety of this equipment depends upon whether the equipment can resist an interior explosion or not, and that warm gases and flashes maintain a long enough distance to be chilled. The weakness in this type of equipment is also here. The equipment with threads and fissures are especially exposed to corrosion and salt deposit. If the small passages are blocked or corroded, the safety is lost and the equipment can not be used. Equipment for high effect, like for example electric motors for pumps, has this design. Two different types of methods are used for cable inlet to explosion safe equipment direct or indirect inlet. The nipple that is used to direct the cable inlet for explosion proof motors consists of many parts that must be installed right to maintain safety.
This type of cable inlet is mainly used on English or American equipment. This type of equipment is delivered finished bored and threaded. There are big problems in locating the right compensation, if the nipple need to be changed, because the replacement must be the same type as the exchanged. It is not allowed to make adjustments for a new nipple, as such a modification may weaken the strength of the enclosure. Bulkhead pipes from a flammable to a gas proof area must be compressive. Special cable boxes filled with a special expanding type compound must be used for this purpose. At the indirect cable inlet to explosion proof equipment a connection box of an intrinsically safe design is used.
7.3.4 Increased safety English: Increased safety German: Erhohte sicherheit CLC symbol: e This design secures against high temperatures and formation of flame/sparks in the equipment during normal operation. The design is mostly used on illumination and connection boxes and in combination with other types of enclosures. Explosion proof electrical motors of type d are normally equipped with an indirect cable inlet of increased safety type. Increased safety can be used in some flammable areas. This is approved for zone 1 ashore. The special conditions onboard limits the user areas some for this equipment. 7.3.5 Intrinsically safe English: Intrinsically safe German: Egensicher CLC symbol: i
During normal operation or by error, an intrinsically safe enclosure will not be able to produce spark or heat by such a character that ignition of a flammable gas mixture is operative. Current and voltage limits by transistor relays or zener batteries maintain the safety. Intrinsically safe equipment only uses voltage of 8-12 V and current of about 5 milliampere. Test equipment that produces high voltage can not be used on intrinsically safe areas. The circuits can be destroyed and at worst form sparks in the hazardous area. All intrinsically safe equipment must be delivered with certificate from an approved test institution. Installation of the equipment must be executed with care. To secure misconnection or interference of the equipment, the cables must lie in their own canals, their own terminal blocks and preferably of blue colour. The distance to other cables should be at least 50 mm to protect the intrinsically safe circuit. Intrinsically safe relays and barriers must be placed in a gas proof area. Intrinsically safe enclosure divides into two categories:
ia Equipment not capable of causing ignition of flammable mixture in any combination of two imaginable errors with the equipment. The whole circuit is approved. ib Equipment not capable of causing ignition of a flammable mixture regardless of what simple error arises. Only the barrier is approved.
7.3.6 Other designs On installations ashore other explosion proof designs are used. Equipment can be filled with oil or sand to prevent penetration of flammable gases. This type of equipment is marked Ex o for oil and Ex q for sand. Excess pressure design, Ex p, prevents penetration of the surrounding atmosphere with help of a protective gas with higher pressure than the environment. Such equipment requires surveillance of the pressure. If the pressure falls below a precise limit the equipment dies. A complete electrical plant with this design is not relevant. But the principle use, as for example, for an electric motor on a gas vessel. Special design, Ex s, appears in combination with another design. Ex s is not a pure intrinsically safe design. 7.3.7 Classification of Ex equipment IEC, International Electro technical Commission, is an assembly with the task to make standardised recommendations about electrical equipment. This recommendation is published as IEC Recommendations. CENELEC, The European Committee for Electro technical Standardisation, has the task from IEC to make a standard for electrical equipment in hazardous areas. Equipment classified and marked by this standard will be approved in all of EU. The standardisation of flameproof equipment will in time ease the work by choosing the
right equipment at installation and replacement, plus execute necessary control on installed equipment. The equipment in use today is marked by a number of different standards. We will concentrate on the three most relevant:
CLC VDE USA
that signifies the European standard that signifies the former West-German standard that signifies the USA’s standard
Flameproof equipment is tested and classified considering the different gases. The gas ignition energy, spontaneous ignition temperature and explosion progress are the primary factors. THE DIFFERENT GAS QUALITIES ARE DIVIDED INTO THE FOLLOWING EXPLOSION GROUPS:
CLC VDE USA Example of gas: I 1 Gr. D Methane (pit gas) II A 1 Gr. D Methane (industrial) II A 1 Gr. D n-Pentane II A 1 Gr. D Petroleum gases II B 2 Gr. C Ethylene II C 3b Gr. B Carbon disulphide II C 3n Gr. A All gases Equipment marked explosion group “I” is used in mines. The other equipment is for use in the industry. The different flammable gas mixtures have different spontaneous ignition temperatures. To secure electrical equipment, which comes in contact with the relevant gas mixtures and has a surface temperature lower than the gas mixture spontaneous ignition temperature; the equipment is marked with a temperature class.
Spontaneous ignition Temp: >450 oC >300 oC >200 oC >135 oC >100 oC >85 oC
CLC
VDE
USA
Example of gas:
T1 T2 T3 T4 T5 T6
G1 G2 G3 G4 G5
T1 T2 T2 D T3 T4 A T5
Acrynolitrile (481 oC) n-Pentane (309 oC) Acrolein (278 oC) Acetaldehyde Carbon disulphide
For CLC-classes, the maximum surface temperature follows the spontaneous ignition temperature of individual gas. The former West-German VDE-classes operate with both a spontaneous ignition temperature and a maximum surface temperature of the equipment. The surface temperature is stated a bit below the spontaneous ignition temperature. As an example, a gas that falls under the temperature class G1 is allowed a maximum surface temperature of 360oC. From the European Standard, all electrical equipment in hazardous areas must be marked, for example Eex d IIB T3 where:
EEx d IIB T3
The equipment is tested and classified according to the European Standard for utilisation in hazardous areas. The enclosure is flameproof. The equipment falls under explosion group IIB The equipment is tested and will at maximum have a surface temperature of 200 oC.
7.3.8 Protection of electrical equipment Installation of electrical equipment must be of a precise strength and resistant to the environment it is exposed to. Demands are made on the equipment from classification companies. A normal demand is that the equipment must bear the effect of 1 kg that falls down on the equipment from a height of 0,7 meters. Tests are also executed regarding chemicals. If the equipment is on open deck and lacks resistance to sun and rain, a cover must be installed above. Normally electrical equipment is constructed to bear a surrounding temperature of between 40oC and – 25oC. A table indicating what the different protection extents imply:
Grad: Physical contact: Protection against dust and foreign objects: IP 00 No protection No protection IP 20 Against contact Against constant with fingers. particles larger than 12 mm in diameter. IP 41 Against contact Against constant with tools particles larger than 1 mm in diameter IP 43 Against contact Against constant with tools particles larger than 1 mm in diameter. IP 54 Against contact Against damaging with tools dust. IP 55 Against contact Against damaging with tools. dust IP 56 Against contact Complete with tools. protection.
Protection against water penetration: No protection No protection.
Runny water.
Water spray.
Water jet. Water jet. Water jet.
Most classification companies require that equipment on open deck must have a protection that corresponds to IP 56. As it may be difficult to obtain all electrical equipment with this protection, exception for equipment used ashore is made. A minimum protection of IP 44 is required. This equipment must be covered with, for example, a tarpaulin when the vessel is sailing. 7.3.9 Portable electrical equipment Demands on portable electrical equipment for use in hazardous areas are the same as for fixed installations. 7.3.10 Maintenance and installation Ex-material must not, as a rule, be repaired onboard. A flameproof enclosure with damaged wards must be scrapped. One must not be tempted to make new and larger wards. There is no control of the mechanical strength of the enclosure after such a repair. Repair of flame splits must not be executed. The best way to look after, and thereby to extend the length of life for this expensive equipment, is regular inspection. External protection against corrosion is of importance. Flameproof electric motors where end covers or bolts are wrecked by rust, are not safe and will entail duty of replacement at inspection. The flame splits are opened for cleaning and application of special “grease” that protects the surface against corrosion. Rejection of large electric motors often occur in connection with the ordered 5-year control where the flame splits are so corroded that it is no longer proper to repair these.
A flameproof enclosure does not prevent gas, and thereby water, from penetrating into the equipment. Pay special attention to the accumulation of water in such equipment in regions with large temperature variations. Choose Ex e in cable insertion to Ex d equipment if possible. A normal PG nipple is then sufficient and installation/control/maintenance is much easier. Deck light in hazardous areas is flameproof, which increases safety. This equipment is cheaper, easier to maintain, and easier to control than a flameproof lighting fixture. All electrical equipment must be grounded. Indirect cable insertion forms no problems, since all earth connection takes place inside the connection box. When dismantling equipment, the cables must be disconnected at the feed line end’s connection box or the exposed ends should be terminated temporarily at the connection box of the same design as the dismantled equipment. New installation or changes of electrical equipment demands approval from classification companies. If the equipment is removed from its area, the same rule is regarded. Electrical motors that are coiled normally must have new certification. 7.4 SEAWATER SYSTEM Seawater is utilised for the following purposes in the cargo tank area: • ·
Cooling and condensing of cargo in heat exchangers and condensers for cooling plant. • · Heating of cargo in a heat exchanger during discharging/ loading. • · Condensing of Freon in Freon condensers. • · Cooling/ heating of glycol in a heat exchanger. • · Fire fighting in a deluge system.
Seawater for the above-mentioned purpose is delivered from pumps in the machine room. Dedicated pumps are normally used for the individual groups of utilisation areas. LPG-pumps deliver water to heat exchangers in the cooling plant and cargo equipment. All vessels have dedicated pumps for fire fighting. The seawater wires to the users should be on deck. If these are placed in hold space, the consequences of leakage may be expensive. Water from the cooling plant is normally put out through hold space amid ship on both sides. The valve is operable from deck over board. 7.4.1 Glycol system Enclosed is an example of a glycol system onboard a gas vessel. Glycol is utilised in systems where the danger of seawater freezing is large. By mixing 60% glycol in water, the freezing point is let down to about –55oC. Another good glycol quality is that it’s not as corrosive as water. A cargo-cooling compressor has both cold and warm areas. Parts of the suction side can get down to –60oC while the warm outlet side can be more than 100oC. A “cooling system” for such a compressor has the duty to both warm the cold areas and to cool the warm areas of the compressor.
7.4.2 Example of a glycol system
7.4.3 Example of Water spray
08-
Safety and environment
8 SAFETY AND ENVIRONMENT 8.1
FIRE FIGHTING IN GENERAL
There are two conventions in particular that deals with safety at sea. One is the “International Convention on Load Line, 1996, that was adopted at an IMO conference in 1996. The other is the “International Convention for the Safety of Life at Sea” as Amended/SOLAS 1974. This international convention was signed in London on the 1.st November 1974. It was 68 different nation at this conference, where the purpose was to come to a decision, as quickly as possible. Both conventions are valid in Norway and are included in the required inspection of ships. The Safety convention and all rules are in the “SOLAS 1974” with supplement 1 and 2 were translated to Norwegian. It is this convention that establishes firefighting arrangements etc., with which we have to comply. The Safety convention is a comprehensive convention that intervenes in many areas regarding safety of human life at sea. It starts with the construction of the ship to maintain a as high level of safety as possible due to divisions, stability of the machinery and electrical installations. There are detailed rules for fire, protection, fire discovery and fire extinguishing and of life saving equipment. In addition to “SOLAS 1974”, national authority and classification companies will include further requirements of equipment and arrangements. There are regulations to consider, international as well as national. We will, in particular, study what involves oil tanker and gas tankers in this part of the compendium.
8.2
MANAGEMENT TASKS & TACTICS – FIRE EMERGENCY PREPARDNESS
8.2.1
Fire Emergency preparedness onboard is comprised of the following:
· · ·
- Sufficient and adequate equipment. - Organisation and management. - Training and practice. · Organization and management are essential factors, which deserves a great deal of attention. The leader of the fire fighting must, in any case, consider the situation and depending on a number of circumstances execute adequate initiatives. The leader of the fire fighting should be able to take care of his/her responsibilities in the best possible way. Essential to this, training and practice must be fulfilled. 8.2.2 Fire onboard - Management’s duty: A fire burst onboard represents a threatening and critical situation. To prevent disaster, a quick and determined effort from the whole crew on board is needed. For most of the people, fire is an unfamiliar event and it is therefore natural that such a threatening occurrence can lead to unpremeditated actions and panicky contributions to the situation.
When this happens, it is the management’s first duty to, as soon as possible, activate the different teams in accordance with the fire instruction plan. Fire resistance arrangements onboard the specific vessel should be utilised to the fullest extent. If a fire should occur, the management will be confronted with a lot of problems that all seem to be equal in importance. It is important to prioritise when dispersing the tasks. This means that those tasks that seem to be most important must be delegated to the most competent unit or team in the emergency squad. The squad will have to do their best to solve the problems in a satisfactory way. In many cases, the first decisions must be made based on few and uncertain pieces of information about the situation. Any hesitation from the management about which approach to use, will promote the feeling of fear and insecurity among the crew. Since the crew has been trained in relevant practical skills, the management must also be prepared and trained for the problems they are expected to solve. The ship’s
fire instructions must be considered as a tool. The benefit and effect that this tool will give depends on how the management decides to utilise it. There is nothing that can really replace the valuable experiences you will get by managing extinguishing operations in real fire situations onboard. As this, of course, is practically impossible to accomplish as part of a training programme, other methods have to be tried out. Typically the standby crew (e.g. fire brigade, first aid teams, civil defence) will need to make quick decisions and judgements of the situation. This type of responsibility requires special training. Imagine a situation and try to picture the conditions and based on that try to find out how you can, as best as possible, use the resources you have available. This is one way to manage a situation. However, you have to be aware that in a real situation, the approach to the problem cannot be changed to fit your own perception. By using similar methods onboard, consider imagined fire situations and at leisure find out how to handle the situations, so that the management of the ship can prepare their fire fighting duties. Even though you have worked through a lot of imagined situations, and one day there is a fire, there will never be a situation similar in detail to one of the imagined situations. On the other hand there will most likely be a situation similar to something you had been through before. In any case you will be better prepared, at least mentally, to manage the situation.
8.2.3 Plans of Action The more people know the main guidelines for fire fighting situations onboard each particular ship, the better the chance for a successful response. Therefore it is of urgent importance that the management group (The Captain, The Chief Engineer and The Chief Officer) is fully aware of the existing plans. When considering these imagined situations where you find the best solutions, several point of views will improve the plans. The management group together should work out the plans for the actions for different kinds of fire situations. Therefore, the managers will be informed about the plans, which will make it easier for them to manage accordingly. In hectic situations, as a fire, it will be easier to change an existing plan rather than making a new plan from scratch. The plan will be easier to execute, if more people know about its contents. If training is arranged according to appointed plans, the crew will get familiar with the plans in addition to variations in training. Realistic and well-planned training exercises are good practice, as well as, it is interesting and instructive. Successful fire fighting is a result of good planning, good leadership and a well-trained standby crew. 8.2.4 Tactics By tactics we really mean line of action. It is a calculated way to act out a plan of action where we want to use the crew available, in such a way that maximises the effect achieved. The intention with tactics is to reach the goal you have set. You have to be aware of what you want, what is the result you aim for. In a fire situation, it should be easy to conclude that you want to extinguish the fire, as soon as possible, with as little mess as possible, without any risks to the fire fighters.
8.2.5 Select an Action When planning a line of action, choose tactics, try to clarify the situation first (reconnoitre). The more details you know about the situation, the easier it is to evaluate the situation. In a critical situation, decisions have to be made quickly. The next step in the planning process will be the evaluation of the situation. Based on the information known, you have to try to determine how the fire will grow. Here it is important to prioritise, as there could be parts of the fire that has to be stopped no matter what. Meanwhile, other things have to be held off, as long as possible. There are may be some parts that can be temporarily disregarded. With the evaluation of the situation as a basis the disposals of resources are being made. The extents of the contribution depends on how important the effort is, how demanding the work to be done is, and how quickly it has to be effectuated. You should always be prepared to change tactics if unforeseen difficulties occur. Wellprepared tactics considers all known factors whether there are only a few, or many and detailed at any stage.
8.2.6 Conditions for actions One condition for actions is that you have resources available. The following estimate can be put up to show the connections in an action of extinguishing onboard: Personnel + Officers = Equipment + technical arrangements = Training + Practice = Reconnoitre + Evaluate situation + = disposal Instructions + communications =
Crew. Material/Force Techn./resources Tactics/action Management/effect
A crew organised in fire protection and with sufficient and adequate equipment forms the force. To be able to perform their duties, the force has to master the necessary skills. Technique is to use the equipment in such a way that a maximum effect is achieved. Extinguishing technique covers the correct use and handling of the particular parts of the extinguisher equipment and the fire technical installations. This also includes practical skills, methods of practice and routines, knowledge of how the particular equipment works, effect, capacity and limitations.
8.2.7 Extinguishing Tactics Extinguishing tactics make use of resources available so that maximum effect in an action is achieved. It also makes a sufficient effort at the right place at the critical moment. Offensive tactics is a well-known expression, it means that you will use all resources in the fight to win back the terrain and to get the situation under control. Defence tactics are when you use the whole force to last as long as possible to prevent being forced to back out, avoid loss of terrain, try to hold the position, as long as possible, while waiting for backup. In the following, you will find some situations listed where you will have to consider the influence these situations have on the actions to be taken. 8.2.8 The site of the fire It might be difficult to get access to the site of the fire, as well as it might be unjustifiable to send people in there. Alternative extinguishing methods might cause large damage to the environment. The risk of loosing lives should be considered more important than avoiding a large loss of materials. 8.2.9 The extent of the fire The extent of the fire can be so large that the possibilities for extinguishing with available equipment are small. An extinguishing effort with the capacity available might reduce the opportunity to successful back out. 8.2.10 Force available A well-trained force, which is familiar with the use of the equipment and the facilities, will be an advantage in a difficult situation. 8.2.11 Mobility A well-trained force, which can quickly attack the fire, has a greater chance for success. 8.2.12 Communication It is very important to have a good communication because it will make it easier to manage the operation. The force can easily be re-directed and there is a quicker determination of how the situation is developing. 8.2.13 What is on fire? There may be combustibles in the fire zone that can produce poisonous gases when heated. Fire fighters must be well protected. We will further mention other situations, such as where and in which directions the fire can spread, changing the ship’s course to achieve the best possible conditions, the fire-technical arrangements intact, element of risk in the fire zone, e.g. oil tanks, transportable gas holders, chemicals etc.
8.3
FIRE PREPARDNESS
Fire preparedness is the capability the crew has to fight a fire with the help of the equipment available on board. To manage a fire situation, preparedness promotional efforts are done. Fire preparedness is the result of a number of arrangements and different efforts, for example fire protection organisation, strategic placing of equipment, instructions, maintenance of equipment, training, exercise. Remember the preparedness is not stronger than the weakest link. Practical (technical) exercises are meant as a test to see if the crew has the necessary skills. The exercises are also designed to train in the skill of being prepared. Tactical exercises will reveal the management’s capability to evaluate situations and delegate the right effort at the right time. The practical and technical skills together will contribute to an effective force. It is therefore very important that realistic and varying exercises are exercised on board. The technical will cover the quality of the “tool” at disposal, while the tactical will cover what capability one has to utilise the strength at his disposal. 8.3.1 Alarm instructions Central part of fire preparedness on board is the safety plan part on the fire fighting organisation. The ship’s alarm instructions provide the emergency plan if there is a need for a united and systematic effort of the crew. Main features in the emergency plan should include special distribution of the crew, duties when fire fighting, plus another special distribution, if preparations for abandon the ship become a reality. All emergency plans organise the crew into practical teams or units, plus instruct of the duties that everyone has when the organisation is active. Emphasise the importance of knowing the alarm instructions well, on board your specific ship. There can also be other situations that can be covered by the preparedness organisation, for example man-over-board, tank accident, and personal injury and helicopter preparedness. 8.3.2 Layout of the instructions The layout of instructions for the individual team in the fire preparedness organisation offers the same challenge as splitting up the crew into teams. At the outburst of fire, there are a number of duties to execute regardless of the site and situation of the fire’s location. For example: starting of auxiliary engine, starting the pump, set pressure on the fire lines, stop the fans, close ventilation to the fire area, switch off the electrical plant in the fire area, prepare for manoeuvring, change course and speed, clarifying the extinguishing equipment etc. The purpose of the instruction is to allow the first effort’s execution without waiting for special orders. In connection with the distribution and the instructions for the individual units in the fire protection organisation, there are a number of other instructions that needs attention: 1. 2. 3. 4. 5.
1. 2. 3. 4. 5.
-
Instructions Instructions Instructions Instructions Instructions
for the navigator on duty. for the engineer on duty. for the extinguishing leader. in utilising the main extinguishing plant. for the officer on duty in port.
6. 6. 7. 7.
- Instructions for helicopter landing. - Instructions for the person who detects fire.
All instructions must overlap one another so that all conditions are covered. The purpose with the instructions is to eliminate doubts of who does what. At the same time secure that what is being done is rational and in right time. One can compare the fire protection organisation to machinery with many wheels that all must overlap one another in such a way that the machinery is functioning. The instructions are the force that will run the machinery. The symbol for teamwork characterises a well drilled fire crew.
The individual links in the fire protection organisation approach by the instructions on board. This is regarded to the captain, extinguishing leader, extinguishing team, technical team, engine room team, instruction for engineer on duty, bridge instruction, instruction for navigator on duty, radio station, security team, first aid team, life boat team, evacuation team, instruction for officer on duty in port. In addition to instructions for the individual fire protection organisation, there are also instructions for special situations. It is important for all those who will operate a CO2 plant that they follow the instructions clearly and categorically, because before triggering of the plant can take place, it must be ascertained that no one is in the area that the plant covers. Get to know the fire alarm on board and follow your instructions when this sounds, and get to know the CO2 alarm and follow the instructions. If the CO2 alarm sounds, one should act as though the fire alarm has sounded, even before the fire alarm. We will pay particular attention to the advice upon fire discovery, which gives all of the crew practical advice and directions of how to react if you are the person discovering the fire. The principal points, in such an instruction, are: · Immediately shout of alarm in order to call on more people. · First try to extinguish the fire, provided this is possible and without risk.
· Close doors and try to prevent, as much as possible, an influx of air to the area. · Try to inspect rooms nearby the fire area where personnel might be, that could be overwhelmed by smoke, or in bed. · Realise the element of risk, such as toxic gas, explosions etc., and where escape equipment is possibly placed. · Upon alarm the significance of all to show up at their post, as quickly as possible, to state if someone is missing. This will avoid time loss resulting from unnecessary searches. Be very strict in having all present at their post. Those who don’t show up are basically missing. · If fire bursts out in port, immediately try to call the local fire department. · Organise in the normal procedure to put fight the fire when there is fire in port.
Alarm instruction layout, crew distribution, layout and content of the special instructions will vary by the type and size of the ship, the number of crew members, extent of equipment, plus other relevant conditions. Common for all alarm instructions is the desire to, as much as possible, have a flexible preparedness plan that gives the possibility for all to contribute his best in the management of a number of different situations. 8.3.3 Some important guidelines: · The entire force on board is available and able to be on the spot rapidly. · Because of continuous duty, fire is detected at an early stage. · Equipment and facilities are nearby. · The area is restricted. · Alter the course in favour of the fire fighting. · The amount of equipment is limited. · Mobilise maximum force immediately. · The crew/force is naturally experienced with the surroundings. · Simple checks if someone goes missing. · The most flammable areas are covered by fixed fire fighting equipment. · The retreat possibilities are very restricted. When onboard during fire, it is similar to being “on the roof of the burning building”. The escape routes are limited and few. Because of the extraordinary conditions on board, limited retreat possibilities and the great chance of blocking off the rescue units in a fire situation, it is natural that retreat route security and clarification of these routes has high priority. As the conditions on board are extremely special compared to ashore, the manager must treat the retreat as a very important point in his procedural duty. Further the fire control officer must observe the four following main tasks:
1. 2. 3. 4.
1. 2. 3. 4.
-
Rescuing life. Extinguishing fire. Restriction of fire. Executing the retreat.
8.3.4 Rescuing of life In most cases, it is not direct contact with the fire that causes the loss of life. In a fire, smoke and the damaging gases that develop cause most deaths. As soon as someone is determined missing, the manager must be informed so a search can be carried out. Notice that this search involves risk for the rescue crew, plus it directly delays the extinguishing activity because of impaired capacity and conditions that may have vital importance to the outcome. It effects fire fighting and the security work drastically when a delay is caused by the search for personnel who not are in any danger. It is therefore an unalterable requirement that all the crew show up at their posts according to instruction, as quickly as possible, and that missing personnel are immediately revealed.
8.3.5 Extinguishing of fire The faster the extinguishing activity is effectuated, the greater the chance of a successful result. In choosing an extinguishing method, quencher remedy and capacity, the goal must be total elimination. One must also consider the amount of damage the extinguishing agent will cause to the area. However, put out the fire before causing any larger damage. In some parts of the vessel, one can choose between permanently installed extinguishing equipment and manual efforts. On parts of the ship, a manual effort is the only alternative. Permanent equipment should be used in an area where the fire risk is large and has a large risk of spreading. Any manual combating involves a large risk for the extinguishing force. The decision about what to utilise in a specific situation must be well substantiated. 8.3.6 Securing the retreat Due to the limited possibilities on board, it is natural upon securing the retreat will involve clarifying if is necessary to abandon the vessel. The fire may block the access to rescue units. In addition, it is very important that the psychological effect will influence all of the crew in the knowledge that the retreat is prepared and protected in case of ending the fire fighting. 5.1 8.4 FIXED FIRE FIGHTING PLANS & FIRE FIGHTING REMEDY 8.4.1 Manual call point plant Fixed fire detection’s plants, discovery and alarm equipment should be installed on vessels that are regulated by SOLAS. Approval type for these detection’s plants takes place according to a determined procedure by posting the plant’s documentation. This documentation should contain user instructions, procedures for routine testing on board, fault location procedures, power supply information, connection of detector loop, alarm organs, fan failure, door magnet, assembly work, function description, accordingly all requirements in accordance to the documentation claim. The plant is tested to determine if it fulfils the regulations required. The manual call point plant should at all times be according to the regulations in force. Some of the criteria follow: · It should give optical and acoustic alarm at fire. · It should indicate where fire breaks out. · It allows for fault warning. · The central unit automatically goes over to reserve power to supply upon voltage failure. · Positive indication on the panel by interruption of functions. Otherwise according to the approval companies, it is important to notice that the plant should have two independent power sources. If one “falls out” the other will operate the plant with full power. However, please refer to the regulations regarding complete approval.
8.4.2 Safety plan The fire control draft or as called on board; the “safety plan” illustrates the safety installations and equipment on board. The draft shows the vessel sidewise and a sketch of each deck top wise. It indicate zones with isolated bulkheads and fire doors, manual call point plants with detectors, alarm buttons and alarm bells, the fixed main extinguishing plant and where on board these can be remote controlled. Valves to stop engines, machinery, and from where one can remote operate these are also indicated. It indicates where the ventilation plant with fans, ducts and damper is and from where one can stop the plant. All portable extinguishing equipment, protection equipment and utility equipment appear on the draft, and where on the vessel the equipment is kept. It also displays all decks, rooms, and all emergency exits. Symbols for marking equipment are utilised to make the draft well arranged. Also, on the draft is a list with an explanation of the different symbol. Colouring is often utilised to keep the symbols apart. This draft is available for all on board. To effectively utilise the different fire technical installations, thorough knowledge of the individual plants is required, plus how to use them. The gangway during the port stay should keep a copy of the safety plan. If anything occurs during the stay and local help is required, the local fire department can quickly approach the plan, and from an early stage, have knowledge of the preparedness plan. All are advised to thoroughly study the “safety plan” in detail. 8.4.3 Fire pumps A fire pump in the engine room is connected to the fire pipeline network. In addition, there is a separate fixed emergency fire pump installed in a distance from the engine room. One can either operate the emergency fire pump by its own diesel engine; it can be hydraulically driven or electrically driven by power from the emergency power unit. Oil, for at least 12 hours of running power, is kept nearby the emergency pump, in addition to oil for the fuel tank itself in case it a should be filled at any time. Fire pumps, which are able to produce more pressure than the pipeline network is designed for, are at all times equipped with a safety valve. All centrifugal pumps, for instance, are supplied with non-return valves. 8.4.4 Fire pipeline network The fire pipeline network branches all over the vessel and has a number of hydrants - hose connections with valves. The pipeline network is divided into sections with a cross over, arranged in a way that if damage occurs on a part of the system, the damaged part is shut off without shutting off the entire pipeline network. Properly study the pipeline network on board to understand how the network is divided, plus where the shut-off valves are placed. If parts of the network are damaged, it is possible to bypass the damaged part by help of hoses from hydrant to hydrant. Hydrants are placed such that two water jets at the same time can reach any part of the vessel, one jet from a hose length, the other from two hose lengths. On the main
line of the tank area there should be one shut-off valve for each 40 metres. This is, of course, fitted to the size and type of the vessel. 8.4.5 Main fire extinguishing plants (For gas and chemical carriers) 8.4.6 Dry chemical system Powder is elected as extinguishing remedy on the tank deck of gas carriers and chemical tankers. A number of minor stationary powder aggregates can be placed on deck or a powder central unit with pipes forward to a number of powder monitors and hose stations on deck. One or several powder containers are placed with a capacity calculated for the specific vessel with accompanying pressure bottles in the powder central unit. The plant can be released from each powder post by opening the valve of the releasing bottle. The gas is lead into tubes to the releasing mechanism of the pressure bottles in the powder central unit. It opens the valve of the powder tube that proceeds to the powder post being released. Several posts can be utilised at the same time, but each post must be triggered in the same way. Stationary dry powder systems are normally delivered with powder (NaHCO3 – - calcium hydrogen carbonate) for natrium hydrogen carbonate or KHCO3 extinguishing fire in class B or E. That is all types of liquid like: petrol, alcohol, acetone, oil, painting etc., and different types of gases like methanol, methane, butane, propane etc. Dry powder systems utilise N2 (Nitrogen) or CO2 (carbon dioxide) as propellant gas. The gas is kept in pressure cylinders. A gas pressure regulator reduces N2 –gas or CO2 – gas (200kg/cm2) to 20 kg/cm2 before it goes via the riser in to the powder aggregate. The riser’s gas taps are very important, as the powder together with the propellant gas must be able to “float” as a liquid through the pipe system and the powder jet. The stationary powder post (monitor) should have a capacity of at least 10kg/second. Manual equipment, “hand hoses”, should have a capacity of at least 3,5kg/second, but not too large for one man to operate. The length of a hand hose should not exceed 33 m. It is very important that the hose is pulled out to its full length before setting the pressure. The extension should be at a minimum of 10 metres for both stationary and hand based equipment. The plant’s powder capacity should be of the size that utilises all posts. The delivery of powder should progress at a minimum of 45 seconds.
Below is an example of this with the following data: 4 stationary and 4 hand stations: Stationary: (4 pcs. x min.10kg./s x min. in 45s) Hand based: (4 pcs. x min.3,5kg./s x min. in 45s) kg. Minimum powder capacity: kg.
=
1800 kg. = 630 = 2430
8.4.7 Technical description The powder type NaHCO3 and KHCO3 has an extinguishing effect based on a reaction inhibitor along with some cooling of the fuel surface and the gas face. Powder is not electrically conductive in dry conditions. To avoid humidity in the powder, a waterrepellent material is added usually silicon. Dry chemical systems consist of a mechanical part that includes a powder aggregate with valves, release mechanism, pipe system and jets. Everyone must memorise maintenance routines and test routines, based on the plant on the specific vessel. (This is part of the fire drill onboard).
8.4.8 Water - spray system (Gas and chemical carriers) In addition, certain ship types should be equipped with a “water-spray system”, as an object for a cooling, fire preventive and crew protective effect. We refer here to the IGC-code, chapter 11, point 11.3.1, what areas the plant should cover. The plant onboard the specific ship is designed according to this.
The system should have the capacity to cover the designated area with at least 10 ltr./m2 pr minute on horizontal surfaces, and 4 ltr./m2 pr minute on vertical surfaces. If parts of the line are damaged, shut-off valves must exist on the main line so that the line can still be utilised. This is operable by shutting off the line to the damaged area. The alternative is that the system is devisable into several sections that can be operated independent of each other. The delivery pumps should have such a capacity that they can deliver simultaneously with full capacity to the whole plant. The plant should contain a material that is resistant to corrosion. There has to be a possibility of remote start of the water delivery pumps, plus remote control of the plants shut valves from a place outside the cargo area. We recommend studying the plant on your vessel, how it is operated, where the remote control is, plus the inclusion of this in the fire drill executed onboard. 8.4.9 Main extinguishing plant (oil tanker) 8.4.10 Foam in general A system consisting of gas or air bubbles bound in a water coating (membrane), is called foam. Constant foam is when the wall/membrane consists of a constant material, such as pumice stone, gas concrete and foam rubber are examples of constant foam. When the wall has a coating, we are talking about floating foam, such as soapsuds. Different types of floating foams are used for fire extinguishing. On new gas and chemical carriers we also find foam utilised for fire extinguishing. 8.4.11 Producing foam In order to produce foam that will extinguish fire, you need: water, a frothing material that dissolves in water in anatomised condition, and a non-flammable gas mixed with the solvent. The foam is shaped when gas/air is mixed into the foam/frothing liquid and into the water by help of mechanical equipment. The result is mechanic foam. 8.4.12 Mechanical foam Different types of pumps, sprinklers and foam pipes are used. The foam liquid is dissolved (or emulsified) in the water. After this, the air is mixed in by mechanical means. Normal equipment produces bubbles, which have a diameter of 0,1mm to 1,5mm.
Extinguish effect Foam has a suffocating effect and acts as a cooling extinguishing agent. The suffocating or the cooling effect can be more or less the dominating effect, but depends on what material is burning and what sort of foam is used. By extinguishing a burning liquid with a surface temperature higher than +100o C, the cooling effect is the dominating force. This is caused by evaporation of the liquid that penetrates into the surface’s layer of the burning material as the foam collapses. By extinguishing fire when the temperature in the surface is below +100oC, the extinguishing effect is connected with the heat-insulating foam and, above all, a differentiation effect. When the foam cover has spread outward across the liquid’s surface, the heat rays from other, still burning parts of the liquid surface, is not able to penetrate through the area covered with foam. Therefore, combustible gases are no longer formed, evaporation ceases and the fire dies out. Foam plant 8.4.13 Foam is chosen as the main extinguishing agent for the tank area. A foam plant consists of a foam central unit with a foam tank, foam pump that is also connected to an emergency generator, distribution manifold, foam jets, automatic valves, and a pipe system connected to fixed monitors on the tank deck. The capacity of the plant should be big enough that the whole tank area could be covered with foam. If the vessel has an inert gas plant, the foam capacity must have a volume that can deliver foam for a minimum of 20 minutes. The demand is at a minimum of 30 minutes if the ship is not equipped with inert gas plant. The main foam line from the foam central unit to the monitors should contain shutoff valves within determined requirements, in order to bind the line in case of damage. The foam line going to each monitor has a delivery valve installed to supply
foam. The valve can also be used to regulate the amount of foam supplied in order to achieve the right mixture condition between foam and water. A foam jet pipe is attached to the monitors. Study the plant installed on your vessel, and understand how this plan is operated. This equipment (the foam plant) is mandatory for oil tankers. Mobile foam equipment is also available on many ships, gas and chemical carriers also. This consists of a fire hose with a foam nozzle unit, small foam containers (20 litre), a foam ejector, a small hose for the transmission of foam from a foam container to a foam hose pipe, and protection equipment. This equipment is prepared for use with fire hoses and a foam nozzle unit connected to the fire line. A foam ejector with a tap for supplying foam liquid is installed between the fire hose and foam nozzle unit. Water pressure is established, foam liquid is sucked (ejector function) from the foam container via hose connection between the foam container and ejector.
CO2 – plant 8.4.14 The engine room and pump room are protected with a bar plant that utilises CO2 as an extinguishing agent. CO2 (carbon dioxide) is a colourless, non toxic, scentless, corrosion free, non-electrical leading gas with qualities that extinguish fire quickly and effectively with a recommended gas concentration, which does not damage electronic equipment and requires no clearing/cleaning after use. CO2 plants are delivered either as a “Total Flooding System”, where the entire room is filled with gas, or as “Object Protection”, where a part of the room is filled with gas. The room on vessels with “Total Flooding System” is filled with a CO2 -content corresponding to 40% by volume, that is 0.72 kg/m3 with gas. CO2 plants consist of CO2 cylinders with valves, a cylinder rack, releasing mechanism, accumulating manifold, pipe system and jet. Extinguishing effect 8.4.15 CO2 has a suffocating extinguishing effect in that the oxygen content in the air is reduced to the point where combustion becomes impossible. Familiarise yourself with your vessel’s plant. Choosing extinguishing agents 8.4.16 If an active extinguishing method is chosen, the extinguishing agent must be properly suited for the actual situation. The extinguishing agent should:
· · · · · ·
- extinguish the fire, as quickly as possible. - not cause the fire object any additional damage. - not harm the environment. - not cause damage to the user. - not demand risky operation. - give protection to the user.
8.4.17 Water Water must be in direct contact with the fire to gain an extinguishing effect. The effect emerges when water changes to vapour. Water takes heat from the fire via evaporation; the vapour displaces air and consequently the oxygen. Since water does not evaporate, it can add to the object’s damage. To obtain maximal evaporation, the water must have as large surface as possible when hitting the heat zone. A scattered jet obtains this, the water comes in shape of drops, the smaller the drop, the larger the surface. Drops with 1 mm diameter have a surface of 0.126 cm2, the same water amount in drops of 0,1 mm in diameter have a 1,26 cm2 surface. The smaller the drops are, the shorter the air throw is needed. With a drop size of 0,2 – 0,3 mm, the most practical proportion between air throw and surface is obtained. By throwing, as much as possible, atomised water into the warmest part of the fire zone, the largest effect is obtained. Powder 8.4.18 To have any effect, the powder must be lead down to the fire object. To obtain an extinguishing effect, one must reach a position from where the powder can reach the fire itself. The powder works partly by suffocating and partly by poisoning the flame, it also has a little bit of a cooling effect. Danger of re-ignition is therefore large until the temperature is brought down below the self-ignition temperature. The powder has no direct damaging effect on the object. In sensitivity instruments and in electronic equipment, powder has a disturbing effect on the functions. 8.4.19 Carbon dioxide Carbon dioxide is utilised either as a “total flooding” agent where the whole area on fire is filled with CO2 in large concentration, or is used as a spot extinguishing agent. The gas has little cooling effect, so that re-ignition is a danger. One must utilise total flooding, as early as possible. Evacuation of the area is necessary, and before releasing the plant, one must check that no one is missing. The gas has no damage on the environment. In utilising spot extinguishers, the thermal effect may cause damage to sensitivity instruments. Foam 8.4.20 Foam has both a cooling and suffocating effect. Light foam contains less water than heavy foam, and thereby the extinguishing agent causes less damage. One can
utilise light foam to cover larger areas and thereby suffocate the fire. One can also utilise foam as secondary remedy to prevent re-ignition, or prevent ignition from oil leakage, etc. When selecting an extinguishing agent, one must consider the type of fire:
· only a glow, or glow and flames, (fire in solid material), where the fire core has high temperature. · only flames (fire in liquids) where gas vaporised from the surface is on fire, relatively low temperature in the liquid itself. · fire in alive, electrical components (glow/flame fire in isolation materials, painting, lubricating oil etc.). The main rule when choosing an extinguishing agent is: · a. glow fire and glow/flame fire is extinguished by cooling (damp extinguishing remedy). · one extinguishes a flame fire by suffocating. · one extinguishes an electrical fire with a dry extinguishing remedy. One must also consider the surroundings (as little extinguishing damage as possible), special circumstances (alive plants), danger of re-ignition (need of cooling), special material (chemicals, explosives, dangerous goods, swelling), protection of the fire squad (protect the users). One must also evaluate the practicality of utilising a combination of mutual extinguishing remedies. For example, at first extinguish the flames with powder, thereby cooling with water, and possibly cover with foam to prevent spontaneous ignition. Portable fire extinguishing apparatus 8.4.21 There are a number of transportable fire extinguishing apparatus (handextinguishing apparatus) placed in different places onboard. The placement and type of apparatus is in accordance to the regulations based on the specific vessel. These apparatus are marked by symbols on the vessel’s “safety plan”. Knowledge of placement and use is introduced in the fire exercises onboard. WATER Normally there are 10 litre apparatus placed in different locations onboard. 10 litres is a very limited amount, and has a period of use lasting approximately 60 seconds. Some types have a united jet, while others alternate between united and spread jets.
Powder There are mostly 12 kg powder apparatus onboard, except for where something else ahs been determined, for example 25 kg, 50 kg or 6 kg. This is clearly found in the “safety plan” onboard. A 12 kg powder apparatus has a period of use lasting approximately 20 seconds; a 50 kg apparatus has a period of use of approximately 60 seconds. The apparatus has good air throw, and will provide the user good protection. To utilise powder extinguishing equipment at full effect, a well-drilled technique is demanded. Add this to the training exercises onboard. Carbon dioxide There are carbonic acid apparatus of 6 kg stationed on board. These apparatus have a very limited capacity and no air throw. The protection for the user is poor. The period of use is approximately 20 seconds. These apparatus are suited for spot extinguishing of relatively small fires. One should have high goals regarding knowledge in utilizing, of function and capacity of the fire extinguishing apparatus on board your vessel.
8.5 PERSONAL SAFETY EQUIPMENT 8.5.1 FIRE FIGHTER EQUIPMENT The requirement onboard oil tankers, as well as onboard gas tankers less than 5000 m3, are 4 sets of fire fighter equipment. Onboard gas carriers of more than 5000 m3, a minimum of 5 sets of fire fighter equipment is required. Each set consists of: · · · · ·
One breathing apparatus (BA) with an air capacity of minimum 1200 litres. Protection suit including boots and gloves. Fire resistance safety line with belt. Safety lamp. Fireman’s axe.
The equipment is specified in SOLAS, chapter 11-2, rule 17. National, and classification companies requirements may come in addition. This is of course considered for each vessel and the equipment is at all times in accordance to existing requirement and rules. 8.5.2 THE BREATHING APPARATUS BA SET The breathing apparatus onboard must be of an accepted type, and must fulfil the requirements in accordance to the regulations. Frequent training exercises on board will familiarise you with the apparatus and use of a breathing apparatus. The requirement for the air content in bottles is minimum 1200 litre clean air. Spare bottles are required for each apparatus, so that each apparatus has a period of use of 2 hours minimum. If a consumption of 60 litre air pr. minute (equivalent to hard work) is assumed, one obtains, as follows: · · ·
Min. 1200 litre: 60 litre pr. minute = 20 minutes pr. bottle. 2 hours (120 min. : 20 min.) = 6 bottles Totally (1200 litre x 6 bottles) = 7200 litre air pr. apparatus.
A compressor is also required to fill up the breathing bottles after use. The period of use for the apparatus is dependent upon the consumption of air pr. time unit, for light or heavy work and the bottle size. EXAMPLE An air bottle of 7 litres and a pressure of 200 bar give (7 x 200) = 1400 litres clean air. Assume that heavy work is to be carried out with a consumption of 60 litres pr. minute. Disposal time will then be (7 x 200): 60 = 23,3 minutes. The apparatus has an audio signal that alerts the user that it’s time to abandon the area. This audio signal is released at approximately 40 bars, and leaves us with (40x7) = 280 litres air for retreat. The time for retreat is (280: 60) = 4,66 minutes. We then have 4,5 minutes to abandon the area.
It is recommended to train in the use of the apparatus in order to learn the special breathing technique for such an apparatus. The definition of breathing technique is a rational way of utilising the air to obtain a maximum safe user time. The user is dependent upon the state of the apparatus when residing in smoke or gas filled rooms. Before entering such rooms, the user himself must control the apparatus. After strapping the bottle on his back, a special routine must be followed regardless of the duration and regardless of others. The procedure varies with each apparatus. It is of great importance to knowledge of the apparatus onboard your vessel. The control will mainly be, as follows: 1. Bottle pressure: Open the bottle valve 2-3 shifts and read the manometer pressure. Then put on the mask, the panorama mask is put on with all straps extended. Set the chin in the mask and lead the straps behind the head. Tighten the lower strap, then the upper straps and finally the top strap. 2. Retreat signal: Shut the bottle valve and breath carefully until the retreat signal appears. The manometer indicates approximately 40 bars when the signal appears. If there is a change over valve on the apparatus, ensure that this is in the right position. The change over valve is activated when the retreat signal appears and has an opening effect for spare air to release. 3. Mask’s seal ability: Suck the mask empty of air. The mask will then be pressed against the face, if not, tighten the head straps. If it is airtight, open for air again. 4. Pneumonic automatic: The bottle valve is set fully open, breath a few times to make sure that the pneumonic automatic unit is working. 5. Flushing button: The button on the pneumonic automatic unit is activated and air will flow into the mask. Sometimes extra air in the mask is needed. 6. Bottle pressure: Control again the pressure manometer to make sure that the bottle is full. Place the manometer back in the clip. All tests must be repeated for each exercise, or change of bottle, to make sure of satisfactory operation at all times. 8.5.3 FIRE STATIONS The fire stations are marked on the safety plan, and also the content of all required equipment at the stations. In addition to mentioned fire fighting equipment, the content must include personal protective equipment, fire hoses, jet nozzles that can switched from jet to fog dispersement, keys to hose coupling and an extra fire axe. Other equipment included is an electrical drill with 5/8” drill steel together with an extension cord. It is smart to obtain a smaller drill steel to drill a pilot hole, if this is a matter of necessity. A portable oxyacetylene torch that renders it possible to make a quick carving of a manhole or other openings to ease access is also included. This equipment is marked on the safety plan, where it is placed onboard and at the right number according to type and size of vessel.
Everyone is encouraged to know the seriousness of exercises onboard, being prepared in a realistic and objective way can be, as a matter of fact, very interesting and informative. Anxiety is relieved because confidence leads to safety. 8.5.4 INTERNATIONAL SHORE CONNECTION SOLAS chapter II-2, rule 19, deals with the international “land coupling” that is required onboard all vessels in accordance to this rule’s specification. The existence of this is shown in the safety plan. It must be readily available during harbour stay. It is used as connection between the vessel and harbours equipment, in case of fire during the stay. Familiarise yourself with the escape and protection equipment onboard the gas tanker/chemical tanker, masks with filter for all onboard, appropriate protection equipment placed for easy availability. This protection equipment contains large aprons, special gloves with long cuffs, appropriate footwear, coveralls made of chemical resistant material, tightened glasses or face shields. Clothes and equipment must protect all body parts. An escape apparatus with oxygen mask that makes the carrier independent of the oxygen in the air at a minimum of 15 minutes must be available on all vessels. This is only for use of escape. Specifications state how much of this equipment exists onboard. The safety plan informs where it is located. 8.5.5 BOARDING TANKS Before boarding tanks and closed spaces, one must take measurements, make sure that dangerous gas is non-existent and that the oxygen content is satisfactory. After tank ventilation, take great care in the measuring and the certainty of the atmosphere before entering. Ensure that the pump room is well ventilated by running the fans for awhile before entering. There is monitor supervision of the pump room atmosphere onboard many vessels today. This takes place by automatically testing the pump room atmosphere at different sampling points. Then it is analysed and indicated on the control room monitor. This is also equipment you need to be familiar with. 8.5.6 PERSONNEL INSIDE TANKS When entering tanks, it is very important to prepare equipment and make it easily available in case of emergency. Preparation includes a complete set of fire fighting equipment (also an extra fresh air mask in case it is needed for a rescue action), lines, communication, and crew on deck for supervising. The knowledge of the number and names of crew in the tank at all times is very important in order to be as prepared as possible for any rescue action.
8.6 IN GENERAL What is health? In short, it is when the physical is in balance with the non-physical, and the harmonisation here has a natural function. The result is good health. To maintain this, knowledge about harmonisation is the vital factor in health. Health is different for each one of us based on individual tendencies and external/internal influences that mark (or chooses to mark) our life. All crewmembers that sign on a vessel should have been through a medical check in order to have a regular status of his/her health condition. Life at sea is a special place to work, it is important that the general health condition at all times is good. What can be done to maintain a good general health condition on board? The answer is built into the safety and protection of personnel on board. You can also take care of one another in a good manor by being aware of the risks that may have direct and external effect on health, regarding the special cargoes carried onboard your vessel. Primarily, you can take good care of your own machine, the body, by paying attention to the “fuel”, which contains the nutrients the body needs. It is very important that the “chef” onboard has basic knowledge about anatomy/physiology, in order to assemble the right raw material into the right article of food, in the form of nutrient rich meals. Together with good cleanliness and comfortable surroundings in the galley, the best basis is given. If the meals are “spiced-up” with a nice atmosphere, in addition to existence of an inter-human working environment and well-being, the result is solidarity, well-being at work, increased efficiency, less sickness absence and saved costs. A positive mental attitude toward life is also important and, at the same time, improves or maintains health. This is an important factor of the human’s total health. For example, one can turn a “I will not, cannot” behaviour pattern into a “I can, I will” attitude. This will increase co-operation between the people onboard. With an attitude change based on positive thinking, the result is noticeable onboard, due to well being, solidarity, better performance, and a healthier atmosphere for the whole crew. The human resource is the only resource. 8.6.1 THE BODY The doctrine of how the body is built is called anatomy. The doctrine of the body’s function is called physiology. This will be roughly illustrated to achieve a synopsis of how the “machine” functions. 8.5.2 The cell This is the smallest, independent unit of the body and the basis for all living organisms. All the processes in the body are caused by the chemical reactions that take place in the cells. Cells in different tissue and organisms co-operate in their duties. The cell has a water content of approximately 70% in addition to proteins, carbohydrates, fat and inorganic material. All the cells have the same basic structure and a number of mutually basic qualities. Simultaneously each part of the cell has its function. We all utilise nutrients both to achieve energy and as “building stones”. In
new cell components, glucose (grape sugar) is the most important energy source. It is important to have nutrient rich and varying diet. 8.6.3 TISSUE Cells that look alike remain lying to form tissue. All surfaces of the body are covered with epithelial tissue (type of tissue that mainly covers all surfaces, the cavity and channels of the body). Connective tissue and support tissue forms the tissue network in the body and keeps tissue and organs together. There is an innumerable of tissues, for example osseous tissue, muscular tissue and nerve tissue. The cell cooperation is controlled by chemical signals. These signals consist of two types, nerve signals and hormone signals. These two systems co-operate for an appropriate reaction. This is fully necessary for our survival. The hormone system controls the activity of many internal organs, the nerve system controls muscles and glands. Several organ systems co-operate to keep the composition of tissue fluid constant. The blood renews this tissue fluid. The blood must circulate the whole time. The duty of the lymph artery is to drain excess tissue fluid. 8.6.4 THE DIGESTIVE ORGANS These demolish and absorb nutrient material. It is very important that the nutrient content satisfies the body’s need. 8.6.5 THE RESPIRATORY ORGANS These absorb oxygen and partly carbon dioxide. Respiration is an exchange of gases between the blood arteries and the air in the lungs. The blood absorbs oxygen into the body’s cells and partly the excess carbon dioxide that arises. The respiratory organs consist of the bronchia and the lungs. Gas exchange between blood and air takes place in the lungs. 8.6.6 THE URINARY ORGANS These regulate the composition of the tissue fluid. The urinary organs consist of the kidneys, urinary tract, bladder and the urethra. The kidneys' most distinct duty is to separate water from waste. The resultant urine is processed in the urinary tract and bladder then empties via the urethra. 8.6.7 THE BLOOD CIRCULATION The blood circulation carries materials between the organs in the body. The tissue fluid is constantly renewed from the blood in the capillaries. The heart is a pump that makes blood circulate. “Heart valves” help the blood run in the right direction. The heart musculature sends blood through the coronary artery (the heart’s arteries). The heart is the most persevering muscle in the body. The blood flows from high pressure to lower pressure. The pulse is thereby a regulator in the blood. The blood pressure is the pressure inside the artery, which is part of the blood circulation. The blood acts, as a sort of, transportation system. Blood sends tissue circulating through
the artery system. This contains blood cells floating in a protein rich fluid – blood plasma – with two main types of cells, red and white. The red cells are important for transportation of oxygen from the lungs to the different body tissues, while the white, in different ways, participates in the body’s defence against disease. It is important to remember that one cannot mix different blood types. Blood cells are formed in the bone marrow. 8.6.8 THE HUMAN ORGANISM AND THE SURROUNDING WORLD Our senses tell us about the surroundings. The main senses are sight, hearing, touch, smell and taste. There are also important senses in the muscles, the joints and the equilibrium organ in the inner ear. Each sensory organ has its best reaction to a certain type of stimulation, but has a different reaction to long lasting stimulation. 8.6.9 THE SKIN The skin forms an essential boundary to the surroundings, and is the body’s largest “breathing organ”. The skin consists of different tissue with different qualities and covers the body surface, like an almost impenetrable protective film. The skin is an important sensory organ with large adaptability. 8.5.10
THE IMMUNE SYSTEM
This system protects the body and consists of several parts. There is no possibility of living a normal life without this defence, as its duty is to render harmless infective agents or other strange material. In addition to combating infection from outside, this defence system also fights against any internal cell changes. 8.6.11
THE BODY’S MOTION APPARATUS
This consists of skeleton and muscles. The skeleton is the body’s framework, consisting of almost 200 large and small bones tied together by link and ties. The muscles can move the bones by shortening (contracting) using an impulse from the nervous system. This was a short description of the “human machine”. When experiencing something exciting, frightening, unexpected, stressing, surprising or likewise, energy runs through us like an electrical blow. In such situations, one experiences the effect of endocrine hormones. The part of us that consists of feelings, thoughts, vibrations, intuition, ergo not physical parts of us, are also an important part of us (in many cases a decisive part). It is very important to provide stimulation and nutrients to these parts, as discussed below. 8.6.12
THOUGHT, ACTION, RESULT, FEELING
Positive thoughts and attitudes together with a healthy diet form the basis for good health. We can do a lot ourselves by choosing the right things, as we are free to choose.
We now take a look at your work place, onboard a vessel, and the influence this has on your health. We will also discuss what external influences can be found in the atmosphere and the injuries/incidents that may occur on board. Onboard different types of vessels carrying different types of cargo, danger to health from external influences are considered regarding the vessel’s protective equipment and routines. This protective equipment is placed practically and can be utilised, as necessary. Familiarise yourself with the equipment onboard your vessel and use it! With a sudden injury or illness on board, medical advice and guidance can be gathered from Radio Medico – the radio medical service for vessels at sea. It is important to have all the important information when help is needed for a serious condition onboard, such as: · Age · Sex · Weight · Duration of the illness · Extent of the injury · Symptoms · Patient's comments (complaints) · Clinical findings (sign of a specific illness) · How the injury happened · Character of the pain (grumbling, stabbing, squeezing) · Whereabouts of the pain · Face colour, limpness, drowsiness, temperature, pulse, breathing trouble, nausea, blood, mucus, urination, etc. All of the above is important. There is a “hospital” onboard containing ordered equipment for treatment and medication. The ship medical directions regarding the ship’s hospital deal with the maintenance, supply, inspection, etc. It is important to know how to protect oneself against harmful skin contact, skin absorption and respiratory absorption of dangerous gases in the atmosphere surrounding us, such as entering tanks and closed spaces. Help given in the first minutes of an emergency situation is crucial. All must endeavour to have respectable first aid skills. 8.6.13
FIRST AID
First aid is used with sudden unconsciousness, stopped breathing and lack of air. (Call for help, but do not abandon the patient, immediately start helping.) A Air: Try to free the air flow, lie the patient on a flat surface, bend the head backwards, remove any dentures, vomit, etc. B Breathing: If the patient is not breathing, start resuscitation with 3-5 breaths/insufflations. Use the “Pocket Mask” as an option. Hold the head curved backward, check the pulse on the neck. If pulse is felt, continue with 12 respiration’s per minute C Circulation: With deadly paleness and no pulse, give 2-3 powerful knocks over the heart. If this has no effect, start external heart compression once per second.
8.6.14
ONE RESCUER
Alternate 2-3 respirations and 15 heart compressions. 8.6.15
TWO RESCUERS
One respiration for each heart compression. When compressing; press the breastbone down 4-5cm. 8.6.16
ABC
The method stands for air, breathing, and circulation. The priority of first-aid training and practice is of great importance. The better you are at first aid in an emergency; the chance of a good outcome is greater. 8.6.17
HEART PROBLEMS
Heart problems can be suspected if sudden, strong pain behind the breastbone is experienced. For cardiac arrest, use the ABC. 8.6.18
SHOCK INJURIES
Description of shock is acute circular failure. This may be caused by reduced blood volume from bleeding, shock by drop of blood pressure or reduced pump functions from a cardiac infarction. If a big incident occurs, shock must be calculated. The symptoms are fast pulse, coldness, pail and difficulty in breathing. Supply oxygen, warm blankets and fluids. 8.6.19
HEAD INJURIES
All knocks against the head must be taken seriously. The symptoms are headache, nausea and dizziness. Flat bed rest for 2-3 days. Limited fluid intake and be sure to supervise. 8.6.20
POISONING AND ETCH INJURIES
Refer to the IMCO’s book “Medical First Aid and Guide for use in accidents involving dangerous goods”. This refers to the data sheets on the different cargo onboard. (This is illustrated later on in this part). Poisoning and etch injuries appear in connection with cargo contact, as air absorption, swallowing or skin absorption (skin contact). The symptoms are pink coloured skin, smell of almonds on the breath, headache, dizziness, nausea and vomiting. Remember that in connection with cargo contact, the emergency squad should efficiently use protective equipment, gloves etc. Supply oxygen and follow the instructions on the data sheet for the cargo in question.
8.6.21
FIRE INJURIES
In fire injuries, ensure a stabile lateral position for the patient, if possible. Supply oxygen and fluid. With fire injuries, quick help is double the help. Quickly cool for at least 20 minutes. Estimate the extent of the injury. The patient mustn’t freeze. Provide warm blankets and abundant fluid. The patient should rest, be under supervision, and have their pulse checked. Check the medical box for proper use of medication and bandages. 8.6.22
FROST INJURIES
Localised frost injuries on the skin’s top layer begins with a prickling feeling, then ascends to white spots on the skin. Careless handling of pipeline and cranes onboard vessels, which carry strongly cooled gases, can lead to localised frost injuries. Important: Frozen hands and feet must not be warmed up actively with warm water. Cover frozen skin parts with a soft woollen garment. Do not massage or rub. It helps a lot to warm up frozen skin with warm skin 8.6.23
BONE, JOINT & SOFT PART INJURIES
A lot of injuries are sprains, fracture and soft part injuries. Use the ICE method, as the proper first aid, in such injuries. ICE means ice, compression and bandage, and elevation. I – stands for ice. Ice the injury in order to lower the injured spot’s temperature. By doing so, the bleeding is reduced in the underlying tissue. Swelling and pain will also be reduced. C - stands for compression bandage or compression. If cooling the injury is not sufficient, compression around the injured spot is recommended in order to counter the pressure from haemorrhage and reduce swelling and pain. Confer with the patient regarding the tightness of the bandage. E – stands for elevation and rest. To decrease the blood pressure and reduce the seepage of blood on and around the injured place, raise an injured arm or foot to approximately heart height and rest for 1-2 days. 8.6.24
INFECTIONS
Refer to the vessels medical cupboard regarding remedies for infection. 8.6.25
INTAKE OF POISON MATERIALS
Poisonous materials can be taken in by inhaling (gas, dust), skin penetration, skin absorption (gas and liquid) and swallowing (gas and fluid). If any of this occurs, different reactions will occur depending on the kind of material, how much, etc. Refer to the material’s data sheet regarding treatment. Blood is most important, since it is the higher brain centre that is first affected from lack of oxygen. A poisonous material emerges quickly to the brain cells and deprives them of oxygen. This may cause unconsciousness, at worst death. By inhaling small concentrations,
we are exposed to localised effects (nasal, throat, and lung) or poisonous gas absorption into the blood. Through skin penetration, gases and fluids are quickly absorbed into the blood and the effects depend on the characteristic of the material, the velocity of the penetration and poisonous elements. If material is swallowed, this is easily absorbed by the mucous membrane in the mouth. 8.6.26
THE EYES
The eyes are very exposed to any spill or contact to cargo. There is normally irritation, burns and tears from harmful exposure. It is of utmost importance with a very fast first aid and abundant rinsing with water. With all injuries and illness it is of the utmost importance to administer first aid and contact competent medical help if any doubt of the outcome exists. Enclosed is a data sheet for Propane, which illustrates the layout and the content of information. There are such sheets for all types of dangerous cargo, which are made readily available and visible onboard. The data sheets tell us about the cargo’s character, the emergency procedure for a cargo fire or cargo spill. There is also information about health hazards, fire, explosion, chemical data, reaction data, physical data and the condition of the material in freight. Information regarding the quality of material is required with the freight of the material.
8.7 ENVIRONMENT AND POLLUTION History Pollution is not a problem recently discovered. When people settled down in small towns and gathered in communities they experienced, as time went by, the difficulties in getting rid of garbage and so on. Civilisations dumped all sorts of waist; garbage, dead animals and so on, in open ditches just outside the small towns. This can be directly connected to large epidemics and disasters, which have arisen during different époques throughout history. Around year 1800, the first attempts were made to make simple constructions of drain arrangements and some kind of garbage removal arrangements. During this period of time the rate of illness and disease showed to be higher in the crowded small towns compared to the countryside. When the Industrial Revolution ended large crowds of people gathered in proportionately small areas. The local environment was extremely loaded because of this. The development of new substances and materials increased rapidly - and lack of knowledge concerning the effect and damages to the environment due to these new substances and so forth led to free flow of the new substances both into the sea and the air. In some cases, both cause and result of these uncontrolled outlets almost immediately appeared. Even today, the reasons for these damaging effects on the environment are questioned. In later years, people really have become aware of the environmental effects this pollution causes. A media focus on oil pollution disasters, industrial leakage, and so on, has contributed to the development of very strict regulations and demands to preserve security that protects us from environmental pollution. However, there is still a long way to go.
8.8
EFFECTS ON ECOLOGY
8.8.1 POLLUTION IN GENERAL AND ITS EFFECT ON ECOLOGY Note that pollution is usually related to human activity. Phenomena, such as radiation due to natural radioactivity in the earth, volcano eruptions and the like, are not usually considered as pollution. They exist, however, in areas where the environment is burdened. This is nature’s own way to balance and renew itself.
Any pollution has a main source and a receiver. The main receivers are air, sea, and soil. The most effective way of spreading pollution is through air. But eventually the pollution always falls to the ground and into the sea. The earth is most resistant to pollution as a receiver, but the problems appear because this pollution almost without restrictions has free flow to pollute sea and waters. Compare the human body with its own immune system to the environmental system (Eco-system), and you will find that all basic “building blocks” are linked together in some way or another with the same influence and with the same purpose. Every part is equally important in obtaining the ability to function as a whole unit. 8.8.2 DEFINITION OF POLLUTION: Substances and materials spread through air - sea - and soil that cause damage and malfunction due to human activity. Many factors contribute to pollution, such as the chemical, physiological or biological characteristics. Life on earth is dependent on solar energy. Plants turn solar energy, water and carbon into plant tissues. This is called the first tropic level. The herbivores (vegetable-only eating animals) cannot exploit solar light directly in their growth or tissue change. Herbivores use the plants to produce tissue. This is called the second tropic level. The energy loss caused by transmission from the first level to the second level is calculated to be at approximately 90%. An even greater loss appears at the next level, which is the third tropic level. This level includes the humans and the animals, which survive by eating animal meat. The demolishing link in this process is the carrion eaters and small organisms, which demolish dead plants and animal materials into simple organic and inorganic compounds, which the plants need to grow. An Ecology System appears as a result of developing and adapting to each other as a species in nature throughout millions of years. Accurate balance and stability is obtained and smoothly functioning. This system is an everlasting process and is continuous throughout time and space. An Ecology System can endure huge changes and variations in nature, but faced with artificial factors and synthetic substances spread by human actions, important parts (areas) in this process can be demolished. The reason is simply that no natural mechanism exists to keep the process active and in balance. In numerous cases, these unwanted non-natural substances are spread throughout the nature process creating disharmony and malfunctions both geographically and ecologically. 8.8.3 HEAVY METALS AND ITS INFLUENCE IN NATURE: Heavy metals are basic elements. These elements exist in some relation or another in nature, and further on in raw materials used by Industry. Some of the most polluting heavy metals are lead, quicksilver, cadmium, nickel and vanadium. Heavy metal is supplied to water, partly by natural flow, through human activity, through the atmosphere, directly to water and spreading on the ground. These heavy metals affect not only single organisms, but also the Ecology System using their ability to function with the other organisms to obtain and keep harmony and balance. Therefore pollution of heavy metals can influence and lead to direct malfunctioning
and cause changes in the composition of a species. This creates disharmony in the Eco-system. Natural, clean and nutrient-rich water demonstrates a well-adjusted and balanced Ecology System.
8.9 POLLUTION OF AIR AND SEA AND THE INFLUENCE OF SHIP TRADE 8.9.1 VIEW OF THE MOST IMPORTANT AIR POLLUTION: Burning sulphurous fossil fuel forms sulphur-dioxides and compounds of this gas. The gas responds to air and transforms into sulphur acid. Nitrogen oxides are also formed by combustion of fossil fuel, and release nitrogen mono oxides, which again transforms into nitric acid and nitrogen oxides. Carbon mono oxides formed by uncompleted consumption of organic material can further react to air and transform into carbon dioxide. Further, a number of gases are released with the gas freeing of cargo tanks and cooling plant. These are CFC – gases (chlorous fluor carbons). Carbon dioxide and CFC - gases function as a glass roof in a hothouse, the heat radiation from the sun is easily received and is harder to let go. This is the hothouse effect in a nutshell. Sulphur and nitrogen oxides in outlets (pollution) cause huge destruction of soil and sea. The consequences of this are recognised in areas where the forest is dead and fishing lakes are empty. Below is a bird, which represents just one of the many members of nature, well worth fighting for. Protect and preserve all parts of our Ecology System. Below are some figures, which show the outlet/pollution from internal combustion engines in the Norwegian coasting trade. In accordance to the investigations performed by the Governmental Pollution Inspection and representative figures were presented in 1985: Air pollution Chemical names Amount in 1000 tons Nitrogen oxides NOx 85,5 Carbon monoxide CO 11,75 Sulphur dioxide SO2 12,57 Hydro carbons HC 7,59 Various particles 1,35 8.9.2 THE FIGURES IN THIS INVESTIGATION REPRESENT ONLY NORWAY AND ITS COASTAL TRADE. The following shows the consumption of fuel in domestic waters: Marine Fuel
514 000 tons
Heavy Fuel Total
163 000 tons 677 000 tons
The average sulphur content in different bunker types: Gasoline Marine Fuel Heavy Fuel Heavy Fuel in the fishing trade
0,1 kg/mt 5,4 kg/mt 43,7 kg&mt 44,0 kg/mt
The outlet of NOx from the coastal trade represents 45% of the outlet from the mobile sources and 38% of the total outlets/pollution in Norway. 8.9.3 WHAT CAN BE DONE TO REDUCE THE OUTLET FROM MACHINES? Through International Conventions, Norway made a commitment to reduce the outlets of nitrogen oxides by 30% within the year 1998. The Norwegian domestic trade represents approximately 40% of our total outlet. In order to reduce outlet along the Norwegian coastline, efforts in following areas were made: 1) Engine technical methods, which concern construction improvements of engines and the engine equipment. 2) Fuel technical methods as a use of alternative fuel and higher graded demands to the compounds of the fuel. 3)
Purifying technical methods as catalyzer and absorbents.
4) Other technical methods as better shaped hulls and alternative propulsion systems. 8.9.4 POSITIVE ENVIRONMENTAL PROPULSION In all these fields with intense investigation, new solutions in future engines and maximisation of already existing engine types is put in priority. Concerning economical views, the investigation has shown increase in costs calculated to be 0,1 % - 0,9% regarding the cargo trade, when the actual reducing actions are executed. 8.9.5 WATER POLLUTION: The total pollution from the fleet world wide is enormous and represents one of the greatest threats to the environment today. The world’s great oceans are continuously exposed to pollution. This causes tragic consequences to animals, fish and all life at sea. The consequences for mankind will be just as tragic, unless this development is reversed. The Ocean has, is and will always be an important food supply and reservoir for human life. If life at sea comes to an end, this will of course lead to the
increase in lack of food and then starvation for millions of people. Various species are threatened of extension as a result of the pollution. The most “significant” oil disasters caused by oil tankers that caused damages and destruction of coastlines and the oceans are: “Tory Canyon” (Southern England 1967), “Amoco Cadiz” (Normandy 1978) and the “Exxon Valdez” (Alaska 1989). These disasters influenced the public opinion and led to new laws and regulations. The last mentioned accident was the direct event leading to new strict American regulations, OPA. 8.9.6 OPA90 THE AMERICAN “OIL POLLUTION ACT OF 1990”. In USA, the accidents involving “The Exxon Valdez” and “Mega Borg” were in focus and were well covered by the media and press, which influenced public opinion. This resulted in the OPA90. The media distributed pictures of the rich animal life and the magnificent coastline in Alaska covered with oil and showing the suffering of dying seals and seabirds. This presentation made a strong impression, which made the U.S. Congress realise that the existing International Conventions had to be reviewed and bettered, in order to protect and take care of the American interests. American lawyers developed the OPA90 and the Congress supported the proposed Act. 8.9.7 THE MAIN ITEMS IN OPA90: 1.
The threat of unlimited responsibility.
2.
Demand of double hull.
3. Direct access to the means in P & I - Companies, in case of indemnity due to accidents. 4. Higher graded demands meant for the crew regarding narcotics and alcohol testing. 5.
Use of pilot in sensitive waters.
Entering American waters OPA requires drill (training) according to OPA90 regulations. The drill (training) should be logged and reported due to the ship owners/operators policy. OPA90 regulations are in force for all kind of ships. 8.10 MARPOL 73/78 The IMO Convention to prevent pollution from ships of 1973, with The Amendment Protocol of 1978 - came in force October 2nd, 1983. The Convention is named MARPOL 73/78 and is introduced in the Norwegian Shipping Control Regulations, section 21 (page 757 and in 1996 edition).
This is the most important pollution agreement ever adapted and it has been of great significance contributing to pollution prevention regulations from ships. 8.9.8 MARPOL 73/78 CONSISTS OF TWO PARTS -MARPOL-73, which is The International Convention about preventing ship-pollution. (Marine Pollution 73). -TSPP-78, which is a 1978 Protocol to MARPOL -73 (Tanker Safety and Pollution Prevention 1978). The MARPOL Convention is extremely important to be familiar with. The Convention consists of 20 articles, 2 Protocols and 5 Enclosures: The 5 Enclosures are as follows: Enclosure I - Oils Enclosure II - Chemicals Enclosure III - Damaging elements in wrapped form, barrels, tanks, containers and so on. Enclosure IV - Sewage Enclosure V - Garbage MARPOL 73/78 - Enclosure I (Oils). The following information referred to is from MARPOL 73/78, enclosure I: 8.9.9 DISCHARGING CRITERIA The amount of oil pumped from an oil tanker on a ballast voyage in open sea, is set to be 1/30 000 of the loading capacity for all “new ships” (ships contracted later than 31.12 1975 or delivered later than 31.12 1979). Ships older than these can pump as much as 1/15 000 of the loading capacity. Nevertheless - no rule allows discharging more than 30 litres pr. nautical mile during the voyage. Several exceptions exist other than the before mentioned dates, but it is too extensive to view in this text. In the MARPOL regulations 73/78, which should be on board all ships, you can find a complete definition of what “new” and “old” ships are. As mentioned before a “new” ship of 200 000 dwt could legally discharge a total of (200 000 : 30 000) = 6,67 tons of oil during a ballast voyage- if a maximum limit 30 litres pr nautical mile is in compliance. 8.9.10
SPECIAL AREAS
In respect for the maritime environment, there are some areas regarding Prohibition Law for Oil Pollution. These areas include among others, the Eastern Sea, the Mediterranean, the Black Sea, the Persian Gulf and the Red Sea and have the definition as follows:
“The Baltic Sea” area means the Baltic Sea proper with Gulf of Bothnia, the Gulf of Finland and the entrance to the Baltic Sea bounded by the parallel of the Skaw in the Skagerrak at N57o 44,8`. “The Mediterranean Sea” means the Mediterranean Sea proper including the gulfs and seas therein with the boundary between the Mediterranean and the Black Sea constituted by the N41o parallel and bounded to the west by the Straits of Gibraltar at the meridian of W5o 36`. “The Black Sea” area means the Black Sea proper with the boundary between the Mediterranean and the Black Sea constituted by the parallel N41o. “The Gulfs” area means the sea area located north west of the rhombi line between Ras al Hadd (N22o 30`, E059o 48`) and Ras al Fasteh (N25o 04`, E061`). “The Red Sea” area means the Red Sea proper including the Gulfs of Suez and Aqaba bounded at the south by the rhombi line between Ras si Ane (N12o 08,5`, E043o 19,6`) and Husn Murad (N12o 40,4`, E043o 30,2`). The definition of clean water means a content of maximum 15 ppm (parts pr. million) of admixture consisting of water/oil. This “clean water” can be pumped over board even in the special areas. Oil tankers should be equipped to load a new cargo on top of the saved oil deposits after tank washing and so forth. The system is known as LOT, Load On Top. 8.9.11
AIR POLLUTION
IMO agrees tithe commitment of air pollution regulations for ships, but not the stringent demands of the rules. Norway, for instance, has the attitude that the presented environment regulations allow for too large sulphur outlets. After seven years of negotiations, IMO probably will adapt regulations concerning pollution from ships in the autumn 1997. The dispute will concentrate on how restricted these regulations will be. This will mainly concern the introduction of upper limits in the sulphur content in bunkers. Sulphur contributes to acid rain. A limit of5% has been proposed and Norway has proposed a limit of 3,5%. An analysis done by DnV proves that bunkers for sale worldwide contain hardly more than 4,2%sulphur. Average level is 2,7% sulphur. Norwegian dealers believe realistic limits to be around 4%. During the negotiations the Baltic countries suggested that the Baltic should be a special area and should have even stronger demands and regulations to sulphur pollution than these presented by IMO regulations. The same proposition will be requested concerning the Northern Sea area. Here, the sulphur limits in air is suggested set to 1,5%. The ship owners will then have the opportunity to select for themselves what type of oil to use that contains low sulphur values or rinse purifiers for the outlet of sulphur. The NO pollution will be evaluated in this conference as well. Even if IMO in autumn should vote for enforcing pollution regulations, it will still be an open question when these regulations will be set in force. The term for this to happen is one of the disagreements yet to be resolved. Keep yourself updated in this field concerning air pollution and the regulations enforce in the nearest future!
8.9.12
TANK WASHING WITH CRUDE OIL
All new crude oil tankers (20 000 dwt or more) and already existing ships (ships delivered before 1.6. 1982 with 40 000 dwt or more) have to be supplied with crude oil washing equipment, described as COW (crude oil washing). Existing ships between20 000 dwt and 40 000 dwt do not need to have this washing equipment, but should have inert gas equipment for use in cargo tanks during operations. These demands are noted in the MARPOL protocol of 1978 to MARPOL 73/78 and are useful in reducing oil pollution at sea. The National Governments have the obligation to put this in force and check that the regulations presented by IMO are in compliance. The Norwegian Maritime Directorate will perform inspection on the Norwegian trading fleet to control that these regulations are executed. When it comes to COW, classification companies are elected to evaluate the documentation and reports after inspection and testing of the equipment on board. 8.9.13
ODME
On board all ships carrying oil there are demands for the installation of Oil Detection Monitoring Equipment that will survey all pumping of ballast and slop water over board. The equipments simply described: An analysis instrument, which analyses the content of oil in the ballast water before pumping it over board. A control unit that calculates the received information and records this on a printer. A sampling pipe from the high over board line. A flow meter on the high over board line. To operate headmen some information has to be manually programmed. This is the ship’s speed, flow rate, year, month, day and time. Study this manual for the specific ship and note that the manual must to be available at all times. 8.9.14
BALLAST HANDLING
Ships constructed for SBT (segregated ballast tanks) have the advantage of already minimised oil pollution. Ships without have CBT (clean ballast tanks) where the cargo tanks are used both for departure and arrival ballast. IMO’s regulations on the tanks used for departure and arrival ballast state that the tanks must be crude oil washed. (COW). Before arrival at discharging port, a discharging plan including crude oil washing should be worked out and it should also contain departure condition that shows the loaded departure ballast. The amount, the quantity and where the departure ballast is placed are viewed in part 10 in the compendium. This departure ballast is called dirty ballast. After departure and in waters allowing water washing of tanks used for arrival ballast, this washing operation should be executed according to regulations in force. Arrival ballast tanks must be crude oil washed beforehand according to the regulations in force. This procedure is done at the last discharging port. The water
washing is performed using the specialised washing system on board meant for crude oil washing - the only difference is that the medium now is water. Set pressure on the line system with water from the sea, and via crude oil washing machines, arrival ballast tanks are water washed. Supplied water is stripped from the actual tank and into the primary slop tank. Via the balance line, the liquid is transferred over to the secondary slop tank, and further the separated water is pumped over board via ODME. To finish water washing of arrival ballast tanks, lines and pumps have to be thoroughly washed. Here the same washing procedure from primary to secondary slop tank is performed and pumped over board via ODME. Now it is time to change ballast. In other words, clean ballast should be pumped in arrival ballast tanks, while dirty ballast should be pumped out from departure ballast tanks. The dirty ballast is pumped over board via ODME. The last oil deposits are stripped into the primary slop tank and after a settling time (when oil and water separates completely), the clean water is pumped over board via ODME. At last the secondary slop tank is pumped into the primary slop tank. This way the ship arrives at the loading port with slop in one of the slop tanks. This saved slop contains the oil deposits from stripping and water washing and the load on top. This is available if it is requested from the charter. This is specified in the charter party for the present voyage. Note: The line system has to be properly cleaned before arriving at the loading port, where the clean ballast is pumped over board. When the ballast changing, line washing and slop handling is completed, the ship contains only the arrival ballast. The arrival ballast is the so-called “clean water”, which is a mixture of water/oil containing a maximum of 15 ppm. This arrival ballast is pumped over board via ODME. 8.9.15
THE OIL RECORD BOOK
This is an extremely important book and must be recorded with accuracy, in order to comply with the regulations in force. All ships should have one oil record book or more in order to record operations, which involve pumping, transferring, ballast handling, bunkers, slop handling, any kind of cargo handling whatsoever. All items mentioned must be recorded in the oil record book. The oil record book must be available at all times to show authorities when arriving at any ports. Remember to keep this book updated and in necessary order at all times. Be environmentally conscious in all aspects on board
09-
Gas Measurement
9 GAS MEASUREMENTS 9.1 PROPERTIES OF PERTOLEUM 9.1.1 True vapour pressure (TVP) All petroleum products and crude oil are essentially mixtures of a wide range of hydrocarbon compounds. The boiling points of the compounds range from –162oC (methane) to well in excess of +400oC, and the volatility of any particular mixture of compounds depends primarily on the quantities of the more volatile elements. The volatility is characterised by the vapour pressure. When transferring a petroleum product to a gas-free tank it begins to vaporise, that is, it liberates gas into the space above it. This gas has also a tendency to re-dissolve in the liquid. The pressure exerted by this gas is called the equilibrium pressure of the liquid, usually referred to simply as the vapour pressure. The vapour pressure of a pure compound depends only upon its temperature. With a mixture of compounds, the vapour pressure depends on the temperature, elements and the volume of the gas space in which vaporisation occurs. The true vapour pressure (TVP) or bubble point vapour pressure is the equilibrium of vapour pressure of a mixture when the gas/liquid ratio is effectively zero. The highest vapour pressure is possible at any specified temperature. As the temperature of a petroleum mixture increases, its TVP also increases. If the TVP exceeds atmospheric pressure, the liquid begins to boil. The TVP of a petroleum mixture gives a good indication of its ability to give rise to gas, but unfortunately it is a property which still is extremely difficult to measure.
9.1.2 The Reid Vapour Pressure (RVP)
Testing is a simple and generally used method for measuring the volatility of petroleum liquids. Measurement of the RVP is conducted at 37,8oC (100oF). The greater the RVP value, the more volatile is the oil. Normally crude oil has a RVP of between 0,1 and 0,8kg/cm2. A sample of liquid is put into the test container at atmospheric pressure. The volume of liquid should be one fifth of the container’s total volume. Then the container is sealed and immersed in a water bath, which is heated to 37,80C. The container is then shaken in order to mix the liquid properly and the rise in pressure due to vaporisation can be read on the attached pressure gauge. This pressure gauge gives a close approximation in bars. Because the liquid’s vapour pressure is at 37,8oC, RVP is useful for generally comparing the volatility of a wide range of petroleum liquids. However, it has, small value as a means of estimating the likeliness of gas evolution in specific situations, mainly because the measurement is made at the standard temperature of 37,8oC and at a fixed gas/liquid ratio. For this purpose TVP is much more useful. As mentioned, in some cases, correlation exists between TVP, RVP and temperature. For safety measures against fire on ships, the Norwegian Maritime Directorate in the Regulation of December 3rd1979 uses 61oC as limit value for flash point and 2,8kg/cm2 for vapour pressure at 37,8oC. The oil referred to in this regulation is:
· Mineral oils with a flash point below 61oC, such as kerosene, benzene, gasoline and crude oil or other flammable liquids with a flash point below said limit. · Mineral oils with a flash point of 61oC or higher, such as marine gas oils, fuel oil, diesel oil, lubricating oil, which give off flammable gases when heated. · Oils and fats of animal or vegetable origins, such as whale oil, groundnut oil, linseed oil etc., which give off flammable gases when heated. The liquid chemicals referred to are: · Chemicals with an absolute vapour pressure lower than 2,8kg/cm2 at 37,8oC. The condensed gases referred to are: · Chemicals with an absolute vapour pressure of 2,8kg/cm2 or higher at 37,8oC.
9.1.3 Flash Point The flash point for an oil product is the temperature at which it is possible to ignite the vapour above the liquid. In other words, the flammable gas concentration above the liquid is close to the lower explosive limit. Determination of the flash point is done with a special apparatus and according to specific rules. A sample of liquid is gradually heated in a special pot and a small flame is repeatedly and momentarily applied to the surface of the liquid. The temperature is recorded when a small flame initiates a flash or flame across the liquid surface, thereby indicating the presence of a flammable gas. In this test the space above the liquid is kept closed except for the brief moments when trying ignite the liquid’s surface. This test is called “Closed cup Flash Point”. When we do the test with the liquid surface permanently open to the atmosphere, the result of such a test is called “Open cup Flash Point”. Because of the greater loss of gas to the atmosphere in the open cup test the open cup flash point is always a little higher (about 6oC) than the closed cup flash point. The restricted loss of gas in the closed cup apparatus also leads to a much more consistent result than can be obtained in open cup testing. For this reason, the closed cup method is generally favoured. However, open cup test figures, still may be found in the registration of various national administrations, in classification society rules and other such documents. If the temperature is increased further beyond the flash point, the liquid will obtain a temperature so high that the evaporation will take place fast enough to support a flame. This is called “The Burning Point"“.
9.1.4 Burning Point of some hydrocarbons Product
Burning point in degree Celsius
Asphalt
+204
Benzene
-50
Benzene
-11
Butane
-35
Crude oil
-10/+30
Diesel oil
+70
Ethan
-125
Fuel oil (no. 1&2)
+38
Fuel oil (no.4&5)
+54
Fuel oil (no.6)
+65
Hexane
-28
Methane (LNG)
-175
Mineral oil
+193
Naphtha (mixtures) +38/+60 Paraffin wax
+320
Pentane
-40
Propane
-105
Lub.oil (motor oil)
-149/+232
Propylene
-108
Ethylene
-150
For refined products, the flash point increases from light to the heavy hydrocarbons, for gasoline it is about –50oC and for kerosene over +60oC. The flash point for liquids is used in rules and regulations for transportation and storage. Crude oil from various sources may have quite different flash points, usually between –10oC and +30oC.
9.1.5 Flammability The burning process means that hydrocarbon gases react with the oxygen in the air to produce carbon dioxide and water. This reaction gives enough heat to form a flame which goes through the mixture of hydrocarbon gas and air. When the gas above a liquid hydrocarbon is ignited, the heat that is produced is usually enough to evaporate sufficient fresh gas to maintain the flame and the liquid is said to burn. In fact, it is the gas that is burning and continuously being replenished by the liquid. 9.1.6 Flammable Limits A hydrocarbon gas mixture and air cannot be ignited and burned unless its
composition lies within a range of gas-in-air concentrations, known as the “flammable range”. The lower limit of this range is known as the “LEL” (lower explosive limit). The “LFL” (lower flammable limit) is also used. This level means that hydrocarbon concentration has an insufficient amount of hydrocarbon gas to support and propagate combustion. The mixture is “too lean”. The upper limit of the range known as the “UEL” (upper explosive limit), or also known as “UFL” (upper flammable limit). This level means that the hydrocarbon concentration has an insufficient amount of air to support and propagate combustion. The mixture is “too rich”. Between these two areas, the mixture is flammable and results in a fire or explosion, if ignited. With hydrocarbon gases from crude and sediments, it is usually assumed that the upper explosion limit lies at about 10% by volume of hydrocarbon gas-in-air and the lower explosion limit at about 1% by volume of hydrocarbon gas.
9.1.7 Explosion limits in % flammable gas in mixture with air.
PRODUKT
LEL, volume UEL, volume % %
Methane (LNG)
5,3
14,0
Ethan
3,1
12,5
Propane
2,1
9,5
Butane
1,5
9,5
Pentane
1,5
7,8
Hexane
1,2
7,5
Hepthane
1,2
6,7
Octane
1,0
3,2
Nonthane
0,8
2,9
Dechane
0,8
5,4
Hydrogen
4,0
75,0
Hydrogen sulphide
4,3
45,0
Carbon monoxide
12,5
74,0
Crude oil
1,5/2,5
8,0/11,0
Benzene
1,4
7,6
Naphtha
0,9/1,1
6,0/6,7
Propylene
2
12
Ethylene
2,5
34
VCM
4
31
9.1.8 Air The mixture of gases found in the atmosphere is given the name air. The ratio of mixture between various gases is the same, independent of time and place, except for the water vapour content, which can have great variations. ELEMENTS in air Nitrogen, N2
78,09%
Oxygen, O2
20,93%
Argon, A
0,93%
Carbon dioxide, CO2 0,03% Other gases
0,02%
AIR
100%
There may be a significant amount of water vapour in the air. Different results are measured depending on whether water or moisture is removed or not. The amount of water vapour, which the air may contain, will depend very much on the temperature. The air is saturated with water vapour when the air contains a maximum amount of water vapour at a certain temperature. Saturated air being cooled will release the excess water in droplets. At high humidity and high temperature, there will be a reduction of oxygen and other gases that is caused by the increased water vapour content. The atmospheric pressure will influence the measurement result when using gas measure instruments. For example, when using a portable oxygen analyser that is calibrated to read 21% oxygen by volume in clean air at atmospheric pressure, the reading will increase as the atmospheric pressure increases. To compensate for the changes in atmospheric pressure, the instrument has to be calibrated with clean air from time to time. The instruments used for measuring hydrocarbon gases will also be influenced by the atmospheric pressure, depending on the instrument’s measuring principle. TEMPERATURE
WATER VAPOUR CONTENT
-200C
0,1 volume %
00C
0,9 volume %
200C
2.3 volume %
400C
7,3 volume %
600C
19,7 volume %
800C
46,7 volume %
1000C
100 volume %
The risk of fire or explosion is drastically increased if air is replaced by pure oxygen. As known, oxygen leakage during welding has resulted in several fatal accidents. However, when reducing the oxygen below 21% by volume, the fire and explosion hazard is reduced. When reducing the oxygen content to below 10,8% by volume, fire and explosion cannot take place even though both hydrocarbon gas and ignition sources are present. 9.1.9 Hydrocarbon gases Crude oil is formed from plants and animals residues and contains several thousand different chemical compounds. Most of these materials consist of only the element hydrogen (H) and carbon (C) called by the common name hydrocarbons. The simplest hydrocarbon is methane, which is the main element of natural gas. Butane, propane and ethane are also composed of hydrogen and carbon atoms and they are all called hydrocarbon gases. For example butane, C4H10 means that this gas contains a total of 4 carbon atoms and 10 hydrogen atoms. Hydrocarbons with up to 4 carbon atoms are liquefied gases at room temperature and atmospheric pressure. From 5 to 16 carbon atoms the hydrocarbons are liquids, and above 16 carbon atoms, the hydrocarbons are solid materials such as wax and asphalt. When the crude oil is taken out of a well, hydrocarbon gases and solid materials are dissolved in the oil. When reducing the pressure, gases will bubble out. To separate these liquefied gases the crude must pass through one or more processing units (stabilisers). The crude oil is called “stabilised crude”, but even stabilised crude oil will give off hydrocarbons from the surface. Methane gas is lighter than air. Ethane gas has approximately the same density as air. The gases butane and propane from higher hydrocarbons are heavier than air. The gas mixtures given off from crude oil, sludge and sediments are all heavier than air. Until such gas mixtures have been mixed with air inside inert gas, the highest hydrocarbon concentration will appear near the bottom. “Spiked crude oil” (also called “enriched” or “tailored” crude) is crude oil, which has had hydrocarbons, added in gas or liquid form. The spiked crude may contain rather large amounts of added hydrocarbons and therefore emit heavy gasses under certain conditions (during loading, crude oil washing, discharging).
9.2 TOXICITY HAZARDS The toxic hazards to which personnel are exposed in tanker operations arise almost entirely from exposure to various kinds of gasses. TLV (Threshold Limit Value) has been in use within the industry for a number of years, and is often expressed as a “Time Weighted Average” (TWA). The use of the term “PEL” (Permissible Exposure Limit) is becoming more commonplace and refers to the maximum exposure to a toxic substance that is allowed by an appropriate regulatory body. The PEL is usually expressed as a Time Weighted Average, normally averaged over an eight hour period, or as a “Short Term Exposure Limit” (STEL), normally expressed as a maximum airborne concentration averaged over a 15 minute period. The values are expressed as parts per million (ppm) by volume of gas in air. 1 ppm corresponds to one-millionth part by volume pollution in air. Compared with a value quoted in percent by volume, we find that 1% by volume = 10000 ppm. List of TLV (PEL) are adjusted from time to time, so take into consideration the experience gained. Keep the list up to date at all times.
9.2.1 Ingestion There is a very slight risk of swallowing significant quantities of liquids during normal tanker operations. The oral toxicity from petroleum is low, but if swallow it causes acute discomfort. Liquid petroleum may be drawn into the lungs during vomiting resulting in serious consequences.
9.2.2 Skin contact Petroleum products cause skin irritation and remove essential oils from the skin, leading to dermatitis. Oil can also cause serious skin disorders from repeated and prolonged contact. The effects of a gas mixture from crude oil include headache, eye irritation, reduced sense of responsibility and dizziness similar to drunkenness. Higher concentrations may lead to paralysis, numbness and death. To avoid direct contact, always wear appropriate protective clothing and equipment!!!!!
9.2.3 Petroleum gases The toxicity of petroleum gases has a wide variation depending on the major hydrocarbon constituent of the gas. For a short period of time the human body can tolerate a somewhat higher concentration than the corresponding TLV. Toxicity can greatly be influenced by the presence of some minor compounds, such as benzene and hydrogen sulphide. A TLV of 300ppm, corresponding to about 2% LEL is established for gasoline vapours. Such a figure may be used as a general guide for petroleum gases, but must not be used for gas mixtures containing benzene or hydrogen sulphide. The following are typical effects found at higher concentrations: Concentration
% LEL
Effects
0,1% vol. (1.000ppm)
10%
Irritation of the eyes within one hour.
0,2% vol. (2.000ppm)
Irritation of the eyes, nose and throat, dizziness and 20% unsteadiness within half an hour.
0,7% vol. (7.000ppm)
70%
1.0% vol. (10.000ppm)
Rapid onset of “drunkenness” which may lead to 100% unconsciousness and death if exposure continues.
2,0% vol. (20.000ppm)
Symptoms as of drunkenness within 15 minutes.
Paralysis and death occur very rapidly
9.2.4 Benzene Aromatic hydrocarbons include benzene, toluene and xylene. These substances can be found in varying amounts, in many typical petroleum cargoes, such as gasoline’s, naphtas, special boiling point solvents, turpentine, substitutes, white spirits and crude oil. Benzene primarily presents an inhalation hazard. It has poor warning qualities. Benzene can be absorbed through the skin and is toxic if ingested. For handling cargo that contains benzene, use the described operation procedures for this kind of hydrocarbon.
9.2.5 Hydrogen sulphide (H2S) If the vessel is carrying sour crude, it is absolutely essential to check the tank(s) atmosphere for hydrogen sulphide before entering. A lot of crude oil comes out of the well with high levels of hydrogen sulphide, but is usually reduced by a stabilisation process before the crude oil is delivered to the vessel. This stabilisation may, however, decrease over time. The nose has no trouble detecting the smell from hydrogen sulphide at low concentrations, which is like the smell of rotten eggs, but the sensory cell in the nose is immediately put out of function if higher concentrations are inhaled. The effects of the gas at concentrations in air in excess of the TWA (Time Weighted Average) are, as follows: Concentration Effects Eye and respiratory tract 50 - 100 ppm irritation after exposure of one hour. 200 - 300 ppm
Marked eye and respiratory tract irritation after exposure of one hour.
500 - 700 ppm
Dizziness, headache, nausea etc. Within 15 minutes, loss of consciousness and possible death after 30-60 minutes exposure.
700 - 900 ppm
Rapid unconsciousness, death occurs a few minutes later.
1000 - 2000 ppm
Instantaneous collapse and cessation of breathing
Persons over exposed to H2S vapour should be taken to clean air, as soon as possible. The adverse effects of H2S can be reversed and the probability of saving the persons life improved, if prompt action is taken. For handling cargoes containing hydrogen sulphide follow the operation procedures described for such a cargo.
9.2.6 Toxic Elements in Inert Gas Inert gas’s low oxygen content is the main hazard. Inert gas produced by combustion, either in a steam boiler or in an inert gas generator, contains a various amounts of toxic gases, which may increase hazard to the personnel exposed to it. Follow the precautions to protect personnel against toxic hazards. These precautions do not include the requirements for direct measurement of the trace flue gas element’s concentration. This is because when gas that is freed from a tank, the hydrocarbon gas concentration is about 2% by volume to 1% LEL. Until there is a steady 21% by volume oxygen reading, it is sufficient to dilute these elements to below their TLV’s.
9.2.7 Nitrogen Oxides Flue gas contains approximately 200ppm (0,02%) by volume of mixed nitrogen oxides. Nitrogen oxide (NOx) is generally removed in the water scrubber in the inert gas plant. The NOx gas is colourless with a weak smell at its TLV of 25ppm. Nitrogen dioxide is even more toxic with a TLV of 3ppm.
9.2.8 Sulphur Dioxide
Flue gas produced by the combustion of high sulphur content fuel oils typically contains about 2,000 ppm of sulphur dioxide (SO2). Inert gas system water scrubbers remove this gas with an efficiency, which depends upon the design and operation of the scrubber, giving inert gas with sulphur dioxide content usually between 2 and 50 ppm. Sulphur dioxide produces irritation of eyes, nose and throat and may also cause breathing difficulties in sensitive people. It has a distinctive smell at its TLV of 2 ppm.
9.2.9 Carbon Monoxide
Carbon monoxide (CO) is normally present in flue gas at a level of only a few parts per million, but at abnormal combustion conditions and slow running it can give rise to levels in excess of 200ppm. This gas is an odourless gas with a TLV of 50ppm. It is insidious in its attack, restricting the blood to absorb oxygen, causing a chemically induced form of asphyxiation.
9.2.10 Oxygen Deficiency For several reasons the oxygen content in enclosed spaces may be low. On oil tankers the most obvious one is that the space is in an inert condition. Also it can be due to a lack of oxygen based on chemical reactions, such as rusting or the hardening of coatings. When the available oxygen decreases below 21% by volume, breathing tends to become faster and deeper. Symptoms indicating that an atmosphere is oxygen deficient may not give adequate notice of danger. Most persons would fail to
recognise the danger until they were too weak to be able to escape without help. This is especially so when escape involves the exertion of climbing. Entry into spaces with oxygen less than 21% by volume must never be permitted.
9.3 INERT GAS In principle, inert gas is used to control the tank atmosphere in order to prevent the formation of flammable mixtures. Inert means inactive and the primary requirement for an inert gas is low oxygen content. The composition of inert gas can vary. The following table provides an indication of typical inert gas components from flue gas, expressed as a percentage by volume: Inert gas
Before scrubber
Nitrogen, N2
Approx. 80% Approx. 80% vol. vol.
Carbon dioxide, CO2.
Approx. 14% Approx. 14% vol vol.
Oxygen, O2
2 - 5% vol.
Water Approx. 5% vapour, H2O vo
After scrubber
2 - 5% vol. 20oC: approx. 2% vol. 40oC: approx. 7% vol.
Carbon monoxide,
CO Approx. 0,01% vol.
Approx. 0,01% vol.
Nitrous gases, NOX
Approx. 0,02% vol.
Approx. 0,02% vol.
Sulphur dioxide, SO2
Approx. 0,3% vol.
Approx. 0,005% vol.
Ash and soot
300mg/m3
30mg/m3
Density
1.044
1.044
When hydrocarbon gas burns in air, the oxygen in the air reacts while the nitrogen gas is inert and does not take part in the reaction. Examples of inert gases are nitrogen, carbon dioxide or combustion gases. On a crude oil tanker, the production of inert gas is done with flue gas from the ship’s boilers or by a separate inert gas generator. The flue gas being produced, before being transferred to the cargo tanks, is first cooled and cleaned of soot and
corrosive gases. This prevents fire and explosion. The maximum permissible oxygen content in the inert gas delivered to the cargo tanks is 5% by volume (all kinds of tankers). Approximately the content of carbon dioxide in the inert gas is 14% by volume depending to some extent on quality of the oil being burned and on the air supply. The carbon monoxide contained in the supplied inert gas is approximately 0,01% by volume, but if the excess air is reduced too much in hopes of reducing the oxygen content, the concentration of carbon monoxide could increase significantly. The concentration of nitrogen in inert gas will more or less be the same as for the concentration in air, broadly speaking, about 80% nitrogen by volume. A small amount of Nitrous Gases (NO and NO2) is formed, following the reaction between nitrogen and oxygen in the air at higher temperatures. It will be approximately 0,02% NOX by volume.
The concentration of sulphur dioxide in the inert gas depends on the sulphur content of the oil being burned. It will be approximately 0,3% by volume in the flue gas. After passing the scrubber, depending on the efficiency of this, the content is reduced to approximately 0,005% by volume. Flue gas contains soot as high as 300mg/m3, but is reduced to below 30mg/m3 after passing the scrubber. The oxygen concentration in flue gas will be different, before the scrubber, than in the inert gas, after the scrubber. Some ships use the same fixed instrument for measuring the oxygen content in the flue gas, before passing the scrubber, and the inert gas, after the scrubber. This is done by providing a choice of sampling lines from two different places into the same instrument. The main problems are in the flue gas measuring with greater reading and guarding against instrumentation error. It is strongly recommended to have a separate oxygen-measuring instrument for inert gas, after the scrubber. When recalculating inert gas through the scrubber beware of the oxygen content increase due to the evolution of oxygen from the seawater. The figure to below shows an example of design of a scrubber for cooling and cleaning of the flue/ inert gas.
9.4 GAS INDICATORS 9.4.10 Sampling lines and pumps It is very important to realise that the quality of the sampling hose has influence on the measuring result, and that correct use and maintenance are important. If the hose is not properly chosen, it is likely that a poor quality hose will absorb hydrocarbon gases. Make sure that the quality of hoses being used on your ship is approved and in good condition. Examples of hoses which have proved acceptable: 1. Teflon inner hose, neoprene outer hose. This hose’s inside diameter is 3mm, which corresponds to an inner volume of about 7cm3 per meter length. 2. “Tanol” (Trade mark of MSA). This hose is marked: “Tanol” - synthetic rubber sampling line, low solvent absorption, anti-static. Note: In an enclosed container use adequate electrical bonding. The inside diameter is 5mm corresponding to an inner volume of about 20cm3 per meter length. When ordering a measuring hose make sure you are getting an approved one. Always ask the deliverer for a certificate, which shows the authorisation. It is very important to -m
9.4.2 Pumps The hand pumps used are often in a rubber form with a volume of 40cm3 or more. When using long hoses, it is important to know the number of pump strokes from the sampling point that are necessary for the gas to reach the instrument. The number of strokes depends on the hose length, as well as, the inside diameter of the hose. The number of strokes may vary from 6 to 15 for a hose length of 30 metres, depending on the inner diameter. The numbers mentioned are based on a pump volume of 40cm3. Some types of instruments are fitted with built-in pumps. Follow the user instruction for such a pump.
9.4.3 Cleaning of hose If the sampling hose gets dirty with oil on the outside, immediately clean it with a dry cotton rag. If the hose is dipped by accident in oil and oil is drawn into the hose, discard the hose because it is very hard to clean it. Always follow this rule: Each gas measuring instrument has its own hose only for using with the specific instrument. Do not mix hoses with hoses, which belong to another instrument.
9.4.4 Leakage, plugging, contamination Always check the hose, instruments and pumps before use, in order to detect any leakage, plugging or contamination. Follow the procedure check for the instrument being used. Place a finger on the hose opening and check that the hand pump remains squeezed together for about 1 minute. If there is a built-in pump, the flow indicator gives an alarm. See the illustration to your right.
Carry out measurements with and without the sampling hose to check that the hose does not influence the measurement by absorbing or releasing gases. For this purpose use clean air and a calibration gas, depending on the type of gas measuring instrument being checked. Also carry out a leakage test on the instrument, and if applicable, on a drop catcher or other optional equipment that has been fitted. See the illustration to your right.
9.4.5 Maintenance Make it a rule to always purge the hose by pumping clean air through it after use. And blow the measuring hoses with compressed air from time to time to remove water droplets and dust. As the analysers are of vital importance, they must be carefully maintained and tested strictly in accordance with the manufacturer instructions.
9.4.6 Filters Normally used in hydrocarbon gas meters are cotton filament type filters, catalytic or non-catalytic. Additional filters are not normally needed. In extremely moist or wet conditions, for example during tank washing, excessive water can be removed from the gas sample using materials that retain water, but do not affect the hydrocarbons. Materials for this purpose are granular calcium chloride or sulphate. If required, soda asbestos will selectively retain hydrogen sulphide without affecting the hydrocarbons. However, it also retains carbon dioxide and sulphur dioxide and must not be used in tanks, which are inerted with scrubbed flue gas. The use of water retaining filters is essential when using an oxygen analyser, especially the analysers based on the paramagnetic principle. This is because the presence of water vapours in the sample can damage the measuring cell. Use only manufacture recommended filters.
9.4.7 Calibration gas Always have the appropriate calibration gas for the instruments on board. This calibration gas has to be the right type and the availability has to be good. Also, knowledge how to use the different types of calibration gas must be properly understood. Always follow the manufacture's recommendation when ordering calibration gas. Also demand a certificate on the ordered calibration gas to be sure that you are receiving a gas of high quality. Explosimeters use a mixture of hydrocarbon gas and air, approximately 50% LEL or lower, as a calibration gas. (It is important to have a certificate on the specified hydrocarbon gas, showing the exact percent of LEL). Various types of hydrocarbon gas measuring instruments may have different requirements of calibration gas. Make sure you have the right one on your vessel. Oxygen analysers used at low concentrations usually use nitrogen as the calibration gas in order to get a zero adjustment and dry air is used for the 21% O2 by volume adjustment.
9.4.8 Attention
Those using the measuring instruments on board must have sufficient knowledge about the instrument, and all such instruments must have the operating instructions attached to the instrument. Also keep a log for each instrument, where records are made of the calibration performed, replacement of parts or other repairs, faults and irregularities. Always have additional spare parts in supply, which may have to be replaced from time to time. If the instrument not is in use for a long period of time, remove the batteries; even the leak proof ones. Warning For the sake of safety, all instruments must be operated and serviced by qualified personnel only. Read and make sure you fully understand the instruction book before using or servicing the instrument.
9.4.9 Volume % hydrocarbon gas measuring instruments
We are going to discuss various principles for the measurement of hydrocarbon gases given off by crude oil. In order to measure hydrocarbon gases in a mixture with other gases, for example inert gas, an instrument is used, which measures the absorption of infrared light. Such infrared absorption instruments are found both as laboratory instruments and as instruments for fixed installations on board ships. In the early 1970’s, when trying to find portable gas measuring instruments for the determination of % by volume HC in a tank atmosphere, there were few commercial instruments, which appeared suitable. An interferometer was modified and the Riken
Interferometer Type 17HC, with the measuring ranges 0-5% by volume and 0-30% by volume HC, was developed in collaboration with Riken Keiki Fine Instrument Co., Japan. At this time, only a few ships had an inert gas system on board. The instrument was used for measuring hydrocarbon concentrations in air, which were higher than the lower explosive limit, to check for freeing gas with air before tank washing in a “too lean” atmosphere. Later on, the instrument also came to be used for the measurement of hydrocarbon gas concentration in an inerted atmosphere.
9.4.10 Riken portable indicator Model 17HC 9.4.10.1 Operating Principles This instrument measures volume by percent of hydrocarbon gases above crude oil using an optical registration at the speed of light, which passes through the air and gas/air mixture respectively. The gas in question is sucked into two chambers that are placed in sequence and equipped with glass end walls enabling the light to pass through. The volume percentage is registered on a double scale that is graduated 05 and 0-30 and is read through an adjustable lens. With Riken 17HC one can measure concentrations of hydrocarbon gases by utilising the difference between the speed of light through air and the gas. The difference increases with increasing hydrocarbon gas concentration. The refractive index for a gas is an expression of the ratio between the speed of light in vacuum and in the gas. The speed in the gas will depend on pressure and temperature. The refractive index is normally quoted at a pressure of 1 atmosphere and either 0oC or 20oC. Compared with the refractive index for the various hydrocarbon gases, the hydrocarbon mixture index used by Riken is closest to butane. The instrument is tested for working within a temperature range of +113oF (45oC) to –22oF (-30oC). Hotter gases should be cooled down to come within this range. The interferometer was originally chosen to determine hydrocarbon gases in air. The conditions become more complicated if the interferometer is used for measuring hydrocarbon gases in inert gas. There will be a difference between the zero adjusts for air without hydrocarbon gas and inert gas without hydrocarbon gas. Oxygen and nitrogen have rather similar refractive indexes, but there will be a positive deviation in relation to air when the oxygen content decreases from 21% by volume. If the oxygen content is reduced from 21% by volume to 5% by volume, the reading on the interferometer increases from 0% by volume HC to 0,5% by volume HC. Carbon dioxide has a higher refractive index than air, so the reading on the interferometer for 1% by volume CO2 is approximately 0,15. Inert gas, which contains close to 14% by volume CO2 and approximately 5% by volume O2, will therefore give a reading on the interferometer of 2,5. (Approximately 2,0 is due to carbon dioxide and about 0,5 is due to low oxygen content). When the interferometer is used for measuring hydrocarbon gas in inert gas, a correction is therefore necessary for the difference between zero setting in clean air and zero setting in inert gas.
Previously a method was used whereby carbon dioxide was removed from the gas mixture before the introduction to the interferometer. The gas mixture was passed through a tube filled with soda lime, as an absorption material. Experience has shown that the absorbent often is not very efficient, so that measurements with the interferometer have given too high values. It is therefore recommended to correct for the difference between zero setting in clean air and in inert gas by using a method, which does not include the use of the external filter. Inert gas contains 1214% CO2. To remove such a large concentration by means of the external filter has proved difficult. Instead of using the filter the measurement is read directly and the values read are reduced by 2,5%. If there is a risk of sucking in water vapour/condensate, one can use a moisture collector (which usually accompanies the instrument) and install it between the suction hose and the instrument. When measuring hydrocarbon gases in an inerted tank atmosphere with an interferometer without the soda lime, the reading must be corrected by subtracting 2,5 from the values read. For example, the correct value will be 2,5% by volume HC for a reading of 5,0. Optical diagram
9.4.10.2 How to use the instrument Function of parts: 1. Inlet port to which the sampling tube is connected. 2. Outlet port to which the aspirator tube is connected. 3. Push button switch to illuminate the scale. 4. Screw off cover to protect zero setting from any disturbance in handling the instrument during tests. 5. Zero adjusting knob for setting interference fringe to zero position in fresh air. 6. Cock to change the sampling route either HIGH RANGE or LOW RANGE. 7. Eyepiece lens and protecting push on cover (on chain) to the right. The lens can be focused for personal vision by turning in either direction.
8. Aspirator bulb. 9. Screw on covers, replaceable moisture absorbent cartridge and single cell flashlight battery. 10. Cover for electric bulb for the light source.
9.4.10.3 Preparation: 1. a) Secure auxiliary filter in leather strap. Connect rubber tube to gas inlet port (1) through auxiliary filter. 2. b) Connect rubber aspirator to gas outlet port (2). 3. c) Place cock (6) in position 5 and squeeze aspirator (8) at least five times in fresh air to clean gas chamber. 4. d) Press the switch (3) and observe interference fringe through eyepiece. 5. e) Remove protective cover (4) of zero setting knob (5). Adjust the right one of two black lines, just on the zero position of scale, by rotating the zero setting knob. 6. f) Put the cover back on, in order to protect the knob from any accidental movement. Reading: 1. Suck the gas to be examined into instrument by squeezing aspirator about 5 times or more if extension tube is used. 2. Press the switch and examine amount of shift of marked black line through eyepiece, which gives percentage of gas on 0 - 5% scale. 3. If the marked black line or fringe is beyond scale, gas concentration is higher than 5%. In such case, change cock position to 0 - 30% scale. 4. Suck clean air into instrument by squeezing aspirator 3 to 5 times. 5. Press the switch and examine amount of shift of marked black line through eyepiece, which gives percentage of gas on scale 0 - 30%. After reading: Place cock position to 5 and clean gas chamber with fresh air.
9.4.10.4 Taking readings: In gaseous atmosphere draw in test sample by squeezing bulb at least 4 times for each meter of sampling hose in use. Press the switch (3) and observe new position on scale of RIGHT HAND EDGE of INDEX STRIPE. The reading indicates the percentages of hydrocarbon gas. Repeat for further gas tests.
9.4.11 Percent LEL measuring instruments & Explosimeters Most types of instruments giving concentration of flammable gas in air in %LEL use catalytic combustion as the measuring principle. Such instruments are usually called exsplosimeter. A catalyst is a substance, which helps a chemical reaction to take place. Exsplosimeter normally use platinum metal or platinum alloyed with other metals as a catalyst. To make the reaction take place, the catalyst has to be heated to a high temperature. Certain types of Explosimeters use a platinum wire as a catalyst and the reaction between flammable gas and the oxygen in the air takes place on the surface of the metallic wire. The temperature of the wire may then be 1000oC. Other types of Explosimeters have a coating on the outside of a heated metal wire, and it is the coating which catalyses the reaction. The reaction takes place somewhat easier on this coating, and a temperature of 500oC may be sufficient. The part of the instrument where the reaction takes place is normally called a sensor or detector.
The flammable gas to be measured is burned on the surface and the heat generated results in a temperature increase. The electrical resistance of the metallic wire increases with the temperature. The change in resistance is proportional to the increasing temperature and to the concentration of flammable gas in the air. This applies only to a lean mixture below the lower explosive limit. The instruments are usually designed in such away that they first have to be adjusted to zero with clean air. Then the atmosphere that should be measured is sucked into the instrument where the sensor is located and a reading is made. Finally, clean air is sucked in again and the zero setting checked. Some types of instruments are intended for monitoring and are designed so that the sensor is located at the spot where the measurement is to be performed. Explosimeters are calibrated with a certain gas, for example butane. It should be marked on the instrument, which gas is used for calibration gas. To some extent the explosimeter will also be suitable for measurement of other flammable gases, and many manufactures of instruments quote the correction factors for various gases other than the calibration gas. The most frequently used calibration gases for commercial explosimeters are methane, propane, butane, pentane, hexane or nonane. For ships carrying crude oils, it is recommended to use butane in air or alternatively propane in air. Theoretical calculations of the sensitivity of an explosimeter for various flammable gases show that the reading for 100% LEL of the gas mixture is proportional to the heat of combustion, to the diffusion coefficient of the flammable gas and to the gas concentration at the lower explosive limit. The diffusion coefficient is an expression for the speed at which the molecules can move to the catalyst surface where the reaction takes place, and the lighter molecules move faster than the heavy ones. For example, the methane molecules move faster than the propane molecules. Theoretical calculations of sensitivity have been performed for nearly 100 different flammable gases, and the value for hydrocarbon gases are given in the table below:
Type of Sensitivity HC gas Methane 100 Ethane
68
Propane 55 Butane, 59 n Butane, 52 i Pentane, 46 n Hexane 37 Heptane 38 Octane
38
Nonane 31 The above figures are given in arbitrary units. As an example, an exsplosimeter calibrated with propane will theoretically give a deflection for 100% LEL of hexane which is (37:55) x 100 = 67% LEL. There is however, some difference between theory and practice. In practice there will not be the same conversion factors for different types of Explosimeters, since the details of how the instruments are designed are of great importance. There may also be a large difference from one instrument to another instrument of the same type, which is greatly dependent on how good of a control the manufacturer has over own production. From what we have seen so far, explosimeters calibrated with butane should show higher values for methane, lower values for pentane, hexane and the other heavier hydrocarbon gases. There is a complicating factor, however, in that methane is a gas, which requires a more efficient catalyst and/or a higher catalyst temperature. On the market there are some types of explosimeters with low sensitivity for methane and several types of explosimeters which have been investigated showing that the sensitivity to methane may drop after a short period of time of using the instrument. However, it still gives a correct reading for the heavier hydrocarbon gases. For explosimeters being used on board LNG-carriers, methane must be used as the calibration gas. Explosimeters to be used on ships carrying crude oil, butane is recommended to be used as calibration gas, alternatively propane. This is because the gas mixture given off by crude oil contains relatively small amounts of methane gas and the gas given off from sediments and oil residues contain quite negligible concentrations of methane. Be aware that the exsplosimeter will give somewhat misleading low values for the hydrocarbon gases that are heavier than the calibration gas.
The catalyst will, when used gradually, lose its ability to bring about combustion, and all types of explosimeters have, to a greater or lesser extent, the regrettable characteristic that the sensitivity is reduced. All explosimeters must therefore from time to time be checked with its calibration gas. Certain gases may poison the catalyst, and it is known that hydrogen sulphide from sour crude may act in this manner. A poisoning will lead to the properties of the catalyst being temporarily or permanently damaged so that the sensitivity of the instrument to flammable gases is greatly reduced or vanishes altogether. The bestknown catalyst poisons are silicones and vapours from leaded gasoline, which give a solid deposit on the outer surface of the catalyst. We have mentioned that the reading of the explosimeter depends on the concentration and diffusion coefficient of the flammable gas. This only applies when we have a lean mixture of flammable gas in air. For high concentration of flammable gas, the reading will instead depend on the concentration and diffusion coefficient of oxygen. Very high concentrations of flammable gas, in relation to oxygen, at the catalyst surface may result in the combustion reaction being completely prevented, so that the explosimeter gives reading of close to zero for such a high concentration. High concentrations of flammable gas and/or low concentrations of oxygen give misleading, ambiguous readings and may also damage the catalyst in that a sooty layer is formed. Therefore, never use the explosimeter at concentrations of flammable gas higher than 100% LEL, and never at lower oxygen concentrations than approximately 10% O2 by volume
9.4.12 Riken, portable combustible detector, model GP-204 9.4.12.1 General description
The model GP-204 hand held portable gas detector is a compact battery operated portable instrument used for taking an air sample and indicating the presence and concentration of combustible gas. Samples of the air under test are drawn by means of a rubber aspirator bulb and analysed for combustible gas content on a heated platinum filament in a Wheatstone bridge measuring circuit. A built-in meter indicates combustible gas content in units of explosibility. Power for operation of the instrument is provided by built-in dry cells. A probe and extension hose permit sampling from remote locations and the instrument fits in a compact leather case with an over the shoulder-carrying strap. The model GP-204 is suitable and recommended for testing tanks, manholes, vessels other spaces to determine presence or absence of combustible gas in pressure cylinders, pipe lines and other closed systems. It is a valuable aid to safety of operations whenever combustible gases or vapours are handled.
”D” is exposed to the gas. ”C” is isolated from the gas. The resistance ”D” increases during catalytic combustion. Samples of air , which may contain flammable gases or vapours, are sucked through the instrument by means of a suction bellow. The content of flammable Gases effects a heated platinum filament (D = detecting element) which forms part of a Wheatstone bridge measuring circuit as shown in the circuit diagram on the right hand side. Besides the measuring filament “D”, this circuit includes a compensating filament “C” and two fixed resistance’s “R1 & R2”.
The flammable gases or vapours in the air are oxidised and burn at the surface of the measuring filament “D”, and the evolution of heat causes a change in the resistance of the platinum wire which gives rise to an imbalance in the Wheatstone bridge. This corresponds to the content of flammable gases in the sample.
9.4.12.2 Operation In a gas hazardous area the instrument should always be in the carrying case and strapped to this. Before taking the instrument to the hazardous area, check the battery voltage. To check the voltages, put the switch in “VOLT ADJ:” position. Meter should rise to the “check” position near top of the scale. Lift and turn VOLT ADJ. Control clockwise to determine maximum voltage setting. If the needle cannot be set beyond the VOLT ADJ mark, batteries need recharging or replacing for full capacity. Do not attempt to use instrument at all if reading cannot be set up to the mark or beyond the mark. Do not replace batteries in a hazardous area; bring the instrument to a safe area before changing taking place. If the voltage is satisfactory, continue with the next steps of preliminary adjustment, as follows: 1. Confirm operating of pilot light/meter illuminating lamp. 2. With sample inlet in fresh air, squeeze bulb several times to flush out any remaining gas from the instrument. 3. Check zero setting by turning the switch in “ON” position. Meter should read close to zero. If not, lift and turn the “ZERO” knob to bring the reading exactly to “0”. 4. Couple the sampling hose to the instrument’s inlet pipe, which is located on the left-hand end, and also connect the probe to the end of the hose. 5. Admit a sample of some combustible gas to the end of probe and confirm that the meter rises upscale. Instrument is adjusted and ready to use. Now it may be turned off and carried to the job area. To run a gas test, proceed as follow: 1. Turn the instrument to VOLT ADJ. position, adjust voltage if necessary 2. Turn the instrument to ON position, zero adjust if necessary. 3. Hold probe within space to be tested. Squeeze bulb several times (4 times for each metre of sampling hose being used) while watching the meter and observe maximum reading. 4. After completion of test, remove probe from test space. Flush the instrument with fresh air and turn it off. The sampling hose being used for this instrument should not be used for sampling with other instruments. Make it a rule that a specific measuring instrument has its own sampling hose.
9.4.12.3 Interpretation Meter readings are taken on a scale graduated 0 – 100% LEL. The abbreviation LEL stands for Lower Explosive Limit and represents the lowest concentration which can be ignited by a source of ignition, hence the lowest concentration which can produce an explosion. This quantity is also spoken as the LFL – Lower Flammable Limit.
The mode GP – 204 is calibrated before shipment to read directly in percent of LEL of iso-butane in air, based on the known LEL for iso-butane of 1,8% by volume. This 1,8% by volume will produce a reading of 100% LEL and lower concentrations will be read in proportion. Other combustible gases will read approximately correctly in terms of explosibility, but for the maximum accuracy a calibration curve for various gases has to be used. This curve is delivered together with the instrument. This curve is drawn in terms of percent LEL for both co-ordinates. See the table below.
9.4.12.4 Maintenance Calibration and adjustment - In addition to the normal operating controls found on the top of the panel, the following auxiliary controls are available. Calibration potentiometer - This adjustment is used to set meter reading to the desired level, while sampling a known concentration of combustible gas. In the GP204 the top plate must be removed by taking out the screws in each corner. The calibration potentiometer is a slotted-shaft control located above right upper corner of meter. Turn clockwise to increase meter reading. Element replacement - The element assembly, consisting of an active filament and a
similar but enclosed reference filament, should be replaced if zero cannot be set within range of “ZERO ADJ.”, or if reading cannot be set high enough on a calibration gas, using calibration potentiometer. 1. Loosen the two panel hold-down screws, remove and invert top panel. 2. With switch off, loosen (do not remove) the three screws holding the terminals for red, black and white wires. Pull wires from terminal. 3. Remove the two Phillips head screws holding cross-shaped element retainer in place. Pull out both filaments and replace with new ones in same position. 4. Check that gaskets are in place on element before installation. Be sure that the active (black wire) filament is in the cavity with the flame arrestor. Install wires on terminals as before. 5. Turn instrument on and adjust zero. 6. If a calibration gas is available reset span.
9.4.12.5 Batteries
The model GP-204 is furnished with two standard size “D” dry cells. These dry cells (UM-1/1,5 size D/R 20 Maxell 100) will give 3 hours (maximum) of operating time. When meter cannot be set as high as the “Check” line with switch in “VOLT ADJ:” position and “VOLT ADJ.” knob all the way clockwise, batteries require replacement or recharging. To replace batteries, remove instrument from hazardous area. Take the instrument out of the leather case, and loosen the coin slotted captive screw found in centre of bottom plate. Remove bottom plate, exposing batteries in their spring contact holders. Pull old batteries out and install new ones in the same position. Observing polarity as marked on holder.
9.4.12.6 Sample system Hose The hose used is Teflon lined synthetic rubber jacketed and immune to absorption or attack by any combustible vapours or solvents. Keep hose clean and are sure that couplings make airtight contact. Check occasionally by holding finger over hose inlet. Bulb should remain flattened after squeezing if there is no leak. Extension hoses in various lengths are available. Flame arrestor The active filament is installed within a sintered bronze porous metal cup, which acts as a flame arrestor to retain explosions that may occur when sampling explosive gas/air mixtures. The flame arrestor may be removed by taking out the four screws that hold the plate in which the elements are installed. If flame arrestor is dusty, wet, oily or corroded, it must be cleaned or replaced. Preferred cleaning method is by washing in detergent solution, rinsing from the inside out, and drying thoroughly in air. Before re-installing flame arrestor in instrument, be sure that the reaction chamber cavity and incoming lines are clean and dry. Meter Lamp The meter lamp is on whenever the instrument is on, and provides illumination to permit reading meter in dark places. If lamp fails, replace it as follows:
1. Remove four screws holding top plate to the top panel. Take off top plate exposing lamp. Loosen set of screws, which lock lamp wires to terminal and pull the lamp out. Install new lamp in the same position.
9.4.12.7 Precautions and notes on operation
1. Heated samples - When sampling spaces such as hot tanks that are warmer than the instrument remember that condensation can occur as the sample passes through the cool sample line. Water vapour condensed in this way can block the flow system and corrode the flame arrestor. A water trap can be used to control this, and is available as an accessory. If heated hydrocarbon vapours of the heavier hydrocarbons (flash point 90oF or above) are present, they may also condense in the sample line and fail to reach the filament. Thus an erroneous low reading may be obtained. 2. Element poisoning - Certain substances have the property of desensitising the catalytic surface of the platinum filament. These substances are termed “Catalyst Poison” and can result in reduced sensitivity or in failure to give a reading on samples containing combustible gas. The most commonly encountered catalyst poisons are the silicone vapours, and samples containing such vapours even in small proportions should be avoided. Occasional calibration checks on known gas samples are necessary, especially if the possibility exists of exposure to silicones. A calibration check on a known iso-butane gas is the most dependable as an indication of normal sensitivity. A convenient calibration accessory is available and described under “Accessories”. 3. Rich mixture - When high concentrations of gas are sampled, especially those above the LEL, considerable heat is liberated at the filament. This heat may cause damage to the filament or tend to shorten its life, so sustained testing of samples beyond the meter range should be avoided. When sampling rich mixtures, the following instrument action may be expected. · Mixture up to 100% LEL reading on scale. · Mixtures between LEL and Upper Explosive Limit (UEL) readings at top of meter. · Mixtures above UEL – When a sample is introduced, the meter is sent to the top of scale, then comes back down on scale or below, depending upon concentration. Very rich mixtures will give a zero or negative reading. The alarm circuit thus insures that a very rich sample will not be overlooked, as it could otherwise be with a simple indicating instrument. 4. Oxygen deficient mixtures - Samples, which do not have the normal proportion of oxygen, may tend to read low if there is not enough oxygen to react with all combustible gas present in the sample. As a general rule, samples containing 10% oxygen or more have enough oxygen to give a full reading on any combustible gas sample up to the LEL. 5. Oxygen enriched mixtures - Samples having more than the normal proportion of oxygen will give a normal reading. However, they should be avoided because the flame arrestor used is not dense enough to arrest flames from combustible gas in oxygen, which can be much more intense than those in air can. Do not attempt to
use the model GP-204 on samples of combustible gas in oxygen. 6. Accessories - Additional lengths of extension hoses may be used for sampling from deep tanks and spaces. The polyurethane hoses are satisfactory for most samples including natural gas, hydrogen, and gasoline vapours. Where there is danger of water being drawn into the instrument, a water trap should be used. This glassbodied trap, with sintered metal filter, couples to the indicator inlet and will collect water that is drawn into or condensed in sample hose. Inspect trap periodically while in use, and empty or clean bowl and filter whenever visible water or dust accumulate. Regular sample hoses connect to inlet of trap when it is installed on the instrument.
9.4.13 Servomex, oxygen analyser, type OA 262 WARNING! To ensure safe operation in hazardous applications, the analyser must be used to comply with the conditions of certification, relevant standards and codes of practice. Failure to do so may invalidate the certification. Any modification to the standard analyser, or repairs or servicing using parts that are not specified or approved by Servomex, will invalidate certification. In case of doubt contact Servomex or their agents.
9.4.13.1 General description
The Servomex portable oxygen analyser type 262A is a robust lightweight instrument built for industrial, marine and laboratory applications. The oxygen content of the gas is indicated directly on a 70mm scale taut band meter after suitable zero and span adjustments. The ranges, 0-100, 0-25, and 0-10% are selected by a rotary switch on the front panel. Battery checks are also selected with this switch. This analyser is used on marine applications throughout the world. The front panel controls are symbolic, such that engineers from many different nations can understand them. All analysers are supplied with a hand aspirator and silica gel dryer. Batteries are not supplied with the analyser. The 262A is powered by dry cells batteries which are housed in a waterproof compartment at the rear of the analyser. The analyser is supplied with a filter, elements of which are and simply replaced from the front of the instrument.
9.4.13.2 Hazardous area and shipboard use Hazardous area - For hazardous areas the 262A is certified by BASEEFA as intrinsically safe code Ex ia s IIC T4 to SFA 3012, SFA 3009. Certificate BAS No. 74149. Instruments up to serial no. 2983 are approved by “Factory Mutual” for use in class 1, division 1, groups B, C and D hazardous locations. Report 25243 dated August 30th, 1975 applies. Seaworthiness - Lloyds has approved the analyser as suitable for shipboard use.
Certificate Lon. 409515.574 applies. The Norwegian Maritime Directorate (Sjøfartsdirektoratet) has also approved the analyser for use on board ship. (Reference letter A-44140/75.AGR/MI dated 24.10.75 applies). Specification
Specification
Oxygen ranges
0-10%, 0-25%, 0100% O2. Selected by front panel switch. Indication on front panel meter. Range:
Accuracy
0-100% O2. +/- 3% F.S.D.0-25% O2. +/- 3% F.S.D.010% O2. +/- 3% F.S.D.
Effect of ambient temperature .
The analyser will operate between the temperature of –10oC to 50oC (14 to 122oF). The accuracy will be maintained for a temperature change of +/- 10oC (18oF) of the calibration temperature
Effect of tilt
0,01% oxygen per degree.
Weight (net)
3kg. (6,5Ib).
Sample pressure
Maximum inlet pressure, 2 psi. (14kPa).
Flow rate pressure
0 to 3 I/min, depending on sample.
Materials contact with sample gas
Acetal copolymer, Glass micro fibre, Nickel, Platinum, Polypropylene, Pyrex glass, Quartz
glass, Stainless steel 316, Synthetic rubber, Viton.
Calibration gases
Zero on O2 free nitrogen (N2). Span on clean dry air or high purity O2 if desired.
Accessories
Waterproof case with shoulder strap. Drying tube. Two hexagon wrenches (2,5 and 3mm).
Case material
Polypropylene. The case is splash proof and sealed against ingress of water, provided the sealing gaskets around the front panel and battery compartment are in good condition.
9.4.13.3 How the Servomex oxygen analyser works The physical property, which distinguishes oxygen from most other gases, is its paramagnetism. Faraday discovered this in 1851, who demonstrated that a magnet attracted a hollow glass sphere at the end of a horizontal rod supported by silk fibres when filled with oxygen.
In portable oxygen analysers, the convenience and sensitivity of Faraday’s arrangement are increased by having a sphere at both ends of the bar, forming a
“dumb-bell”, which seals the gas surrounding it. The dumb-bell is suspended in a symmetrical non-uniform magnetic field, and being slightly diamagnetic, it takes up a position away from the most intense part of the field. When the surrounding gas contains oxygen, the dumb-bell spheres are pushed further out of the field by the relatively strongly paramagnetic oxygen. The strength of the torque acting on the dumb-bell will be proportional to the paramagnetism of the surrounding gas: it can therefore be used as a measure of the oxygen concentration.
The only common gases having comparable paramagnetic susceptibility are NO, NO2 and CO2. A magnetic oxygen analyser cannot therefore be used where these gases occur in the mixture other than in trace amounts. It is important to note, however, that in the direct method of measuring susceptibility no other physical property of the gases has any significant effect.
The heart of the Servomex analyser is a measuring cell using these principles, but having a rare metal suspension in place of the delicate materials used in earlier designs. The “zero” position of the dumb-bell is sensed by a split photocell receiving light reflected from a mirror on the suspension. The output from the photocell is amplified and fed back to a coil wound on the dumb-bell, so that the torque, due to the oxygen in the sample, is balanced by a restoring torque, due to the feedback current. The measuring system is thus “null-balanced”, and has all the inherent advantages of this type of system.
Because of the extremely linear relationship between the feedback current and the susceptibility of the sample, a proportional output voltage can be developed, and various ranges can be obtained by means of a switched attenuator. Linearity of scale also makes it possible to calibrate the instrument for all ranges by checking at two points only. For example, accurate calibration is obtained by using nitrogen for zero and air for setting the span at 21%
9.4.13.4 Operating procedures Installation and changing of the batteries. The following batteries are required: 3 of 1,5V Type IEC LR6 (HP7) 1 of 9V Type IEC 6F22 (PP3). It is recommended that alkaline batteries be used.
The batteries are housed in a waterproof compartment at the bottom of the analyser. This compartment is opened using the 3mm-hexagon wrench supplied with the analyser. A battery strap is provided for easy removal of old batteries. The batteries must be installed with the correct polarity, as indicated by + and - signs moulded into the plastic holder. Various resistors are potted into a recess in the battery compartment. Under no circumstances should these components be removed or tampered with. The stud of a 1,5V battery is “+” and the base “-”. These batteries will not make contact if fitted the wrong way round. The 9V battery has a terminal clip that can only mate when the battery is correctly positioned. Care must be taken, when fitting new batteries, not to damage the gasket sealing the edge of the battery compartment. If the analyser is to be stored for a longer period of time, remove the batteries. Do not replace batteries in a hazardous area
9.4.13.5 Battery checks
Check that the batteries are fully operational: Select switch position “B1”. The reading should be greater than 60 on the 0-100 scale. Change the 9V battery if the reading is low. Select switch position “B2”. The reading should be greater than 60 on the 0-100 scale. Change the 1,5V batteries if the reading is low.
9.4.13.6 Calibration Frequency of calibration - Check the zero adjustment weekly. If there is a large difference in ambient temperature between the point of measurement and the last calibration, it is advised that calibration should be rechecked. The span adjustment should be checked daily when in use, due to variance in atmospheric pressure. Set Zero - Switch the control to 10% range. Introduce oxygen free nitrogen into the instrument at a pressure between 1 to 2 psig. (7 to 14 kPa). Stop the gas flow. Adjust the screw for zero adjustment so that the meter reads 0% oxygen.
9.4.13.7 Span Switch the control to the 25% range. Introduce dry air into the instrument at a pressure between 1 and 2 psig (7 to 14 kPa). The hand aspirator and a drying tube are convenient for this. Stop the gas flow. Adjust the screw for the span adjustment so that the meter reads 21% oxygen on the 0-25% scale. When changing from air or oxygen to nitrogen or vice versa, ensure that the filter, cell and sample lines have been purged thoroughly. One minute with the standard hand aspirator should be enough. With long sample lines a pump is recommended. When using the instrument for higher concentrations of oxygen it is recommended that pure oxygen is used on the 0-100% range for optimum accuracy.
To prevent possible damage, it is not recommended that air or pure oxygen be put into the analyser when it is switched to the 0-10% range.
9.4.13.8 Measuring sample gas
Connect the hand aspirator to the sample inlet by means of the drying tube. Connect sample tube to the aspirator and place in space to be checked. Check the battery voltage. Set switch to range required. Pump the hand aspirator until the reading is steady. Ensure that sufficient sample gas has been taken to flush out the sample lines. CAUTION. The drying tube must always be used, unless the sample is known to be dry. The analyser will be damaged if water or liquids are allowed to get into the instrument. However, the crystals can be regenerated by removing from the drying tube and drying in an oven at about 110-1200C.
9.4.13.9 Maintenance WARNINGS Only qualified personnel who are familiar with good workshop practice should do maintenance of the analyser. Replacement parts should be to the quality specified by Servomex in the part lists. The use of inferior replacement components may degrade the performance of the analyser and invalidate any certificates, which may apply.
9.4.13.10 Replacement of measuring cell 1. Remove the six hexagon socket stainless steel screws holding the front panel into the case and keep them in a secure place. 2. Remove the chassis by placing one hand over the front, and turn the analyser upside down. This will prevent the chassis falling out accidentally. Should the chassis not come out very readily, bring the analyser sharply down on the flat of the hand, which is guarding the front. Never substitute a hard surface for a hand. 3. Unscrew the nuts on the cell supporting the gas connections (use non-magnetic spanners). 4. Unsolder the electrical leads. Apply minimal heat to the pins on the cell. 5. Remove the two hexagon socket screws, which retain the cell and slacken the third retaining screw, which is situated between the inside of the lower magnet space and the chassis wall.
6. Withdraw the measuring cell and replace it with a new cell type 286. When fitting a new cell, ensure that the ball of the dumbbell, which is nearest to the cell window, is nearest the front panel. 7. Tighten the remaining screws in the reverse order described for the removal of the cell. 8. Solder the electrical connections to the solder pins on the cell. Black to the pin with a black spot near it and yellow to the pin with a yellow spot. 9. Reconnect the cell gas connections. 10. Adjust the zero and span of the analyser. Should the analyser not zero or the adjustment is at one end of its travel, readjust the photocells. It may not be possible to span the analyser, in this case change R23 on the printed circuit board 00262905, to a value, which gives a reading with air between 20 and 22 % oxygen. For circuit diagram, see the instruction manual.
9.4.13.11 Replacement of photocells
1. The photocells are located to the side of the magnet assembly, just in front of and above the measuring cell. 2. Release the two screws, which fix the retaining plate to the photocell mount. 3. Remove the screws and plate and manoeuvre the photocell mount through the springs of the support. 4. Unsolder the leads. 5. Replace the new photocells on their mount in reverse order. 6. Leave the two retaining screws slack and pass nitrogen into the analyser. 7. Ensure the zero adjustment is at the centre of its travel and move the photocells until the analyser reads as near to zero as possible. 8. Tighten the screws and make a final zero adjustment. 9. Adjust the span. 10. Confirm with the analyser’s instruction book. 9.4.13.12 Replacement of LED 1. Remove the two screws, which hold the photocell mount to the control magnet assembly. 2. Allow the photocell assembly and mount to lay away from the magnet. 3. Remove the two screws holding the LED mount. Withdraw the LED and mount and unsolder the leads to the LED. 4. Remove sleeving from old LED and discard lamp. Replace with new LED and sleeve and solder the leads. 5. Replace the LED and secure the retaining strip. 6. Replace the photocell assembly and mount. 7. Replace the cell. 8. Adjust the zero and span. 9. Confirm with the analyser’s instruction manual. For replacing the amplifier board, meter, filter block and circuit description do confirm with the instruction manual. Any doubts about the analyser or its equipment, contact the manufacturer or any of the manufacturer's agents.
9.4.14 Riken portable oxygen indicator, Model Ox - 226
1. Summary Riken portable oxygen indicator, Model OX-226 and OX-227 provide a quick, convenient method for determination of oxygen content of any atmosphere. It is intended primarily as an indicator of oxygen deficiency, with good readability from 0 – 25%. The instrument is routinely calibrated on normal atmospheric oxygen concentration (21%). These models are most suitable and recommended for testing tanks, manholes, vessels and other spaces to determine safety from the standpoints of oxygen deficiency before entering and while work is in progress. 2. Principle The oxygen cell operates by an electro-chemical process in which a voltage is set up between two electrodes. Under a test where one electrode is exposed to the atmosphere, a change in oxygen concentration on this electrode produces a proportional change in the cell’s output voltage. Therefore, an increase in oxygen concentration will “speed up” the electro-chemical process, producing a higher output voltage, and a decrease in oxygen concentration will “slow down” the process, lowering the output voltage. The centre electrode is exposed to the atmosphere by means of a Teflon membrane placed directly in contact with the polished top surface. This Teflon membrane serves two functions simultaneously. First, it has the ability to pass oxygen molecules freely, thus placing the electrode in direct contact with the atmosphere and secondly, it keeps the electrolyte contained in the cavity between the two electrodes. 3. Measurement procedure a). Preparation - Connect the sampling hose (6) to the gas sampling probe (7) and then connect it to the gas inlet of the instrument. b). Voltage checks of battery - Turn the control switch (1) to “Batt” zone and check the meter needle marks inside of “Batt” zone. If the case of model OX-226, the
battery drop can be heard as a buzzer sound. c). Span adjustment - Turn the control switch (1) to “25” and make span adjustment by spanning adjusting knob so as to bring the meter needle to 21%. When making span adjustment of Model OX-227, try it with 0-25% range.
4. Measurement After finishing the above procedure items 1, 2 and 3, the instrument is ready to run. Introduce the sampling probe to the source and start measurement. In the case of Model OX-226, when the oxygen concentration is less than 18% by volume, alarm light (4) illuminates and it gives us the warning of oxygen deficiency by buzzer sound. Caution 1. Check the flow pump by the flow monitor during operation. 2. Operate the instrument in leather case when in use. 3. The replacement of batteries and recharging procedure must be done in nonhazardous areas. 5.Maintenance procedure The replacement of batteries and recharging procedure. a). Take off the leather case from the instrument and turn the battery box knob (11) to “open” position. b). Pull out the whole battery box and replace the batteries with new ones. c). When the replacement of batteries is finished, put back the battery box in correct position and turn the battery box knobs (11) to “Lock” position with finger press. Replacement procedure (Ni-Cd battery). When Ni-Cd batteries are used for the instrument, detach the label (12) of charging inlet and insert the exclusive charger to the charging jack, and plug the charger into AC 100V. The recharging takes 15 hours. Replacement of sensor. When the meter needle can not be adjusted to 21% by turning the span adjusting knob and the indication of meter needle gets unstable, this is the sign to replace the sensor. In this case, take off the bottom screws of the instrument and remove the cover. The cover comes off by sliding it sidewise. Turn the sensor to left and adjust
the mark to “open”. Now the sensor can be removed. Insert the new sensor and turn it in clockwise direction to the mark “lock”. Place the cover back.
Replacement of filter The filters are filled in the gas-sampling probe and in instrument. When they appear dirty, replace them with new ones.
Take off the tip of the sampling probe by turning the metal part of roulette and replace the cotton filter with a new one.
Pull out the filter holder (10) of the instrument’s flank and take out the filter. Replace it with a new one. Zero adjustment As the zero adjustment is factory set, there is no need of zero adjustment procedure in normal operation. But, when it is high sensitive type instrument such as Model OX227A with 0-5 and 0-25% etc., make zero adjustment. Induct 100% clean nitrogen and turn the adjusting screw to bring the needle to zero.
9.4.15 Detector tubes for health hazardous gases
Health hazardous gases may be detected through chemical colour reactions, and several manufacturers make metering pumps and accompanying detector tubes for a great number of various gases. Probably the most convenient and suitable equipment to use for measuring very low concentrations of toxic gases on board tankers are chemical indicator tubes. These tubes consist of a sealed glass tube containing a proprietary filling which is designed to react with a specific gas and to give a visible indication of the concentration of that gas. To use the device, the seals at each end of the glass tube are broken, the tube is inserted in a bellows-type fixed volume displacement handpump, and a prescribed volume of gas mixture is drawn through the tube at a rate fixed by the bellow’s expansion rate. A colour change occurs along the tube and the length of the discoloration, which is a measure of the gas concentration, is read off a scale integrated with the tube. In some versions of these instruments, a hand operated injection syringe is used instead of a bellow pump. It is important that all the components used for any measurement should be from the same manufacturer. It is not permissible to use a tube from one manufacturer with a hand pump from another manufacturer. It is also important that the manufacturers’ operating instructions are carefully observed. Since the measurement depends on passing a fixed volume of gas through the glass tube, if an extension hose is used it should be placed between the glass tube and the hand pump. The tubes are designed and intended to measure concentrations of gas in the air. Thus measurements made in a ventilated tank, in preparation for tank entry, should be reliable. Under some circumstances errors can occur if several gases are present at the same time, as one gas can interference with the measurement of another. The manufacturer should be consulted for guidance. For each type of tube the manufacturer must guarantee the standards of accuracy laid down by national standards. Tanker operators should consult the regulatory authority appropriate for the ship’s flag.
9.4.16 Dräger Multi Gas Detector In our experience, detector tubes and metering pumps made by “Dräger” are the most frequently used. A more detailed description is given in the instruction book for “Dräger Multi Gas Detector”.
Various chemical substances are used for tube fillings, depending on the gas to be analysed. For some gases there are several types of tubes, so that there are tubes for measuring very low concentrations and for measuring larger concentration ranges. In some cases two scales will be marked on the tube, corresponding to different numbers of pump strokes. It is important that the pump is checked to see if it is tight before it is being used, sealing the opening with an unused detector tube does this. The bellows should then use more than 10 minutes to expand for the pump to be satisfactory. Cleaning the valves, according to the instructions accompanying the instrument may usually eliminate any leakage that has arisen. To avoid corrosion, the pump must be purged with air by performing a number of pumping strokes each time after use. To perform measurements with difficult accessibility, an extension hose may be used. The detector tube is placed in the suction of the hose.
9.4.16.1 Opening of the tubes
Both ends of the tube are opened in the hole, which is provided for that purpose in the pump. A breaking socket accompanying the apparatus can also be used for this. This prevents glass fragments from falling down.
9.4.16.2 Installation of the tube in the pump
The opened sampling tube is inserted into the pump head so that the arrow on the tube points toward the pump. The tube must be attached firmly and tightly in the pump head so that false air is not sucked in.
9.4.16.3 Suction of a gas sample The bellow is pressed together completely and is then released. During the compression the air is squeezed out of the bellow through an exhaust valve. The suction action of the pump takes place when the compression springs inside the bellow expand after the compression. The air (to be measured) flows through the sampling tube and into the bellow while this again expands to its original volume. The suction movement comes to an end when the distance chain is tight once again, and at this stage 100cm3 has been sucked through the tube. The operating instructions, which accompany each packet of tubes, give i.e. the approximate time for each pump stroke, for example 15 - 25 seconds. The time will depend on how tightly the powder is packed in the tube. The specified number of pump strokes, indicated in the operating instructions, should be used for each sampling tube.
9.4.17 MSA – Detector Tubes and Kwik-draw Pump Features 1. Quick and inexpensive to use. 2. A reliable method of testing more than 120 hazardous gases and vapours. 3. Kwik-draw pumps offer accurate one-handed automatic stroke counter and unique end of stroke indicator on deluxe version. 4. Tubes are printed with easy-to-read scales. 5. Specialised kits are available for use in HAZMAT work and underground storage tank applications. Description SA’s Kwik-Draw and Kwik-Draw Deluxe Pumps can be used with an assortment of MSA detector tubes to spot-test the atmosphere for a wide variety of toxic substances. Kwik-Draw Pumps are designed for one-hand operation and consistent delivery of a sample draw volume of 100 millilitres (ml). The pumps are constructed with a shaft-guided compression system for a more consistent and replicable flow rate and volume per stroke than may be available with hand-guided pumps. MSA offers detector tubes for measuring more than 150 gases and vapours.
Kwik-Draw Detector Tube Pumps Kwik-Draw Pumps allow detection of gases and vapours with the squeeze of a
handle. To obtain a precise (100ml) sample volume, the user simply grasps the handgrip and pushes the knob. The pump’s compression system provides the guiding action to drive a spring-loaded bellow pump. An internal easy-to-read stroke counter shows the exact number of strokes performed and provides a positive stop when the stroke is fully compressed. A second model, the Kwik-Draw Deluxe Pump has a unique end-of-stroke indicator that “winks” after the precise volume of air is drawn, confirming that enough air has been sampled for a successful reading. Detector tubes. MSA/Auer detectors are made of glass, have break-off tips and are filled with treated chemical granules for sampling a variety of substances. Most MSA/Auer detector tubes are packaged 10 in a box. For ordering information, see the Detector Tube Summary Chart which follows the Detector. After selecting the appropriate tube, the user would break off the tubes’ end tips and attach the tube to the sampling pump. After air is drawn through the tube by the pump, the chemical layer in the tube changes colour if the test gas or vapour is present in the air. The length or shade of the colour-change, indicates the concentration of the gas or vapour in the air. A scale is printed on each tube for interpretation of data. Controlled Interchange ability of MSA/AUER Detector Tubes and Pumps with Other Manufacturers’ Tubes and Pumps. As long as a pump meets the following criteria, it may be used with any detector tube designed for use with that kind of pump. Pumps meeting these criteria are interchangeable. 1. The characteristics of the pump- volume per stroke, sampling time and flow – must be within the same accuracy range. 2. The detector tubes must have an outer diameter of 7 mm and be factorycalibrated with a pump that meets the criteria of (1) above. 3. The manufacturer of tubes and pumps must operate under a certified quality assurance program. Based on these criteria, the following pumps are interchangeable: · MSA's Kwik-Draw Pumps. · AUER's Gas Tester II H Pump. · Dräger's Model 31 Bellow Pump. · Dräger's Accuro Pump. Sampling Pump Operation and Maintenance. The Kwik-Draw Pump is designed to measure concentrations of gases and vapours when used with AUER/MSA Detector Tubes. Description - The Kwik-Draw Pump is a one-handed, manually operated bellow pump of 100cc capacity. Tube Holder - This rubber part permits mounting of detector tubes, remote sampling
lines or other detectors. Filter Disc - This porous plastic disc mounted in the rubber tube holder protects the pump from dirt and dust particles, which may alter the flow or damage the pump. Exhaust Valve - Located under the valve cover, this valve closes as the bellow reinflates, and readily opens on the exhaust stroke so that blow-back through the tube holder is negligible. Stroke counter - For convenience, a stroke counter is incorporated into the pump handle. End-of-stroke indicator - As the bellow begins to re-inflate, and after the knob is released, the indicator eyeball turns high visibility green. As the vacuum decreases, the eye begins to roll back to black. The stroke is over when the eye is all black. Note! - Kwik-Draw Pump (part no. 488543) does not have an end-of-stroke indicator. Operation · Using the breaker on the pump, break off both tips of the detector tube. · Using a twisting motion, insert the tube into the rubber tube holder. The arrow on the tube should point toward the pump. · Re-zero stroke counter. · With all four fingers on the handle, depress the knob with your palm. Note! Watch the stroke counter to ensure proper sample volume, the counter will only advance if a full pump stroke is taken. · Release the knob. · As the pump re-inflates, the end-of-stroke indicator turns to high-visibility green. The stroke is over when the eye returns to the all black state. Note! If your pump does not have the end-of-stroke indicator, wait 30 seconds after full bellow inflation to ensure that all 100cc of the sample has been drawn through the tube. The detector tube must be held in the sampling area during this period. · To evaluate the stain, follow the instructions provided with the detector tubes. Remote sampling Remote sampling is accomplished by putting the pump, connecting tube, remote sampling line and detector tube together, in this order. Maintenance Under conditions of normal use, this pump should require little maintenance. Depending on the frequency of use, periodic cleaning and checks for correct
performance as recommended. Tube holder - Replace tube holder when it shows signs of wear or loss of elasticity. If filter is not clogged or cracked, save the filter discs for re-use in new tube holder. Filter disc - Periodically remove the filter disc for cleaning or replacement. Remove filter disc from tube holder by rolling flange part of tube holder down and away from the disc. Gently tap or blow on the surface to remove any foreign matter. Replace disc so previously exposed surface is once again facing away from pump. Shaft If shaft becomes dirty or if bellow inflation is jerky, remove shaft by unscrewing, then clean with auto wax. Valves 1. With the valve cover removed, check the valves for dirt or debris. 2. Remove dirt with a gentle puff of air or by using a soft brush. 3. Replace valve(s) if necessary. Pump performance test After extended idleness and periodically during use, check the pump for proper performance with the following test: 1. Plug pump inlet by inserting an unbroken detector tube into tube holder. 2. Deflate pump fully, release, and wait 10 minutes. The pump is leak-free if the distance from the bellow to the frame is ½ inch or greater after 10 minutes. If the pump leaks check the tube holder and, if necessary, the valves (see Maintenance). After repair, re-test for leakage. Warning! Use of a pump that leaks may result in the under-estimation of a hazard and could result in property damage, injury or death. Read the instruction book following the Detector!
9.5 FLAMMABILITY COMPOSITION DIAGRAM In 9.4, we saw that when measuring oxygen content we use the instrument “Servomex OA-262”. For measuring hydrocarbon gases in percent of volume the instrument, we use the “Riken 17HC”, and an explosimeter, “Riken GP-204” is used for measuring combustible gas below LEL in air. Finally, the “Drager Multi Gas Detector” is used for measuring low concentrations of toxic gas. When an inert gas, typically flue gas, is added to a hydrocarbon gas/air mixture, the result increases the lower flammability limit hydrocarbon concentration in order to decrease the upper flammability limit concentration. The effects illustrated on the following two diagrams, one for crude oil and one for propane gas, should only be regarded as a guide to the principles involved, and
should not be used for deciding acceptable gas compositions. Every point on the diagram represents a hydrocarbon gas/air/inert gas mixture, specified in terms of its hydrocarbon and oxygen contents. The first flammability diagram is for hydrocarbon gas above crude oil where the UEL is 11% and the LEL is 1,5%. The left side of the diagram (vertical) gives the hydrocarbon gas value. The bottom line of the diagram (horizontal) represents the oxygen content from 0% to 21%. Note that 21% oxygen represents 100% clean air and, as mentioned before, 10,8% by volume of oxygen is the minimum oxygen content present in a mixture's ignition. Also when measuring 21% oxygen, the atmosphere contains 100% clean air, so the ratio will be close to one to five. For example when measuring the oxygen content at 20,5% volume, 0,5% volume oxygen is still missing in the atmosphere in order to call it clean air. When multiplying 0,5% with 5, the result will be 2,5%, and the amount of clean air in the atmosphere will be (100-2,5)= 97,5%. The missing 2,5% from the clean air contain unknown gas concentrations. So, proper tank venting is extremely important. The following example is based on a cargo of crude oil. During discharging the cargo tanks where refilled with inert gas. The quality of the supplied inert gas was in accordance to the regulations in force. After discharging is completed the tank atmosphere contains a mixture of inert gas and “cargo gas”. During the ballast voyage, the arrival ballast tanks are cleaned. Water washing takes place in an inerted atmosphere and before any tanks are vented with air, they have to be re-inerted, in order to avoid entering the flammable zone.
Follow example “1” on the this page: After water washing is completed the measurements in point “A” give the following values: HC=15%, O2=3%. These values are plotted on the diagram and give the point “A”. Point “B” represents 21% oxygen and 0% HC. From point “B” a line is drawn against the left side of the diagram by keeping sufficient clearance from the flammable zone. The inert gas that is supplied contains an O2 of 4,5%. Point “C” on the diagram is the point, which the re-inerting is heading toward. All measurements will follow a straight line from point “A” to point “C”. Where the line from point “A” crosses the line from point “B” the following measurements are found: HC= 2,6% and O2= 4,2%, point “D”. In point “D” we stop the inerting and start venting with air. All measurements taken from now on will follow a straight line from point “D” toward point “B”. If we had started to vent with air at point “A”, all the measurements taken would have followed a straight line from point “A” toward point “B”, through the flammable zone involving a great deal of danger. Avoid all contact with the flammable zone! At point “D” there it is still too early to use the explosimeter, because the content of hydrocarbon gas is above LEL. Continue to use the instrument, which measures hydrocarbon gas by volume until the HC-gas content is below 1,5% by volume. This is the LEL value in this example. Also, to repeat, the hydrocarbon concentration of
1,5% by volume corresponds to 100% LEL. In point “E”, the measurements are HC= 1% and O2= 15,3%. The hydrocarbon gas concentrations are now below LEL and the explosimeter can be used. It is also possible to calculate the explosimeter reading ahead of the measuring by using the formula where measured HC gas is multiplied by 100 and divided by LEL. In our example, the explosimeter in point “E” will show (1 x 100): 1,5 = 66,67% of LEL. After sufficient venting the measuring in point “B” will be HC= 0% and O2= 21%. The tank is ready to enter. After inspection/repairing, the tank(s) must be re-inerted before arrival at loading port in order to achieve the required tank atmosphere according to the regulations in force. Example “2” .
The above example is based on propane cargo. In the example, nitrogen and air are used for tank purging and air venting. Follow the diagram for propane. After some time purging with nitrogen, a measurement is taken at point “A”: HC= 12% and O2= 0%. Just like in example “1”, a line is drawn from point “B” toward point “C” on the left side of the diagram by keeping sufficient clearance from the flammable zone. Continue to purge with nitrogen until reaching point “C” where the measurements are HC= 3,75% and O2= 0%.
Stop purging with nitrogen at point “C”. Start dilution with air. All measurements will now follow the straight line toward point “B”. Take a measurement at point “D” which is HC= 2,4% and 02= 8%. When the LEL for propane is 2,1% by volume, it is too early to use the explosimeter. At point “E”, the measurements are HC= 1% and 02= 15,25%. The explosimeter will in point “E” show (1x100) : 2,1 = 48% of LEL. After sufficient dilution, point “B” end up with measurements of HC= 0% and 02= 21%. After inspection/repairing etc., the tank must be treated according to routines and regulations.
10-
Cargo Pumps
10 Cargo pumps 10.1 Classification and selection of pumps There are a number of different pump types. Each type has its own special quality and therefore certain advantages and disadvantages. The selection of pumps is determined by a thorough study of the capacity needs and under which operational conditions the pump will operate. The following factors are important when you evaluate these conditions:
· Estimated back pressure · Capacity requirement · Capacity range · Requirement for installation and arrangement · Expenses for purchase, installation and maintenance · Availability of parts and service · Suction terms · Characteristics for the liquid to be pumped
Selection of the right pump for a determined purpose qualifies a close co-operation between the customer and the producer of the pump. The customer has a special responsibility to clarify all conditions concerning the pump installation, so the producer can choose the best pump from his product range with the best match. When you choose a pump you must find out how much the pump needs to deliver under a specific condition. Definition of capacity range is important. Demand for capacity or capacity range and expected discharge pressure must be specified. The capacity requirement is determined by the intended use of the pump. The discharge pressure is determined by various conditions where the pump’s delivery pipeline design, the capacity of the pump and the liquid’s characteristics, is the essential. Alternative installation locations of the pump are limited due to special demands from Class and Shipping Authorities and also from lack of space. Purchase and installation cost is important. Future maintenance expenses, availability of parts and service now and over the next years, are also important and must be included in the evaluation of alternative pump supplies. The liquid’s properties and which other arrangements you have to consider, often
limits the options. Density, viscosity and boiling point are important properties to consider. The liquid temperature and corrosive properties are important factors when pump material is selected. The pump’s suction condition is determined from where the pump is located in relation to the liquid to be pumped. A given suction pipe creates a certain resistance that will have influence on the pump capacity. The main principle is to minimise resistance on the suction side by decreasing the suction pipe length, have the largest diameter possible and few as possible restrictions in form of bends, valves and so on. The different types of pumps are divided into two main groups, displacement and kinetic pumps. The displacement pumps displace the liquid by reducing the volume inside the pump. An example is a piston pump where the piston is moving up and down inside a cylinder or when the screws revolve inside a screw pump. Kinetic pumps (kinetic energy is equal to “movement” energy) increase the liquid’s velocity through the pump. The diagram below gives a brief view of the different available groups and types of pumps. The diagram would be more comprehensive if the pumps were divided in all details according to number of rotors, design of pump inlet/outlet and flow directions.
A kinetic pump like the centrifugal pump increases the liquid’s velocity in the pump by means of a rotating impeller. A displacement pump, like the piston pump, mechanically displaces the liquid in the pump, either by help of a piston or screws. Resistance on delivery side gives a liquid pressure rise (pump delivery pressure). One should be aware of this difference for these two pump types. The pressure rise on a kinetic pump is restricted by the increase in velocity over the pump, which is controlled by the pump design. All kinetic pumps therefor have a designed or built-in limitation for maximum discharge pressure. The displacement pumps limitation depends only on available power and the constructional strength. In
contrast to a kinetic pump, such a pump will operate against resistance with all its available power. A closed-delivery valve on a displacement pump is damaging. The same closed delivery valve for a kinetic pump will not bring any immediate danger.
Piston pumps and screw pumps have good suction capacity and are used where these characteristics are required. The weakness of these pumps is the complex construction and the relatively low capacity.
Centrifugal pumps are simply constructed with few parts and no valves. There are no immediate problems if the outlet of the pump is closed. These qualities result in relative low purchase and servicing costs. Operation at high speed makes the pump small in proportion compared to the capacity and flexibility in relation to the pump’s location.
The most negative side of using a centrifugal pump is the lack of self-priming capacity. This weakness is improved by constructional efforts and positioning, which consolidate the free flow of liquid. Location of a pump, for instance below the liquid level, can reduce the flow resistance. High viscosity liquids are therefore particularly difficult to pump due to this condition. A centrifugal pump’s efficiency is high only within a small range. This is the reason it is especially important to have a clear understanding of what capacity range the pump will operate under, in connection with the selection of a centrifugal pump. The differential pressure over each impeller is relatively low. Using so-called multistage pumps where several impellers are mounted in serial, increase the pump’s capacity to deliver against higher backpressure. A centrifugal pump will, without a non-return valve on delivery side, give complete back flow at the time the pump stops. For all operators of centrifugal pumps, this relationship is important to know
Examples of various pump types
Double-suction split-casing centrifugal pump
10.2 THE EJECTOR The ejector design is simple and is used for stripping and as bilge pumps in hold and interbarrier spaces. The ejector has no revolving or reciprocating parts and is thereby especially easy to maintain.
The propellant (driving water), is forced through a nozzle into a mixer tube. The velocity of the propellant will naturally increase as it passes through the nozzle. Due to the propellant’s velocity and direction, plus the friction force between the propellant and the liquid, the surrounding liquid will be sucked into the ejector’s mixer tube. The mixer tube is connected to an expanding tube, the diffusor. Here some of the kinetic energy supplied to the liquid in the mixer tube is transformed into potential energy. The capacity depends on the friction force between the two mediums, suction head, delivery head and the propellant’s velocity. The ejector has the advantage that it does not lose the suction capacity even if it sucks air or vapour. The ejector’s efficiency is between 30% and 40%. Even if the propellant’s efficiency is up to approximately 70%, the total efficiency for the whole ejector system is far less than compared to a pump system, such as a centrifugal pump. Another
drawback with ejectors is that the propellant is mixed with the pumping liquid. This implies that if the ejector is to be used in cargo transfer operation, the cargo itself must be used as propellant liquid.
The ejector is frequently used as a bilge pump in hold spaces. A common arrangement for a hold space is as follows: The ejector is usually submerged in a bilge sump and the propellant is normally supplied from a seawater pump. Onboard gas carriers where the hull is the secondary barrier, the ejector may also be used to pump cargo from hold space. In that case, the liquefied cargo itself must be used as a propellant. Tips · Be aware that the ejector has a limitation on the propellant’s pressure. Higher pressure than recommended by the supplier may result in reduced suction capacity. · Start the ejector by opening all valves on delivery side first, and then adjust the correct propellant pressure. The ejector’s suction valves should be opened last, which will prevent the propellant’s flow back into the tank that is to be stripped. · Stop the ejector by using the opposite procedure.
As the drawing shows the ejector is positioned 3 meters above the liquid level. The liquid level in the slop tank is 15 meters above the ejector and the propellant's pressure is 8 bars. The ejector’s capacity can be found by use of the performance curve for the specific ejector. In the performance curve the ejector capacity is set as a function of the propellant pressure. Observe that this curve has curves for different suction lifts. The different performance curves are marked with different suction lifts. The ejector’s suction lift in this example is 3 meters; this specific curve shall be used. You can find the capacity of the ejector by drawing a vertical line from 8 bars on the scale for a delivery head of 15 meters and up to the performance curve with a suction lift of 3 meters. From this point of intersection, draw a horizontal line to the left and over to the ejector’s capacity side. The found capacity in this case is 600 m3/h.
The ejector’s Performance Curves
10.3 The centrifugal pump 10.3.1 The theory of the centrifugal pump The sketch below indicates a radial section of a rotor-blade wheel for a centrifugal pump. For plainness, we observe the liquid as an amount of small particles and see what occurs with one of these on its way through the rotor-blade wheel. We presume further that the rotor-blade wheel is filled with liquid when this is rotating.
The inlet at the end of the blade will have a precise velocity and direction, marked ua. The direction is the key to the circle of the point. The size of the vector is given by the angular velocity of the rotor-blade wheel, w. The connection between the sizes, can be expressed as: ua = w x r when w = 2 x p x T The liquid particle will, at point A, be affected by power, from the blade marked Fua. The energy works in the same direction as the velocity vector ua. It means that the particle performs power F that is equal, but in the opposite direction as Fua, which is toward the blade housing. When the blade influences a liquid particle, the particle achieves a certain velocity. The velocity is causing the particle’s centrifugal force, F, which has a direction leading straight out from the centre. This force gives the liquid particle certain acceleration, a., the relation between proportions may be expressed as:
Fsa = m x a or a = Fsa/m As we see from the sketch, the energy forces Fsa and F1 to produce energy, marked F. When the rotor-blade wheel is rotating, the liquid particles will move lengthways along the blade because of the centrifugal force. Since the blade governs the liquid, the relative velocity will have the same direction as the blade. The relative velocity factor will try to accelerate the particle, but the liquid’s inner friction (viscosity) resists and reduces the centrifugal force influence. If the only consideration is the liquid’s viscosity, the relative velocity will achieve a certain proportion when the inner friction compensates for the centrifugal acceleration. The system is in balance and a certain relative liquid velocity is achieved. The relative velocity is a direct expression of the flow through the rotor-blade wheel (the pump).
The liquid particles are influenced by the relative velocity vector (V) and the blade’s velocity vector (U). The resulting velocity vector gives the particle’s absolute velocity (C). This vector (C) determines the particle’s track through the rotor-blade wheel.
In the point “D” the liquid particles leave the rotor-blade wheel. As a result of the relative velocity vector (Vd) and the rotor-blade wheel’s velocity vector (Ud), an absolute velocity (Cd) has been achieved with direction and proportion.
The blade’s deflection will determine the relative direction of the liquid that comes out of the rotor-blade wheel. When it is the blade that set the liquid in motion, you see that the liquid’s absolute velocity can never exceed the rotor-blade wheel’s peripheral speed.
The rotor-blade wheel sets the liquid in motion; i.e. the liquid is supplied with kinetic energy, Wk. This energy is transformed to potential energy, Ws, and/or pressure energy, Wt. The connection between these energy forms may be expressed as: Kinetic = Potential energy = Pressure energy or: Wk = Ws = Wt
If the pump is connected to a high riser, the liquid will stabilise at a certain level. All kinetic energy is transformed to potential energy. The centrifugal pump’s lifting height, H, is then: ½ mv2 = mgH
H = v2/2g
The pump’s lifting height is dependent on the liquid’s mass and consequently, the liquid’s density. If a centrifugal pump is running against a closed valve, all the kinetic energy will be transformed to pressure energy. The pressure after the pump is then: ½ mv2 = mp/r p = ½ v2r Also, the pressure after the pump is proportional with the liquid’s density. If we are placing a number of rotor-blade wheels in serial in order to prevent energy loss. The liquid’s absolute velocity out of the pump will be to equal to the peripheral speed. The theoretical maximum lifting height, Ht1, for the pump will be: Ht1 = Cd2/2g when Cd = Ud, we will obtain: Ht1 = Ud2/2g
The volume flow is determined by the liquid’s density. When the lifting height is at a minimum, the volume flow will be at a maximum. The theoretical pump characterisation, QHt1, which emerged, will be linear.
But in our example, we only have one rotor-blade wheel. The liquid’s absolute velocity will be less than the peripheral speed. A new pump characterisation, which has adapted real velocity, is shown on the diagram to you right (marked QHt2). In all pumps a certain loss will always occur. The friction between the liquid, the rotor-blade wheel and deflection loss can be empirically set to: h1 = k1 x Q2 k1 = an invariable determined by the specific rotor-blade wheel. In addition, it is entrance loss and shock loss, which are theoretically set at: h2 = k2 x (Q - Qs)2 Out of the above formula: h2 = 0 when Q = Qs
This will occur when the liquid’s relative velocity into the blade has the same direction as the inlet blade. The loss curves h1 and h2 give a resultant loss curve, h3. From the theoretical pump characterisation QH1, 2 and the resultant loss curve, h3, a theoretical calculated pump curve emerged, marked QH. 10.3.2 The centrifugal pump’s mode of operation A centrifugal pump consists of a rotating impeller inside a pump casing. The liquid inside the impeller is affected by the “blades”, and will be lead through the “blades” due to the centrifugal force. Energy in forms of kinetic energy (velocity energy) is added to the liquid. New liquid is constantly lead into the impeller and put into rotation. A flow through the pump is established.
If the delivery pipeline from the pump is open to the atmosphere and has sufficient height, the liquid will adjust itself to a precise level given by the energy, which was added to the liquid through the impeller. Here, all kinetic energy is transformed into potential energy.
The difference in liquid level is called net delivery head. A pump’s delivery head is dependent on the individual pump’s construction. If the level in the tank is lowered, the liquid level in the delivery pipeline will be correspondingly lower. Net delivery heads (H1, H2, H3) will be equal for the same pump provided that flow disturbance does not occur on the pump’s suction side. However, the pump’s delivery pressure is dependent on the liquid’s density and delivery head. In this case, the liquid is water with a density (r) of 1000 kg/m3 and the head (H) is 100 meters, the manometer pressure (pm) after the pump will be read at: pm = r x g x H = (1000 kg/m3 x 9,81 m/s2 x 100 m) pm = 981000 Pa = 981 kPa pm = 9,81 bars One can see from the previous example that the delivery head of the pump is obtained from the pump itself, and that the delivery head is independent from the pump’s position or location. It is therefore natural that the centrifugal pump’s capacity always is given as a function of the pump’s delivery head. If you bend the discharge pipe from the previous example, like the illustration below, the liquid will flow out of the pipe. Only a part of the added energy in the pump will “lift” the liquid. The rest of the energy is still in the form of kinetic energy. From the previous taught experiment, one can predict that the capacity of a centrifugal pump will be highest at minimal delivery head. The capacity curve (Q-H curve) will, in practice, follow this assumption, but the curve is not linear due to loss of energy in the pump. If you ignore the pipe resistance, the capacity Q in this situation is determined by the delivery head (H). The delivery head here is the static height or the static backpressure, which the liquid has to lift.
In a real pipe system, bends and valves will create a resistance due to friction against free liquid flow. This resistance varies with the velocity and viscosity of the liquid, and is called the dynamic backpressure. The total pipe resistance, composed by the static and the dynamic backpressure, is called a system characteristic curve. The intersection point between the system characteristic curve and the capacity curve is called the actual operation point. It was previously mentioned that disturbances on the pump’s suction side would have influence on the capacity.
The conditions on the inlet side are very important for the centrifugal pump’s operation. A centrifugal pump has normally no self-priming qualities, meaning that the pump is not able to suck liquid from a lower level. Additional vacuum equipment connected to the pump will, however, improve the pump's self-priming qualities. When the inlet pipe and impeller is filled with liquid, the pumping process will be able to continue without this equipment. The liquid’s viscosity may ensure a continual flow into the pump. Too high resistance in the inlet pipe will cause the same operational disturbance. If the flow into the pump is less than the outlet flow, due to too high pipe resistance and/or too high viscosity, these factors will have considerable influence on the pump’s capacity.
If you start a pump, submerged in water like the sketch indicates, the pump will have a specific capacity at a specific delivery head. If you gradually lift the pump, the pump will, at a specific height, have a perceptible reduction in the capacity. When this occurs, the height of the pump above liquid level is called Net Positive Suction Head or NPSH. The explanation of this phenomena is that when the pump is lifted up out off the water, the pipe length and the resistance at the inlet side increases. The increased resistance creates constant negative pressure on the inlet side of the pump. The liquid that accelerates from the centre of the impeller and out to the periphery increases this negative pressure. When the negative pressure reaches the liquid’s saturation pressure, the liquid starts boiling and a large quantity of vapour is created in the pump. The output from the pump become irregular, and will stop at huge vapour volumes. We say that the pump cavitates. A centrifugal pump operates satisfactorily with approximately 2% gas in the liquid. But cavitation will always cause damage to the pump. The gas bubbles created in the liquid on the pump’s suction side will collapse when the pressure rises inside the impeller. The consequences of cavitation are: · Vibrations and noise · Reduced efficiency · Pitting or cavity erosion inside the pump house As we have observed, the cavitation is destructive and must be avoided or controlled. To ensure limited or non-generation of vapour one must make sure that the liquid at the pump inlet has sufficient overpressure to avoid evaporation. The resistance at the pump inlet side should be made as low as possible. This can be
done by constructing the pipeline as short as possible, limiting the number of bends and selecting a maximum diameter on the pipeline. The pump should be positioned at the lowest possible level, and preferably below liquid level at the suction side. A pump’s NPSH is variable and dependent on the flow. When the flow increases, the negative pressure generated inside the pump increases. A reduction of the flow will reduce the negative pressure. Reducing the pump’s capacity may therefore control and reduce the cavitation. A centrifugal pump’s capacity is adjusted by throttling the delivery valve. Throttling increases the pumps discharge pressure (backpressure) which causes reduced capacity. The capacity may also be adjusted by changing the revolution on the pump. Adjustments of the pump’s revolution move the capacity curve up or down. Reduction of the revolution moves the curve parallel downwards, an increase in revolution, upwards. Note that these relations are valid only if the flow conditions are unchanged. 10.3.3 The Pump performance diagram
All manufacturers supply a pump performance diagram with the pump delivery. The curves in the diagram are results from practical tests in the manufacturers workshop and specifies:
· Type of liquid used in the test (generally water) · Number of revolutions · Type and size of impeller · The optimal operation point The operation point is normally set at the best possible efficiency, simultaneously within the pump’s predicted capacity range. It is important to be aware that the pump’s diagram is made for a special liquid with specific properties. The capacity curve will be real for all liquids, provided the free flow to the pump inlet is not restricted due to for example too high viscosity. The power consumption curve will of course depend on the fluid’s density. A pump’s condition is of course vital for the curve accuracy. There are a lot of methods to check the centrifugal pump’s condition. Monitoring the pump’s delivery head, capacity, power consumption and development of these is obvious. Detection of many minor operational disturbances may be difficult and not necessarily observed. Establishment of routines ensures continuous control of vibrations. Visual inspection of the pump and regular maintenance is important to prevent break down.
10.3.4 Example on pump diagram
10.4 The deepwell pump Cargo discharge pumps onboard gas carriers are generally deepwell pumps. The deepwell pump’s main parts are the pump, shaft, and mechanical seal, coupling and motor. The pump is located at the lowest level inside the cargo tank pump sump. The shaft is located in the discharge pipe from the pump to the mechanical seal and connected to the coupling. The coupling connects the shaft and motor together. On top of the cargo dome the top unit with an electric or hydraulic motor are located.
Due to the length of the shaft, the pump has a limited rotation speed. This leads to a reduced delivery head for a single stage pump. The pump is therefore built with multiple stages. Shaft bearings are located between each stage in the pump unit and with even intervals along the shaft. These bearings can be made of PTFE preservative carbon that have self-lubricating qualities. The top unit consists of an axial bearing, a double mechanical shaft seal, revision ring and an upper ball bearing. The axial bearing is a roller bearing that holds the entire weight of the shaft and impellers. The bearing house is usually provided with cooling ribs to maintain an acceptable oil temperature. The purpose of the double mechanical shaft seal is to prevent leakage of cargo into the environment, and leakage of air into the cargo. The oil in the seal lubricates the seal surfaces, and
“quarantee” an operation without leak or other problems. If we got a leak in the seal the oil must not came in contact with the cargo. The revision ring in the mechanical seal functions normally as a deflector that will lead a leakage of seal oil into a special chamber. This chamber can be drained. A second function for the revision ring is to prevent leakage of cargo vapour along the shaft when renewal of the mechanical seal. If the axial bearing nut is loose, the shaft will drop down and the revision ring will land on a seat below.
The top carbon shaft bearing, must like the other shaft bearings, be lubricated by the cargo itself. One can thereby not avoid contact between the cargo and the seal oil in the top unit. Control of compatibility between the seal oil and actual cargo is therefore important. The seal oil should not pollute the cargo or generate hazardous reactions when mixed with the cargo.
10.4.2 General tips for operation of a deepwell pump: · Check pump unit guides clearances inside the cargo tank when possible. · Check for lose bolts and nuts inside the cargo tank when possible. · Always carefully check the cargo tank and sump for rags and other lose objects before the hatch is closed. · Always check the motor’s running direction before the coupling is fitted. Wrong direction may result in shaft damage. · Always regularly check the “anti rotation device”, if fitted. · Always check lubrication oil level and seal oil level, pressurise before the pump is started. · Regularly check seal oil and lubrication oil levels when the pump is running. · Always turn the pump before start. · Check and adjust the pump’s safety device. · Running of deepwell pump without liquid is the most common reason for breakdown. Stop the pump when the tank is liquid free, do not force operation of the pump and do not run the pump against a closed delivery valve for too long a time. · These pumps are operating without vibration and related noise. Stop the pump immediately if this occurs.
10.4.3 Design of the double mechanical seal Example of pump parts
10.5 Submerged pumps Submerged pumps are multistage centrifugal pumps that are often used as discharge pumps on large LNG and LPG tankers. The motor and pump are submerged down in the tank sump or as close to the tank bottom as possible. The motor is connected directly to the pump with a short shaft on this type of pump. The liquid that is pumped lubricates and cools the pump’s bearings. It is therefore essential that the pump is used only when there is liquid in the tank. The liquid is pumped up through the tank’s discharge pipe and up to the liquid line.This type of pump is equipped with electrical motor. The cables to the electric motor are either made of copper or stainless steel. If copper is used in the cable, the cables must be sheathed with stainless steel to prevent damage on the cable from corrosive cargoes. When transporting Ammonia, the cable and engine must be sheathed with a thin layer of stainless steel. It is important that the stainless steel sheathing is kept unbroken, and we must avoid a sharp bend on the cable to protect the stainless steel sheath. One must at all times check the resistance of the cable insulation before starting the pump. Submerged pumps are also installed as portable pumps. The discharge pipe is then the steering pipe for the pump. At the bottom of the discharge pipe it is a non-return valve that opens when pump is lowered and shut when the pump is taken up. Before opening the discharge pipe it must be gas freed, this is done either with inert gas or Nitrogen.
10.6 The boster pump If the backpressure during discharging is too high for the deepwell pump(s), a booster pump is connected in serial with the deepwell pump(s) to increase the system’s ability to pump against high backpressure. There are normally two booster pumps installed on deck on gas carriers. The booster pump is normally a centrifugal pump, installed horizontally or in vertical position. Horizontally installed pumps have an axial inlet and radial outlet in the same centre line. Vertically installed booster pumps have radial in and outlet “in-line”.
The booster pumps onboard gas carriers have mainly an individual and compact design. They are constructed with focus on reliability, simple maintenance and long lifetime. The shaft seal, with double mechanical seal with sealing liquid, is similar to the arrangement of the deepwell pump. The bearing is lubricated by oil; the liquid lubricates the inner shaft bearings.
The following general regulations are existing for working a booster pump: · Turn the pump shaft regularly when the pump is not used for long periods. · Turn the shaft before starting the pump. · Do never start the pump if the pump is not filled with liquid. · Regularly check the seal oil level. · Never run the pump against the closed delivery valve for more than approximately 30 seconds.
· Adjust the flow by throttling the delivery valve, the inlet valves should always be fully open. 10.7 Parallel operation of centrigal pump The capacity requirement is many times higher than the performance of one centrifugal pump. All available pumps are then lined up and run in parallel operation. The diagram below indicates two equal pumps in parallel operation. We assume a symmetry pipeline for both pumps.
The pump’s capacity curve (1) indicates the relation between the delivery head and the flow rate for one pump. As both pumps are equal, the pumps’ individual capacity curves are represented by the curve marked “1”. When equal pumps are run in parallel the delivery head for the system will be equal the delivery head for one pump. The capacity will meanwhile increase in proportion to the number of pumps. If, for example one pump has a capacity of 100 m3/hrs at a head of 100 meters, two pumps in parallel will supply 200 m3/hrs and three pumps 300 m3 /hrs at the same head. If the pumps are of different types or equal pumps are run at different speed, their individual performance curves will be different. This is no problem provided the operation point is outside the pumps’ individual operation point. If, for example the operation point is altered by increasing the back pressure, this may lead to that one of the pumps are run without output flow. If this situation occurs and is maintain, the pump may be damaged.A deepwell pump is special vulnerable in this situation because these pumps are dependent of a liquid flow through the pump. The only indication on such condition is that the power consumption no longer is in agreement with the operation. If there are no check valves after the pump and the backpressure increases further, you will have a back flow trough the pump into the cargo tank. Even though the type of the pump is the same in a parallel operation, variation in rotational speed, asymmetric piping, variance in opening if valves, cavitation and variance of the pumps condition, may form the same condition. 10.7.2 Two equal pumps are run in parallel operation. Two equal pumps are run in parallel operation. The resulting performance curve of
the pumps (1+2) is constructed like previous described. We then see on this actual pump situation, a pump alone will deliver 120 m3/hrs against a head of 120 meters. When you start pump number 2, the capacity for the system increases to 140 m3/hrs against a head of 150 meters. Starting pump number 2 will not double the capacity because a higher volume flow creates higher dynamic resistance. The system’s capacity will increase if other pumps are started up in parallel operation. The increase in capacity will however be relatively less for each pump added.
10.8 Serial operation of centrifugal pumps If the backpressure while discharging is too high and the capacity hence too low, a booster pump in series with cargo pumps will improve the capacity. The following illustration shows two cargo pumps run in parallel (1 and 2) in series with a third booster pump (B). Some ship has a dedicated pump for this purpose, also-called booster pump.
The system’s maximum capacity is the number of pumps in parallel operation, times maximum capacity for one pump. The maximum head is the maximum head for one pump (1 or 2), plus maximum head for the booster pump (B).Notice that cargo pumps no.1 and no.2 must ensure sufficient flow of liquid to the booster pump (B). One must emphasise that there are made no special demands for a dedicated booster pump’s NPSH-performance. If one of the pumps in parallel stops, there is a risk that the supply of liquid to the booster pump became too low and will result in cavitation. If so occur, cavitation can be reduced or stopped by throttling the delivery valve on the booster pump. 10.8.2 Two equal pumps run in parallel operation. Two equal pumps run in parallel operation. The capacity is 100 m3/hrs with a head of 160 meters. The master and chief officer evaluate the operation and decide to line up for a booster pump in serial with the cargo pumps. The new capacity is calculated by constructing a new resulting performance curve for all three pumps, as previous described. The new operation point will appear in the intersection between the resulting performance curve, and the system characteristic curve. The capacity will increase to 140 m3/hrs at a head of 340 meters. If the cargo is propane at a temperature of 20oC, and with a density of 502 kg/m3, the corresponding delivery pressure will be: r x g x H = 502 kg/m3 x 9,81 m/s2 x 340 = 167.437 Pa » 16,7 bars
10.9 Pressure surge and liquid pressure When a valve on a liquid line is closed too quickly, the pressure inside the line very quickly increases to a hazardous high level. Quick changes to the liquid flow in a pipeline may lead to a pressure surge resulting in a rupture in the pipeline system. This surge pressure can be recognised by a “knock” in the pipeline. This type of pressure peak is generated very quickly in pipelines, faster than a common safety valve is capable to relieve. The consequence may be breakdown of the pipeline system and thereby high risk of pollution, fire and personal injury. Pressure surge may appear if: The emergency shutdown valves are activated and closed too quickly. (ESD/Emergency Shut Down) Fast closing/opening of manual or remote operated valves. Fast variation of the volume flow resulting that a non-return valve starts hammering.
When a pump is started or stopped. The pressure in a cargo transfer system has three components; the hydrostatic pressure, the cargo tank pressure and the pressure generated by the pump. The hydrostatic pressure and the cargo tank pressure will mainly be constant and we will refer to these as “static pressure”. If the flow suddenly is varying due to, i.e. too fast valve closure, the moving liquid that have a specific velocity and mass will hit a “wall” inside the valve (the valve seat or the valve flap). The kinetic energy of the moving liquid will immediately convert into potential energy by compression of the liquid against the valve seat. How fast the pressure peak is generated depends on the velocity and the density of the liquid. A pipeline of 250 meters and 150 mm in diameter is used for water transfer at a capacity of 400 m3/hrs. The total mass of the moving liquid inside the pipe is 4400 kg and moves with a velocity of 6,3 meters/second. If a valve is closed very fast, the kinetic energy will convert almost immediately to potential energy. The pressure surge may reach approximately 40 bars within 0,3 seconds. If the liquid is a condensed gas or crude oil, vapour may be present. These vapour bubbles will collapse when the pressure increases. The collapsed bubbles will generate pressure waves that will also be transmitted through the pipeline system. In an opposite case where the pressure is decreasing rapidly, a volatile liquid will start boiling. The above mentioned cases illustrate why it is especially important that the valves and pumps are cautiously operated so neither dangerous pressure peaks nor pressure drops are generated. The enclosed diagram on the next page is from ITC Tanker Safety Guide. It shows a normal cargo operation and pressure in the pipeline. The maximum pressure is at the pump outlet. This pressure is the sum of the hydrostatic pressure and the pressure generated by the pump. Due to friction in the pipeline, the pressure will gradually decrease toward the cargo manifold. If the ESD valve is activated and the valve is closed too quickly, the liquid flow will stop quite quickly. The liquid’s kinetic energy will convert into potential energy immediately when the liquid hits the valve seat. A pressure peak is generated and will be transmitted at the speed of sound (the only way possible) back towards the pump. When the wave of pressure reaches the pump, some of the pressure will unload through the pump, but the resistance here will also operate as a “wall”. The pressure is rebuilt and reflected back towards the ESD valve again. A new pressure peak is generated with additional increased pressure. A “knock” will occur each time a pressure top is generated against a “wall”. All personnel that operate valves or pumps must be observant of these phenomena and of the liquid pressure, which may occur consequently. The progress and the length of the pressure surge depends on the system. If the wave of pressure is allowed to move between two valves without pressure relief, a maximum pressure surge will be generated.
The most vulnerable parts in the system are the shore connections and loading/discharging arms. The operative personnel normally work nearby the manifold area. A rupture in this area may easily lead to personnel injury. (Please note that control of cargo hoses is dealt with in this chapter). Maintenance and testing of the ESD-valves’ closing time is the most important of the above mentioned causes. Closing time of the ESD-valves, which is too short, may lead to generation of a dangerous pressure surge. Always consult the terminal representatives about the required pipe line period and ESD time.
10.9.1 Development of pressure surge
The closing time of ESD valves should be as short as possible to prevent overflow and spillage. But not so fast that a risky pressure surge occurs. Necessary time for a safe closure of valves can be calculated based on the expected maximum pressure
surge when the pressure wave has passed forward and backward through the pipeline. The speed of the sound is set to 1320 m/s. If the pipeline is 2 km, the calculated time for maximum pressure surge at closure of the ESD valve is: T = (2 x L) / Speed of sound = (2 x 2000 m) / 1320 m/s = 3 s The maximum pressure surge will occur 3 seconds from closure of the ESD valve. This time is called a “pipeline period”. It is assumed that the safe closing time is five times a pipeline period, so the closing time should at minimum be: 5 x 3s = 15 seconds 10.9.2 Cargo hoses The cargo hoses are normally the weakest part in a pipeline system transferring cargo. The responsibility for the cargo hose condition on board lays with the ship. It is important to be aware of this fact, in case a cargo hose is lent out to a third party or is used in transferring cargo between ships. Hoses for cargo should be cleaned and dried before storing. The storage area should be dry and out of the sun, if you want to take care of the hoses as long as possible. Poor cleaning and storage is generally the cause of damage and consequently replacement. A cargo hose prototype is tested with the products, pressure and temperature for which the hose type is approved. The cargo hoses should be tested yearly at a pressure that is 1,5 times the maximum working pressure. The normal procedure for yearly testing is: The hose is laid out on deck, blinded off and filled with water. The hose is pressurised with 1,5 times the working pressure. The hose is checked for leakage. Electric bonding is checked. Linear expansion for the hose is measured (measure for strength) The test result is logged. If there is no leakage, bonding is okay and the linear expansion is less than the limitation set by the manufacturer, the hose is approved.
10.10 capacity calculation All centrifugal pumps are delivered with pump performance diagrams. The diagram is an important tool for insight in the factors that have influence on operation of pumps. Enclosed is a pump curve with a drawn system curve for an actual unloading situation. The operation point is in the intersection between the pump curve and the system curve. The system curve is composed of a static- and a dynamic curve. The static backpressure (H), is corresponding to a level distinction of 20 m between the liquid in the shore tank and the cargo tank onboard. The dynamic backpressure is calculated from a stated differential pressure in the pipeline of 10 mlc at a flow rate of 500 m3/h.
The pipeline resistance is dependent of the flow rate in the pipe, and the dynamic backpressure (pipe resistance) can be expressed as: H = c x Q2 (see “The Affinity Laws”)The constant c, is calculated from above mentioned expression. The dynamic curve is a result of calculation of the dynamic backpressure at different flow rates with the same c-value. The system curve is constructed by adding the static- and the dynamic backpressure at the same flow rate. We then see from the diagram, that the pump delivers 80 m3/h at a head of 120 mlc. The pump’s delivery pressure is dependent of the density of the liquid pumped. If the liquid is propylene at a temperature of –44oC, the pump’s delivery pressure (p) will be: p=rxgxH = 607,4 kg/m3 x 9,81 m/s2 x 120 m = 715.031 Pa = 7,15 Mpa = 7,15 bars The cargo tank pressure is 0,2 bars at a temperature of –40oC. Observed delivery pressure (manometer pressure) is thereby approximately 7 bars.If the backpressure increases, by for example throttling of the manifold valve, the dynamic backpressure
will increase. An increased dynamic backpressure is visualised by a more steep system characteristic curve. The operational point will move up along the pump curve. The pump’s delivery head increases and the capacity is reduced. The “new” delivery head, and hence the capacity, can be calculated without construction of a new system curve.If the backpressure increases to 8 bar, the new delivery head will be: p=rxgxH H = p / r x g = 800 000 Pa / 607,4 kg/m3 x 9,81 m/s2 = 134 m A delivery head of 134 m corresponds to a capacity of approximately 50 m3/h. 10.10.1 An extended capacity calculation The method of calculation above is a simplified procedure, but gives sufficient means for most practical calculation. An extended calculation of capacity where one considerate all factor influencing a pump’s working conditions, must necessarily be more complex.When describing the centrifugal pump’s physical relations, it is natural to focus on the whole system’s energies. The system’s energy balance is then: Energy on the suction side + Energy added the pump = Energy after the pump. In the following example we have a partly filled tank where a submission is working. The pump is supplying liquid a stated energy, which is lifting the liquid into the tank and gives is velocity, which creates a flow rate through the pump. The energy level on the suction is decided by the height of the liquid, and the liquid pressure. This is expressed as: (m x g x X) + (m x po/r) The energy supplied into the pump is expressed as: mxgxH where “H” is the lifting height of the pump.
The liquid is lifted out of the cargo tank. The energy level after the pump is compound of static energy (liquid are lifted a given value Y), kinetic energy (given backpressure p2) and velocity energy (velocity of the liquid c2). The balance of the energy is then: (m x g x X) + (m x po/r) + m x g x H = (m x g x Y) + (m x p2/r) + 1/2 x m x c22 where the real pump head H is: H = (p2 - p0)/(r x g) + (y - x) One can see how the tank pressure and levels affect the simplified calculation of the lifting height of the pump. When the liquid level in the tank is lowered toward the end of the unloading, the lifting height increases with hence following reduction in the capacity. A higher tank pressure gives lower lifting height, and with that larger capacity. 10.10.2 Affinity equation The affinity equation is an expression that demonstrates the proportionally relation between flow rate, lifting height, effect and number of revolutions for a centrifugal pump. If one of these element changes, this will influence on the rest. The equation can be used to calculate theoretic consequences of the changes on the existing centrifugal pump.
10.10.3 Equation 1 The velocity of the liquid from a pump, is an expression for flow rate through the pump, and can be defined as: c = Q/A where: c = velocity of the liquid in m/s Q = volume flow through the pump in m3/hrs A = cross section of the pipe in m2 10.10.4 Equation 2 The velocity of the liquid from a pump is dependent from the impellers velocity, the number of revolutions. Higher number of revolutions of an impeller gives larger velocity of the liquid, which again gives larger capacity. This is defined as: Q1/Q2 = c1/c2 = n1/n2 where: Q1 = Capacity before changes Q2 = Capacity after changes c1 = velocity of the liquid before changes c2 = velocity of the liquid after changes n1 = revolution of impeller before changes n2 = revolution of impeller after changes 10.10.5 Equation 3 Lifting head of a pump are given by the following expression: H = c2/2g where: H = pump lifting head in mlc c = velocity of the liquid in m/s g = gravitation in m/s2 We then se that the pump lifting head is proportional with c2 and equation 2 can be changed to: H1/H2 = c12/c22 = n12/n22 10.10.6 Equation 4 The theoretical need of power (P) for a pump is: P = r x g x Q x H where: = density of the liquid in kg/m3 Change of the operation of the pump gives the following connections: P1/P2 = (c1/c2)3 = (n1/n2)3 Peripheral speed (v) for a pumps impeller is: v = (p x d x n)/60 where: d = impeller diameter
When the impeller diameter and the peripheral speed is variable the following equation can be used: v1/v2 = d1/d2 = n1/n2 When the velocity of the liquid is proportional with impeller speed can we use the following equation: c1/c2 = v1/v2 = d1/d2 = n1/n2 = Q1/Q2 or: Q1/Q2 = (n1 x d1) / (n2 x d2) 10.10.7 Equation 5 Above mentioned expression can be summarised as: H1/H2 = (c1/c2)2 = (n1 x d1)2 / (n2 x d2)2 where the condition in the alteration of the pump effect is: P1/P2 = (d1/d2)5 x (n1/n2)3 One shall notice that the three last expression for flow rate, lifting height an effect, only effect small changes of the rotor-blade wheels diameter. This is due to that the loss of the pump is not directly proportional with the flow rate. Large changing of the diameter will give bigger effect of the flow rate, lifting height an effect. 10.10.8 Equation 6 A combination of equation 4 and 5 can the relation between lifting head and volume flow give the hydraulic operation of the pump as follows: H1/H2 = (Q1/Q2)2 = constant or H = k x Q2 10.11 Displacement pumps Pumps are very old machines. The first types of pumps (screw pump and piston pump) are more than two thousand years old. A pump’s purpose in transport of liquids, usually are to pump from a low level to a higher level. Its purpose can also be to pump a liquid into a tank, which contains higher pressure than the surroundings. The pump increases the liquid’s energy. The increased energy is potential energy; the liquid is transported from a low level to a higher level. This is the kinetic energy, the liquid’s flow has increased or as pressure energy, if the liquid is pumped into a tank with a higher pressure than its surroundings. As an example, the feed water pump to a boiler is working using these principles.In addition to the mentioned increase of energy, the pump also has to maintain the energy, which is lost due to streaming in the system. As mentioned, kinetic pumps constantly have liquid streaming through the pump with pressure increasing simultaneously. In displacement pumps, a certain volume of liquid is branched off and moved from the pump’s delivery side. Then a pressure increase occurs. Screw pumps and piston pumps will be viewed further in this
chapter. A wide range of displacement pumps is available, such as the lamella pump, ring pump, propeller pump, etc. 10.11.1 Piston pump The piston pump is used for relatively small amounts of liquid with large delivery heads. When the piston is pulled upwards, a vacuum occurs inside the pump housing. The suction valve will then open and liquid streams into the pump. When the piston is pressed downward the pressure will increase, the suction valve will close, the delivery valve is set open and the liquid sent out of the pump. The liquid does not stream in a smooth flow as in a centrifugal pump, but accelerates and slows alternately. This is of inconvenience with long pipelines. The valve is a weak point. They are sensitive to liquid pollution and they also increase the resistance against streaming. Usually, the piston pump is double acting. Because of the pump’s movements, the pump must have a relatively slow piston speed. The piston pump may sustain almost unlimited pressure. However, the limitation is the automotive power and the material strength. The piston pump does not have to be filled with liquid before starting. Make sure that all the valves on the delivery side are open before starting. The efficiency of piston pumps is higher than, for instance, centrifugal pumps. The piston pump is a well-known pump on board an oil tanker. This is the pump, which is used to pump cargo deposits ashore at the end of the discharging operation. These oil deposits from cargo tanks, lines and cargo pumps are pumped ashore through a small diameter line. 10.11.2 Screw pump The screw pump consists of two screws or more, where one of them is activated. The screws are placed inside a pump house. A common and well-known screw pump is the Swedish manufactured so-called IMO pump. This pump consists of one active screw placed in the middle and two symmetrical side screws. The screws tighten to each other and to the housing, but have no metallic contact. When the screw rotates, the threads are filled with liquid. The liquid is displaced by axial through the pump. In this pump, the side screw rotates in the opposite direction of the middle screw. These screws are working like an endless piston which constantly moves forward. The liquid is not exposed to rotation. The pump is self-priming, running almost soundless and with little exposure for wear and tear when pumping clean liquids. The screw pumps are used a lot as a lubricating pump, but are also used as a stripping pump on oil tankers.
11-
Cargo Handling Routines
Routines when cargo handling 11.1 LEGISLATION AND RULES All transportation of liquefied gas is controlled by international and national legislation. The international legislation is set up by IMO, which is the UN’s maritime division. All new laws and legislations are renewed and updated when the old rules are no longer appropriate for it’s original intent. As we gain more experience and knowledge about accidents, the laws and rules will be revised to fit what has been learned. The various cargoes that is allowed for transport on gas carriers and the types of gas carriers that may carry the them is updated as we gain better knowledge about the products. Over time we can observe the various types of cargoes to see if there are any changes regarding health hazards or environmental pollution. Some cargoes have changed TLV from 400 ppm to 1 ppm. The reason for this is the experience we have got with the cargo, and it is very seldom that new products came into the market with 100% guaranty that they are safe in all manners. What kinds of cargoes we can carry and which types of gas carriers that may carry the various cargoes depend on the toxicity of the cargo. The toxicity ratings of the various cargoes will always change over time as we gain more information about their potential health hazards. One example is VCM “Vinyl Chloride” that had a TLV of 50 ppm in the early 1980s, but in the 1990s the TLV had been changed to 2 ppm. What types of cargoes a gas carrier may transport, depends on what precautions have been taken in the vessel’s design and construction, to prevent cargoes from pollute the environment. In order to ratify IMO rules and legislation’s, a given number of the IMO member states must to abide by them. All flag and port states may have there own set of rules and legislation that differ from the IMO rules. The IMO has set up a minimum of standards that all gas carriers must be constructed and classed according to.
11.1.1 International rules There is three international conventions to protect the environment and those are SOLAS (1974) with protocol of 1978 and appendixes to 1991, MARPOL 73/78, and STCW 78/95. SOLAS «Safety of Life at Sea» contain rules and legislation’s on safety certificates such as: Safety Construction Certificate, Safety Radio Certificate, and Safety Equipment Certificate. These three certificates cover the safety of crew, ship and safety equipment. Over time when as we gain more knowledge, the rules and regulations will be updated to avoid similar accidents from happening again in the future.
11.1.2 Certificates and documents we are required to have onboard according to SOLAS 74/92 appendix 3:
Intact Stability Booklet
SOLAS 1974 Regulation II-1/22
Minimum safe Manning document SOLAS 1974/89 Regulation V/13b Cargo Ship Safety Construction Certificate
SOLAS 1974 Regulation I/2 as amended by GMDSS amdts.
Cargo Ship Safety Equipment Certificate
SOLAS 1974 Regulation I/2 as amended by GMDSS amdts.
Cargo Ship Safety Radio Certificate
SOLAS 1974 Regulation I/2 as amended by GMDSS amdts.
Document of compliance with the SOLAS 1974 regulation II-2/54.3 special requirements for ships carrying dangerous goods Dangerous goods manifest or stowage plan
SOLAS 1974 regulation VII/5(3)
MARPOL «International Conference on Marine Pollution» contains rules and regulations that are designed to protect the environment from pollution on a short and long term basis. MARPOL specifies what we are allowed to pump or throw overboard, and also what equipment we must have onboard to prevent pollution of the environment.
MARPOL defines clean water as water with less than 15 ppm of contamination.
11.1.3 Certificates and documents related to MARPOL 73/78 required to be onboard according to SOLAS 92 appendix 3: International Oil Pollution MARPOL 73/78 annex 1 Prevention Certificate regulation 5 Oil Record Book MARPOL 73/78 annex 1 regulation 20 Dangerous goods manifest or MARPOL 73/78 annex III stowage plan regulation 4
STCW 78/95 «Seafarers’ Training, Certification and Watch keeping» contains rules and regulations on qualification certificates of officers and rating onboard vessels. STCW 78/95 was implemented 1st February 1997 and is based on two parts. Part A contains the minimum standard requirements for qualification of all personal on various types of ships. Part B contains the minimum requirements of what all personal need to know in order to receive a certificate for their various ranks on all types of ships. 11.1.4 Certificates and documents related to STCW 78/95, we are required to have onboard according to SOLAS 92 appendix 3 Certificate for masters, officers or STCW 78/95 article VI ratings
IMO «IMO Gas Code» contains rules and regulations that are meant to make the transport of liquefied gases as safe as possible for persons onboard as well as the environment. There are three gas codes issued by IMO. ·
Code for existing ships carrying Liquefied gases in bulk.
·
Code for construction and equipment of ships carrying liquefied gases in bulk.
· IGC code International code for construction and equipment of ships carrying liquefied gases in bulk.
The gas codes contain requirements for the construction of gas carriers. This includes requirements on stability, leakage after collision/ grounding and cargo compartments. In the gas code we also find a list of products that are classified as liquefied gases, as well as the requirements covering how those cargoes are to be transported. The gas code also covers toxic cargo and what types of safety devices are required to carry those cargoes. The vessels Certificate of Fitness states which cargoes the vessel is permitted to carry. We are required to always have onboard the personal safety protection equipment for the type cargo we are carrying, as specified for in the vessels Certificate of Fitness. Especially important are instruments for atmospheric measurements and personal safety equipment.
11.1.5
Local rules
Some port states have local rules that are stricter than the IMO rules and are designed to protect local waters. All countries try to protect own territorial waters against pollution. Local rules may be stricter on clean water, shore line pressure, navigation after sunset etc. There are three countries that have especially strict rules and have a large amounts of import and export of liquefied gases and those are: USA, Italy and Japan. In the USA the US Coast Guard issue a Certificate of Compliance on all gas carriers that are to load or discharge in US waters. According to US rules and regulations, all gas carriers that visit US waters are to be inspected by the US Coast Guard in accordance to the vessels Certificate of Compliance. Gas carriers that fail the US Coast Guard inspection will not be permitted to birth before the vessel has fulfilled the standards set by the Certificate of Compliance. One main difference between IMO and USCG is that in US they have a higher standard of security regarding strength on pressure vessels. That means that most of the gas carriers have one safety relief valve setting according to IMO and a lower one according to USCG. Gas carriers that are built according to IMO gas code have no problem in attaining a Certificate of Compliance. The owner of the vessel must send the USCG diagrams of the mid ship section, GA plan, fire and safety plan. Those drawings and plans must be sent to the USCG in long before the vessels first arrival US waters, if possible when the vessel is new. Italy issues all vessels that carry gas in Italian waters an Italian safety certificate called “RINA”. The vessels Class Company can issue the RINA certificate if it is approved by RINA. The RINA certificate is renewed and surveyed together with the vessels IMO Certificate of Fitness. Japan has their own rules and certificates on gas carriers. Those rules cover for the most part measurements of vessels. That means there are other net and gross weights on the vessel in Japan than on the IMO load line certificate. Japanese authorities will survey all gas carriers that handle cargo in Japan. If the documentation on the vessel is found unsatisfactory by the authorities, they can require that the vessel must go in dock for new measurements. In Norway it is the Norwegian Maritime Directorate that makes all the local rules, those rules are applicable for all vessels in NOR or NIS. The Norwegian Maritime Directorate uses the Class Companies to issue certificates and to conduct surveys on those vessels. Then we have some local rules around the world that does not allow gas carriers to navigate after dark. Some ports have regulations concerning the wind speeds. The ship/ shore checklist must state if there are any restrictions on wind speed or the
height of waves. Information about the terminals is found in the “Guide to port entry” or from the agents, terminals or your own company.
11.1.6 Publications We required to always have onboard the latest edition of publications related to cargo, cargo handling, SPM, ship to ship transfer etc. The most important publications we must have onboard are: SOLAS 74/92 latest edition, MARPOL 73/78 latest edition, STCW 78/95 latest edition, the latest IMO gas code for the type of gas carrier we are on, ICS Tanker safety guide liquefied gas, and ICS Ship to ship transfer guide for liquefied gases. Vessels that are registered in either NOR or NIS must have the latest edition of the Norwegian Maritime Directorate rules. In addition we must have the guidelines and publications from SIGTTO, ICS, OCIMF and USCG. In the Exxon vessel inspection guide we can find references to all publications they require us to have onboard.
11.2
LOADING ROUTINES
Before we can commence loading we need a confirmed loading order from the owner. In the loading order we will find the quantity to be loaded, at which temperature we will receive the cargo, and what temperature we are to discharge the cargo. Further we find information about the load port and discharge port. It may also state what size there is on the terminal lines and flanges. The quantity to be loaded is given in metric tons. There are three different ways to state the quantity, either:
4000 mt +- 5% MOLOO 4000 mt +- 5% MOLCO 4000 mt
Then we can load from 3800 mt to 4200 mt on owners option Then we can load from 3800 mt to 4200 mt on charterer option Then we have to load 4000 mt
MOLOO means that it is the owner, represented by the captain that states the quantity to be loaded. MOLCO means that it is the charter that states the quantity. The cargo loading temperature is given either fully refrigerated, ambient or on a given temperature. Ambient means the temperature is equal to the air temperature if
the shore tank is located on the surface. It also states what temperature the cargo is to be discharged at. The maximum allowed filling limit when loading is 98% and it is the safety relief valve setting and the cargo temperature that give the filling limit. To find the filling limit we can either use the operation manual for the vessel or the cargo density table. When using the density table, we can calculate the temperature of the cargo from the absolute pressure, in this case 4 bar. When we have the basic information on the cargo, we must start planning the loading. We then calculate the filling limit in each cargo tank and then plan the loading rate. The loading rate is determined by three factors: cargo temperature, the ambient temperature, and whether we do or do not have vapour return. When we are loading and we need to run the cargo plant, there are various ways to run the plant. It depends on the temperatures and the flexibility of the plant. If we are set up to load two different cargoes e.g. ethylene and propane, then we must separate the cargoes from each other. We call that segregation of cargo tanks, cargo cooling plants and lines. Taking out small pipes on the cargo lines does segregation, those pipe parts are called “spool pieces”. The spool pieces are taken out of the lines and the main line is blanked of flange covers. We also have to segregate the cargo cooling plant e.g. two plants are used for ethylene and one is used for the propane. It is stated in the Certificate of Fitness how the lines and compressors can be segregated. If we have 4 cargo tanks there could be a possible segregation with cargo tank 1+2 and 3+4. When we are loading a partial cargo we must try to use the manifold that is linked directly to the tank that we are loading. If we are going to load on tanks 2,3 and 4, then we use the manifold for tanks 3+4. All spool pieces are marked according to the diagram. Many of the spool pieces have the same diameter but have a different length, so try to keep your spool pieces orderly. Normally the spool pieces are mounted on the cargo lines, so there should not be any problem keeping them organised. Before we can commence loading we have to cool down the tank shell as mush as possible, the optimal is less than 10oC above cargo loading temperature. The resulting temperature of the tank shell depends on how much time is used, the amount of cargo remaining on board, and the arrangement of the lines to the cargo tanks. We need thermometers on the outside of the tank shell in order to achieve the proper temperature. If we are going to load fully refrigerated propane, we must try to get the temperature on the outside on the cargo tank shell below –35oC before we start loading. When the cargo tank shell is chilled down before arrival, the time used for loading will be reduced. Before we arrive port the cargo tank shell must be chilled down, and cargo lines and spool pieces must be readied. Only then will we be able to reduce the time used in port.
When loading, the liquid is either pumped or pressured onboard from the shore tank. If we are loading by pressure, the vessel’s cargo tank pressure must be lower than the shore tanks pressure. This way of loading is mostly used on fully pressurised vessels. When we are loading by pumps we must follow the cargo tanks pressure to hold it below the safety valve set point. After completion of the loading, we need to free the loading hose/ arm of liquid by use of hot gas. The hot gas is produced by the vessel’s cargo compressor or from the shore tank or terminal’s compressor. On some terminals they use nitrogen to free the hose/ arm of liquid. When using this method we must keep a close watch so we do not get so much nitrogen into the cargo tanks. Before we commence loading cargo it must be issued a checklist, load planning, loading log, ship/ shore checklist and a time sheet. The vessel’s plans for loading have to be discussed and agreed to by the terminal loading master and safety officer. Read carefully the checklists and pay special attention to any notes about maximum pressure or minimum temperatures on the loading hose/ arm. All deviations from the planning have to be noted in the deck logbook and discussed with the loading master/ safety officer. It is very important that we are familiar with both the vessels and the terminals emergency routines, so that we all know what to do if there is a cargo leak or uncontrolled venting. When we transfer cargo from other vessels, we must follow our company quality manual and the ICS Ship to Ship Transfer Guide.
11.2.1 Loading without vapour return When loading without vapour return, only the liquid line is connected to the terminal. On some terminals we also have to connected a vapour line, but it is only for emergency use, and goes directly to flare. The cargo liquid is pumped or pressured to vessel’s cargo tanks through the liquid lines. To avoid high pressure in the vessel’s cargo tanks we need to pressure control the cargo in the tanks. To control the pressure we use the cargo cooling plant. During the entire loading process we must check the tank pressure, and we must do our utmost to avoid uncontrolled venting. Uncontrolled venting happen when the cargo tanks pressures rise to the set point of the safety relief valves and they open. The cargo tank vapour will then be led to the vessels vent mast. To avoid uncontrolled venting we have to reduce the loading rate or stop loading if we can not increase the cooling capacity.
11.2.2
Loading without vapour return but use of cargo cooling plant
When commencing loading, we always start with a slow rate to check that there is not any leakage on the terminal lines/ arms/ hoses or the vessel lines/ valves. When we are sure that there is no leakage’s and the cargo tank shell is close to the same temperature as the cargo, we can then increase the loading rate slowly to the agreed maximum rate. While we are increasing the loading rate we must watch the cargo tank pressure carefully.
In order to avoid having too much pressure in the cargo tanks when loading, we can either reduce the loading rate or stop loading, if the cargo cooling plant is on maximum capacity. If we can increase the cooling plant capacity, we have to do it before reducing the loading rate. It is important for the vessel to load at the rate that is stated in the charter party or is agreed to by the loading master. If we have to reduce the loading rate due to foul gas, we will then have to clarify it with the loading master and it must be noted in the deck logbook. When we reduce the loading rate the cargo temperature from shore will increase. On all types of gas carriers, it is important to check the cargo tank pressure all the time while loading. We have to do our outmost to avoid uncontrolled venting. 11.2.3
Loading with vapour return
The safest and fastest way to load is when we have vapour return, and that can be done on all types of gas carriers. When we are loading with vapour return the liquid hose/ arm is connected to the vessel’s liquid manifold and the vapour hose/ arm connected to the vessel’s vapour manifold. The cargo liquid is pumped or pressured onboard through the vessel’s liquid lines and to the cargo tanks that are to be loaded. The cargo tanks excess pressure is evacuated through the vessel’s vapour lines to shore.
Before we commence evacuating any vapour from the vessel we must be sure that the vapour is returned to the shore tank and not to flare. If the vapour is evacuated to flare, the vessel will be charged for the amount of vapour that is burned. We must be aware that if we evacuate more vapour than is agreed to, the terminal can develop problems with the shore tank pressure.
11.2.4
Loading with cargo cooling plant and vapour return
In addition to the vapour return, we use the vessel’s cargo cooling plant. If we have an indirect cargo cooling plant, we cool down the cargo tank shell or the vapour phase in the cargo tank. If we have a direct cargo cooling plant we condense the vapour in the cargo condenser and the condensate is pressured to the cargo tank.
While we are loading we must try to keep the cargo tank pressure as low as possible, in doing so the vapour return will help us great deal. When vapour return exists, we can increase or reduce the amount of vapour to shore by throttling the vapour manifold valve. How mush vapour we can send to shore must be agreed upon with the loading master before commencement of loading. It is the terminal’s capacity to receive vapour that determines the rate of vapour the vessel can send to shore while loading. Before we open the vapour manifold valve to send vapour to shore we must be sure that the vapour goes back to the shore tank and not to flare. If the vapour is sent to flare, the vessel will be charged for the amount that is burned in the flare. On fully pressurised gas carriers we must not send to shore so much vapour that the cargo is chilled down to less than –10oC. As an example, if we load propane and the cargo tank pressure is taken down to near 0 bar, the cargo temperature will be about -42oC.
After the loading is completed the terminal loading hose/ arm has to be freed of liquid. To evacuate the liquid from the loading hose/ arm we either use the vessel cargo compressors and blow hot vapour, or the terminal uses nitrogen and blows onboard. If the terminal is using nitrogen, we must minimise the amount of nitrogen to the cargo tanks. Try to blow the line into one cargo tank only. If we get to much nitrogen in the cargo tanks, we will develop too high condenser pressure and our cargo compressors may stop, and in the worst case we may have an uncontrolled venting. When loading fully refrigerated or semi-refrigerated gas carriers we must try to evacuate as much vapour to shore as possible in order to get the lowest possible cargo tank pressure. When loading with a high rate, the shore tank liquid level and the cargo temperature will be reduced and the vapour phase increased. When the liquid level and the cargo temperature is reduced, the terminal can take more vapour from the vessel. The maximum loading rate depends on the cargo temperature, temperature of the cargo tank shell before commence loading, cargo cooling plant capacity, size of the loading lines and the ambient temperature.
11.3
ROUTINES WHEN COOLING OR HEATING CARGO AT SEA
Cargo procedures while the vessel is at sea depend on what has been stated in the charter party. Some cargoes are to be discharged fully refrigerated, while others at a given temperature. In addition to the charter party, the vessels cargo handling equipment performance is important, especially the capacity of the cargo cooling plant, capacity of the cargo heater, insulation on the cargo tanks and the length of the sea voyage.
11.3.1 Cooling of the cargo while at sea On fully refrigerated gas carriers we do not have any choice, we must maintain a low pressure and temperature of the cargo. How to run the cargo cooling plants at sea depends on the plant itself, type of cargo, cargo tank insulation, the ambient temperature and the length of the sea voyage. The fewer cargo plants and hours we are running the plants, the lower fuel consumption do we have, and maintenance cost will be reduced. As long as the cargo plants are running, we have to watch that we don’t get vacuum in the cargo tanks. We must try to maintain a positive cargo tank pressure e.g. 0,01 bar or higher, this is to avoid air leaks into the cargo tank. Some cargoes like ethylene and butadiene, will be contaminated if the content of oxygen is to high. To avoid a vacuum in the cargo tank there may be a pressure switch on the suction side of the cargo compressor or on the cargo tank connected to the cargo compressor, that stops the compressor when there is to low suction pressure.
On semi-refrigerated gas carriers we have a few more options how we may handle cargo at sea, than with a fully refrigerated gas carrier. We may either maintain the cargo temperature or cool down the cargo, depending on the charter party. The number of cargo cooling plants we have to use depends on the cargo, the ambient temperature, capacity of the cooling plants, length of the sea voyage and the insulation of the cargo tanks. We must try to cool down the cargo to the discharge temperature as soon as possible and then maintain the temperature the rest of the voyage. We will then be able to discharge the cargo earlier if we are sent to another port of call. As long as the cargo is onboard it may be sold to another customer, so we must be prepared to discharge the cargo earlier than planned. We must always keep in mind that cooling down the cargo demands a lot of energy. Read the charter party/ loading order carefully and run the cargo cooling plants as economically as possible. Always check the weather forecast and air temperatures for your voyage. The seawater temperature has a major influence on your cooling capacity. Higher seawater temperature results in reduced cooling capacity. Pressure is defined as the movement of molecules, and when a vessel is pitching or rolling the molecules will move faster and the result is higher cargo tank pressure. Try to cool down the cargo as mush as possible before you enter into bad weather. When the cargo cooling plants is running we must fill out the cargo-cooling log. In the cargo-cooling log we must record the various pressures, temperatures and ampere for each of the cargo cooling plants. We must also record when we have changed condensate and to which tank we have pumped the condensate. This to avoid overfilling of cargo tanks and interrupt the cargo cooling process. On fully pressurised gas carriers we can only control the cargo tank pressure if there is cargo compressor onboard, if not, we have to vent the vapour to the atmosphere. The cargo tank steel of fully pressurised gas carriers are normally designed for minimum temperature of –10C°. That means we normally control the cargo temperature and pressure and do not lower it. 11.3.2 Heating of cargo on the voyage The charter party or loading order must state if we have to heat the cargo on the sea voyage before discharging. Depending of the vessel’s cargo equipment, there are normally three different ways to heat the cargo. We can pump the cargo through the cargo heater to another cargo tank. We may use the cargo compressor to blow hot vapour down into the liquid in the cargo tank. We can also use indirect cargo plant and pump e.g. ethanol in coils either outside of the cargo tank shell or in the vapour phase inside the tank. On fully refrigerated gas carriers there are no options for heating the cargo at sea. If the vessel is equipped with cargo heater we may heat the cargo when discharging.
On semi-pressurised gas carriers we may heat the cargo during the sea voyage if we have the cargo equipment for it. The most effective way to heat the cargo is to use the cargo pumps and the cargo heater. Then we pump the cargo from one tank through the cargo heater and then to another cargo tank. If we have submerged pumps this is normally not a problem. If we use deep well pumps, we must keep in mind of the shaft bearings and the shaft itself when the vessel is pitching or rolling. As we pump liquid from one cargo tank to another we must check the sounding on the cargo tank we are pumping to. Check the sounding in the other cargo tanks also, and record the cargo heating log temperatures and soundings on all tanks. When we check all the tanks while we are heating the cargo we can avoid overfilling any of the cargo tanks. 11.4
ROUTINES WHEN DISCHARGING
Before we may commence discharging any cargo, the captain must get a written permission from the owner to discharge the cargo. Certain information is needed before arriving at discharge port, such as the discharge temperature given in charter party, backpressure on the terminal, and size of the terminal lines and flanges. The captain can receive this information either from the agents or directly from the terminal. When all the information is received we must make a discharging plan. The discharging plan must contain which cargo tanks are to be discharged, and the sequence and rate of the discharging. If the cargo has to be heated and we need to use a booster pump, and agreement must be made with the terminal as to the minimum/ maximum cargo temperature and maximum backpressure. We must also prepare the correct reducer to fit the terminal flanges. The cargo pumps we plan to use have to be checked according to the manufacturer’s specification ref. operation manual. Check closely the oil and mechanical seals. The vessels discharge plan must be discussed with the terminal before commence discharging. When the discharge plan is agreed to, fill out the ship/ shore safety checklist. The discharge plan and all checklists must be signed and followed by the vessel and the terminal. All deviations from the discharge plan or checklist must be noted in the deck logbook and cleared with the terminal. There are different methods for discharging a gas carrier, and they depend on the cargo equipment onboard and the facilities at the terminal. There are two main methods used in discharging a gas carrier, either by pumps or by pressure. If shore backpressure is very high we must use a booster pump in addition to the ordinary cargo pumps or cargo tank pressure. When we use cargo pumps to discharge the cargo we must first check that the pump is free by turning the pump shaft by hand. When we can turn the shaft by hand we may then start the pump.
When the terminal and the vessel are ready to commence discharging, we start the process by using only one pump. The delivery valve on the pump is only slightly open as well as the one back to the cargo tank. When the line is chilled down we then close the valve down to the cargo tank and open the pressure valve a little bit more. Check that there is no leakage on the vessel’s lines and valves and as well as the terminal lines. When both the vessel and terminal lines are chilled down we can increase the discharge rate according to the discharge plan and on the terminal advice. Do not increase the discharge rate more than what the terminal has asked for, in the worst case the terminal will stop discharging and the vessel is charged for the delay. While discharging, it is always the terminal that sets the rate and required the cargo temperature. If the terminal asks the vessel to reduce the rate then we must comply. To reduce the rate we either throttle the pressure valve on a pump or stop a pump. If we throttle the pressure valve, we then generate pressure in that cargo tank. To avoid increased cargo tank pressure it is best to stop the pump. If the vessel refuses to reduce the rate the terminal can stop the discharging and then hold the vessel responsible for the delay or damage caused by the high discharge rate. If the shore tank pressure is to high the terminal must flare the excess pressure, or they can arrange a vapour return to the vessel. Reasons for the high shore tank pressure may be either a discharge rate that is high, to high temperature of the cargo, or the cargo compressors on shore may have to small capacity. If the vessel must use the cargo cooling plant while discharging, permission to do so must be stated in the charter party or agreed to by the receiver so that the vessel will be compensated for the extra fuel consumption. Always while discharging we must record in the deck logbook if there have been any deviations from the discharge plan. Onboard we must do the utmost to avoid any claims from the terminal.
11.4.1 Discharging by cargo tank over pressure When we must discharge the cargo tank by pressure, the tank that receives our cargo must have a lower pressure than our own. This way of discharging is the simplest, but when the liquid level decreases, the cargo tank pressure also decrease. That means we have to compensate for the lost pressure, and that is done either by hot gas from the vessel or shore, or by nitrogen from shore. This way of discharging can be done by semi and fully pressurised gas carriers.
11.4.2 Discharging with centrifugal pumps The most common pumps on gas carriers are centrifugal pumps, either of deep well or submerged type. Most gas carriers have one pump in each cargo tank and they are also normally equipped with one or more booster pumps on deck. When discharging fully refrigerated and semi-pressurised gas carriers we normally only use the pumps in the cargo tanks. If we have to discharge against high backpressure then we also need to use the booster pump either onboard or at the terminal. While we are running the cargo pumps we have to check that the pump is running the correct direction and the liquid level is reduced in the cargo tank. Record in the discharge log pump pressure and ampere and discharge volume every hour. On new cargo pumps there is an anti rotating device to protect the pump from running the wrong way. Most of the pumps also have a non-return valve on the pressure side to avoid leaks if the pump has or are stopped. Check the oil level and the mechanical seal when the pump is running. The booster pumps must be used if the backpressure is higher than maximum pressure for the pumps in the cargo tank. Booster pumps can be set up either in a series or parallel, depending on backpressure and the rate agreed upon. We must never increase the pump pressure higher than the pressure limit on the terminal discharging hose/ arm. When transferring cargo from one vessel to another we must follow the company quality manual and the ICS Ship to Ship Transfer Guide. While transferring cargo either ship to ship or ship to shore there must be a good communication between the two parties. All changes from the discharge plan or checklists must be reported in the deck logbook.
11.4.3
Discharging through the cargo heater
Sometimes we may have to discharge the cargo with a higher temperature than we have in our cargo tanks. In those cases we use our cargo heater while discharging. When using the cargo heater we must first check that the heating medium is circulating. Seawater or oil is used as a heating medium, however some terminals do not accept water as heating medium. When we have checked that the heating medium is circulating we may then pump cargo through the heater. Normally we can by-pass the heater with some cargo, but watch the temperature of the liquid manifold. Never pump cargo to shore with lower temperature than the minimum temperature given in the checklist. In the worst case the shoreline may be damaged. If we are using water as heating medium we must keep the seawater temperature from dropping below 5oC as it goes out of the heater. If the water is freezing, the cargo heater may be damaged and start leaking.
The cargo temperature on the vessel’s liquid manifold depends on the amount of cargo that is pumped through the cargo heater. The manifold temperature is adjusted by bypassing the cargo heater. Less cargo through the heater will result in lower temperatures and higher flow “discharge rate”. If the water temperature is 5oC or less we must not use the cargo heater unless we have facilities to heat the water before we pump it to the cargo heater. Some terminals do not allow the use of cargo heaters that utilise water as heating medium; they require oil as medium. If we must discharge at a terminal that does not accept our heater type, we then must try to heat the cargo at sea. 11.5
ROUTINES DURING CHANGE OF CARGO
All gas freeing operations requires a statement in the charter party or direct orders from the company operation. Gas freeing is very expensive whether we gas free a single tank or the whole vessel, and we must try to avoid those unnecessary costs. Normally we have to gas free all the cargo tanks when we are changing cargo and there must be visual inspection made of the cargo tanks. Other reasons for gas freeing cargo tanks are for example times when we have to repair anything in the tank or when we have to go in dry-dock. The tanks are gas free when they are free of flammable, toxic or neutral gases and the tank atmosphere is pure air. When we have to gas free cargo tanks, we must discharge as much liquid as possible so that we can reduce the time used to liquid free the tank. To gas free cargo tanks we use the vessel’s cargo compressors, inert gas generator and the cargo fan. When we have received a confirmation from the charterer or owner that we are to change cargo, we must set up a plan for the cargo change. The time we need to gas free the vessel depends on the cargo ROB and what type the next cargo is. Some products we may carry, such as propane and butane can be loaded on each vapour phase, we only need to be liquid free. Other products such as ethylene and butadiene require that we are gas free before gassing up with the cargo to be loaded. There is two temperatures we have to keep in mind when gas freeing. Those are the temperature of the cargo tanks when start the gas freeing process, and the expected air temperature when we commence blowing air. The reason that those two temperatures are important is that we have to heat the tank shell the same amount of degrees as the difference between the two temperatures. As an example if we start with –99oC in the cargo tank and the ambient temperature is 19oC, we must heat the cargo tank shell 118oC. It is always the heating process that takes the longest time when gas freeing.
11.5.1 Gas freeing The first we have to do when gas freeing is to liquid free the cargo tanks. To liquid free the tanks we can either blow hot gas down in the pump sump or pressurise the tank and empty the tank through the empty blow line. It depends on the type of gas carrier we are on and the pipes in the cargo tanks. When we blow hot gas, we blow it either through the condensate line, empty blow line or the pump. If we blow through the pump we must check that the pump does not start to rotate. When the hot gas is blown down in the pump sump the liquid will be boiled off. As long as we have liquid the cargo tank pressure will increase as we blow hot gas. When there is no more liquid left in the cargo tank, pressure will be stabilised and only the tank shell temperature will increase. We must try to have as high pressure in the tank as possible when we liquid free the tank. On fully refrigerated gas carriers we must watch the cargo tank pressure at all times to avoid an uncontrolled venting. Continue to blow hot gas down in the cargo tank until we have about 5oC above the seawater temperature. Read the temperature on both sides of the cargo tank shell. Keep in mind that the cargo tank shell can have a thickness of 20 mm or more. We then stop the cargo compressors when we have reached the planned cargo tank temperature. Then we have to vent off the cargo tanks pressure, either through the vapour manifold into the water or to the vessels vent mast. If the vessel is equipped with purge tanks or we have a gas recovery plant we can use the cargo compressors and condensate the overpressure to the purge tank. The purge tank is a small tank “pressure vessel” located either on deck or in a hold space. When we have reached atmospheric pressure in the cargo tank we can commence to inert or purge the cargo tanks with nitrogen. If we are using inert gas the oxygen content by volume has to be less than 5%, that is an IMO requirement. The inert gases have to be as dry and warm as possible before we send it to the cargo tank. If the vessel is equipped with a heater on the inert gas line we must use it to make use of the density difference between the inert gas and the cargo vapour. When we have difference’s in the density it is easier to achieve a good displacement purging. The differences in density tell us if we must blow the inert gas through the cargo tank vapour or liquid line. We must try to hold as low cargo tank pressure as possible while we are inerting to avoid turbulence in the cargo tank. Start the inerting with as low rate as possible, that way we will get the most effective purging. Displacements purging means the cargo atmosphere is pressed out by the inert gas. Just after we have commenced the inerting we have to measure the cargo tank atmosphere for HC vapour in the part of the cargo tank we have blown inn the inert gas. During the whole inert operation we have to measure the cargo tank atmosphere for HC vapour content until we reach the planned content less or equal to LEL or the limit stated in the company QA manual. While we are inerting, the oxygen content will not be higher than the oxygen content we have set on the inert gas generator e.g. 2% by volume. The HC content will be
reduced as long as we are inerting and we do not stop the inerting before we have reached the LEL for the actual cargo. As an example the LEL on propane is 2% by volume, so we have to inert until we reach 1,5% by volume. When we have less HC content than LEL on the actual cargo we can commence ventilating the cargo tank with air.
11.5.2
Example on displacement purging
In the IMO regulations we must use a safety factor of 2 as the margin for error on measurement and instruments. That means that when we draw flammability diagram, the line for critical mixture with 2% oxygen by volume gives 4% HC content by volume. We then have to inert until we read 2% HC by volume before we commence ventilating with air. It is important that we also inert all liquid lines, condensers and cargo compressors before we stop the inerting, to have a neutral atmosphere in them. Equipment we can use when venting cargo tanks with air are cargo compressors, cargo fan, inert gas blower, booster compressor or portable cargo fans. The kinds of equipment we use depend on the vessel’s cargo equipment. If the vessel is equipped with a vent heater we must use it to get as warm and dry air as possible. While we are venting with air we have to measure the oxygen content and also check that the HC content is reduced to 0% by volume. Before we stop the air ventilation we have to measure the cargo tank atmosphere for 0% by volume of CO - CO2 (Carbon monoxide - Carbon dioxide) and the oxygen content must be 21% by volume. CO Carbon monoxide CO is a very toxic gas and extreme caution should be taken prior to entering a tank that has been previously inerted and ventilated.
The HC content in the cargo tanks can not be higher then when we started venting with air, unless we have forgotten to purge any lines or other cargo equipment’s. The humidity and temperature of the air we use for venting gives us the necessary temperature of the cargo tank shell. If the cargo tank shell has a temperature that is much lower than the air, the air will condense on the steel and we will get water in the tank. Temp. 4oC 5oC 6oC 24oC 25oC 26oC
Water 6.30 g/m3 6.75 g/m3 7.22 g/m3 22.20 g/m3 23.50 g/m3 24.80 g/m3
For example at an ambient temperature on 25oC and 75% relative humidity and an average cargo tank shell temperature of 5oC, will produce 10,75 g/m3 of water condensation. Temp Water 4°C
6.30 g/m3
5°C
6.75 g/m3
6°C
7.22 g/m3
24°C
22.20 g/m3
25°C
23.50 g/m3
26°C
24.80 g/m3
At 25oC the maximum content of water is 23,5 g/m3 and 75% humidity gives us then 17,625 g/m3. At 5oC the maximum water content can be 6,75 g/m3 that give 17,625 g/m3 – 6,75 g/m3 = 10,87 g/m3 water. If we are venting with 10000 m3/h, 108,7 kg
of water will be released an hour. We then understand how important it is to heat the cargo tank shell to same temperature as the ambient temperature and that we have to use dry and heated inert gas and air. If we get water into the tank while venting we have to use either an ejector or rags to dry up the water. If we had heated the cargo tank shell to 30oC before venting with air at 25oC the relative humidity in the tank atmosphere will be approximately 57%. At 30oC the maximum content of water is 31 g/m3 and we had 17,625 g/m3 that gives us 17,625 g/m3 divided by 31 g/m3 = 56,8% and we do not develop any problems with water. While we have a visual inspection of cargo tanks we must use that time to sweep and clean up dust and check that there is not any foreign substances on the tank top. Check that all bolts and nuts on the pump and cargo tank lines are tight. Also check that the pump is in the correct position, if a pump is damaged it is very costly to repair and gas free the tank. 11.5.3 Gassing up cargo tanks To gas up a cargo tank means that we change the cargo tank atmosphere from air to either a neutral, toxic or cargo atmosphere. There are different methods and ways to gas up cargo tanks and they are dependent on the specifications from the charterer and what type of cargo to be loaded. If the vessel is set up to load a cargo that requires low dew-point and a low content of oxygen, we must then use nitrogen to purge out the humidity and oxygen from the cargo tank. If there isn’t any required limit to the humidity and oxygen content can be more than 1% by volume, we can then use inert gas to purge out the humidity and oxygen from the cargo tank. Before loading ammonia we can gas up the cargo tanks directly with ammonia vapour if the terminal and charterer agree on that, otherwise we have to use nitrogen. Normally we purge with nitrogen alongside a terminal or jetty, it can also be done at anchorage with a barge. Some new gas carriers have their own nitrogen plant onboard and can purge while they are at sea. Before the nitrogen is blown down into the cargo tank we must try to heat it as much as possible, up to 60oC or more. Nitrogen vapour expands when it is heated and the warmer we have the nitrogen, the lower the consumption and time used is. To heat nitrogen onboard we can use either the superheater feed by steam or an electrical heater. While we are purging with nitrogen we must have as low tank pressure as possible, less than 0,02 bar. When we start purging we must start with a low rate to avoid turbulence in the tank. Normal method is displacement purging and uses the different density to push out the old atmosphere from the cargo tank. Just after we have commenced purging we must measure the oxygen content in the part of the tank we blow in the nitrogen to see the reduced content of oxygen. Also check the oxygen content in the middle of the tank just after commence purging to be sure that it isn’t turbulence in the atmosphere.
The purging rate must be discussed and cleared with the loading master before we commence purging. Always start with a low rate and increase to maximum when we are sure that there is no turbulence in the atmosphere. We have to calculate for each purging what is the most economical, either the lay time or the use of nitrogen. There is a big differences in harbour fees and nitrogen prices around the world, so what was the cheapest in one port could be the most expensive in other ports. Before we have completed purging we have to purge through all lines and all cargo equipment to be sure that we do not have any air left in the cargo systems. When the surveyor has, according to the specifications given from the shipper or charterer approved the cargo tanks and cargo equipment, we can then commence purging with cargo vapour. Some gases as ethylene and butadiene require less than 0,1% oxygen by volume. For liquefied gases as methane, ethylene and ethane most shippers require a dew point of less than –45oC. The charter party always specifies the maximum content of oxygen and the maximum dew point. 11.5.4 Examples on parallel purging
Purging with cargo vapour is mainly done at a terminal or at anchorage, it depends on where we are in the world and what type of cargo we are purging with. The method that is most friendly to the environment is to conduct the operation in a place where it is possible to condensate the cargo and nitrogen vapour. The most common method is to load some cargo from shore and then do the operation out on the road. At some terminals we are allowed to do the operation alongside and send the cargo vapour to the terminal flare.
We must always use the difference in density while we are purging; the lightest vapour to be purged must go into the top of the cargo tank. Then we take the vapour to be purged out through the bottom liquid line in the cargo tank. 11.5.5 Example on nitrogen serial purging
Which method we should use, either serial or parallel depends on the experience and the lines onboard. On one vessel it can be easiest to purge parallel, another vessel get the best results from using serial purging. When we are gassing up we have to avoid opening any lines after the cargo tanks are completed gassed up. That means on most vessels we must gas up using the parallel method. The more flexible the vessel is built, the easier it is to purge and gas up the vessel.
11.6
Formulas to be used when changing atmosphere in an tank
There are some formulas that we can use to calculate the consumption of nitrogen or inert for changing atmosphere in tanks and the time to be used for the same purpose. The formula is a bit different, if we use inert versus nitrogen. 11.6.1
Using nitrogen
When we use nitrogen, the oxygen content is 0%. That means we should purge 21% oxygen from the air to a given maximum content of oxygen in the tank e.g. 0,2%.
11.6.2 Numbers of volume changed is ln (original O2 content/ desired O2 content) ln Original O2 content Desired O2 content Numbers of volume changed
Natural logarithm The original content of O2 in the tank that we should purge The specified O2 content given in the charter party The number of times the specified tank capacity needs to be completely purged of nitrogen
If we have one tank at 1000 m3 capacity and the O2 content, according to the charter party, should be less than 0,2%, we start with air in the tank. The calculation will be as follows: Number of volume changed = Number of volume changed =
ln ( 20,8% / 0,18%) ln ( 115,56) 4,75
The desired O2 content has been set to 0,18%, to be less than 0,2%. The nitrogen consumption will then be 1000m3 * 4,75 which equals 4750m3 nitrogen. This is the minimum required nitrogen. When ordering nitrogen add 10% to the minimum needed (5225m3) nitrogen. 11.6.3 Using inert When using inert we also use the formula with ln “natural logarithm” but we must calculate the O2 content in the inert gas also.
11.6.4 Numbers of volume changed is ln (original O2 content/ desired O2 content) ln Original O2 content Desired O2 content O2 content in the inert Numbers of volume changed
Natural logarithm The original content of O2 in the tank that we should purge The specified O2 content given in the charter party The O2 content that we set the inert gas generator to give, never above 5% The number of times the specified tank capacity needs to be completely purged of nitrogen
Take an example with the same tank at 1000m3. The charter party states maximum 2% O2 and the O2 content on the inert is set to 0,5%. We start with air in the tank. Number of volume changed = Number of volume changed =
ln ( 20,8% - 0,5% / 2% - 0,5%) ln ( 13,53) 2,61
We have to subtract the inert gas O2 content from the original and desired O2 content. The total consumption of inert will be 1000m3 * 2,61 = 2610m3. 11.6.5
Use with allowed vacuum
On vessels that have facility to have some vacuum on their tanks they can use their compressors to create the minimum allowed pressure in their tanks. If your vessel can have 30% vacuum in the tanks, it means a 0,7 bar absolute pressure. When you have 30% vacuum you have already quit 30% of the oxygen, which means you have 20,8/100*70 = 14,56% oxygen left. When we come alongside we pressurise the tanks with nitrogen to 1 bar absolute. Then we can continue the purge normally. To pressurise the tanks to 1 bar absolute we need 30% of the total capacity of our tanks. If we should purge one tank on 1000m3, we need 300m3 of nitrogen.
12-
CARGO CALCULATION
12 CARGO CALCULATION 12.1 CALCULATION OF MAXIMUM ALLOWED LIQUID VOLUME In this part, we will take a look at the different methods in calculating cargo onboard. The quantities of cargo we will load are specified in the charter party and this information is given directly from the charter or from the operation in the owner’s office. When we load and transport liquefied gases there are some variables that we have to have in mind, the setting of the safety valve’s “relief valve”, the cargo temperature when loading and at which temperature we should discharge the cargo. The type of gas carrier and the equipment we have onboard is also important in the flexibility of our transport. 12.4.1 Maximum filling limit Maximum filling limit is the maximum volume liquid we are allowed to load in the cargo tank. In chapter 15 of the IMO gas code, we find that the maximum filling can be 98% of full tank volume. Filling limit depends on the set point of relief valve and the density of the actual cargo. Formula for maximum volume liquid is as follows:
Filling limit = ρR / ρL * 98% ρR
Density of reference temperature on the relief valve setting
ρL
Density for actual cargo temperature
This means that if the relief valve setting is low, we can load more than if the setting is high. If there is a possibility to take off one or more of the pilot valves, we can increase the liquid volume loaded. We then have to calculate the difference between the pilot settings. The time used for loading will also increase if we have a lower set point on the cargo tank’s relief valves. What we always have to avoid is an uncontrolled venting. Uncontrolled venting is when we get such a high pressure in the cargo tank that the relief valve opens. If we look at some examples e.g. propane and the first example relief valve setting is 4,5 bar and the other example relief valve setting is 0,5 bar. Cargo temperature is – 35oC. Relief valve setting 4,5 4,5 bar + 1 bar = 5,5 bar ≅ 5oC bar Relief valve setting 0,5 0,5 bar + 1 bar = 1,5 bar ≅ bar 32oC Cargo temperature – 35oC
= kg/m3 = kg/m3 = kg/m3
523,3 570,2 573,7
In our example with 1000 m3 tank, we can see that the difference is about 45mt. With 4,5 bar setting we can load 513,259mt and with 0,5 bar setting we can load 558,900mt. If the freight rate is 80 USD/mt we then miss USD 3651. If we are on a gas carrier on 10 000 m3 the loss of income will then be USD 36 510. When we reduce the set point on cargo tank relief valves, the time used for loading and discharging will increase. What we have to avoid is letting the cargo tank fill 100% with liquid. On semi-refrigerated gas carriers, normally the lowest relief valve setting is 0,5 bar. There are two or more pilot valves e.g. 3,6 bar and 5,2 bar. If we change the relief valve setting, we have to mark that on the cargo tank and also note it in the decklog book. On fully refrigerated gas carriers the relief valve setting is about 0,25 bar and there are often facilities for putting one extra weight on the pilot, normally 0,2 to 0,3 bar. That means we have a relief valve setting of 0,45 bar. The extra setter is allowed to be used only while loading or gas freeing. In all cargo calculations in this compendium, we use T0 = 273oC and atmospheric pressure to 1,013 bar if nothing else is stated. In all calculations we have to use pressure in kilo Pascal (kPA) that gives 1,013 bar ⇒ 101,3 kPa. For the cargo calculations, we use densities from thermodynamic properties edited by Ocean Gas Transport. When the vessel is at sea and we get a telex that we are to set up to load propane at –30oC in Fawly. Our cargo tank relief valve set point is 4,5 bar. To find out how mush we can load, we then have to take a rough calculation. We can then use density for propane at –30oC and for 6oC, this only to get an overview of how mush we can load. At 6oC ρ is 522,0 kg/m3 and at –30oC ρ is 567,9 kg/m3 Then we get 98% * ρR/ρL ⇒ 98% * 522,0/567,9 = 90,07% To calculate the accurate filling limit, we have to know the actual cargo temperature and we must use density table. However, as long as we do not know the exact cargo temperature, we use the nearest values in the table. When we know the exact temperature of the cargo, we can calculate more accurately. Relief valve setting is 4,5 bar and atmospheric pressure 1,013 bar gives absolute pressure 5,513 bar. In the thermodynamic table we find: 5,45 bar ⇒ 5oC 5,61 bar ⇒ 6oC We have to interpolate between 5,45 bar and 5,61 bar to find the correct reference temperature and the correct density. The reference temperature is 5,39oC and reference density is 522,79 kg/m3.
Then we use -30oC and we find density to 567,9 kg/m3.
Filling limit = ρR / ρL * 98% = 522,79 kg/m3 / 567,9 kg/m3 x 98% = 90,22% In this example the filling limit will be 90,17% when we load propane at a temperature on -30oC. If the loading temperature is colder than -30oC the filling limit will be less than 90,17% and higher if the temperature is above -30oC. 12.4.2
Example 1
Cargo Propane Temp in oC
-30
Temp. reference rel. valve R 5,39 Tank #1, 100% Volume Relieve valve set point Atmospheric pressure Absolute pressure valve Filling limit = Filling limit
1182,18 4,5 1,013
relieve 5,513 rR 522,793
o
C density o C density m3 bar bar
567,9
kg/m3
522,79
kg/m3
bar / rL x 98 % / 567,900 x 98,00 % 90,22 %
When we have loaded propane on –30oC to the limit 90,17% we are then sure that if the pressure in the cargo tank increases to 4,5 bar and the temperature in the liquid increases to 5,39oC the liquid volume will be 98%. When we have calculated the filling limit we can find the maximum volume of liquid that we can load.
VL = 0,98 * V * ρR/ ρL VL
Volume liquid
V
100% Volume of the cargo tank
ρR
Density of reference temperature on the relief valve setting
ρL
Density for actual cargo temperature
When we have found the correct filling limit, we can find the maximum volume to be loaded. We have to find the cargo tank at 100% volume and multiply with the actual filling limit. If we have a cargo tank on 1182,18 m3 volume at 100%, we find the maximum volume to be loaded by multiplying with 90,17% filling limit.
Cargo tank 100% volume in m3
Filling limit in %
Volume to be loaded in m3
1182,18
90,17
1065,943
When we do this calculation we use the formula: VL = 0,98 x V x rR / rL. 12.4.3
Example 2
Filling limit
522,793
Filling volume = Filling limit * Cargo tank 100% vol. Filling volume 90,22 % Or Filling Volume VL=
/ 567,900 x 1182,18
98 % 90,216 %
m3 =
0,98 x 1182,18 x 522,793 / 567,9 =
1065,972 m3 1066,517 m3
After we have found the filling volume, we find the ullage or sounding in the vessels ullage/sounding table.
Ullage Sounding
Sounding is the level from tank bottom to the liquid surface. Ullage is the level from liquid surface to deck level. In the following examples, we use sounding.
In this example, we find the correct sounding to be 8,1662 meters. We have to do this calculation on each cargo tank before we start loading. In this example, the filling volume is 1065,943 m3 and that is in between 8,16 meters and 8,17 meters, so we have to interpolate to find the correct sounding. 12.4.4
Example 3
Filling volume = Filling limit * Cargo tank 100% vol. Filling limit 90,17 % 1182,180 m3 = 1065,943 m3 sounding in m 8,16 8,17 8,1662
volume in m3 1065,25 m3 1066,36 m3 1065,94 m3
When we have found the correct sounding/ullage we have to find which corrections we must use to get the actual sounding/ullage. The corrections can be found in the sounding/ullage table for each vessel.
12.4.5 Corrections There are normally four corrections to be used: the correction on the float, correction on the sounding tape, list and trim correction. The float correction depends on the liquid density; with a higher density the float becomes lighter in the liquid. The tape correction depends on the temperature in the vapour phase. List and trim correction depend on how the vessel is in the water. We have to study the corrections carefully so we use the correct sign character. Spherical floats have the highest corrections on float. All corrections we do, we find in the sounding/ullage table for each cargo tank. On the next page, we found an example of a spherical float. Example of a spherical float
The table for float correction is calculated against different densities and when we have a cargo with density in between the table values, we have to interpolate to find the correct correction. Out of the table above we can see that lighter liquid will give a higher correction. The Float correction table Specific gravity (kg/dm3) Corrections in meter
If we have cargo density 0,55 kg/dm3, we have to interpolate between 0,50 and 0,60 and the correction will then be 0,160 meter. Small floats will give the lowest corrections. A tank equipped with spherical float will have higher corrections than tanks equipped with a flat float. A correction on the sounding tape depends on the temperature in the vapour phase in the tank. High temperature and a small vapour volume give a small correction, low temperature and big vapour volume gives a higher correction. Correction on trim is either a correction to be added or multiplied to the measured sounding/ullage or the volume table is calculated with the trim directly. The trim correction is higher on long tanks than on short tanks. This means that small transverse tanks have a trim correction near to zero and long tank has higher corrections. Correction on list is either correction to be added or multiplied to the measured sounding/ullage or the volume table is calculated with the list directly. The list corrections are highest on wide transverse tanks and small on narrow longitudinal tanks. How we should use the corrections are explained in each sounding/ullage table. Earlier in this chapter, we found the corrected sounding to be 8,1662 meter. We will now continue using this example to find the sounding that we will read on the sounding tape. Normally the corrections are used directly on the sounding measurement, but when we calculate the other way we have to use the correction’s signs the opposite way. 12.4.6
Example 4
Corrected sounding Trim correction from table List correction from table Sounding w. 20oC Correction for vapour temperature Float correction from table Read sounding
8,1662 -0,021 0 8,1872 -0,001 0,1564 8,0318
To find the correct corrections we have to know the density of the cargo, in this case, propane at –30oC and density 567,9 kg/m3 = 0,5679kg/dm3, aft trim on 0,5 meter zero list and –15oC in the vapour phase. When we are completely loaded on this tank, we will have a sounding of 8,0318 meter. The 98% maximum filling is to prevent liquid getting in the relief valve, if the tank pressure reaches the relief valve setting. On vessels with relief valve setting of 0,5 bar we do not have any possibilities to heat the cargo at sea. On semi-refrigerated or fully pressurised vessels, we have opportunity to heat the cargo while the vessel is at sea. When we are heating the cargo, we have to follow the tank pressure carefully to avoid uncontrolled venting.
Vessels with a low relief valve setting can have a higher filling limit than vessels with a high relieve valve setting. The sketch below shows how the filling limit changes with the cargo temperature, as long as the relief valve’s set point is the same.
12.1 12.1 CALCULATION OF CARGO WITH USE OF ASTM-IP TABLES In this chapter we will look at the tables and corrections we use when calculating weight of cargo onboard gas carriers. We then start to look at how we calculate weight in air at 15oC by using the correct tables. The tables we are using are the ASTM-IP-API tables for light hydrocarbons. Density is mass divided by volume. The mass has either kilo (kg) or metric ton (mt) as unit. Volume has either cubic meter (m3) or litre (lt) as unit. Unit for density is either kg/m3 ⇒ tonn/m3 or kg/dm3 ⇒ kg/lt. Density and specific gravity is often given in vacuum, then we need tables or calculations to convert to weight in air at 15oC. 12.4.7 Liquid calculation We start calculation of the liquid in air and then we look at the vapour calculation. For LPG cargoes and some chemical cargoes it is normally accepted to calculate the weight in air at 15oC, as we do in the crude oil trade. We then get either specific gravity 60/60oF or density at 15oC from shore and we have to use the ASTM-IP-API tables. In table ASTM-IP no. 21, we find density at 15oC when the gravity 60/60oF is given. In table ASTM-IP no. 54, we find the reduction factor to the actual cargo temperature compared with density at 15oC. In table ASTM-IP no 56, we find the factor to be used to find weight in vacuum from weight in air. If we take an example with propane, liquid temperature is -25oC and specific gravity 0,5075, we will calculate the weight in air at 15oC.
We then start with table ASTM-IP-API no. 21 to find density at 15oC from specific gravity 60/60oF 0,5075. We look in the column for Specific gravity 60/60oF 0,5073 kg/lt 0,507 and find density at 15oC to We then look in the column for specific gravity 0,5083 kg/lt 60/60oF 0,508 an find density at 15oC to The density has now increased with
0,0010 kg/lt
Our Specific gravity is 0,5075, we then have to 0,5078 kg/lt interpolate as follows 0,5073 + (0,0010 / 0,001 x 0,0005) that give
12.4.8
Example on table ASTM IP-API 21
Specific
API
Gravity
Gravity
60/60oF
60oF
Density 15oC
0,506
-
0,5063
0,507
-
0,5073
0,508
-
0,5083
Specific gravity 60/60oF
0,5075 that gives
0,5078 kg/lt
We have now find the density at 15oC to 0,5078 kg/lt which is equal to 507,8 kg/m3, which we use in table ASTM IP no.54 to find the reduction factor to –25oC. In table ASTM IP no.54, we look in the column for actual liquid temperature –25oC. The table is divided in three columns and we have to interpolate between the 0,505 and 0,510 columns.
12.4.9
Example from table 54
Table 54C Observed
Density 15 oC 0,500
0,505
0,510
temperature, o
C
Factor to reduce volume to 15 oC
When we do the interpolation, we find the reduction factor to 1,10432. When we have different temperatures on the different cargo tanks, we have to do this calculation on each tank. Below, we have an example on table ASTM IP no. 54
12.4.10
Example on table ASTM IP-API 54
Table 54C Observed
Density 15 oC
temperature, 0,500 o
C
0,505
0,510
Factor to reduce volume to 15 oC
-26
1,111
3
1,108
3
1,105
-25,5
1,109
2
1,107
3
1,104
-25
1,108
2
1,106
3
1,103
-24,5
1,107
2
1,105
3
1,102
-24
1,106
3
1,103
2
1,101
-25
0,5078
1,10432
The next correction is the shrinkage factor, which is a thermal factor on the tank steel. Shrinkage factor is normally 1 at 20oC and is less than one when the steel is colder than 20oC. The shrinkage factor is the correction for the thermal expansion on the cargo tank steel. It is the correction between 20oC and the actual steel temperature. With different steel, we have different shrinkage factors, but on one vessel the shrinkage factor is similar on all cargo tanks if they are made of equal steel. Aluminium and invar steel have a shrinkage factor near 0 and mild steel has higher factor. Shrink factor for a vessel depends on the material of the cargo tank. There is a shrinkage table on each vessel. Only vessels with equal quality of steel and tank thickness have equal shrinkage factors. When we calculate cargo, we use shrinkage factor both on the liquid and the vapour. 12.4.11
Example on shrinkage factor at different temperatures Temp. Sh.fact.
Temp. Sh.fact.
Temp. Sh.fact.
20
1
-21
0,99879
-62
0,99759
19
0,99997
-22
0,99876
-63
0,99756
18
0,99994
-23
0,99873
-64
0,99753
17
0,99991
-24
0,99870
-65
0,99750
16
0,99988
-25
0,99868
-66
0,99747
The last table ASTM-IP no 56 is used to find mass of liquid and vapour in air from mass in vacuum or vice versa. We have to use the liquid density at 15oC, which in this example is 0,5078 kg/ltr, and find the factor for propane to 0,99775. We have to multiply this factor with the mass in vacuum to get mass in air. If we have the mass in air we must divide with the factor. When the cargo calculations are completed, on the bill of lading and the other cargo papers we have to note if the loaded mass is in vacuum or air. We must always use liquid density at 15oC on the actual cargo to find the correct factor. 12.4.12
Example on table ASTM IP-API 56 Table 56 Density at 15oC kg/ltr 0,5000 to 0,5192 to 0,5422 to 0,5674 to Factor is
0,5191 0,5421 0,5673 0,5950
Factor for mass in vacuum to mass in air 0,99775 0,99785 0,99795 0,99805 0,99775
We can look at one example where we have loaded 1089,556m3 propane with specific gravity 60/60F 0,5075 and liquid temperature is –25oC. From table ASTM-IP-API no. 21 we find the cargo density at 15oC to 0,5078 kg/ltr. ⇒ 507,8 kg/m3 From table ASTM IP no. 54, we find reduction factor from 15oC to –25oC to 1,10432. From table ASTM IP no. 56, we find factor from mass in vacuum to mass in air to 0,99775. From the cargo tank shrinkage table, we find shrinkage factor to 0,99868 at –25oC. The calculation gives us 610 994 kg in vacuum at 15oC that gives us 609 619 kg in air. We have to note on all cargo documents that the mass is in air and also note the specific gravity 60/60F.
12.4.13
Calculation of the liquid’s mass Volume loaded 1089,556 m3 Shrinkage factor for -25oC 0,99868 Corrected volume at -25oC 1088,118 m3 Reduction factor to 15oC
1,10432
Volume at 15oC
1201,630 m3
Density at 15oC
507,8
kg/m3
Mass in vacuum at 15oC
610188
kg
Factor from table 56
0,99775
Mass in air at 15oC
608815
kg
12.4.14 Calculation of vapour We will now calculate mass of the vapour in air at 15oC. We always have to calculate the density of the vapour as the density change with the pressure. When we are calculating the mass in air on the vapour we need the following values, 288 K which is equal to 15oC, 101,325 kPa which is equal to 1,013 bar. Molar volume of ideal gas at 288 K is 23,6382 m3/kmol. We also need molar weight of the actual cargo and for propane it is 44,1 kg/kmol. Then we use the actual cargo temperature and pressure. We can take an example with Propane with vapour temperature at –18oC and cargo tank pressure at 1,5 bar. • • • • • •
· Ts is standard temperature 288 K · Tv is average temperature on vapour in K · Pv is absolute pressure of vapour in kPa · Ps is standard pressure 101.325 kPa ⇒ 1,013 bar · Mm is molecular mass of the product in kg/kmol · I is molar gas volume at 288 K and standard pressure 1,013bar ⇒ 23,6382 m3/kmol ρv = (Ts x Pv x Mm) / (Tv x Ps x I) kg/m3
When we insert the values in the formula we find the following vapour density. • · Ts = 288 K • · Tv = 273 + (-18) = 255 K • · Pv = (Ps + PT ) x 100 = (1,013 + 1,5) x 100 = 251.3 kPa • · Ps = 101.3 kPa • · Mm = 44,1 kg/kmol for propane • · I = 23,6382 m3/kmol
12.4.15
Density calculation of vapour Ts 288 288 K Ps
1,013
101,3
kPa
Pv
1,013
1,5 251,3
kPa
Mm
44,1
44,1
I
23,6382
23,6382 m3/kmol
288 rv =
x
255
kg/kmol
251,3 101,3
x
44,1 5,227 kg/m3
23,6382 =
When we have calculated the vapour density, we have to calculate the mass of the vapour. We continue with the calculation of propane loading. The cargo tank 100% volume is 1182,18m3 and we have loaded 1089,556m3 liquid. The vapour volume is then 100% cargo tank volume minus liquid volume. That gives us 1182,18m3 – 1089,556m3 = 92,624m3. We have a vapour temperature on –18oC, which gives us a shrinkage factor (cargo tank expansion factor) on 0,99888 taken from the vessel’s shrinkage table. The vapour density is in kg/m3 and the mass will then be in kilos. When we calculate the mass of liquid in kilos, we also calculate the mass of vapour in kilos. If we use mass of liquid in metric ton, we have to calculate the vapour in metric ton also. In this example, the vapour density is 5,227 kg/m3, which is equal to 0,005227 mt/m3. In this example, the mass of vapour is 484 kilos. 12.4.16
Calculation of vapour mass at 15oC in kilo Cargo tank 100% volume
1182,180 m3
Liquid volume
1089,556 m3
Gas volume
92,624
Shrinkage for - 18oC
0,99888
Corrected Gas volume
92,520
m3
Density of gas at 15oC
5,227
kg/m3
Mass of gas in vacuum at 15oC 484
m3
kg
To find the total mass of liquid and vapour in the cargo tank, we have to add mass of liquid 610 994 kg + 484 kg = 611 478 kg. Then we use ASTM-IP table 56 and find the conversion factor to mass in air. Cargo density at 15oC is 507,8 kg/m3 with a factor of 0,99775. Then we multiply total mass in vacuum 611 478 kg with 0,99775 which gives us 610 102 kg in air. 12.4.17
Calculation of total mass in air at 15oC Mass of liquid in vacuum at 15oC
610 994 kg
Mass of gas in vacuum at 15oC
484 kg
Total mass in vacuum at 15oC
611 478 kg
Factor from ASTM-IP 56 table
0,99775
Total mass in air at 15oC
610 102 kg
We will take an example on a full calculation and find the total mass in air, the cargo is propane and we have the following information: Molecular mass
44,1
kg/kmol
Liquid temperature
-24
o
Vapour temperature
-20
o
Atmospheric pressure
1,017
bar
Relief valve setting
4,5
bar
Cargo tank pressure
1,550
bar
Spes.Grav.60/60F
0,5072
Liquid density at -24oC
560,6
kg/m3
Density at Relief valve setting
522,756
kg/m3
Trim by stern
1
meter
Sounding
8,152
meter
100 % Volume of cargo tank
1468,180 m3
ROB before loading
3114
C C
kg
With a set point on the relief valve at 4,5 bar we can load maximum 91,38% with liquid temperature –24oC. Maximum filling volume is, as follows: Maximum filling volume = 0,98 x VT x ρR / ρL Maximum filling volume = 0,98 x 1468,18 x 522,7 / 560,6 = 1341,69 m3 We always have to start with the calculation of maximum filling volume. This calculation is based on figures we got before we start loading. If the temperature and pressure changes, while we are loading, we have to recalculate the maximum filling volume. Warmer cargo gives a higher filling volume; colder cargo gives a lower filling volume. When the loading is completed, we do the final calculation. We have to find the maximum filling limit on all tanks. Example on a full calculation on mass at 15oC
12.4.18
PROPAN
Tank #
2 1468,180 m3
1
100 %Volume cargo tank
2
Liquid temperature
-24,0 oC
3
Sounding
8,152 m
4
Float correction
0,158 m
5
Correction for vapour temperature
6
List correction
0,000 m
7
Trim correction
-0,059 m
8
Sounding at 20oC
9
Liquid volume at 20oC
10
Shrinkage factor tank steel at –24oC
11
Corrected liquid volume
12
Reductions factor from table 54C
13
Liquid volume at 15oC
14
Liquid density at 15oC table 21
-0,001 m
8,250 m 1341,373 m3 0,99871 1339,643 m3 1,102 1476,287 m3 507,5 kg/m3
15
Mass of liquid in vacuum at 15oC
749 215 kg
16
Uncorrected vapour volume
126,807 m3
17
Shrinkage factor vapour phase –20oC
0,99882
18
Corrected vapour volume
126,657 m3
19
Tank pressure
1,550 bar
20
Atmospheric pressure
1,017 bar
21
Molecular mass Propane
22
Vapour temperature
-20,0 oC
23
Vapour density at 15oC
5,382 kg/m3
24
Mass of vapour in vacuum at 15oC
25
Total mass of cargo in the tank in vacuum
749 897 kg
Mass in air 749 897 kg x 0,99775
748 210 kg
ROB in air Total loaded in air at 15oC
44,1 kg/mol
682 kg
3 114 kg 745 096 kg
After we complete the cargo calculation, we have a ships figure which is the one the chief officer must calculate and one shore figures, which is the one that the surveyor has calculated. Those two figures will be nearly equal or equal. The one we use in the Bill of Lading is the surveyor’s figure. In our example, we have loaded 745 096 kg in air at 15oC in the actual cargo tank. It must be specified on the Bill of Lading that the mass is in air at 15oC. When we discharge the cargo, we will have 311,4 kg vapour left in the tank. At a minimum, we are allowed to discharge is 99,5% of Bill of Lading, in this example 741 370 kg. It is important for the vessel to calculate which temperature and cargo tank pressures will remain when we finish discharging. In this example, we must have maximum 0,16 bar pressure and vapour temperature –27oC. When we load on an atmosphere from a previous cargo, we call that ROB (Remaining on Board) or heel. That means when we have calculated the total mass of cargo in a tank we have to subtract the ROB. When the discharging is completed, that means we are finished pumping liquid. We have blown hot vapour to shore and tank pressure, and vapour temperature is equal to what we estimated before loading.
It is important to remember that the tank pressure has a big influence on the vapour density. If we transport an ambient cargo, we have to remove the tank pressure before we commence the calculation of the ROB. Tank pressure is removed with the vessel’s compressors and the condensate is sent directly to the discharge line. We can look at two examples on density calculation of a cargo with equal temperature but different tank pressures. We use propylene as example and vapour temperature is –25oC molecular mass 42,08 kg/kmol. The first example tank pressure is 0,3 bar and the other example tank pressure 1,5 bar. The atmospheric pressure is 1,020 bar vessels total volume is 12000m3. 12.4.19 pressures r v 0,3bar =
Example of calculations on vapour density with different tank
288
x
243 Volume
r v 1,5bar =
12000
288 243
Volume
12000
Difference in mass
134
x
101,3
42,08 23,6382
=
x 2,791 kg/m3
33 490
kg
x
42,08
252
x
101,3 x 5,249 kg/m3 29 492
kg
23,6382
=
62 982
kg
Difference in r
2,791 kg/m3
5,249 kg/m3
2,458 kg/m3
With a difference of tank pressure at 1,3 bar on a 12 000 m3 vessel, we get 29 492 kg in mass difference. It is a good routine to always calculate the maximum mass of vapour, which we can have as ROB to reach 99,5% of Bill of Lading before we start discharging. If we are onboard a fully refrigerated gas carrier, we do not have any problem with high tank pressure when we have completed discharging.
12.3 CALCULATION OF CARGO WEIGHT USING DENSITY TABLES When transport of chemical gases and also sometimes LPG cargoes, we use density tables for the actual cargo. We get the density tables from the surveyor, the shipper of the cargo or thermodynamic properties of gases. The weight of cargo is calculated by use of the actual cargo temperature and the density tables are either in vacuum or in air. On clean cargoes, such as, ethylene, propylene, butadiene and VCM, we can use the density tables composed by SGS or thermodynamic properties of gases. We have to be sure that the density tables we are using are either in vacuum or in air and it has to be noted on the Bill of Lading. The density table we are using in the load port has to be used also in the discharge port. The only ASTM table we are using is ASTM-IP table no.56 for converting weight in air to weight in vacuum or vice versa. When the calculation is completed, we have to note that the weight is in vacuum or in air. We always have to calculate the vapour density because the vapour temperature does not match the cargo tank pressure. We should use the actual vapour temperature and actual tank pressure in the calculation of vapour density. First we take a look at how we are calculating the weight of liquid. First of all we have to find out the maximum filling volume on the actual cargo tanks that we have to load.
Maximum filling volume is as follows: Maximum filling volume = 0,98 x VT x ρR / ρL The cargo tank 100% volume is 1182,18 m3, safety valve set point is 4,5 bar ⇒ 5,5 bar ata, liquid temperature is –24oC. Liquid density at 5,5 bar is 523,3 kg/m3 and density at –24oC is 560,6 kg/m3. 98% Vt m3 r SV kg/m3 r c kg/m3 Maximum filling volume = 0.98 1182.18 523.3 560.6 3 Maximum filling volume = 1081.452 m We should calculate the weight of liquid propane, cargo tank pressure is 1,1 bar. We have loaded 1089,556m3 liquid propane, density from density table and –24oC is 560,6 kg/m3. Cargo tank expansion factor at -24oC is 0,99870. Weight in vacuum will then be 605 477 kg, weight in air 604 115 kg.
12.3.1
Example on calculation of weight in air
Loaded volume
1081.452 m3
Correction factor for -24oC
0.99870
Corrected volume
1080.052 m3
Density at -24oC from table
560.6
kg/m3
Weight in vacuum at -24oC
605 477
kg
Factor from table 56
0.99775
Weight in air at -24oC
604 115
kg
12.3.2 Calculation of vapour density and weight To calculate the weight of vapour, we first have to calculate density of the vapour on the actual temperature. The actual vapour temperature has to be in K (Kelvin) and pressures in kPa (kilo Pascal). Another factor we should use is molar gas constant which is 8,31441 J/(mol x K). To find the pressure in kPa “kilo Pascal” we multiply the pressure in bar with 100, that means 1 bar is equal to 100 kPa. In all calculations in this manual, we use 273K as 0oC. When we do the calculations onboard we use 273,15K as 0oC. We should now look at one example to find vapour density on propane with vapour temperature on –25oC, tank pressure is 1,4 bar and the atmospheric pressure is 1.013 bar. Molecular mass on propane is 44,1 kg/kmol. Vapour density at actual temperature formula: (Tank pressure in kPa + Atmospheric pressure in kPa) x Molecular mass molar gas constant x (T0 K + Gas temperature in oC) Tank pressure 1,4 bar is equal to 140 kPa and the atmospheric pressure 1,013 bar is equal to 101,3 kPa. Vapour temperature ∆T in K = 273 + - 25 = 248K Tank pressure plus atmospheric pressure ∆P is equal to 241,3 kPa. D P x Molecular mass Molar gas const. x D T ( 140,0 + 101,30) x 44,1 8,31441 x ( 273,00 + -25,00)
5,16075
kg/m3
We can take another example with ethylene and calculate the vapour density, molecular mass is 28,05 kg/kmol, vapour temperature is –99oC ⇒ 174K and tank pressure is 0,35 bar ⇒ 35 kPa. Atmospheric pressure is 1,012 bar ⇒ 101,2 kPa. D P x Molecular mass Molar gas const. x D T
( 35,0 + 101,20) x 28,05 8,31441 x ( 273,15 -99,00) +
2,638 kg/m3
Now when we have found the vapour density at the actual vapour temperature, we can calculate the weight of vapour at the actual temperature. We have loaded one tank with ethylene, tank 100% volume is 1182,15 m3 and liquid volume is 1088,6 m3. Liquid temperature is –100oC and vapour temperature is –99,5oC shrinkage factor at –99,5oC are 0,99648. Tank pressure is 0,15 bar and the atmospheric pressure is 1.014 bar. Vapour volume Vapour density Vapour weight 99,5oC
(1182.15 1088.60) m3 2.263367 kg/m3 = (D P x 28,05) / (8,31441 x D T) – 210.99 kg = 103,55 x 0,99648 x 2,26337 93.55
at
m3 =
We have now seen how to calculate weight of liquid and weight of vapour and we should now calculate both liquid and vapour. We should calculate one tank loaded with ethylene, relief valve set point is 4,5 bar and atmospheric pressure is 1,020 bar. After loading the vessel we have 1 meter by stern trim with the following values: Vapour Liquid
-95oC and tank pressure 0,345 bar. -100,5oC, density 563,63 kg/m3, liquid volume 1313,348 m3
Maximum filling limit is 89,45%, which is equal to 1313,348 m3 with relief valve setting on 4,5 bar. Total weight of cargo in the tank is 738 009 kg in vacuum. 12.3.3 Calculation of Ethylene set point 4,5 bar Liquid volume 1313.35 m3 Shrinkage factor at -100,5oC
0.99645
Corrected liquid volume
1308.69
m3
Liquid density at -100,5oC
563.625
kg/m3
Weight of liquid in vacuum at -100,5oC 737610
kg
Cargo tank 100% volume Vapour volume Shrinkage factor at -95oC Corrected vapour volume
m3 m3
1468.18 154.832 0.99661 154.308
m3
Vapour density at -95oC 2.583 o Weight of vapour in vacuum at -95 C 399
kg/m3 kg
TOTAL LOADED IN VACUUM
kg
738009
When we change the relief valve set point to 0,5 bar the maximum allowable filling limit then increase to 97,0% that is equal to 1424,127m3. We then get a total weight of cargo in the tank on 799 940 kg, which is 61 931 kg more than with set point on 4,5 bar. First, we have to calculate maximum allowed filling limit.
Set point is
0.5 bar
Filling limit rR/rL x 98%
Absolute pres. Ref. temp. Ref. dens.
1.520 bar -96.53 oC 557.87 kg/m3
97.00 %
12.3.4 Calculation of Ethylene set point 0,5 bar Liquid volume 1 424,13 m3 Shrinkage factor at -100,5oC
0,99645
Corrected liquid volume
1 419,08
m3
Liquid density from table at -100,5oC
563,625
kg/m3
Weight of liquid in vacuum at -100,5oC
799 827
kg
Cargo tank 100% volume
1 468,18
m3
Vapour volume Shrinkage factor at -95oC Corrected vapour volume Vapour density at -95oC
44,053 0,99661 43,904 2,581
m3 m3 kg/m3
Weight of vapour in vacuum at -95oC
113,326
kg
TOTAL LOADED IN VACUUM
799 940
kg
When we are loading on ROB from previous cargo, the total loaded cargo is total weight of liquid and vapour in the tank minus ROB. If we, in this example, had ROB before loading and we surveyed the tank atmosphere at –87oC and tank pressure 0,02 bar, atmospheric pressure 1,019 bar, the ROB will then be 2758 kg in vacuum. Total loaded will then be 799 940 kg – 2 758 kg = 797 182 kg in vacuum.
12.3.4 Weight of ROB before loading at temperature -87oC and tank pressure 0,02 bar Vapour density 1,885 kg/m3 Tank volume 100% 1468,18 m3 Weight of vapour 2 758 kg Shrinkage factor 0,99685 To find the weight in air we can either density at 15oC or we have to calculate a
D T = 186 D P = 104
K kPa
use table ASTM-IP-API 56 if we know the factor. The factor is, as follows:
(1 – (ρ air/ ρ cargo liquid)) / (1 – ( ρ air/ ρ Brass) (1 – (1,2 kg/m3 /ρ cargo liquid)) / ( 1 – (1,2 kg/m3 / 8100kg/m3))
In our example we will get a factor, as follows: (1 - (1,2 kg/m3 /563,625 kg/m3)) / ( 1 – (1,2 kg/m3 / 8100kg/m3)) = 0,997985 Then we have to multiply mass in vacuum with the factor: 797 182 kg x 0,997985 = 795 575 kg in air On a full-loaded tank, we can use the following formula: Mass in vacuum loaded - (Mass in vacuum loaded x r air / r liquid) When we use the values from our last example it will be, as follows Weight in air = Mass in vacuum - (Mass in vacuum x 1,2/ r liquid) Weight in air = 797 182 - (797 182 * 1,2/ 563,625) = 795 kg 485 Before we commence with cargo calculations, we have to be sure that the density given is in air or in vacuum. With most chemical gases, we get the density on the actual liquid temperature in vacuum. Always note on the Bill of Lading that the quantity is either in vacuum or air. On the calculation forms, we calculate both in vacuum and in air. We should now do a full cargo calculation. We start to calculate ROB before loading. Then we do calculations after we have completed loading. The vessel has three twin tanks numbered as follows 1P, 1S, 2P, 2S, 3P and 3S. Cargo tanks 2 and 3 are equal and tank 1 is a bit smaller.
12.3.5
Calculation of ROB before loading
Loading report Cargo Propylene Molecular 42,08 mass Atm.press. 1,015
Port Date
Al Jubail 17.05. 1994
Vessel LPG Seagull
Liquid Tank # Sounding Volume Temp. Pressure r liquid Shrinkage Mass of in meter from in in oC in bar factor liquid in 3 tab. in kg/m kg m3 1P 0,02 0 1S 0,02 0 2P 0,02 0 2S 0,02 0 3P 0,02 0 3S 0,02 0 Total mass of 0 Liquid Vapour Tank #
1P 1S 2P 2S 3P 3S
!00% vol. in m3
Vapour volume in m3
Temp. in oC
1182,18 1182,18 1468,18 1468,18 1468,18 1468,18
1182,18 1182,18 1468,18 1468,18 1468,18 1468,18
-27 -27 -27 -27 -25 -25
r Shrinkage Mass of vapour factor vapour in in kg 3 kg/m 2,129 0,99862 2 514 2,129 0,99862 2 514 2,129 0,99862 3 122 2,129 0,99862 3 122 2,112 0,99868 3 097 2,112 0,99868 3 097 Total mass of vapour 17 465 Total mass in vacuum 17 465 ROB Total loaded in vacuum Total loaded in air
12.3.6
Calculation of mass after loading
Loading report Cargo Propylene Molecular 42,08 mass Atm.press 1,020
Port Date
Al Jubail 18.05. 1994
Skip
LPG Seagull
Liquid Tank #
Sounding Volume Temp. Press r liquid Shrinkage in meter from in oC in bar in kg/m3 factor tab. in m3 1P 8,74 1123,83 -39 0,6 601,2 0,99826 1S 8,76 1125,55 -39 0,6 601,2 0,99826 2P 8,72 1400,11 -39 0,6 601,2 0,99826 2S 8,73 1401,20 -39 0,6 601,2 0,99826 3P 8,76 1404,41 -38 0,6 600,0 0,99829 3S 8,75 1403,35 -38 0,6 600,0 0,99829 Total mass of Liquid
Vapour Tank # 1P 1S 2P 2S 3P 3S
100% Vapour Temp. in oC vol. in volume m3 in m3 1182,18 58,35 -35 1182,18 56,63 -35 1468,18 68,07 -36 1468,18 66,98 -36 1468,18 63,77 -34 1468,18 64,83 -34 Total Total
Mass of liquid in kg 674 675 840 840 841 840 4
471 503 281 936 205 570 712 967
r vapour Shrinkage Masse of in kg/m3 factor vapour in kg 3,445 0,99838 201 3,445 0,99838 195 3,459 0,99835 235 3,459 0,99835 231 3,431 0,99841 218 3,431 0,99841 222 mass of vapour 1 302 mass in vacuum 4 714 269 ROB 17 465 Total Loaded in vacuum 4 696 803 Total loaded in air 4 687 973
12.3.7
CALCULATION OF LIQUID TO BE USED FOR GASSING UP
There are some parameters we have to have in mind to find out how much liquid we need to take onboard for gassing up our cargo tanks. The first is the temperature of the liquid we will take onboard then the temperature of the cargo tank steel and what volume we should gas up. To change cargo and gas up costs lot money, to minimise the cost we have to use all the available cargo equipment onboard in the most efficient way. We have to be sure that the amount of liquid we order for gassing up is enough to gas up and to commence cooling down the cargo tanks. If we have some ROB in one tank, we can begin gassing up at sea if the tanks are surveyed and approved by a surveyor. If we don’t have any ROB or not enough, we have to order liquid to gas up the rest of the volume to be gassed up. To minimise the consumption of cargo for gassing up, we need to heat the cargo, as mush as possible. The amount of cargo lost when gassing up depends on the people onboard, cargo equipment and the time we use for gassing up. For cargoes with a heavy vapour, such as VCM, propane butane and propylene, the loss of cargo is near to 0 when gassing up correctly. The only way to reduce the loss of cargo is to control tank pressure when loading coolant, measure and check when commence heating the coolant for gassing up. 12.4.1 Volume of liquid to be used for gassing up We have a cargo tank with volume 1182,18 m3 that we have to gas up. Our experience is that we need two times the tank volume for gassing up and commence cooling the tank. We then have to order the following amount cargo, 1182,18 m3 x 2 = 2364,36 m3. We will take onboard propylene liquid for gassing up and it is two vital temperatures we must recognise, tank steel temperature and liquid temperature on the coolant. Liquid temperature on shore tank is –40oC and our cargo tank steel 20oC. The formula is mass = ρ vapour x total volume. From the table thermodynamic properties for propylene superheated vapour, we find the vapour ρ to 1,812 kg/m3 on 20oC and P=1 bar. Then we take the total volume 2364,36 m3 and multiply with vapour ρ 1,812 kg/m3 = 4 283 kg, which is the minimum we need for gassing up and commence cooling the tank. The loss of cargo and number of changes is individual for each vessel and it is our duty to reduce the loss of cargo down to a minimum. Calculation of volume liquid we have to order 12.4.2 Cargo Propylene 100% Tank volume 1182,18 m3 o C 1,812 kg/m3 r vapour at atmospheric pressure from 20 table o r liquid from table -40 C 602,4 kg/m3 Number of changes 2 Total volume to be changed 2364,36 m3 Mass volume = Volume x r for vapour Mass total volume = 4283,26 kg Volume liquid to be loaded = Mass volume / r liquid
Volume to be loaded =
7,110
m3
We have to load 7,11 m3 propylene at –40oC from shore tank to change the vapour atmosphere at 20oC two times. This was a calculation for one tank, if we gas up all tanks, the calculation has to be on the total volume of the vessel’s cargo tanks. After completion of the loading two Bill of Lading will be made, one for what we have used for gassing up and one for the quantity we have loaded. Number of changes with a given amount of liquid 12.4.3 To find the number of vapour changes with a given amount of liquid in either a deck tank or a cargo tank, we then have to know the liquid temperature and the temperature of the cargo tanks we have to gas up. Then we have to calculate the mass of the liquid we have. When we know the mass of liquid and the volume to be gassed up, we know if we then need to order more liquid or if we can complete to gas up and commence cooling tanks with the amount of liquid we have onboard. A cargo tank is completely gassed up when we have more than 97% hydrocarbons in the vapour atmosphere. We must remember that the tank we use for gassing up will have a given amount of mass vapour left ROB, which we are unable to get out. First, we have to calculate the mass of vapour we will have ROB in our deck tank/cargo tank after we have gassed up the other tanks. When we have calculated the mass of vapour we have left, we must subtract it from the amount of liquid we have. How many changes we need depends on the cargo, the cargo handling equipment we have onboard, temperature of the liquid and temperature of the atmosphere that we should gas up. If we are able to heat the vapour, we should have it as hot as possible to use as less liquid as possible. We can use an example on the calculation of vapour after gassing up. Average temperature on the vapour is –10oC, total tank volume is 2564,36 m3 and tank pressure is 0 bar. We then find the density of the vapour, either calculate the density or use the thermal property table to find it. When we have found the vapour density, we have to multiply it with the tank shrinkage factor and the tank volume. Mass of vapour after gassing up 12.4.4 Cargo Propylene Tank volume #1 P/S 2364,36 m3 o C 1,953 kg/m3 r for vapour at atmospheric -10 pressure Mass of vapour in the tank 4 618 kg We have now calculated that we should have 4 618 kg vapour left in tank #1 P and S when we are not able to get out any more from the tanks. Before we order any liquid, we have to subtract 4 618 kg from the amount of liquid we need to gas up the whole vessel. We can continue with the example and have 15 m3 liquid propylene at –10oC, vapour temperature 0oC and the pressure 3,3 bar in tank #1 P/S. Total volume of the vessel is 8237 m3 and atmospheric pressure is 1,015 bar. That means we have to gas up vessel’s total volume – volume of tank #1 P/S, which is equal to 8237 m3 – 2364,36
m3 = 5872,64 m3 with an average temperature of 25oC. Our experience is that we need 2,5 volume changes to reach 97% hydrocarbons in the vapour atmosphere, 2,5 changes is 5872,64 m3 x 2,5 = 14681,6 m3. It is always stated in the charter party how clean the atmosphere has to be before loading and it depends on which cargo we have to load. Mass we can use for gassing up 12.4.5 Cargo Propylene Tank volume #1 P/S 2364,36 Volume liquid in tank #1 P/S 15,00 Mass of liquid in tank #1 P/S 8 420 Volume of vapour in tank #1 P/S 2349,36 Mass of vapour in tank #1 P/S 18 783 Total mass in tank #1 P/S 27 202 Mass of vapour in tank #1 P/S after gassing 4 618 up Usable mass in tank #1 P/S 22 584
m3 m3 kg m3 kg kg kg kg
We have 27202 kg available in tank #1 P/S, but when we are completed gassing up, we have 4618 kg vapour left, that means we have 22584 kg available for gassing up. Total volume to be gassed up is vessel’s total volume minus volume of tank #1 P/S multiplied with 2,5. That gives ⇒ 8237 m3 - 2349,36 m3 ⇒ 5872,64 m3 x 2,5 = 14682 m3. We have to find the vapour density equal to tank steel temperature 25oC, which is 1,724 kg/m3. Calculation of volume needed 12.4.6 Volume vapour at 25oC of available 13 101 m3 mass o C r for vapour with atmospheric pressure 25 at Total volume vapour 2,5 times tank 14 682 m3 volume Volume to be ordered 1 581 m3
1,724 kg/m3
In this example, we do not have enough liquid to reach 2,5 times for gassing up. There was 1581 m3 vapour short, so we have to order that 1581 m3 x 1,724 kg/m3 = 2726 kg. Calculation of mass vapour at a given temperature 12.4.7 Cargo Propylene Vessel’s total volume 8237 m3 Tank #1 P/S volume Volume to be gassed up Amount changes
2364,36 5872,64 2,5
m3 m3
Tank steel temperature Atmospheric pressure Vapour r at 25oC
25 1,015 1,724
Total volume to gas up 5872,64 x 2,5 14 682 Mass of total volume to gas up 25 309 Available mass 22 584 Mass in kg to load to complete gassing 2 725 up
o
C bar kg/m3
m3 kg kg kg
To hold the temperature of the vapour we use for gassing up, we have to use either the compressors or heaters. If we are able to increase the temperature on the vapour from 25oC to 60oC, we do not need to supply any extra from shore. Calculation of vapour at 60oC 12.4.8 Vapour r at 60oC Total volume to gas up 5872,64 x 2,5 Mass total volume to gas up Available mass Mass in kg difference
1,543 14 682 22 649 22 584 65
kg/m3 m3 kg kg kg
It is important that we continue to heat the tank we are taking the vapour from to hold a positive pressure.
12.5 12.4 ENCLOSES 12.5.1
12.4.1 Enclose 1
Table 21 0,500 - 0,510 Specific Gravity 60/60oF
API Gravity 60oF
Density 15oC
0,500 0,501 0,502 0,503 0,504 0,505 0,506 0,507 0,508 0,509 0,510
-
0,5004 0,5014 0,5023 0,5033 0,5043 0,5053 0,5063 0,5073 0,5083 0,5093 0,5103
12.4.2 Enclose 2 Table 54C Observed
Density 15 oC
temperature, 0,500 0,505 0,510 o C Factor for reduction of volume to 15 oC -43 -42,5 -42 -41,5 -41 -40,5 -40 -39,5 -39 -38,5 -38 -37,5 -37
1,153 1,152 1,15 1,149 1,148 1,147 1,146 1,145 1,143 1,142 1,141 1,14 1,139
4 4 3 3 3 3 3 3 3 3 3 3 3
1,149 1,148 1,147 1,146 1,145 1,144 1,143 1,142 1,14 1,139 1,138 1,137 1,136
3 3 4 4 4 4 4 4 3 3 4 4 4
1,146 1,145 1,143 1,142 1,141 1,14 1,139 1,138 1,137 1,136 1,134 1,133 1,132
-36,5 -36 -35,5 -35 -34,5 -34 -33,5 -33 -32,5 -32 -31,5 -31 -30,5 -30 -29,5 -29 -28,5 -28 -27,5 -27 -26,5 -26 -25,5 -25 -24,5 -24 -23,5 -23 -22,5 -22 -21,5 -21 -20,5 -20 -19,5 -19 -18,5 -18 -17,5 -17 -16,5 -16 -15,5 -15
1,138 1,136 1,135 1,134 1,133 1,131 1,13 1,129 1,128 1,126 1,125 1,124 1,123 1,121 1,12 1,119 1,117 1,116 1,115 1,113 1,112 1,111 1,109 1,108 1,107 1,106 1,104 1,103 1,102 1,101 1,100 1,098 1,097 1,096 1,095 1,093 1,092 1,091 1,090 1,089 1,087 1,086 1,085 1,084
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 2 3 3 2 2 3 2 2 2 3 2 2 2 3 3 2 3 3 3 2 2 3 3 3 2 2 3 3
1,135 1,133 1,132 1,131 1,13 1,128 1,127 1,126 1,125 1,123 1,122 1,121 1,12 1,118 1,117 1,116 1,115 1,113 1,112 1,111 1,11 1,108 1,107 1,106 1,105 1,103 1,102 1,101 1,1 1,098 1,097 1,096 1,094 1,093 1,092 1,091 1,09 1,088 1,087 1,086 1,085 1,084 1,082 1,081
4 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 2 3 3 3 2 2 2 1 1 2 2 2 1 1 2 2 2 1 1
1,131 1,13 1,129 1,128 1,127 1,125 1,124 1,123 1,122 1,12 1,119 1,118 1,117 1,115 1,114 1,113 1,112 1,11 1,109 1,108 1,107 1,105 1,104 1,103 1,102 1,101 1,099 1,098 1,097 1,096 1,095 1,094 1,093 1,092 1,09 1,089 1,088 1,087 1,086 1,084 1,083 1,082 1,081 1,08
-14,5 -14 -13,5 -13 -12,5 -12 -11,5 -11 -10,5 -10 -9,5 -9 -8,5 -8 -7,5 -7 -6,5 -6 -5,5 -5 -4,5 -4
1,082 1,081 1,080 1,079 1,077 1,076 1,075 1,074 1,072 1,071 1,070 1,068 1,067 1,066 1,065 1,063 1,062 1,061 1,059 1,058 1,057 1,055
2 2 2 2 2 2 2 2 1 1 2 1 1 1 2 1 1 1 1 1 1 1
1,08 1,079 1,078 1,077 1,075 1,074 1,073 1,072 1,071 1,07 1,068 1,067 1,066 1,065 1,063 1,062 1,061 1,06 1,058 1,057 1,056 1,054
2 2 2 2 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
1,078 1,077 1,076 1,075 1,074 1,072 1,071 1,07 1,069 1,068 1,066 1,065 1,064 1,063 1,061 1,06 1,059 1,058 1,056 1,055 1,054 1,052
12.4.3 Enclose 3 Shrinkage factor for exercise Seagull Temp. Sh.fakt. Temp. Sh.fakt. 20 1 -21 0,99879 19 0,99997 -22 0,99876 18 0,99994 -23 0,99873 17 0,99991 -24 0,99870 16 0,99988 -25 0,99868 15 0,99985 -26 0,99865 14 0,99982 -27 0,99862 13 0,99979 -28 0,99859 12 0,99976 -29 0,99856 11 0,99974 -30 0,99853 10 0,99971 -31 0,99850 9 0,99968 -32 0,99847 8 0,99965 -33 0,99844 7 0,99962 -34 0,99841 6 0,99959 -35 0,99838 5 0,99956 -36 0,99835 4 0,99953 -37 0,99832 3 0,99950 -38 0,99829 2 0,99947 -39 0,99826 1 0,99944 -40 0,99823 0 0,99941 -41 0,99820 -1 0,99938 -42 0,99817 -2 0,99935 -43 0,99815 -3 0,99932 -44 0,99812 -4 0,99929 -45 0,99809 -5 0,99926 -46 0,99806 -6 0,99923 -47 0,99803 -7 0,99921 -48 0,99800 -8 0,99918 -49 0,99797 -9 0,99915 -50 0,99794 -10 0,99912 -51 0,99791 -11 0,99909 -52 0,99788 -12 0,99906 -53 0,99785 -13 0,99903 -54 0,99782 -14 0,99900 -55 0,99779 -15 0,99897 -56 0,99776 -16 0,99894 -57 0,99773 -17 0,99891 -58 0,99770 -18 0,99888 -59 0,99767 -19 0,99885 -60 0,99765 -20 0,99882 -61 0,99762 -21 0,99879 -62 0,99759
04 Temp. -62 -63 -64 -65 -66 -67 -68 -69 -70 -71 -72 -73 -74 -75 -76 -77 -78 -79 -80 -81 -82 -83 -84 -85 -86 -87 -88 -89 -90 -91 -92 -93 -94 -95 -96 -97 -98 -99 -100 -101 -102 -103
Sh.fakt. 0,99759 0,99756 0,99753 0,99750 0,99747 0,99744 0,99741 0,99738 0,99735 0,99732 0,99729 0,99726 0,99723 0,99720 0,99717 0,99714 0,99712 0,99709 0,99706 0,99703 0,99700 0,99697 0,99694 0,99691 0,99688 0,99685 0,99682 0,99679 0,99676 0,99673 0,99670 0,99667 0,99664 0,99661 0,99659 0,99656 0,99653 0,99650 0,99647 0,99644 0,99641 0,99638
12.4.4 Enclose 4 Correction for expansion of sounding tape tank #1 Temp. Read Sounding in meter o C 6 7 8 9 10 -104 -103 -102 -101 -100 -99 -98 -97 -96 ± -48 -47 -46 -45 -44 -43 -42 -41 -40 -39 -38 -37 -36 -35 -34 -33 -32 -31 -30 -29 -28 -27 -26 -25 -24 -23 -22
-0,004 -0,004 -0,004 -0,004 -0,004 -0,004 -0,004 -0,004 -0,004 -0,003 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,001 -0,001 -0,001 -0,001 -0,001
-0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001
-0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,002 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001
-0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001
-0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 0,000 0,000 0,000
11 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000
12.4.5 Enclose 5 Trim table tank #1, tank 100% volume 1182,18 m3 Sounding and trim meter in Sounding Stern trim Sounding Stern trim 0,5 1 0,5 1 8,04 -0,021 -0,043 8,41 -0,021 -0,043 8,05 -0,021 -0,043 8,42 -0,021 -0,043 8,06 -0,021 -0,043 8,43 -0,021 -0,043 8,07 -0,021 -0,043 8,44 -0,021 -0,043 8,08 -0,021 -0,043 8,45 -0,021 -0,043 8,09 -0,021 -0,043 8,46 -0,021 -0,043 8,10 -0,021 -0,042 8,47 -0,021 -0,043 8,11 -0,021 -0,042 8,48 -0,021 -0,043 8,12 -0,021 -0,042 8,49 -0,022 -0,043 8,13 -0,021 -0,042 8,50 -0,022 -0,043 8,14 -0,021 -0,042 8,51 -0,022 -0,043 8,15 -0,021 -0,042 8,52 -0,022 -0,043 8,16 -0,021 -0,042 8,53 -0,022 -0,043 8,17 -0,021 -0,042 8,54 -0,022 -0,043 8,18 -0,021 -0,042 8,55 -0,022 -0,043 8,19 -0,021 -0,042 8,56 -0,022 -0,043 8,20 -0,021 -0,042 8,57 -0,022 -0,043 8,21 -0,021 -0,042 8,58 -0,022 -0,043 8,22 -0,021 -0,042 8,59 -0,022 -0,043 8,23 -0,021 -0,042 8,60 -0,022 -0,043 8,24 -0,021 -0,042 8,61 -0,022 -0,044 8,25 -0,021 -0,043 8,62 -0,022 -0,044 8,26 -0,021 -0,043 8,63 -0,022 -0,044 8,27 -0,021 -0,043 8,64 -0,022 -0,044 8,28 -0,021 -0,043 8,65 -0,022 -0,044 8,29 -0,021 -0,043 8,66 -0,022 -0,044 8,30 -0,021 -0,043 8,67 -0,022 -0,044 8,31 -0,021 -0,043 8,68 -0,022 -0,044 8,32 -0,021 -0,043 8,69 -0,022 -0,044 8,33 -0,021 -0,043 8,70 -0,022 -0,044 8,34 -0,021 -0,043 8,71 -0,022 -0,044 8,35 -0,021 -0,043 8,72 -0,022 -0,044 8,36 -0,021 -0,043 8,73 -0,022 -0,044 8,37 -0,021 -0,043 8,74 -0,022 -0,044 8,38 -0,021 -0,043 8,75 -0,022 -0,044 8,39 -0,021 -0,043 8,76 -0,022 -0,045 8,40 -0,021 -0,043 8,77 -0,022 -0,045
Sounding 8,78 8,79 8,80 8,81 8,82 8,83 8,84 8,85 8,86 8,87 8,88 8,89 8,90 8,91 8,92 8,93 8,94 8,95 8,96 8,97 8,98 8,99 9,00 9,01 9,02 9,03 9,04 9,05 9,06 9,07 9,08 9,09 9,10 9,11 9,12 9,13 9,14
Stern trim 0,5 1 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,022 -0,045 -0,023 -0,045 -0,023 -0,045 -0,023 -0,045 -0,023 -0,046 -0,023 -0,046 -0,023 -0,046 -0,023 -0,046 -0,023 -0,046 -0,023 -0,046 -0,023 -0,046 -0,023 -0,046 -0,023 -0,046 -0,023 -0,046 -0,023 -0,047 -0,023 -0,047
12.4.6 Enclose 6 Sounding table tank #1, tank 100% volume 1182,18 m3 Sounding Volume Sounding Volume m m3 m m3 8,04 8,05 8,06 8,07 8,08 8,09 8,10 8,11 8,12 8,13 8,14 8,15 8,16 8,17 8,18 8,19 8,20 8,21 8,22 8,23 8,24 8,25 8,26 8,27 8,28 8,29 8,30 8,31 8,32 8,33 8,34 8,35 8,36 8,37 8,38 8,39 8,40
1051,77 1052,91 1054,04 1055,17 1056,30 1057,33 1058,55 1059,67 1060,79 1061,91 1063,03 1064,14 1065,25 1066,36 1067,46 1068,56 1069,66 1070,76 1071,86 1072,95 1074,03 1075,12 1076,20 1077,28 1078,36 1079,43 1080,50 1081,57 1082,63 1083,69 1084,75 1085,80 1086,85 1087,90 1088,94 1089,98 1091,01
8,41 8,42 8,43 8,44 8,45 8,46 8,47 8,48 8,49 8,50 8,51 8,52 8,53 8,54 8,55 8,56 8,57 8,58 8,59 8,60 8,61 8,62 8,63 8,64 8,65 8,66 8,67 8,68 8,69 8,70 8,71 8,72 8,73 8,74 8,75 8,76 8,77
1092,05 1093,08 1094,10 1095,12 1096,14 1097,15 1098,16 1099,17 1100,17 1101,17 1102,16 1103,15 1104,14 1105,12 1106,10 1107,07 1108,04 1109,00 1109,96 1110,92 1111,87 1112,82 1113,76 1114,70 1115,63 1116,56 1117,48 1118,40 1119,32 1120,23 1121,14 1122,04 1122,94 1123,83 1124,71 1125,59 1126,47
Sounding Volume m m3 8,78 8,79 8,80 8,81 8,82 8,83 8,84 8,85 8,86 8,87 8,88 8,89 8,90 8,91 8,92 8,93 8,94 8,95 8,96 8,97 8,98 8,99 9,00 9,01 9,02 9,03 9,04 9,05 9,06 9,07 9,08 9,09 9,10 9,11 9,12 9,13 9,14
1127,33 1128,20 1129,05 1129,90 1130,74 1131,58 1132,41 1133,23 1134,05 1134,85 1135,65 1136,44 1137,23 1138,01 1138,78 1139,54 1140,30 1141,06 1141,80 1142,54 1143,27 1144,00 1144,71 1145,43 1146,13 1146,83 1147,53 1148,21 1148,89 1149,56 1150,23 1150,89 1151,54 1152,19 1152,82 1153,46 1154,08
12.4.7 Enclose 7 Correction for expansion of sounding tape tank #2 Temp. Read sounding in meter o C 6 7 8 9 10 -104 -103 -102 -101 -100 -99 -98 -97 -96 ± -48 -47 -46 -45 -44 -43 -42 -41 -40 -39 -38 -37 -36 -35 -34 -33 -32 -31 -30 -29 -28 -27 -26 -25 -24 -23 -22
-0,004 -0,004 -0,004 -0,004 -0,004 -0,004 -0,004 -0,004 -0,004 -0,003 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,001 -0,001 -0,001 -0,001 -0,001
-0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001
-0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,003 -0,002 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001
-0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,002 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001
-0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,000 -0,000 -0,000
/ #3 11 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 -0,001 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000
12.4.8 Enclose 8 Trim table tank #2 and #3, tank 100% volume 1468,18 m3 Sounding and trim in meter Sounding Stern trim 0,5 1 8,04 -0,029 -0,059 8,05 -0,029 -0,059 8,06 -0,029 -0,059 8,07 -0,029 -0,059 8,08 -0,029 -0,059 8,09 -0,029 -0,059 8,10 -0,029 -0,059 8,11 -0,029 -0,059 8,12 -0,029 -0,059 8,13 -0,029 -0,059 8,14 -0,029 -0,059 8,15 -0,029 -0,059 8,16 -0,029 -0,059 8,17 -0,029 -0,059 8,18 -0,029 -0,059 8,19 -0,029 -0,059 8,20 -0,029 -0,059 8,21 -0,029 -0,059 8,22 -0,029 -0,059 8,23 -0,029 -0,059 8,24 -0,029 -0,059 8,25 -0,029 -0,059 8,26 -0,029 -0,059 8,27 -0,029 -0,059 8,28 -0,029 -0,059 8,29 -0,029 -0,059 8,30 -0,029 -0,059 8,31 -0,029 -0,059 8,32 -0,029 -0,059 8,33 -0,029 -0,059 8,34 -0,029 -0,059 8,35 -0,029 -0,059 8,36 -0,029 -0,059 8,37 -0,029 -0,059 8,38 -0,029 -0,059 8,39 -0,029 -0,059 8,40 -0,029 -0,059
Sounding 8,41 8,42 8,43 8,44 8,45 8,46 8,47 8,48 8,49 8,50 8,51 8,52 8,53 8,54 8,55 8,56 8,57 8,58 8,59 8,60 8,61 8,62 8,63 8,64 8,65 8,66 8,67 8,68 8,69 8,70 8,71 8,72 8,73 8,74 8,75 8,76 8,77
Stern trim 0,5 1 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,059 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,028 -0,058 -0,028 -0,058 -0,028 -0,058 -0,028 -0,058 -0,028 -0,058 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057
Sounding 8,78 8,79 8,80 8,81 8,82 8,83 8,84 8,85 8,86 8,87 8,88 8,89 8,90 8,91 8,92 8,93 8,94 8,95 8,96 8,97 8,98 8,99 9,00 9,01 9,02 9,03 9,04 9,05 9,06 9,07 9,08 9,09 9,10 9,11 9,12 9,13 9,14
Stern trim 0,5 1 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057 -0,028 -0,057 -0,029 -0,057 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,029 -0,058 -0,028 -0,058
12.4.9 Enclose 9 Sounding table tank #2 and #3, tank 100% volume 1468,18 m3 Sounding m Volume m3 Sounding m Volume m3 Sounding m Volume m3 8,04 8,05 8,06 8,07 8,08 8,09 8,10 8,11 8,12 8,13 8,14 8,15 8,16 8,17 8,18 8,19 8,20 8,21 8,22 8,23 8,24 8,25 8,26 8,27 8,28 8,29 8,30 8,31 8,32 8,33 8,34 8,35 8,36 8,37 8,38 8,39 8,40
1311,35 1312,81 1314,27 1315,72 1317,18 1318,63 1320,08 1321,52 1322,96 1324,40 1325,84 1327,27 1328,70 1330,13 1331,55 1332,97 1334,38 1335,80 1337,20 1338,60 1340,00 1341,29 1342,77 1344,15 1345,53 1346,89 1348,26 1349,62 1350,97 1352,32 1353,66 1355,00 1356,33 1357,66 1358,98 1360,29 1361,60
8,41 8,42 8,43 8,44 8,45 8,46 8,47 8,48 8,49 8,50 8,51 8,52 8,53 8,54 8,55 8,56 8,57 8,58 8,59 8,60 8,61 8,62 8,63 8,64 8,65 8,66 8,67 8,68 8,69 8,70 8,71 8,72 8,73 8,74 8,75 8,76 8,77
1362,91 1364,21 1365,50 1366,79 1368,07 1369,35 1370,62 1371,88 1373,14 1374,39 1375,64 1376,87 1378,11 1379,33 1380,55 1381,76 1382,97 1384,17 1385,36 1386,54 1387,71 1388,88 1390,04 1391,19 1392,33 1393,46 1394,59 1395,70 1396,82 1397,92 1399,02 1400,11 1401,20 1402,28 1403,35 1404,41 1405,47
8,78 8,79 8,80 8,81 8,82 8,83 8,84 8,85 8,86 8,87 8,88 8,89 8,90 8,91 8,92 8,93 8,94 8,95 8,96 8,97 8,98 8,99 9,00 9,01 9,02 9,03 9,04 9,05 9,06 9,07 9,08 9,09 9,10 9,11 9,12 9,13 9,14
1406,51 1407,56 1408,59 1409,61 1410,63 1411,64 1412,64 1413,64 1414,62 1415,60 1416,57 1417,52 1418,47 1419,42 1420,35 1421,27 1422,18 1423,09 1423,98 1424,86 1425,74 1426,60 1427,46 1428,31 1429,14 1429,97 1430,79 1431,59 1432,39 1433,18 1433,95 1434,72 1435,47 1436,22 1436,95 1437,67 1438,38
12.4.10Enclose 10 Correction on the float
Specific gravity (kg/dm3)
Correction in meter + 0,220 + 0,187 + 0,170 + 0,150 + 0,136 + 0,127 + 0,121 + 0,114 + 0,110
0,40 0,45 0,50 0,60 0,70 0,80 0,90 1,00 1,10
Cargo calculation table LPG Seagull
ASTM
Port: Last: Tank no. #
Date: Skip: 1P
1S
2P
2S
3P
3S
1182.18 1182.18 1468.18 1468.18 1468.18 1468.18 1 Volume of tank 100 % m3 o 2 Liquid temp. C 3 Sounding read m 4 Float correction m 5 Correction for vapour m temperature 6 List correction m 7 Trim correction m o 8 Sounding at 20 C m o 9 Volume at 20 C m3 10 Shrink. factor tank steel 11 Corrected liquid volume m3 12 Reduction factor from table 54C 13 Volume 15 oC m3 14 Density on liquid at 15oC Mt/m3
tab. 21 15 Mass of liquid in vacuum at 15oC 16 Uncorrected vapour volume 17 Shrinkage factor vapour phase 18 Corrected Vapour volume 19 Tank pressure 20 Atmosphere pressure 21 Molecular weight 22 Temperature on vapour 23 Density on vapour 24 Mass of vapour in vacuum 25 Total mass in vacuum Total weight in air ROB in air
Total loaded in air
Mt m3
m3 bar bar o
C kg/m3 Mt Mt Mt Mt Mt
13-
Cooling Processes and Calculation
13 COOLING PROCESSES AND CALCULATIONS 13.1 MOLLIER DIAGRAM – INTRODUCTION The Mollier diagram is an invaluable in helping to understand refrigeration calculations. In a cooling plant, the cooling media will constantly change its state. Vapour is compressed and gets a higher pressure and temperature, vapour condenses and liquid vaporises. In the cooling process, the aggregate state, pressure and temperature change continuously. To perform calculations of the cooling process, one must know the enthalpy changes taking place. If one is to make enthalpy-tables for a cooling media in all possible states, you will find a large and unpractical table. Diagrams for simply cooling media where the cooling media’s enthalpy under the actual aggregate states, are therefore developed. Such a diagram contains infinitely much more information than a table can have, and gives, in addition, possibility to make a “heat-technical picture” of the cooling process. Before making use of the Mollier diagram one has to learn how the diagram is built, and how the different lines lie in the diagram. This will make it easier to find information in the diagram.
Log p-h diagram or called the Mollier diagram, has a vertical logarithmic scale for pressure (p) and a horizontal scale for enthalpy (h). In the Mollier diagram, one will often think of the two different units for the different qualities. We will consistently make use of the SI-units and refer to these scales only.
The pressure of the Mollier diagram is, as in all heat technical tables, given as absolute pressure. If nothing else is given, we have chosen to set the atmospheric pressure to equal 1 bar, which corresponds to 0,1 MPa. In cooling technical calculations one is only interested in enthalpy changes and not the absolute enthalpy values. The values for enthalpy are therefore chosen from a random reference state. One must take note of this, if one wants to compare enthalpy values from the different tables or diagrams.
In the Mollier diagram, the “sack” is the most bearing curve. The bent line that goes from the lower left corner and upwards the KP (the kinetic point) is called the liquid line. If this line is lengthways, the liquid is always in its boiling point. The line bends from KP and is almost vertically down the middle of the diagram. This line is called the saturation curve. On this line, the state will be saturated gas. At the right of the saturation curve, the gas is super-heated. To the left of the liquid line the liquid is super-cooled. The area between the liquid and the saturation line specifies a mixture of boiling liquid and saturated gas. The dashed line indicates the proportion of mixture between liquid and gas. Halfway between the liquid line and the saturation line there will be equal parts of gas as liquid. In the sketch above, one line is marked “0,1”. Along this line there is 90% liquid and 10% saturated gas. The distance between the liquid and the saturation line indicates how large the vaporisation is. Notice that the vaporisation varies with the pressure and is lessened, the higher the pressure is. In the critical point, liquid can not appear. The Mollier diagram also has lines that indicate density (or specific volume), temperature and entropy. The lines for entropy indicate how pressure, temperature and heat content change in an adiabatic state of proportion. An adiabatic state of proportion is an alteration without heat exchanging with the surroundings. The real compression progress in a compressor will of course deviate some from these lines because of loss in the compressor and heat exchange with the environment.
The lines for density (kg/m3) or specific volume (m3/kg) indicate density with varying pressure and temperature. The line for constant temperature is vertical from the top of the diagram down to the liquid line. The temperature goes from the liquid line horizontal to the saturation line, thereby to bend vertically towards the enthalpy from the saturation line. To maintain necessary training in using the Mollier diagram, we will see in some examples how to obtain useful information from the diagram. This course provides a Mollier diagram for propane utilises the diagram in the following advice. Take note that there are often two scales for pressure and enthalpy. Note which units to utilise for temperature, density and entropy. We will only utilise SI-units in our calculations. In the course enclosure, tables for conversion between the most common units are enclosed. When you plot and draw in the diagram it is recommended to always utilise a soft pencil. The diagram can be used several times. 13.1.1 Example 1 The ship is loaded with propane and the tank manometer pressure is 2 bar. Find the physical state for the liquid in the loading tank. Any state of proportion can settle in the diagram if we can identify two actual crossing lines. As the cargo in the loading tank always lies in its boiling point, the state of proportion must lie somewhere among the liquid line. It is also known that the pressure in the tank and above the liquid is read off at 2 bar on the manometer. This pressure is equivalent 3 bar absolute (if we assume the atmospheric pressure to 1 bar) or 0.3 MPa. The state of proportion here is the point of intersection between the liquid line and the pressure line of 0,3 MPa.
13.1.2 Example 2 The ship is loaded with propane and the tank pressure is read off on the manometer at 2 bar. Plot the state of proportion for the gas above the liquid in the cargo tank. As the gas above the liquid in the loading tank is, at all times, saturated, the state of proportion must lie somewhere on the saturation line. The pressure in the tank and above the liquid is read off to 2 bar on the manometer. This pressure is equivalent to 3 bar absolute (if we assume the atmospheric pressure to 1 bar) or 0,3 MPa. The state of proportion here will be the state of intersection between the saturation line and the pressure line of 0,3 MPa.
13.1.3 Example 3 The ship is loaded with propane and the tank pressure is read off on the manometer to 2 bar. Utilise the Mollier diagram to find how much heat one must supply the tank to evaporate (boil off) 1 kg propane. In example 1 and 2, the state of proportions for the liquid and the gas in the loading tank is stated at a tank pressure of 2 bar. The difference between the liquid enthalpy h1 and the saturated gas enthalpy h2 is the heat quantity that is needed to evaporate 1kg propane at 2 bar pressure.
Calculation of evaporation heat Gas
:
Propane
Pressure (abs.)
=
3
Enthalpy for saturated gas, h2
=
880
kJ/kg
- Enthalpy for saturated liquid, h1
=
490
kJ/kg
Latent heat of evaporation
=
390
kJ/kg
bar
The latent heat of evaporation for propane can also be found in a heat thermal property table for propane. Find this table in the course enclosure, and check that the latent heat of evaporation is the same.
13.1.4 Example 4 The gas carrier is loaded with propane and the tank pressure is read of on the manometer of 2 bar. On a cargo compressor in operation the suction pressure is read off to 1,5 bar, and the suction temperature to –10 oC (14oF). Plot the state of proportion for the gas on the compressor.
As the pressure in the gas in to the compressor is 1,5 bar, the state of proportion must lie on a pressure line equal 0,25 MPA (1,2 bar + 1 bar) in the diagram. The exact state of proportion is plotted where the temperature line of –10oC crosses the pressure line of 0,14 MPa. When the point is plotted, the density of the gas into the compressor can be defined from the density lines that run sideways out to the right in the diagram. A cooling plant’s net cold capacity is expressed as: Qnett = m x Dh where m =the mass of gas that streams through the cooling plant per hour (kg/s) Dh = the difference between enthalpy on the gas that abandons the tank and enthalpy on the condensate that returns back to the tank (kJ/kg) Notice that the density of the gas increases at higher gas pressure and lower temperature. Larger density gives more mass per hour that will flow through the plant. More mass involves larger cold capacity for the plant.
13.1.5 Example 5 A gas cylinder is filled with floating propane. Temperature of the air and propane liquid and gas is 15 oC. We say that the gas over the liquid is saturated. The valve opens and floating propane flows over in an open container. Plot the state of proportion for the liquid in the bottle, before the valve was opened, and for the liquid in the opened container later.
The liquid lies on the liquid line in the diagram. The point (1) is defined either from the temperature line of 15oC (59oF) or equivalent pressure line 0,73 Mpa (7,3 bar). When the liquid is let out of the bottle, the pressure of the liquid to the atmospheric pressure (1 bar) lowers. The fast reduction of the pressure involves a powerful boiling of the liquid because of an “unbalance” between the liquid’s temperature and gas pressure of the liquid. The heat of the boiling is taken from the liquid itself and the surroundings and the liquid gets colder. An enthalpy change during the process will not take place. We can draw the process line (from point 1 to point 2) for any change to the liquid, as a vertical line from the cross-point through the liquid line to the pressure line. Notice that the new state of proportion (2) is inside the “sack” and that a precise share of the liquid has evaporated because of pressure reduction.
13.2 THE COOLING PLANTS COMPONENT
A good processor should have a construction that secures from gas leakage, be applicable for the different media qualities, have large regulation opportunity, obtain least space and give as little noise and vibrations as possible. We separate between four types of compressors in the cooling plant: oil lubricated pistons, oil free pistons, oil lubricated screw compressors and oil free screw compressors. Both types of pistons and oil free screw compressors are used on the cargo side in the cooling plant. On the Freon side, oil lubricated piston and screw compressors are used. Pistons 13.2.1 An oil-lubricated piston has piston rings made of cast-iron. It is therefore necessary to lubricate the cylinder walls. A part of this oil will be lead out of the compressor. An oil separator in the pressure pipeline separates most of the oil. But some of the oil is lead further out in the system in form of oil vapour. The consequence is that one can not have too large demands for defilement of cargo on ships with such cargo compressors. The state of the piston rings and the cylinder liner is conclusive for how much oil leads out of the compressor. In time, mud will extend from the oil settler. This reduces the settler’s capacity. Cleaning of this is therefore one of the assumptions to maintain the oil consumption and the pollution at a fair level. The working valves are usually a plate type valve and are placed inside the compressor. The working valves have work-over intervals from 2000 to 5000 hours. The capacity regulation by the suction valve plates is gradually lifted from the seats and only the gas is pumped in and out of the cylinder. The lifting arrangement is normally performed hydraulically by oil taken from the pressure side of the oil pump. The lubricate oil pump is normally the gear type and normally placed in the extension of the crankshaft. The oil lubricates the bearing, shaft and the cylinder walls. One supplies the crankcase with a coil for heating and cooling. The lubricated piston compressors are normally built as V or W machines. This construction is less space demanding, but also less friendly working-wise. The compressor case is sterling in cast-iron and the valves normally of aluminium. The casings are loose and cast-iron. The principal for oil free piston compressors is that no parts, which are in contact with the gas, are lubricated. The piston runs dry in the cylinder. These construction problems are solved, as follows: The sealing device between piston and cylinder liner is performed by piston rings with self-lubricated qualities. Piston rings of teflon material are often used in these compressors. A labyrinth gland performs the sealing device between piston and cylinder walls. Piston/cylinder is made by a small groove. The clearance is as small as possible. As the piston is in touch with the cylinder walls, there is minimal wear on compressors with this construction. Loss of energy, which is due to friction in a normal
compressor, is about the same as the leakage loss for a compressor with labyrinth gland. Oil free piston compressors are at all times built as double acting. That involves that they are supplied with a cross-head, and the working valves are placed outside the cylinders. The compressor is also built as two and three stage compressors. This means that one cylinder is used as 1st stage, the next to 2nd stage and the third as 3rd stage. The individual stages here will have different cylinder diameters where the first is the largest and the others gradually smaller. The first stage is also referring to as a low-pressure (LT) cylinder and the highest as high-pressure (HT) cylinder. Piston compressors and lubricate oil 13.2.2 The oil will, at all times, be led out with the gas, from the compressor in plants, by oil lubricated piston compressors. Oil that departs from the oil remover is lead back to the crank room or to an exhaust tank. Where is decided by what cargo we have. One can lead ammonia back to the crank room because this is not soluble in the oil. LPG gas is soluble in oil and should therefore first boil out in an exhaust tank. The lubricant free piston compressors have, under normal situations, no draft of oil. Some gas will however leak down in the crankcase on the piston compressors and there mix with the oil. High pressure in the crankcase is an indication of inferior sealing. This indicates that the viscosity of the oil is less. If sealing and oil is in bad shape, oil will also go up in to the cylinder liner and go with the vapour out of the compressor. Pollution of cargo and reduced compressor capacity are the consequences of this.
One recommends using a mineral oil for the butadiene, while synthetic oil is used for the rest of the cargo. Many of the synthetic oils are hygroscopic and will therefore accumulate dampness if they are exposed to damp air. When changing oil types, one
must be sure not to mix the oil types. To be sure that all remnants of the “old” oil are removed, washing the crank room with the “new” oil before refilling is recommended. Linde compressor 13.2.3 Linde is a type of oil free piston compressor with Teflon sealing rings that is used as a cargo compressor in the cooling plant. These are built as V or W machines with a number of revolutions of about 1200 rpm. The piston is kept central in the cylinder by the help of steering on the topside of the piston. A packing seal and oil blocks the connection between cylinder and crank room. Cylinder, cylinder cover and crank house are cooled and heated by glycol. The working valves are plate type and are capacity regulated at 50% and 100%. They are generally performed with a hydraulic lifting of the suction valves. It is important to consider that the length of lifetime of the Teflon material depends upon at which working temperature the compressor is operating. Too high operating temperature will lead to higher wear and thereby higher leakage loss in the compressor.
13.2.4 Sulzer Compressor Sulzer produces oil free piston compressors with labyrinth sealing for use as cargo compressors in the cooling plant. The cylinders are arranged in series and can be used as one to three stage compressors with the number of revolutions from 600 to 1000 rpm. The pistons are kept central in the cylinder by the help of a piston steering and oil lubricated drawback. Labyrinth sealing and an arrangement of oil scraper rings fence the compound between cylinder and crank room. Gas that flows through the labyrinth sealing is lead back to the suction side. The working valves are made of the ring flapper type. The capacity can be regulated to 50% and 100% and is performed with a hydraulic lifting of the suction valve
flapper. Cylinder, labyrinth sealing, crank room and piston rod steering is cooled and heated by glycol.
13.2.5 Delivery rate A compressor delivery rate is the difference between the suction volume and the stroke volume. A high delivery rate is thereby an important factor for the cooling plant capacity. If we compare the piston compressors, the delivery rate to an oillubricated compressor will be clearly better than an oil free compressor. The volume difference is lowest at low working pressure. The pressure ratio over the compressor or a cylinder in the compressor is the ratio between delivery pressure and suction pressure. The placing of the working valves externally and oil free compressor makes the “damaging room” larger and is the cause for a different delivery rate. If the diagram above is actually for an oil free compressor with a read off suction pressure of 0,5 bar and a delivery pressure of 5 bar, the pressure ratio is: 6 / 1,5 = 4 A pressure ratio of 4 gives a delivery rate of about 0,60. If the compressor has a gross capacity of 400 m 3/h, net capacity is: 400 m3/h x 0,60 = 240 m3/h 240 m3/h gas flow through the compressor in this working situation. Comparison between oil free piston compressors with labyrinth sealing and Teflon sealing indicates that the delivery volume is a bit larger for a compressor with Teflon sealing. However, the power consumption rises at a higher delivery pressure percentage, more than occurs with delivery volume.
13.2.6 Screw compressors The screw compressor is divided into two groups: oil lubricated and oil free. The difference is the same as for the piston compressors. In the oil free screw compressors, the parts that are in contact with the gas are not lubricated with oil.
This type of compressor works by the displacement principal. The working parts of the compressor are two rotating screws, parallel placed in a compressor case. The screws, or the rotors, are shaped with male and female profiles that mesh into on another.
The impetus is transferred to the male rotor. The most common combination is four and six lobes for the male and the female rotor. Four lobes on the male rotor mesh into 6 lobes on the female rotor. If the male rotor has a number of revolutions of 3600 rpm, the female rotor number of revolutions is: 4 / 6 x 3600 rpm = 2400 rpm The compressors working progress is, as follows: 1. 1. The lobe on the female rotor reveals the die orifice. When the rotors turn, the chamber gets longer and gas sucks into the compressor. 2. 2. The chamber passes the inlet opening. The gas is now closed between the end casing. 3. 3. Gradually as the rotor twists additionally, the volume is less and the gas compress against the outlet wall. 4. 4. An additional twist of the rotor uncovers the outlet wall and the gas is pressed out of the compressor. In the most common screw compressors on board, the confined gas will compress to a precise volume before blowing down to the pressure side. One says that the compressor has a constant embedded volume proportion. The embedded volume proportion is determined from the proportion: V1 / V2 where V1 is the maximum induction volume per rotor per rotation. V2 is the volume of reticent gas after compression, but before blow down to pressure side. The pressure proportion is not constant, but dependent from ploy for the employed gas and rate of cooling. The connection between the volume proportion and the pressure proportion can therefore be expressed, as:
(V1 / V2)k = Designed compression proportion = Delivery pressure/suction pressure
Practically, this means that when the drift parameters change, the screw compressors with a built-in volume proportion will compress the gas needlessly too much or too little. Too low compression increases back flow of gas and hence reduced efficiency. The oil free screw compressors are used on board as cargo compressors. A synchronising gear is used to keep the rotors from wearing one another. The axial strength recovers when placed on the outlet side. Bearings and gear lubricate by a self-pressure oil system. To prevent oil from forcing into the compressor room, axial sealing is installed between the bearings and the gas area inside the compressor. The compressor case is cooled and heated by glycol. The number of revolutions is kept relatively high, 5000 to 15000 rpm, to keep the leakage loss as low as possible. This means that if electric motors are used as; the gear must be used. Oil lubricated screw compressors are used on board on the Freon side in cascade plants. The female rotor is driven directly by the male rotor in this type of compressor. Ample oil must therefore be supplied to the rotors. The oil has the following purpose: • • • • • •
· · · · · ·
Lubricate the bearings Lubricate, cool and seal between rotors and cases Lubricate, cool and seal between the rotors Cool and lubricate the axial sealing Give hydraulic energy to the capacity regulation Cool the gas
The screw compressors power consumption is unfavourable compared with the twostage piston compressor. Installation of a “superfeed” arrangement compensates some in this condition.
The condensate is super-cooled in a coil placed in a combined evaporator and intercooler. The boil off in the inter-cooler is led back to the compressor (S). The plants cooling capacity will rise (∆Q). The power consumption will also rise, but less than the increased cooling capacity.
The capacity regulation on oil lubricated screw compressors takes place by help of a hydraulic operated drawback that opens a gate to the inlet side. The capacity if often infinitely variable between 10 % and 100 %. The advantages with screw compressors are: • • • • • • • • •
· · · · · · · · ·
no suction or pressure valves few mobile parts lower cost at purchase, installation and maintenance little vibrations and simple foundation even gas flow, no pulsating no sensitiveness for liquid in the gas flow high volumetric efficiency of the actual working area possibilities for infinitely variable capacity regulation low pressure pipe temperature for oil lubricated screw compressors
13.2.7 Lubricating oil system The sketch below indicates a normal oil system for cooling plants with soil lubricated screw compressors.
Despite the fact that the oil separator is installed after the compressor, a larger share of oil is constantly in the freon consequently this leads to poor plant cooling capacity. It is not possible to drain off pure oil on the liquid side and a system is therefore installed as an oil-restoring device in these plants. The oil-restoring device system is often used in these systems. Some of the cold and the oil-rich freon liquid is taken from the pressure side and delivered to the lower inlet of the heat exchanger. A relatively warm freon liquid from the liquid collector meets it here. The oil-rich freon liquid evaporates inside the pipes. Because of the large velocity of the gas out of the heat exchanger, the oil particles are carried with it, as well.
An independent regulation valve provides that a suitable amount of freon is let into the heat exchanger when the liquid is taken from the pressure side. A constant overflow pipe provides that the level in the heat exchanger is constant. 13.2.8 Thermostatic expansion valve Thermostatic expansion valves are used in the cooling plant to regulate the liquid injection in the heat exchangers. The evaporation pressure and the overheating in the heat exchanger control the valve. For the valve to function satisfactorily the valve must be installed as recommended by the supplier and the valve is dimensioned to the plant. In choosing a valve, one must consider the operation terms and the capacity need for the plant.
13.3 ONE-STAGE DIRECT OPERATION Cooling plants with 1-stage direct operation are the simplest cooling plants with the least number of components. The description “1-stage” refers to the compression that occurs in only one step. “Direct operation” means that the cargo vapour is sucked from the cargo tank and compressed directly either against seawater or Freon in an cascade plant.
In this chapter we have chosen to use one-sage direct cooling for our examples, which is used on board many semi-pressurised gas carrier. Propylene is chosen as cargo and cooling media. The simplified sketch of the plant indicates the main components in a cooling plant with possibility for motoring both with 1 stage and 2 stage direct operation. With 1 stage direct operation cargo tank(s), liquid separator, compressor, condenser, liquid collector and regulation valves are utilised. Notice that the compressor has two or more cylinders with different diameters. This is normal when the plant is also designed for 2 stage operation. Following, there is also an intermediate cooler with coil. This is not in use in 1 stage operation and supply lines are therefore drawn as a broken dashed line. On the following log sheet, an actual operation situation for this plat is registered. The cooling process itself is drawn in a sketch of a Mollier diagram for propylene.
Cargo
Air Power o Date: Time: C Amp. % load 21/3
12:00
28
180
100
Sea water Pressure Temp. Temp Tank 1.suct 1.deliv. 1.suct. 1.deliv. In Out o o o bar bar bar C C C oC 3,0
2,8
10,5
-5
80
23
27
The loading tank is filled with propylene. The temperature of the liquid in the tank is – 12oC, this gives a tank pressure of 3 bars. Heat will be transferred to the tank and the cargo because of the difference in temperature between cargo and the surroundings. The condition of the liquid in the tank is in the boiling point line at a pressure of 3 bars. When liquid boils, the gas over the liquid is regarded as saturated. The condition of the gas at this point is, therefore, in the Mollier diagram on the saturation line at an absolute pressure of 4 bar. The point is marked as A. The piston compressor sucks the gas from the tank via a liquid separator. Because of heat leakage, the gas is heated between the tank and compressor. In addition, the pressure in the gas will reduce some because of friction in the pipeline. When gas sucks into the compressor, both temperature and pressure will be changed. If the compressor’s suction temperature is –5oC and read-off suction pressure is 2,8 bars, the condition of the gas (when it sucks into the compressor) can be plotted into the diagram. The condition is in an overheated area where the temperature line of –5oC and the pressure line of 3.8 bars (2,8 bars + 1 bars) crosses. The point is marked as B.
The gas is compressed parallel over both cylinders and delivered to the seawatercooled condenser. The compressor performs a job on the gas and the gas is supplied with energy. Pressure, temperature and enthalpy increases. The compressing process is traced into the diagram as the line from B to C.
The heat is transferred from the gas to the seawater in the condenser. The gas into the condenser now has a temperature of 80oC. The temperature of the seawater into the condenser is read-off as 23oC. The gas cools and condenses against the relative cold seawater under constant pressure. The pressure is maintained by the compressor, which continuously compresses new vapour. The process line in the Mollier diagram is traced from point C to D. Notice that the gas’s overheating at first is removed, and then the gas is saturated. Then the evaporation heat is removed and the gas condenses. When all gas is condensed in the condenser, the state is on the boiling point line. The relative cold gas of about –12oC that was sucked out from the tank has, through the compression, increased the temperature to a relative warmer state than the seawater. This renders a possibility of heat transmission from an originally colder to a warmer medium. One can see that the gas’s enthalpy reduces in the process. A heat quantity is transferred to the seawater that is corresponding to the difference in enthalpy of the gas that sucks from the tank and the condensate enthalpy after the condenser. It is the temperature of the seawater that circulates through the condenser that determines the condensation pressure. The heat exchanger is normally designed so that the temperature difference between seawater and condensate is between 3oC and 8oC. In our example, the read-off condensation pressure is 10,5 bars. In the heat technical table for propylene, one can see that this corresponds to a condensation temperature of 25o C, which is within the normal area. A change in the temperature of seawater and gas over the condenser indicates something is wrong. If for example, the seawater side becomes dirty, the temperature increase of the seawater over the condenser is smaller and the condensation pressure higher. One should notice that such changes of the cooling plant mainly occur over time, something that makes it difficult to intercept the signals. The only useful method to reveal such a development is to log the drift parameter regularly. This routine is of great importance. The suction of the gas is now condensed and the condensate collected in a liquid collector. A regulation valve between the liquid collector and the cargo tank regulates the level in the liquid collector. One can see the process line between point D and E in the Mollier diagram. Notice that there is no enthalpy change. Temperature and pressure is what changes over the valve. When the pressure over the condensate is reduced from condensation pressure of 10,5 bars to tank pressure of 3 bars, the liquid will boil energetically. Boiling requires heat. This heat is mainly taken from the liquid itself. The liquid cools and some of the gas evaporates. A mixture of liquid and gas with the same temperature as the liquid and the gas in the tank is what returns into the cargo tank. The temperature must be the same for the mixture, because the pressure is the same. One can locate the shares of liquid and saturated gas in the Mollier diagram as point E. The regulation valve’s job is to let the condensate back to the cargo tank in a constant and controlled process. The valve should simultaneously keep the liquid locked to secure the compressor’s maintenance of the compression pressure. It is of great importance that the regulation valve has satisfactory activity. If the liquid level and thereby the liquid lock disappears, uncondensed gas will blow through the
condenser and reduce the capability of the plant. If the liquid level rises, so the condenser refills this result in an inferior heat exchange of the condenser and a higher condensation pressure. If the compressor has a capability of 350 m3/h, the net cooling capacity of the plant, Qnet is: Qnet
= m x (h2 - h1)
The capacity of the compressor (V) and the density (p) of the gas into the compressor determines the mass (m), which flows through the plant. The gas’s specific volume (v) is point B in the Mollier diagram to 0,12 m3/kg. This corresponds to a density of (l/0,12 m3 /kg) or 8,33 kg/m3. Qnet = {V /3600 s} x r x (h1 - h2) = {(350 m3/h) / 3600 s} x 8,33 kg/m3 x (240 kJ7kg - -60 kJ/kg) = 243 kW Notice the factor, which expresses the cooling plant’s capability to influence and to control, thereby, the cooling capacity of the plant. Its construction and condition determine the capacity of the compressor. Good control and maintenance influence the capability of the cooling plant. Pressure drops determine the density of the gas into the compressor and temperature increases between tank and compressor. Needless throttling of valves on the suction side or tightened filters, gives a needless low pressure and thereby, a lower density. Inferior isolation of suction pipe or tank gives a larger heat leakage, higher temperature and thereby lower density. Lower density gives less mass through the compressor hence with reduced capability. The enthalpy difference of the plant is determined by the cooling media and other outer conditions like the seawater temperature. These factors are therefore less influential than the others are. By studying the process of the Mollier diagram, one can see that the condition for the temperature of the liquid in the tank and h2 determines the gas at the top of the tank. The other enthalpy point, h1, is determined by the condensation pressure. Notice that a higher condensation pressure removes this point to the right and will give less enthalpy difference than with a low condensation pressure. A lowest possible condensation pressure gives therefore the best capacity in addition to the best operating condition for the compressor. If the capability of the cooling plant is larger than the total heat leakage, the temperature of the cargo will be lower over time. When the temperature sinks, the tank pressure sinks. If the seawater temperature and other operating conditions are unaltered, the process lines EAB are displaced parallel downward with the tank pressure against the process lines E1A1B1.
One can see that a lower tank pressure results in a lower cooling capacity. The mass through the compressor sinks because the density of the vapour is lower with a lower pressure. In addition, the point h2 is displacing gently against the left; this results in some lower enthalpy difference. One must notice that the saturation line of the different material has different inclines. Therefore the influence of this change varies. If the condensation pressure is kept unaltered, the process will be longer for the compression and the outlet temperature from the compressor will be higher. The condition for the gas after compression is removed from C to C1. This has in itself no direct influence on the capacity, as long as the condenser can transfer the necessary heat amount from the gas to the seawater. Previously, we have seen how the piston compressor delivery extent depends on the pressure condition of the compressor. The pressure condition in our example is, as follows: Pressure condition = Delivery pressure/suction pressure = 11,5 / 3,8 = 3 According to the fall of the tank pressure, the pressure condition will rise and result in a constant decreasing delivery extent. A reduction of the suction pressure with 2 bars and the same condensation pressure gives a pressure condition of: 11,5 / 1,8 = 6,4 This increased pressure condition gives an essential reduction in delivery extent and thereby cooling capacity. The influence of reduced delivery extent of the cooling capacity is often larger than the summary of the other factors. The explanation of why the cooling capacity is at the lowest by low tank pressure is some complex. In addition to above-mentioned condition, the heat leakage will, of course, be largest at a low tank pressure.
13.3.1.1 Example 1 The cooling capacity of the vessel is at the moment 234 kW. We want to find the time to cool down the cargo of 1000 MT propylene, from –12o to –30o. At first we set an expression for the heat balance: Net heat abducted in the cooling plant = heat-leakage + heat abducted from the cargo In the vessel’s operation manual, we find that the heat leakage to the cargo tanks at presence air and seawater temperature is 36 kW. The necessary heat amount removed from the cargo to cool the cargo is located in the Mollier diagram or in a heat technical table: Enthalpy for propylene at-12 oC = 61,5 kcal/kg = 257,1 kJ/kg - Enthalpy for propylene at -30 oC = 52,1 kcal/kg = 217,8 kJ/kg =Enthalpy difference = 39,3 kJ/kg Here one must eliminate 39,3 kJ from one kilo propylene to lower the heat temperature from -12 oC to -30 oC. As we have 1000 MT propylene, the heat amount that must be abducted is: (1000 x 103) kg x 39,3 kJ/kg = 39,3 x 106 kJ We insert above-mentioned values of the heat balance and get: (234 kW x T) = (36 kW x T) + 39,3 x 106 kJ where T = cooling time in seconds T = (39,3 x 106 / 198)s = 198484 s T = (198484 / 3600) hrs. = 55 hrs. The calculated cooling time here at unaltered heat leakage and cooling capacity is about 55 hours. In practice, the cooling time will be a bit longer because the cooling capacity and the heat leakage is not constant in the period. Example 2 13.3.1.2 The vessel is loaded with propane and the cargo tank pressure is read to 2 bars. The suction pressure of a cargo compressor is read to 1,5 bars and the suction temperature is –10oC (14oF). Plot the state-point for the gas into the compressor. As the pressure of the gas into the compressor is 1,5 bars, the state-point must lie in a pressure line equal 0,25 MPa (1,5 bars + 1 bars) in the diagram. The exact statepoint is plotted where the temperature line of –10oC crosses the pressure line of 0,25 MPa.
When the point is plotted, the density of the gas into the compressor is determined from the density lines aslant towards right in the diagram. Qnet = m x h Where m = the mass of gas flowing through the cooling plant per time (kg/s) h = the difference between enthalpy of the gas that leaves the tank and enthalpy on the condensate that is returned back to the tank (kJ/kg) Notice that the density of the gas increases at higher gas pressure and lower temperature. Larger density gives more mass per hour that flows through the plant. More mass involves larger cold capacity for the plant. 13.3.3 Example 3 A gas bottle is filled with liquid propane. The temperature of the air and propane liquid and gas is 15oC. The gas above the liquid is regarded as saturated. The valve opens and liquid propane runs into an open container. Plot the statepoints for the liquid in the bottle before the valve was opened and for the liquid in the open container afterwards. The liquid lies on the liquid line and the point (1) is determined either from the temperature line of 15o C (59oF) or corresponding pressure line of 0,73 MPa (7,3 bars). When the liquid is discharged from the bottle, the pressure lowers above the liquid to the atmosphere pressure (1 bars). The quick reduction of the pressure involves powerful boiling in the liquid because of an “unbalance” between the liquid temperature and the gas pressure above the liquid. The heat from boiling is taken from the liquid itself and the surroundings and the liquid gets colder. No enthalpy change will take place during the process. One can trace the process line (from point 1 to point 2) for the alteration that occurs with the liquid, as a vertical line from the point of the liquid line to the atmosphere pressure line. Notice that the new state-point (2) lies inside the “sack” and that a precise share of the liquid has vaporised because of the pressure reduction. 13.4
2-STAGE DIRECT OPERATION
One can see how the cooling plant’s capacity was reduced at larger pressure conditions in the chapter concerning 1 stage direct operation. At 2 stage operation, stated pressure conditions will improve the capacity of the plant. This plant is the same as used in the one-stage lesson. The difference is that the plant now is altered to 2 stage operation with intermediate cooling. Notice how the volume flow now compares to 1stage operation and which valves that regulates this.
The compressor is the same, but only one of the cylinders is used to suck from the tank.
The compressor is traced with different diameters of the cylinders. Most of the piston compressors that are used on the cargo-side have this construction. The largest cylinder is used as 1st stage in the compression and is called the low-pressure cylinder. The smallest cylinder is used in 2nd stage compression and is called the high-pressure cylinder. There is also installed an intermediate pressure container with a coil for sub-cooling of the condensate in this plant. The following table indicates a summary from the cooling plant’s log sheet in an actual stage situation. Cargo Pressure Temp. Air Tank 1.suct 1.deliv 2.deliv 1.suct 1.deliv 2.suct 2.deliv o o o o o C bar bar bar bar C C C C
Sea water Temp. In Out o C oC
27
23
0,7
0,5
3,5
10,5
-25
50
-7
74
27
The compressor’s low pressure cylinder or 1st stage suck vapour from the cargo tank. The vapour leaves the tank with state A and are sucked into the compressor’s 1st stage with state B. The vapour compresses in the compressor’s 1st stage and is delivered to the intermediate cooler (MT) with state C. The intermediate cooler is
partly filled with cargo liquid. The superheated gas is normally led down in the liquid where it is cooled by the relatively cold liquid. The compressor’s 2nd stage suck in saturated vapour from the top of the MT cooler and compresses this vapour and delivers it to the cargo condenser. The process line is marked DE.
The heat is transferred from the vapour to the seawater in the cargo condenser, the vapour condensate and is collected in the liquid collector. The process line is marked EF. The condensate is led via a coil in the MT cooler through the regulation valve and back to the cargo tank. The process line is marked FG. Notice that the state-point is below the sub-cooled area. The pressure in the intermediate cooler determines the temperature of the liquid in it. A pressure of 3,5 bars for propylene corresponds to a temperature of –8oC. The vapour that is sucked inn to the 2nd stage has a temperature of –7oC in the log. The small difference of temperature may be caused by heat leakage in the suction line or reading error. The state-point G is therefore determined from the temperature line corresponding to a pressure of 0,45 MPa (3,5 bars + 1 bars). A to low liquid level in the MT cooler results in a sub-cooling of the condensate and thereby reduces cooling capacity. The heat supplied to the liquid in the MT cooler from the warm vapour from the 1st stage and the relatively warm condensate leads to a lot of liquid evaporation. An independent regulation valve regulates the liquid level in the MT cooler. The vapour is led from 1st stage and down in the liquid of the MT cooler in our 2stage plant. Many plants that also have the possibility for “flash cooling” where the vapour is lead into the top of the MT cooler and give operational advantages. After the coil in the MT cooler the condensate is released back to the cargo tank through the regulation valve. Notice that there is a larger share of liquid now than at 1-stage operation. The state-point for the condensate that returns back to the tank is further to the left in the diagram.
The cooling plants net cold capacity is calculated, as previously, by the expression: Qnet = m x (h2 - h1) The enthalpy difference is larger at 2-stage operation and sub-cooling, which is something that increases the result. But the mass of gas flowing through the plant is also altered when the plant is reorganised from 1-stage to 2-stage operation. A lower pressure condition gives a better delivery extent and thereby larger mass. With 2stage operation, vapour from the cargo tank is sucked with one cylinder only compared with 1-stage where all cylinders is used to suck in the vapour. This has a negative guided influence so that the total increase of the cooling capacity is thereby reduced. 13.3.1 Example 1 To better visualise the difference between 1-stage operation and 2-stage operation, we can look at an example where the vessel has loaded warm propylene and delivered the cargo fully cooled. The plant is started with 1-stage operation because of a very high tank pressure. Subsequent to the cargo cooling, the cargo tank pressure is reduced. The result is increased differential pressure between 1st and 2nd stage and reduced delivery extent. When the temperature of the cargo is at –30oC, one decides to reorganise the operation to a 2-stage operation with intermediate cooling. The following working parameters are registered before and after the re-adjustment: Cargo
Sea water Pressure Temp. Temp Tank 1.suct 1.deliv. 2.deliv. 1.suct 1.deliv. 2.suct. 2.deliv. In Out o o o o o bar bar bar bar C C C C C oC 1,1
1,0
3,7
1,1
0,9
10,5
10,5
-20
54
-22
80
-7
74
23
27
23
27
The compressors capacity at 1-stage operation and a pressure condition of 6 (11,5/1,9) are calculated to 275 m3/h. At 2-stage operation and a pressure condition of 2,4 (4,7/2), this is calculated to 240 m3/h. The cooling capacity at 1-stage operation is calculated from the following values located in the Mollies diagram and technical table: Density on gas at -22oC and 0,9 bar:
4,17 kg/m3
Enthalpy of gas in a cargo tank: 230 kJ/kg Enthalpy of the condensate after condenser: -60 kJ/kg Qnett = {V /3600 s} x r x (h1 - h2) = {(275 m3/h) / 3600 s} x 4,17 kg/m3 x (230 kJ7kg - -60 kJ/kg)
= 92 kW The cooling capacity at 2-stage operation and intermediate cooling are calculated from the following values located in the Mollier diagram and technical table: Density of gas at -20oC and 1 bar: Enthalpy of gas in a cargo tank:
4,17 kg/m3 230 kJ/kg
Enthalpy of the condensate after condenser: -140 kJ/kg Qnett = {V /3600 s} x r x (h1 - h2) = {(240 m3/h) / 3600 s} x 4,17 kg/m3 x (230 kJ7kg - -140 kJ/kg) = 103 kW The two calculated cooling capacities in the example must be evaluated from the accuracy in our measures. With normal measure divergence, one can conclude that the cooling capacities are about equal for this plant at –30oC for propylene and a condensation pressure of 10,5 bars. The decision of reorganising the plant to 2-stage operation at this moment looks correct, if one only evaluates the cooling capacities. One must however notice that normally it refers, in the operating manuals, only to maximum delivery-pressure/maximum differential-pressure over the compressor and not pressure condition. These parameters are simple practical expressions for the compressor’s constructional limitations, which influence the operation. If it is specified, in the operating manual, that maximum delivery pressure at 1-stage is 12 bars and maximum differential pressure is 8 bars, alteration from 1-stage to 2stage operation is determined, as follows: At unchanged seawater temperature, the condensation pressure is 10,5 bars. Limitation of maximum delivery pressure is thereby kept regardless of operations form. The differential pressure at 1-stage will rise at falling tank pressure. If the condensation pressure is a constant 10,5 bar, 1-stage will have a differential pressure of 8 bars when the suction pressure p0 is: po = (10,5 - 8) bar = 2,5 bar If the pressure drop of the suction line is constant at 0,1 bars, the plant must alter to 2-stage operation at a tank pressure of 2,6 bars, which corresponds to a cargo temperature of –15oC. From constructional limitations one can see that the cooling plant should be altered to 2-stage operation at –15oC and not at –30oC. Example 2 13.3.4 When we are loading cargo that is warmer than compared with the safety valve’s set point, the loading time is determined by the capacity of the cooling plant. To maintain the tank pressure during the opening pressure for the valves, the cargo that is loaded must be cooled down. Bad plant condition and poor plant operation reduces the cooling capacity and results in longer loading time. The same gas tanker, as in the example above is to load 4000 MT hot propylene of +15oC. The loading tank’s safety valves have an opening pressure of 4,5 bars. It is determined to maintain the tank pressure of 4 bars during the loading.
In the following log two different operational parameters are registered from the cargo cooling plant. In the first alternative, the compressor’s suction valves are throttled to a suction pressure of 1,5 bars. In the second alternative, the suction valve is less throttled and the suction pressure is 2,5 bars.
The following operational parameters is registered before and after the alteration: Cargo
Alt.
Tank bar
Pressure 1.suct 1.deliv. bar bar
Temp. 1.suct. 1.deliv. o o C C
1
4,0
1,5
8,0
5
81
2
4,0
2,5
8,0
0
77
The two cooling processes are plotted into a Mollier’s diagram for propylene. Notice that the enthalpy difference is the same regardless of suction pressure. In the Mollier diagram, we find specific volume and density for the gas into the compressors at the two suction pressures: Specific volume for propylene: v/2,5 bar and 0oC:
0,14 m3/kg
= 7,143 kg/m3
0,22 m3/kg
= 4,546 kg/m3
Specific volume for propylene v/1,5 bar and 5 oC:
As the cooling capacity expresses: Qnet = m x (h2 - h1)
One can see that the difference in the cooling capacity will be proportional with the change in the mass flowing through the plant. The percentage difference in the cooling capacity thereby is expressed, as follows: {(7,143 - 4,546) x 100 / 7,143}% = 36 % The cooling capacity is reduced by about 36% if the suction pressure is reduced from 2,5 bars to 1,5 bars for this re-condensation plant during the above-mentioned conditions. This also means that loading time will increase with 36%.
To visualise this influence better, we calculate the loading time for the vessel. We assume that the cargo tanks with ROB, steal and isolation are cooled down to -5oC before commence loading. The heat leakage to the cargo tanks (QTr) is stated in the vessel’s operational manuals to 144 kW and three identical cargo cooling units drive in the re-condensation plant, where the compressor’s capacity in each cooling unit is set to 400 m3/h. We locate how much heat has to be removed from each kilo propylene liquid to cool this down to +15oC to –5oC in the heat technical table: Enthalpy for propylene v/15oC:
76,8 kcal/kg
= 321,5 kJ/kg
- Enthalpy for propylene v/-5oC:
65,7 kcal/kg
= 275,0 kJ/kg
= Necessary abducted heat, ∆h
= 46,5 kJ/kg
Necessary heat that must be removed from the cargo (QL) is: = m x Dh QL = (4000 x 103)kg x 46,5 kJ/kg = 186 x 106 kJ We find the enthalpy values of the gas out of the tank (h2) and the enthalpy of the mixture that is returned to the tank (h1), in the Mollier diagram: h2 = 240 kJ/kg h1 = -80 kJ/kg The cooling capacity for the entire re-condensation plant at a suction pressure of 2,5 bars and 0oC is: Qnetto
= m x (h2 - h1) = {V /3600 s} x r x (h1 - h2) = {3 x 400 m3/h / 3600} x 7,143 kg/m3 x (240 –80)kJ/kg = 762 kW
The loading time T1, at this operation situation is then: = QL / (Qnetto - QTr) T1 = 186 x 106 kJ
/ (762 - 144) kW
= 300971 s = 83.6 hours The cooling capacity for the entire re-condensation plant at a suction pressure of 1,5 bar and 5oC is: Qnetto
= m x (h2 - h1) = {V /3600 s} x r x (h1 - h2) = {3 x 400 m3/h / 3600} x 4,546 kg/m3 x (240 –80)kJ/kg = 485 kW
The loading time, T2, at this operation situation is then: T2
= = = =
QL / (Qnetto - QTr) / (485 - 144) kW 186 x 106 kJ 545455 s 152 hours
We can see that the influence of unnecessary throttling on the suction side of the compressor gives large deflection of the cooling capacity and thereby loading time. The tank pressure during the loading period also has influence on the total loading time. If a high tank pressure is kept close up to the safety valve’s opening pressure, less heat will be abducted from the loaded cargo and loading time will be shorter. 13.5 CASCADE PLANT A cascade plant onboard a gas tanker is a cooling plant composed by two coolant circuits working in serial with each other. Both circuits are complete cooling plants that are built in many different configurations. Both 1-stage and 2-stage operations are used depending on the cargo that should be cooled and choice of components in the cooling systems. One of the cooling circuits consist in a closed cooling process where the cargo directly condenses contra freon or other cooling media. The other cooling circuit consists in a closed cooling process where the freon or another cooling medium condenses contra seawater. The cargo side of the cascade plant is the same as in the 2-stage plant example, with exception for the condenser. The seawater-cooled condenser is now replaced with a freon-cooled condenser. On most cascade plants we have both a freon condenser and seawater cooled condensers in the cargo cooling plant. This gives
flexibility and good operation economy. The freon side of this plant is a simple 1stage plant with screw compressor.
Freon compressors are of oil-lubricated type. Large amounts of oil will follow the compression gas out of the compressor. If this is not separated and returned to the oil receiver, the share of oil in the freon circuit will be too high. The oil will reduce heat transfer in the heat exchangers and create operation interruptions. The oil separator has two functions where it separates oil from the gas and at the same time is a system tank for the oil system. The following summary from a cooling plant’s log indicates an actual operation situation for this plant. The cargo that is cooled is ethylene. Ethylene cycle Pressure Temp. Tank 1.suct 1.deliv. 2.deliv. 1.suct. 1.deliv. 2.suct. 2.deliv. o o o o bar bar bar bar C C C C 0,6
0,5
5,3
17,3
Freon cycle Pressure Temp. 1.suct 1.deliv. 1.suct. 1.deliv. o o bar bar C C 0,2
12,5
-36
60
-42
70
Sea water Temp In Out o o C C 28
30
-60
50
The cooling process of the cargo side is plotted into a Mollier diagram for ethylene and is, as follows:
The cooling process on the freon side of the cascade plants is plotted equal.
The condensation pressure of the loading side depends on the temperature of the freon liquid circulating through the loading condenser. The pressure in the liquid separator again depends on the suction pressure of the freon compressor and determines the freon temperature. (We assume that the freon side has the right filling). Notice that the condensation pressure for ethylene lies at 17,3 bars, which corresponds to a temperature of –27oC. The pressure in the liquid separator is 0,2 bars, which corresponds to a temperature of –37oC. This gives a temperature difference of 10oC in the cargo condenser, usually too high for this type of heat exchanger. If the freon pump don’t deliver sufficient liquid to the cargo condenser, the cause is probably one or a combination of the following: • · • · • ·
reduced heat transmission caused by incrustation in the loading condenser reduced heat transmission caused by too large share of oil in the Freon liquid too high condenser pressure caused by unknown gas on the loading side
Cascade plants are used both for LPG and ethylene. The plant exists both on atmosphere gas carrier LPG and for semi pressured gas carriers LPG/ ethylene. This
plant is an example of a usual cascade plant that can also be used to re-condense ethylene. Normally screw compressors are used both on the loading side and on the freon side in the cooling plant onboard big atmosphere pressure gas carrier LPG. There are many different configurations of cascade plants. The freon side is frequently equipped with super-feed or intermediate pressure container with subcooling. Some plants have freon pumps, which pump freon through the cargo condenser, where others use thermal expansion valves. Freon compressors can be piston compressors or screw compressors. Screw compressors can be built with one or two stages. Plants with and without MT-containers, with and without flash cooling, with and without de-super-heaters, 2-stage compression with and without sub-cooling, exist on the loading side. The compressors on the cargo side have to be oil free piston or screw compressors. The configuration possibilities are many and the variation in plants from vessel to vessel is what one faces onboard. The understanding of the cooling process and knowledge to your vessel’s “special” plant is a basic assumption for safe and economic operation of the plant.
14-
Insulation and Heat Transfer
14.1 INSULATION AND INSULATION MATERIAL
There are three different methods in transporting heat: thermal conductance, convection and radiation. The insulation material’s’ main task is to reduce heat transmission from thermal conductance. Most insulation material’s insulating qualities emerge from stationary gases, bad thermal conductance capability and thereby good insulation capability. The thermal conductance capability is expressed by a material’s thermal conductance number (thermal conductivity) and states the heat quantity measured by Watt, which is transported through to surface of 1 meter thickness when the temperature difference is 1K.
Thermal conductivity Natural pure metals
From 8 to 400
W/m K
80
W/m K
2,4
W/m K
From 0,10 to 0,60
W/m K
Wood
From 0,1 to 0,3
W/m K
Glass
1
W/m K
Polypropylene
0,12
W/m K
Sand
0,35
W/m K
Natural gases
From 0,008 to 0,048
W/m K
Stationary air
0,024
W/m K
Freon 22
0,012
W/m K
Iron Ice at -20 oC Natural liquids
One can see that the best heat conductor or worst insulation materials are pure metals. The worst heat conductors or best insulation materials are stationary gases. It is of most importance that the gases are kept as stationary as possible, because the total thermal transmission is higher if convection also arises. This is in practice solved by trying to catch the gas inside the smallest possible cavity, or by keeping the gas inside a net of thin fibres. The thermal conductance figure will mainly increase at higher temperature because of larger convection. The best insulation material regarding thermal transmission is a composition of a firm material and a gas with the separately lowest thermal conductance figures. First of all, it is the thermal conductance figure that is of interest when looking at the important qualities for an insulation material. Lower thermal conductance figures render possible thinner insulation and thereby place saved.
The upper temperature limit for the material’s relevance normally has no importance for an insulation material that should be used for cool insulation, but one prefers that the material tolerate highest possible temperature considering fire. The insulation material’s lower temperature is of major importance. In the plastic insulation condensation can occur at a lower temperature and thereby increase the thermal inductance figure for the material. Thermal expansion and elasticity are two qualities of great importance for plastic insulation materials, for example cargo tanks on gas tankers. Changing temperature for tank shells and insulation can lead to periodical expansions and compressions. As the thermal expansion co-efficiency for the insulation material can be 4 to 5 times larger than the steel, cracks may easily arise if the insulation material don’t have good elasticity. A plastic insulation material like polyurethane has good adhesion firmness to the steel priming and good elasticity and is thereby resistant to cracks. An insulating problem area is around the loose tank cradle of cylinder cargo tanks. All of the expansion and compression movement of longitudinal direction of the tank takes place here. Large demands are made both to material and for the insulation to be good and long performance. A special developed insulation with especially good elasticity is suitable for such areas. All insulation materials that are used onboard must be fire resistant. Insulation materials made of plastic are added or built up in such a way that in case of fire, fireextinguishing vapour are released, and they are self-quenching. The insulation material will only burn as long as a foreign fire source is present. One must not underestimate the risk of fire in the plastic insulation and the consequences by such a fire. Fire can easily arise in such materials in connection with weld work. Ignition of the insulation on the cargo tanks has occurred with fatal consequences. The large amount of thick, black and poisonous smoke that is formed by such a fire has prevented escape from hold space and serious poisoned injures. Where the material is exposed to strains the material’s compressive-strength is important. The strain points will, for example, be large in the support points for an insulated pipeline. To evade lasting compression or crumbling of the insulation material, special compressive-strengthen materials are used in these areas. Of the chemical qualities for insulation materials, it is the corrosive qualities that often are underestimated. When the insulation material of glass or mineral wool got a high humidity, it will have a strong corroding effect on metals. It is difficult over time to avoid this onboard. Even stainless steel pipes corrode in such an environment and if this is allowed to proceed, an expensive replacement is soon the result. The capability to resist humidity or diffusion resistance is an important quality that must be evaluated in choosing insulation material, as well as, in planning maintenance. When insulating between a warm and cold side, for example, a cargo tank, the cold side will be tight (tank shell) and the warm side will be surrounded by air with high humidity. Because of a higher saturation pressure on the warm side in proportion to the cold side, moist air will be forced through and will condense against the cold side. The thermal conductance figure will, because of the humidity, increase and the water will freeze and destroy the insulation. The damage extent will accelerate if such a process is continuously unchanged. If the diffusion resistance for stationary air is set to 1, the proportional diffusion resistance for mineral wool will be about 1,5 and for polyurethane it will be about 60.
This indicates that insulation of polyurethane is about 40 times more resistant against moist air to leak through than an insulation of mineral wool. Regardless of which insulation material that is used, moist air will penetrate into the material and destroy it.
A diffusion-tight damp-latch on the warm side is, at all times, imperative on a chill insulation. The most commonly used materials and methods are: • • • • •
• • • •
Thin aluminium foil glued on the insulation. This method is suitable for insulation non-exposed for mechanical wear and tear, for example, some loading tank constructions. Galvanised and stainless steel sheet that is fastened by pop cones and the joint seals with jointing compound. This method is expensive, but strong against mechanical wear and tear and necessary for “foaming”. Glass-fibre armed polyester. This method is more moderate, but gets easily fragile. Most suitable for repair One or more layers with asphalt emulsion armoured with multiple layers of glass fibre fabric. Moderate method and easy to maintain, but weak for mechanical wear and tear. Sprayed mastic with or without armouring. Moderate method, but week against mechanical war and tear and requires more maintenance on exposed places. Most suitable for repair and sprinkling on insulation not exposed to rough weather. The insulation material on gas ships may be divided into the following three different groups based on structure and material: Cellular plastic, which is expanded plastic raw material, built up in a cellular structure. Wallboard, which is built up of a net with thin fibres. Expanded volcanic perlite, which is built up on a cellular structure.
14.1.1 Polyurethane There are a number of insulation materials, which are built up of raw plastic materials. The most used is polyurethane. Mixing isocynate and polyole, normally in the proportion 1:1 makes polyurethane. Isocyanate has a resemblance to thick oil and polyole has a resemblance to clear liquid. A chemical reaction that is exothermic is actuated when mixing the liquids. If one adds some water, carbon dioxide is formed, which because of the reaction-heat evaporates and “blows” up the material. Foam with about 90% closed cells and a very low thermal conductance number appear. Polyurethane-foam based on CO2 has a relatively long time of expansion.
The qualities of the polyurethane-foam can be improved by using different freon materials as a blowing agent. Because of the freon material’s lower boiling point, the expansion is quicker. R11 was used earlier as a blowing agent. The thermal conductance capability for R11 is only half of CO2. If applying polyurethane foam with a sprayer, or frothing, freon is used as the blowing agent. Freon evaporates with speed in normal surrounding temperature and one obtains pre-expansion, when the mixture leave the spray. One can mix polyurethane foam for smaller repairs in a bucket before pouring into a mould. As there are different suppliers on the market, it is recommended to check with the supplier about the composition and if water has to be used when mixing. There are also a number of machines for spraying on the market, also disposable spray equipment with smaller containers for iciyanat and polyol. Disposal-spray equipment of this type or a simple modifying of the paint sprayer onboard, is suitable for smaller repairs to the insulation. Polyurethane is also available as half-cups for insulation of pipes. It is important that the dimension of the cup fit to the outer diameter of the pipe so that air leak is avoided. The polyurethane cups can easily be cut for matching the pipe bend, bend and valves. It is recommended to lay two layers with half cups on larger pipes, so that the connections are displaced in proportion to each other. The half cups and the connections are glued, and secure the density and strength and a water barrier is laid either with thin metal plates or mastic. 14.1.2 Polystyrene Polystyrene is produced in two stages. At the first stage polystyrene is pre-expanded with vapour where blisters from 1 to 6 mm are formed. The pre-expanded material is sent up into large silos for de-aeration. After 2 to 4 day in the silo, the pre-expanded material is filled in forms for further expansion and compression of the grains, to shape blocks. Heat from steam or electrical elements are used in this last expansion. The finished blocks are cut up into plates and pipe cups. 14.1.3 Isolation of LNG ships with spherical tanks Cellular plastic is used as insulation material for Moss Rosenberg's spherical cargo tanks. Dow Chemicals in USA and Technical Isolation in Norway developed a new method of mounting insulation, because the insulation materials contraction is 2,5 times more than aluminium and 5 times more than steel at -163oC. Poles of “Styrofoam” of about 3 meters long are welded together and set continuously around the spherical tank, from equator against the bottom and on top of the tank. An external aluminium foil of 0,25 mm is laid on also. The room around the loading tank is filled with nitrogen with a dew point down to –40oC. The purpose is partly to protect the tanks from corrosion and to reduce the “pressure” of humidity against the insulation.
14.1.4 Mineral wool Mineral wool is a collective term of different fibre rich insulation materials. Rock wool and glass wool are two types of insulation material that are used. The production method and user area is equal. A mixture of several types of stones is used to produce rock wool. The stones are melted in a temperature up to 1600oC and are dispatched over a wheel with very high rotation. The melted mixture of stones is hurled out and chilled in long thin fibres. Cementing agent based on plastic is added and hardened with hot air. The amount of cementing agent varies and is determined by the material’s purpose of use. Rock wool plates are elastic and a normal density of about 45 kg/m3. The plates are also delivered with larger density and firmness. Glass wool is produced like rock wool by hurling and chilled melted glass to very fine fibres, only about 0,0025 mm diameter. 14.1.5 Expanded Perlite Perlite is made of a volcanic rock species with perlite structure. The main component is about 71% SiO2 and about 16% Al2O3. The raw material has some water content, which by heating to about 1200oC evaporates and “blows” up the material. One obtains a 10 to 20 times expansion with numerous closed airtight cells. Simultaneously the individual corns loosen form each other, the material “explodes” and forms sharp-edged corns with sizes from 0 to 3 mm with very large mechanical strength. The density is about 60 kg/m3. One can easily fill the whole room around a loading tank with perlite. It is first of all used on atmosphere pressure gas carriers. Maintenance of insulation The cooling plant on a gas carrier is constructed and calculated for thermal leakage from cargo tanks and system when the ship is new. Insulation is exposed for ageing, wear and tear and will in time be reduce if maintenance of the insulation is not kept. If the insulation on cargo tanks and pipelines reduces, increased thermal leakage will occur. Increased thermal leakage involves removal of more heat from the cargo. Load time and time used to cool the cargo increase. Preserving the insulation is good economy. Regular control and systematic maintenance of this from day one will save large future expenses. Protection of water and humidity is of high importance and that’s why we purge hold spaces with dry inert/ nitrogen. With exception of mechanical wear and tear, there is nothing more destroying for the insulation then the humidity. The only way to protect the insulation from humidity is to assure that the water barrier is intact. Re-insulation of the insulation on cargo tanks is especially expensive. The best and cheapest way to preserve this is to be sure that the atmosphere around the loading tanks is dry. It is important to consider the different material’s capability to absorb the heat of radiation when working with external insulation materials. A light water barrier absorbs less heat than dark. In practice, this means that white pipe insulation absorbs less heat than one with red or orange colour on the pipe insulation. One must also notice that pipe insulation faced with stainless steel plates radiates less
heat and thereby is warmer than a galvanised plate. The thermal conductance is thereby larger. Five good advice for maintaining the cargo tank insulation: 1. Held the atmosphere in hold spaces dry by drying the atmosphere regularly, use dry inert gas or nitrogen, if possible. 2. Control the cargo tank insulation regularly. Areas with ice or humidity indicate thermal loss. Note these areas with spray paint to easier locate the areas when these need repairing. 3. Control external insulation regularly and repair wrecked water barrier plates immediately. Areas with ice or humidity indicate thermal loss. Note the areas with spray painting in order to easily locate these when need of repairing. 4. At all times, have necessary materials to repair wrecked insulation onboard, minimum materials to repair damage on water barrier. As some of the insulation materials have limited operating time, the stock onboard must be adjusted to the expected consumption the next month. 5. Before adding new insulation in place, corrosion and pitting must be controlled. The steel must be protected from corrosion before new insulation is put to place.
14.1.6
The qualities of the insulation material compared with other materials
Thermal Density Pressure Heat Fire Diffusion conductance firmness capacity qualities resistance W/m K kg/m3 kg/cm2 J/kg K Stationary air 1 000 1 0,024 1,3 on/20oC CO2 0,015 2,0 840 Freon 22 0,012 4,7 1 090 Polyurethane foam, R11 Polystyrene foam (Isophor) Carbamide foam Phenol foam (Bakelite) Glass wool plates
0,023
40,0
2,00
1 260
0,033
25,0
0,30
1 340
0,035
10,0
0,041
32,0
0,035
20,0
840
Rock plates
0,035
45,0
840
Expanded perlite
0,035
50,0
840
Iron Steel (12 Cr) Stainless steel (19 Cr/10Ni) Water of/ 20 o C Ice of/ -20 oC
80 25 17,3
7 860 7 612 8 020
452 460 510
¥ ¥ ¥
0,56
998
4 180
¥
2,4
920
1 950
¥
wool
1,10
Selfquenching Selfquenching
50 70
1 260
2
1 340
2 Flammable above 700oC flammable above 700oC Non flammable
1,4 1,4 1,2
14.2 CALCULATION OF THERMAL TRANSFER
The cooling plant on a gas carrier is dimensioned by calculated heat transfer to cargo tanks and systems when the ship is new The insulation is exposed for wear and tear and is on many gas carriers partly strong reduced. When the insulation on cargo tanks gets inferior, it will have influence on the capacity of the vessel. Its construction and choice of components give the cooling plant’s capacity. Systematic maintenance will hold this capacity. The amount of heat transferred to cargo tanks and cargo pipes dependent on the insulation’s state, surrounding temperature, heat radiation and movements. Before taking a closer look at the condition around heat transfer on gas carrier, it is useful to form a picture of the heat balance. It is indicated easily by following illustration:
Heat is transferred from the surroundings to the cargo and systems for cargo because of the temperature difference. The transmission heat to the cargo tanks, Qtr.tank is the total transferred heat to the cargo tanks with cargo, steel and insulation. Transmission heat to cargo pipelines QTr.pipe is the total transferred heat to cargo in the pipes, pipe and insulation around the pipes. The heat of compression, QTr.Compr is the heat supplied to the gas in the compressor and the heat of condensation QCond is the heat transferred to the seawater in the loading condenser. When the cooling plant is driven to keep the temperature of the cargo constant, the heat balance is expressed as: QTr.tank + QTr.pipe + QTr.Compr = Qcond or as: Qtr.tank = Qfrom cargo tank- Qreturn to cargo tank The actual amount of heat transferred from the surroundings to a cargo tank or a system can be quantified in several methods. At first we will look on how this can be
done onboard, how to evaluate the result and what results the eventual effectuated effort will have. Most gas carriers are equipped with a graphic description of the calculated heat transfer to the cargo tanks. This is a theoretical calculated description that does not necessarily give the right image of the heat transfer. The older the ship is the larger probability that the calculated heat transfer DOES NOT coincide with reality. Control of the reel heat transfer can be executed onboard. If a loaded cargo tank is closed and isolated from the cooling plant over a period of time, the transferred heat from the surroundings can be measured. The heat transfer is thereby quantified and is comparable with what it was or should be. Before looking on a concrete example, it is of importance to emphasise that when accomplish such measuring, one must evaluate the results from the accuracy of the instruments. As the measuring instruments onboard has normally no more accuracy than + 10%, the period of measure should be as long as possible. Further it is important that if comparing repeated actual measurements, the measures has to be made at the best possible equal condition. We will now take a look at different examples and what we can do. 14.2.1 Example 1 A 12 year old smaller intermediate pressure gas carrier with 6 cargo tanks is loaded with ethylene and has just moved the sailing area from Europe to SEA. The captain rapport that the ship capability to cool down ethylene is perceptible inferior. The cooling rate is now at the lower edge of 0,3oC per day when the temperature of the cargo gets lower than minus 102oC. Inspection of the loading tanks indicate ice more than usual round all of the tank foundation and tank no.2 and 4 has many “ice spots”. Several “ice spots” than before are also observed on suction lines and condensate lines on deck. The cooling plant is checked and driven at optimum, but the cooling rate is more than halved at the same pressure in proportion to when the ship sailed in European waters. As the cooling plants condition is verified good and the plant is verified optimum driven, the bad cooling rate must be the result of the insulation has been worse during the years. The heat transfer has probably increased gradually through the years, but the influence has not been operational visual before the ship altered sailing area. It is obvious that the insulation on tank no. 2 and 4 plus the insulation on the lines on deck are mostly reduced. Repair of the insulation is necessary, but the question is which areas have most influence on the cooling capacity. Clarification of this is of importance when planning and priority of the insulation repair. The amount of heat transfer to the cargo tanks and pipelines must be concretised and compare with the repair costs before making the right decision. Cargo tank no. 2, 3 and 4 are the same type and size. To find out how much heat that is transferred to the “bad” loading tanks in proportion to one of one of the “good”, the tanks are shut for 24 hours.
The measuring instruments that are utilised during the experiment is calibrated and the following sketch for heat transfer is utilised and filled in:
Heat transfer for Cargo tank no.2 Date/time
30.07.94 / 10:30
Cargo tank filling ratio
31.07.94 / 10:30
97,0 %
Mass cargo in MT
592,837
Ambient temperature in oC o
Sea water temperature in C o
Average Hold space temperature in C
24
34
28
29
23
27
Ship's movement
Calm sea
Calm sea
Weather condition
Cloudy
Cloudy
Average liquid temperature in oC Cargo tank pressure in mBar Enthalpy liquid in kJ/kg
-103,1
-102,2
80
165
27,0
29,3
As the weight of gas is relative much less than the liquid weight, only the enthalpy change of the liquid is measured. The enthalpy values exist in heat technical table and the heat transfer to the cargo in tank no. 2 is calculated to: [(mass cargo x enthalpy-change) / (time in seconds)] [(592,837 x 103 x (29,3 - 27,0)) / (24 x 60 x 60)] kW = 15,8 kW Corresponding, the heat transfer is measured and calculated to the cargo in tank no. 3 and 4 to respectively 13,9 kW and 22,2 kW. In the technical description of the ship the calculated heat transfer when the ship was built to 13,1 kW for each tank at the same surrounding temperature and seawater temperature is located. A comparable table can be made and indicates as follows: Calculated transfer before: Cargo tank 13,1 kW 2
Calculated transfer now:
Percentage change:
15,8 kW
20 %
Cargo tank 13,1 kW 3
13,9 kW
6%
Cargo tank 13,1 kW 4
22,2 kW
70%
The calculations confirm the observations and presumptions made before in connection with the inspection of the tank insulation. It is rather no doubt that the insulation on tank no. 4 is essential much more deteriorated than the remaining two tanks. The priority at en eventual re-insulation of cargo-tanks can thereby well substantiate. By comparing present operational parameters for the cooling plant with earlier registered operational data, the temperature increase on the vapour from the cargo tank to compressor has increased essential. Six years ago, during almost the same condition, the vapour temperature rise from –100oC to –60oC from the cargo tank to compressor. Present observed temperature increase is from –100oC to –40oC. The vapour is essential now more over heated than earlier. A compressor’s cold capacity is expressed as: Qnetto = m x Dh Higher temperature on the vapour into the compressor involves lower density and thereby reduced amount of vapour through the compressor per time. The influence of inferior insulation on the suction lines to the cooling plant will have direct influence on the cooling capacity. A comparable table indicates as follows: Density on gas v/1 bars and -60 oC Density on gas v/1 bar and -40 oC Percentage reduction in density
1,7699 kg/m3 1,6181 kg/m3 9%
The reduction of the suction line's insulation involves a direct reduction of the cooling rate of about 10% in this temperature area. The size of the heat loss through condensate lines and liquid lines cannot be measured directly because there will at all times be and unknown and varying mixture of liquid and gas in the pipes. Judgement must at all times be adjusted and from the rapport from this ship, one must assume that the insulation here has the same condition as the suction lines. If this is the case, the influence of bad insulation on the condensate lines will have maximum consequence. The liquid lines are only utilised in a short period (during loading and discharging), while the condensate lines are utilised during all of the cooling period. The increase of the relative heat loss through the insulation on loading tank no. 2 and 4 is now indicated. Likewise is the reduction of cooling capacity because of increased heat transfer to the suction gas established. There is no doubt that both conditions have influence on the operational situation on the ship. But it is difficult to compare these two directly, for thereby to establish which one of them that has the
strongest effect on the ship’s possibility to execute the transport commission. To make the data’s comparable the alteration in the heat transfer to the suction line is quantified. The ship’s three loading compressors have stated a capacity of 680 m3/h at the same operational condition. The alteration of the heat transfer to the suction line in proportion to earlier years is calculated to: Vapour Ethylene Temperature at inlet compressor -60 before, T1 Enthalpy, h1 572,7 Temperature at inlet compressor -40 now, T2 Enthalpy, h2 607,4 Density, r2 1,6181 Number of compressors 3 Capacity per compressor, V 680
o
C
kJ/kg C
o
kJ/kg kg/m3 m3/h
Difference in heat transfer, DF 31,8 kW DF = ((3 x V) / 3600) x r2 x (h2 - h1) One can see that about 30 kW more heat to the suction cables is supplied now compared to earlier year. Simultaneously the measures indicate that the heat transfer to the cargo is about 15 kW more than earlier. (One must here emphasise that the total heat transfer to the cargo tanks will be larger because the steel in the cargo tanks with insulation is supplied heat). Heat transfer to the suction vapour in the suction pipe has a direct influence on the cooling plant's capacity. In addition, too high suction temperature will have a bad influence on the plant’s operational conditions by for example that the pressure pipe temperature may be too high.
