9 INERT GAS SYSTEM (IGS) (STCW Code, section A-V/1 paragraph 10) Learning objec objectives tives which oil tanker must be provided with an inert gas system the inert gas system the inert gas plant the scrubber the inert gas blowers the inert gas pressure regulating valve the non-return devices the inert gas distribution and venting gas-analysing, recording and indicating equipment operations meters, indicators and alarms emergency procedures maintenance and testing 9.1 GENERAL
Fire and explosion are among the greatest threats to the mariner; a tank ship carrying crude oil, refined petroleum,, or chemicals is an even greater threat. Fortunately, a properly designed, petroleum designed, installed, operating operating,, and a nd maintained maintain ed inert gas system (IGS) will complete c ompletely ly prevent fire and a nd explosion in an intact ship tank. Combustion is impossible without oxygen; if there is some way to keep the oxygen below about 8 percent, the ship will be free of danger from explosions in intact tanks. Typically, this is done by adding to t he tank atmosphere a gas that has less oxygen (often 5 percent or less) than air, which has an oxygen concentration of 21 percent. Of course, when a tank is opened, as in a collision, oxygen can enter the tank regardless of the IGS. In the 1920's and 1930's, one American petroleum company company suffered several tank ship fires and decided to inert the cargo tanks of its ships. Reportedly, its vessels have not had any intact cargo tank fires since that time. In the mid1970' s, a series of tank ship accidents led to the International Conference on Tanker Safety and Pollution Prevention (TSPP) of 1978 and the passage of the Port a nd Tanker Safety Act of 1978 (PTSA). International Internation al rules are contained in Chapter II-2, Regulation 62 (Inert Gas System Systems) s) of the International Convention for the Safety of Life at Sea (SOLAS), 1974. The first set of amendments to SOLAS 74 were adopted and came into force on 1 September 1984; under them, a s hip must satisfy applicable requirements requirements to receive a SOLAS Safety Certificate. The second set of amendments to SOLAS 74 come into effect on 1 July 1986; they have only a minor effect on the IGS requirements.
Regulation 60 of SOLAS Chapter II-2 Cargo tank protection 1
For tankers of 20,000 tonnes deadweight and upwards the protection of the cargo tanks deck area and cargo tanks shall be achieved by a fixed deck foam system and a fixed inert gas system in accordance with the requirements of regulations 61 and 62, except that, in lieu of the above
installations, the Administration, after having given consideration to the ship's arrangement and equipment, may accept other combinations of fixed installations if they afford protection equivalent to the above, in accordance with regulation I/5. 2 To be considered equivalent, the system proposed in lieu of the deck foam system shall: .1 be capable of extinguishing spill fires and also preclude ignition of spilled oil not yet ignited; and .2 be capable of combating fires in ruptured tanks. 3 To be considered equivalent, the system proposed in lieu of the fixed inert gas system shall:
.1 be capable of preventing dangerous accumulations of explosive mixtures in intact cargo tanks during normal service throughout the ballast voyage and necessary in-tank operations; and .2 be so designed as to minimize the risk of ignition from the generation of static electricity by the system itself. 4
Tankers of 20,000 tonnes deadweight and upwards constructed before 1 September 1984 which are engaged in the trade of carrying crude oil shall be fitted with an inert gas system, complying with the requirements of paragraph 1, not later than: .1 for a tanker of 70,000 tonnes deadweight and upwards 1 September 1984 or the date of delivery of the ship, whichever occurs later; and
.2 for a tanker of less than 70,000 tonnes deadweight 1 May 1985 or the date of delivery of the ship, whichever occurs later except that for tankers of less than 40,000 tonnes deadweight not fitted with tank washing machines having an individual throughput of greater than 60 m3/h the Administration may exempt such tankers from the requirements of this paragraph, if it would be unreasonable and impracticable to apply these requirements, taking into account the ship's design characteristics. 5
Tankers of 40,000 tonnes deadweight and upwards constructed before 1 September 1984 which are engaged in the trade of carrying oil other than crude oil and any such tanker of 20,000 tonnes deadweight and upwards engaged in the trade of carrying oil other than crude oil fitted with tank washing machines having an individual throughput of greater than 60 m3/h shall be fitted with an inert gas system, complying with the requirements of paragraph 1, not later than: .1 for a tanker of 70,000 tonnes deadweight and upwards 1 September 1984 or the date of delivery of the ship, whichever occurs later; and .2 for a tanker of less than 70,000 tonnes deadweight 1 May 1985 or the date of delivery of the ship, whichever occurs later. 6
All tankers operating with a cargo tank cleaning procedure using crude oil washing shall be fitted with an inert gas system complying with the requirements of regulation 62 and with fixed tank washing machines. 7 All tankers fitted with a fixed inert gas system shall be provided with a closed ullage system. 8 Tankers of less than 20,000 tonnes deadweight shall be provided with a deck foam system complying with the requirements of regulation 61.
9.2 THE I NERT GAS SYSTEM There are several ways to inert a tank. The s implest would be to add a pure non- flammable gas, such as nitrogen or carbon dioxide, to the tank atmosphere. Unfortunately, these pure gases tend to be expensive, the costs of their storage aboard ship tend to be high, and re-supply in some ports is difficult. Thus, most "inerted" ships use the gases from a fuel burner, from the ship's propulsion equipment (flue gas), or from a unit dedicated to producing inerting gas (an inert gas generator (IGG)). The following general description of an IGS includes components that may vary, depending upon the manufacturer. Use of combustion gases as the inerting medium proves advantageous due to its availability as needed and much lower cost, if sufficient fuel and properly adjusted and operated equipment is provided. Its disadvantage is that the raw combustion gases are impure and must be treated before use in the cargo tanks. This is especially important for product carriers, in which cargo purity is critical and some cargoes may react with impurities in the inert gases. Each IGS has several components intended to remove these impurities. For example, sulphur in the fuel appears in the inert gas in the form of sulphur oxides, sulphurous acid, and sulphuric acid. If not r emoved, they will attack the metal of the tank and gradually destroy it. Passing the inert gas through a water-filled device called a scrubber removes most of these acids.
installations, the Administration, after having given consideration to the ship's arrangement and equipment, may accept other combinations of fixed installations if they afford protection equivalent to the above, in accordance with regulation I/5. 2 To be considered equivalent, the system proposed in lieu of the deck foam system shall: .1 be capable of extinguishing spill fires and also preclude ignition of spilled oil not yet ignited; and .2 be capable of combating fires in ruptured tanks. 3 To be considered equivalent, the system proposed in lieu of the fixed inert gas system shall:
.1 be capable of preventing dangerous accumulations of explosive mixtures in intact cargo tanks during normal service throughout the ballast voyage and necessary in-tank operations; and .2 be so designed as to minimize the risk of ignition from the generation of static electricity by the system itself. 4
Tankers of 20,000 tonnes deadweight and upwards constructed before 1 September 1984 which are engaged in the trade of carrying crude oil shall be fitted with an inert gas system, complying with the requirements of paragraph 1, not later than: .1 for a tanker of 70,000 tonnes deadweight and upwards 1 September 1984 or the date of delivery of the ship, whichever occurs later; and
.2 for a tanker of less than 70,000 tonnes deadweight 1 May 1985 or the date of delivery of the ship, whichever occurs later except that for tankers of less than 40,000 tonnes deadweight not fitted with tank washing machines having an individual throughput of greater than 60 m3/h the Administration may exempt such tankers from the requirements of this paragraph, if it would be unreasonable and impracticable to apply these requirements, taking into account the ship's design characteristics. 5
Tankers of 40,000 tonnes deadweight and upwards constructed before 1 September 1984 which are engaged in the trade of carrying oil other than crude oil and any such tanker of 20,000 tonnes deadweight and upwards engaged in the trade of carrying oil other than crude oil fitted with tank washing machines having an individual throughput of greater than 60 m3/h shall be fitted with an inert gas system, complying with the requirements of paragraph 1, not later than: .1 for a tanker of 70,000 tonnes deadweight and upwards 1 September 1984 or the date of delivery of the ship, whichever occurs later; and .2 for a tanker of less than 70,000 tonnes deadweight 1 May 1985 or the date of delivery of the ship, whichever occurs later. 6
All tankers operating with a cargo tank cleaning procedure using crude oil washing shall be fitted with an inert gas system complying with the requirements of regulation 62 and with fixed tank washing machines. 7 All tankers fitted with a fixed inert gas system shall be provided with a closed ullage system. 8 Tankers of less than 20,000 tonnes deadweight shall be provided with a deck foam system complying with the requirements of regulation 61.
9.2 THE I NERT GAS SYSTEM There are several ways to inert a tank. The s implest would be to add a pure non- flammable gas, such as nitrogen or carbon dioxide, to the tank atmosphere. Unfortunately, these pure gases tend to be expensive, the costs of their storage aboard ship tend to be high, and re-supply in some ports is difficult. Thus, most "inerted" ships use the gases from a fuel burner, from the ship's propulsion equipment (flue gas), or from a unit dedicated to producing inerting gas (an inert gas generator (IGG)). The following general description of an IGS includes components that may vary, depending upon the manufacturer. Use of combustion gases as the inerting medium proves
advantageous due to its availability as needed and much lower cost, if sufficient fuel and properly adjusted and operated equipment is provided. Its disadvantage is that the raw combustion gases are impure and must be treated before use in the cargo tanks. This is especially important for product carriers, in which cargo purity is critical and some cargoes may react with impurities in the inert gases. Each IGS has several components intended to remove these impurities. For example, sulphur in the fuel appears in the inert gas in the form of sulphur oxides, sulphurous acid, and sulphuric acid. If not r emoved, they will attack the metal of the tank and gradually destroy it. Passing the inert gas through a water-filled device called a scrubber removes most of these acids.
9.2.1 Sources Possible sources of inert gas on tankers and combination carriers are:
Uptake gas from the ship¶s main or auxiliary boilers.
An independent inert gas generator.
A gas turbine fitted with an afterburner. 9.2.2 Quality A final oxygen level of 8% or less will be more easily achieved if the oxygen content of the inert gas in the inert gas main is considerably less than 8%. Ideally the inert gas should not contain oxygen but this is not possible in practice.
When using flue gas from a main or auxiliary boiler, an oxygen level of less than 5% can generally be obtained, depending on the quality of combustion control and the load on the boiler. When an independent inert gas generator or a gas turbine plant with afterburner is fitted, the oxygen content can be automatically controlled within finer limits, usually within the range 1.5% to 2.5% by volume and not normally exceeding 5%. Whatever the source, the gas must be cooled and scrubbed with water to r emove soot and sulphur acids before being supplied to the cargo tanks . The International Convention for the Safety of Life at Sea (SOLAS 1974), as amended, requires that inert gas systems be capable of delivering inert gas with an oxygen content in the inert gas main of not more than 5% by volume at any required rate of flow; and of maintaining a positive pressure in the cargo tanks at all times with an atmosphere having an oxygen content of not more than 8% by volume except when it is necessary for the tank to be gas free. Existing systems are only required to be capable of producing inert gas with an oxygen content not normally exceeding 5% by volume, and of maintaining the tank inerted at all times except when it is necessary for t he tank to be gas free.
9.3 I NERT GAS PLANT The inert gas system referred to in SOLAS r egulation 60 shall be designed, constructed and tested to the satisfaction of the Administration. It shall be so designed and operated as to render and maintain the atmosphere of the cargo tanks non-flammable at all times, except when such tanks are required to be gas-free. In the event that the inert gas system is unable to meet the operational requirement set out above and it has been assessed that it is impracticable to effect a r epair, then cargo discharge, deballasting and necessary tank cleaning shall only be resumed when the ³emergency conditions´ laid down in the Guidelines on Inert Gas Systems are complied with. The system shall be capable of:
Inerting empty cargo tanks by reducing the oxygen content of the atmosphere in each tank to a level at which combustion cannot be supported;
Throughout this regulation the term cargo tank i ncludes also slop tanks.
Maintaining the atmosphere in any part of any cargo tank with an oxygen content not exceeding 8% by volume and at a positive pressure at all times in port and at sea except when it is necessary for such a tank to be gasfree;
Eliminating the need for air to enter a tank during normal operations except
when it is necessary for such a tank to be gas free;
Purging empty cargo tanks of a hydrocarbon gas, so that subsequent gas- freeing operations will at no time create a flammable atmosphere within the tank.
The system shall be capable of delivering inert gas to the cargo tanks at a rate of at least 125% of the maximum rate of discharge capacity of the ship expressed as a volume.
The system shall be capable of delivering inert gas with an oxygen content of not more than 5% by volume in the inert gas supply main to the cargo tanks at any required rate of flow. The inert gas supply may be treated flue gas from main or auxiliary boilers. The Administration may accept systems using flue gases from one or more separate gas generators or other sources or any combination thereof, provided that an equivalent standard of safety is achieved. Such systems should, as far as practicable, comply with the requirements of this regulation. Systems using stored carbon dioxide shall not be permitted unless the Administration is satisfied that the risk of ignition from generation of static electricity by the system itself is minimized.
9.4 SCRUBBER A flue gas scrubber shall be fitted which will effectively cool the volume of gas and remove solids and sulphur combustion products. The cooling water arrangements shall be such that an adequate supply of water will always be available without interfering with any essential services on the ship. Provision shall also be made for an alternative supply of cooling water. Filters or equivalent devices shall be fitted to minimize the amount of water carried over to the inert gas blowers. The scrubber shall be located aft of all cargo tanks, cargo pump rooms and cofferdams separating these spaces from machinery spaces of category A. 9.4.1 Boiler Uptake These valves are located near the main boiler uptake to isolate the IGS scrubber from the boiler uptake. Alternatively, if a dedicated IGG is used, this valve will be located near the IGG; it is closed when the IGS system is not operating. Associated with each boiler uptake valve is a steam soot blowing system. A spectacle blank is also fitted between the boiler uptake valve and the IGS scrubber to ensure complete isolation of the IGS plant and cargo. This is very important to ensure that inert gas is not introduced into the system during maintenance. 9.4.2 IGS Scrubber
The raw inert gases are hot and c ontain soot, sulphur oxides, sulphurous acid, and sulphuric acid; all of these can be harmful to the cargo and the cargo tanks. The scrubber cools the gases and removes the contaminants by bubbling the gases through large quantities of seawater (there must be two s ources of water for the scrubber). The gases are then sprayed with additional quantities of water, or rise through a packed bed of ceramic forms, plastic shapes, or metal trays through which seawater falls, increasing the efficiency of the water in cleaning the inert gas. The warm, acidic, dirty seawater is then piped overboard. If the scrubber is provided with a heater to prevent the water from freezing, an automatic control system is installed to prevent its overheating. The scrubber also acts as an automatic safety device by preventing a backflow of gas to the boiler uptake or the IGG. 9.4.3 Demister Units
The gas from the scrubber has significant amounts of moisture, both from the burning process and from bubbling through the seawater in the scrubber. The demister is located close to the scrubber to remove entrained water from the IGS gas stream. If this water is not removed, it increases the corrosion rate in the system's piping, valves, and cargo tanks. Water may also contaminate the cargo. The demister may consist of "pads" or "mattresses" of w oven polypropylene or fibreglass, or centrifuge separation (cyclone dryers). There are many designs, which vary considerably.
Figure 9.4.4
9.5 I NERT GAS BLOWERS At least two blowers shall be fitted which together shall be capable of delivering to the cargo tanks at least the volume of gas required by paragraph 3. In the system with a gas generator the Administration may permit only one blower if that system is capable of delivering the total volume of gas required by paragraph 3 to the protected cargo tanks, provided that sufficient spares for the blower and its prime mover are carried on board to enable any failure of the blower and its prime mover to be rectified by the ship's crew.
Two fuel oil pumps shall be fitted to the inert gas generator. The Administration may permit only one fuel oil pump on condition that sufficient spares for the fuel oil pump and its prime mover are carried on board to enable any failure of the fuel oil pump and its prime mover to be rectified by the ship's crew. The inert gas system shall be so designed that the maximum pressure, which it can exert on any cargo tank, will not exceed the test pressure of any cargo tank. Suitable shutoff arrangements shall be provided on the suction and discharge connections of each blower. Arrangements shall be provided to enable the functioning of the inert gas plant to be stabilized before commencing cargo discharge. If the blowers are to be used for gas freeing, their air inlets shall be provided with blanking arrangements. The blowers shall be located aft of all cargo tanks, cargo pump rooms and cofferdams separating these spaces from machinery spaces of category A. 9.5.1 Fan Units
Two or more independent blowers are located near the demister to draw the inert gas through the s crubber and the demister and deliver it to the IGS distribution system at the r equired pressure. Since the greatest need for inert gas is during offloading, the blower capacity is set at 125 percent of the maximum rated capacity of the cargo pumps. This provides a margin of safety to ensure that no air enters the cargo tanks. This capacity may be provided by two blowers of equal size, or by one large and one small blower. Separate inlet and discharge valves are fitted to each blower unit. On most plants, the valves are hand operated, but on some the discharge valves are combined with the main and auxiliary pressure regulating valves.
Figure 9.2
(3) The dry seal operates normally dry and is filled with water when the IG plant is shut down or when tank pressure exceeds the IG pressure. This system requires more operating parts, reacts slower and is not deemed as reliable as the wet type. Dry seals are not approved for use on U.S. vessels. See Figure 9.3 for a more complete description. In the dry type seal, the water is drained from the seal when the IG plant is in operation (gas flowing to the tanks), and filled with water when the IG plant is either shut down, or the tank pressure exceeds the IG blower discharge pressure. Filling and drainage are performed by automatically operated valves controlled by the
levels in the water seal and the drop tank, and by the operating state of the blowers. Vessels must be equipped with seals that are completely passive in operation, so that failure of sensors, control systems, or moving parts cannot cause failure to establish a seal. Active seals, such as the dry seal shown above, are not acceptable. Figure 9.3
9.8 I NERT GAS DISTRIBUTION AND VENTING 9.8.1 General
The inert gas main may be divided into two or more branches forward of the non-return devices. The inert gas supply main shall be fitted with branch piping leading to each cargo tank. Branch piping for inert gas shall be fitted with either stop valves or equivalent means of control for isolating each tank. Where stop valves are fitted, they shall be provided with locking arrangements, which shall be under the control of a responsible ship's officer. The control system operated shall provide positive indication of the operational status of such valves. 9.8.2 Gas Distribution
When the inert gas is clean, cool, and water-free, it is pressurized and sent to the various cargo tanks. The distribution system contains backflow prevention devices, control valves, alarms, and automatic shutdowns. These alert personnel to a malfunction and, when necessary, shut down the system before a dangerous condition is developed in the tanks. Such conditions involve the oxygen concentration, the temperature and pressure of the inert gas, the water flow to and water level within the scrubber, and the backflow prevention devices (water seals). Because IG systems are complex and subject to malfunction, it is important for ships' crews to inspect and maintain them in good working order. 9.8.3 Combination carriers
In combination carriers, the arrangement to isolate the slop tanks containing oil or oil residues from other tanks shall consist of blank flanges which will remain in position at all times when cargoes other than oil are being carried except as provided for in the relevant section of the Guidelines on Inert Gas Systems. 9.8.4 Overpressure or vacuum
Means shall be provided to protect cargo tanks against the effect of overpressure or vacuum caused by thermal variations when the cargo tanks are isolated from the inert gas mains. Piping systems shall be so designed as to prevent the accumulation of cargo or water in the pipelines under all normal conditions. Suitable arrangements shall be provided to enable the inert gas main to be connected to an external supply of inert gas. 9.8.5 Venting, inerting, purging
The arrangements for the venting of all vapours displaced from the cargo tanks during loading and ballasting shall consist of either one or more mast risers, or a number of high velocity vents. The inert gas supply main may be used for such venting. The arrangements for inerting, purging or gas-freeing of empty tanks shall be to the satisfaction of the Administration and shall be such that the accumulation of hydrocarbon vapours in pockets formed by the internal structural members in a tank is minimized and that:
On individual cargo tanks the gas outlet pipe, if fitted, shall be positioned as far as practicable from the inert gas/air inlet. The inlet of such outlet pipes
may be located either at deck level or at not more than 1 m above the bottom of the tank;
The cross-sectional area of such gas outlet pipe shall be such that an exit velocity of at least 20 m/s can be maintained when tanks are being simultaneously supplied with inert gas. Their outlets shall extend not less than 2 m above deck level;
Each gas outlet shall be fitted with suitable blanking arrangements; If a connection is fitted between the inert gas supply mains and the cargo piping system, arrangements shall be made to ensure an effective isolation having regard to the large pressure difference, which may exist between the systems. This shall consist of two shutoff valves with an arrangement to vent the space between the valves in a safe manner or an arrangement consisting of a spool-piece with associated blanks. The valve separating the inert gas supplies main from the cargo main and which is on the cargo main side shall be a non-return valve with a positive means of closure. One or more pressure/vacuum-breaking devices shall be provided to prevent the cargo tanks from being subject to:
A positive pressure in excess of the test pressure of the cargo tank if the cargo were to be loaded at the maximum rated capacity and all other outlets are left shut; and
A negative pressure in excess of 700 mm water gauge if cargo were to be discharged at the maximum rated capacity of the cargo pumps and the inert gas blowers were to fail. Such devices shall be installed on the inert gas main unless they are installed in the venting system or on individual cargo tanks. Means shall be provided for continuously indicating the temperature and pressure of the inert gas at the discharge side of the gas blowers, whenever the gas blowers are operating.
This subsystem consists of a single inert gas main running the entire length of the cargo deck, starting at the deck isolating valve aft and ending at the vent valve forward. One or more pressure/vacuum devices are fitted to the inert gas main to prevent the cargo tanks from being over or under-pressurized. The inert gas main contains a means for receiving an outside source of inert gas when the IGS is not functioning. From the inert gas main, individual branch lines run to the cargo tanks. S top valves or equivalent closures are fitted at each branch line, so that each cargo tank can be isolated from the inert gas system. If there is a connection between the inert gas main and the cargo piping system, there must be valves or similar closures to isolate the systems from one another. Each tank vessel has a venting system that is capable of venting vapours displaced from the cargo tanks during loading and ballasting. There are also pressure/vacuum (P/V) valves to protect the cargo tanks from overpressure or vacuum resulting from thermal variation when the cargo tanks are isolated from the inert gas mains. Some of the possible arrangements are:
(1) A single common venting system, using the IGS deck main and branch lines from each tank and venting to the atmosphere through one or more mast risers or high velocity vents. Precautions must be taken to prevent an arrangement by which tank vents can be blocked off; (2) A common venting system, using a separate vent main and vent lines from each tank, with the system venting to the atmosphere through one or more mast risers or high velocity vents; or (3) Individual vents on each tank, using either standpipes (vent stacks) or high velocity vents.
9.9 GAS-A NALYSING , RECORDING AND INDICATING EQUIPMENT Instrumentation shall be fitted for continuously indicating and permanently recording, when the inert gas is being supplied:
The pressure of the inert gas supply mains forward of the non-return devices; and
The oxygen content of the inert gas in the inert gas supply mains on the discharge side of the gas blowers. The devices shall be placed in the cargo control room where provided. But where no cargo control room is provided, they shall be placed in a position easily accessible to the officer in charge of cargo operations. In addition, meters shall be fitted:
In the navigation bridge to indicate at all times the inert gas pressure forward of the non-return devices and the pressure in the slop tanks of c ombination carriers, whenever those tanks are isolated from the inert gas supply main; and
In the machinery control room or in the machinery space to indicate the oxygen content of the inert gas in the inert gas supply mains on the discharge side of the gas blowers Portable instruments for measuring oxygen and flammable vapour concentration shall be provided. In addition, suitable arrangement shall be made on each cargo tank such that the condition of the tank atmosphere can be determined using these portable instruments. Suitable means shall be provided for the zero and span calibration of both fixed and portable gas concentration measurement instruments. For inert gas systems of both the flue, gas type and the inert gas generator type, audible and visual alarms shall be provided to indicate:
Low water pressure or low water flow rate t o the flue gas scrubber;
High water level in the flue gas scrubber;
High gas temperature;
Failure of the inert gas blowers; Oxygen analyser
Oxygen content in excess of 5% by volume;
Failure of the power supply to the automatic control system for the gas regulating valve and to the indicating devices;
Low water level in the water seal;
Gas pressure less than 100 mm water gauge. The alarm arrangement shall be such as to ensure that the pressure in slop tanks in combination carriers can be monitored at all times; and
High gas pressure forward of the non-return devices. For inert gas systems of the inert gas generator type, additional audible and visual
alarms shall be provided to indicate:
Insufficient fuel oil supply;
Failure of the power supply to the generator;
Failure of the power supply to the automatic control system for the generator. Automatic shutdown of the inert gas blowers and gas-regulating valve shall be arranged on predetermined limits being reached. Automatic shutdown of the gas-regulating valve shall be arranged in respect of failure of the inert gas blowers. In respect of the oxygen content of the inert gas exceeds 5% by volume, immediate action shall be taken to improve the gas quality. Unless the quality of the gas improves, all cargo tank operations shall be suspended so as to avoid air being drawn into the tanks and the isolation valve shall be closed. Alarms shall be fitted in the machinery space and cargo control room, where provided, but in each case i n such a position that they are immediately received by responsible members of the crew. In respect of low water level in the water seal the Administration shall be satisfied as to the maintenance of an adequate reserve of water at all times and the integrity of the arrangements to permit the automatic formation of the water seal when the gas flow ceases. The audible and visual alarm on the low level of water in the water seal shall operate when the inert gas is not being supplied. An audible alarm system or automatic shutdown of cargo pumps shall be provided to operate on predetermined limits of low pressure in the inert gas main being reached. Tankers constructed before 1 September 1984, which are required to have an inert gas system, shall at least comply with the requirements of regulation 62 of chapter II-2 of the International Convention for the Safety of Life at Sea, 1974. Detailed instruction manuals shall be provided on board, covering the operations, safety and maintenance requirements and occupational health hazards relevant to the inert gas system and its application to the cargo tank system. The manuals shall include guidance on procedures to be followed in the event of a fault or failure of the inert gas system.
9.10 OPERATION 9.10.1 Introduction The purpose of an IG system is to keep the oxygen content of the vapour space below the level needed for combustion. For crude carriers, the oxygen content of the inert gas delivered to the cargo tanks should be no more than 5 percent, to ensure that there is not enough oxygen in the tank to support combustion.
On board oil tankers required to have an inert gas system the cargo tanks should preferably at all times be inerted and have a tank atmosphere with an oxygen content not exceeding 8% by volume except when the tanks need to be gas-free. This means that during normal operation of oil tankers the following operational modes frequently take place:
Inerting of empty tanks
Inerting during loading and simultaneous discharge of ballast
Inerting during loaded sea voyage
Inerting during discharging and ballasting
Inerting during tank cleaning
Purging prior to gas freeing and use of the IGS during gas freeing. The vessel used as an example in this paper is fitted with so-called ³purge pipes´ for ventilation and purging. (see Fig. 1 on page 19 of this chapter) On board vessels without purge pipes but with stand-pipes (i.e. vent pipes from deck level and about 2500 mm up vertically from deck level), these pipes are to be opened instead of the purge pipes mentioned in this paper. The vessel is not fitted with SBT (Segregated Ballast Tank) capacity according to MARPOL 73/78, and ballast water has to be carried in cargo oil tanks on ballast voyages.
IG operation panel
9.10.2 Inerting of empty gas-free tanks
Start up the inert gas plant according to instructions. Close all tank hatches and check proper function of O 2 analyser. Open the lids or hatch covers on standpipes or purge pipes on tanks to be inerted. Open fan discharge valve (6) and main isolating valve (10) (see Fig. 1 on page 19 of this chapter) If the gas pressure control valve (7) is on ³automatic´, there is now a risk of ³overload'´ of the scrubber due to a limited pressure drop in pipes and tanks. The ³overload´ of the scrubber may result in ³carry over´ of water or high-temperature trip of the plant. Because of the above, it is therefore sometimes necessary to control the pressure manually to reduce the gas flow during the first part of the inerting, until a minimum gas backpressure is established in the tanks. When this minimum gas pressure for stable operation is established, the inert gas plant can be operated automatically at full capacity. The oxygen content in the tanks should be checked frequently, at least every half hour. The inerting should be continued until the gas leaving the tank has had an O 2 content of less than 8% for a minimum of 30 minutes. This is because of the possibility of local air pockets in the tanks. When all tanks are satisfactorily inerted, the tanks should be put under a slight overpressure, normally 300-600 mm W.G., and the plant closed down according to i nstructions and the tank isolating valves closed.
Fig. 1 indicates the plant in operation Figure 1 Condition: Inerting of tanks filled with air
9.10.3 Inerting during loading and simultaneous discharge of ballast water The vessel is supposed to arrive at the loading port with all cargo tanks inerted. If the ship is fitted with a central gas vent outlet, all tanks to be l oaded are connected to the vent system. In case only local P/V valves are fitted, the valves are checked and adjusted for evacuation of gas through the high-speed valves. It should be checked that all tank hatches are closed and possible float level indicators are operable.
9.10.4 Discharge of ballast
Discharge of ballast can be done either before or during the loading. During discharge of ballast, before loading is commenced, the inerting procedure is the same as during discharge of cargo. See below. At simultaneous loading of cargo and discharge of ballast, the loading capacity is normally higher than the ballast water discharge capacity. This means that the inert gas volume available in the usage space above the oil level in the tanks being loaded is more than sufficient for the inert gas needed in the ballast tanks during discharge. By simply connecting the ballast tank to the cargo tanks with the inert gas lines on deck, inert gas will flow from the cargo tanks to t he ballast tanks. See Fig. 2. The inert gas system can be operated if required, but will normally not be required to deliver any gas to the deck lines. If by chance the ballast discharge rate is higher than the loading rate, the inert gas system must be in operation and the deck pressure adjusted sufficiently high to give a positive outflow of inert gas through the ventilation mast (or the individual P/V valves on the tanks). This is to avoid air being sucked into the tank system by a possible under pressure in the ballast tanks.
Failure of the power supply to the automatic control system for the gas regulating valve and to the indicating devices;
Low water level in the water seal;
Gas pressure less than 100 mm water gauge. The alarm arrangement shall be such as to ensure that the pressure in slop tanks in combination carriers can be monitored at all times; and
High gas pressure forward of the non-return devices. For inert gas systems of the inert gas generator type, additional audible and visual alarms shall be provided to indicate:
Insufficient fuel oil supply;
Failure of the power supply to the generator;
Failure of the power supply to the automatic control system for the generator. Automatic shutdown of the inert gas blowers and gas-regulating valve shall be arranged on predetermined limits being reached. Automatic shutdown of the gas-regulating valve shall be arranged in respect of failure of the inert gas blowers. In respect of the oxygen content of the inert gas exceeds 5% by volume, immediate action shall be taken to improve the gas quality. Unless the quality of the gas improves, all cargo tank operations shall be suspended so as to avoid air being drawn into the tanks and the isolation valve shall be closed. Alarms shall be fitted in the machinery space and cargo control room, where provided, but in each case i n such a position that they are immediately received by responsible members of the crew. In respect of low water level in the water seal the Administration shall be satisfied as to the maintenance of an adequate reserve of water at all times and the integrity of the arrangements to permit the automatic formation of the water seal when the gas flow ceases. The audible and visual alarm on the low level of water in the water seal shall operate when the inert gas is not being supplied. An audible alarm system or automatic shutdown of cargo pumps shall be provided to operate on predetermined limits of low pressure in the inert gas main being reached. Tankers constructed before 1 September 1984, which are required to have an inert gas system, shall at least comply with the requirements of regulation 62 of chapter II-2 of the International Convention for the Safety of Life at Sea, 1974.
Detailed instruction manuals shall be provided on board, covering the operations, safety and maintenance requirements and occupational health hazards relevant to the inert gas system and its application to the cargo tank system. The manuals shall include guidance on procedures to be followed in the event of a fault or failure of the inert gas system.
9.10 OPERATION 9.10.1 Introduction The purpose of an IG system is to keep the oxygen content of the vapour space below the level needed for combustion. For crude carriers, the oxygen content of the inert gas delivered to the cargo tanks should be no more than 5 percent, to ensure that there is not enough oxygen in the tank to support combustion.
On board oil tankers required to have an inert gas system the cargo tanks should preferably at all times be inerted and have a tank atmosphere with an oxygen content not exceeding 8% by volume except when the tanks need to be gas-free. This means that during normal operation of oil tankers the following operational modes frequently take place:
Inerting of empty tanks
Inerting during loading and simultaneous discharge of ballast
Inerting during loaded sea voyage
Inerting during discharging and ballasting
Inerting during tank cleaning
Purging prior to gas freeing and use of the IGS during gas freeing. The vessel used as an example in this paper is fitted with so-called ³purge pipes´ for ventilation and purging. (see Fig. 1 on page 19 of this chapter) On board vessels without purge pipes but with stand-pipes (i.e. vent pipes from deck level and about 2500 mm up vertically from deck level), these pipes are to be opened instead of the purge pipes mentioned in this paper. The vessel is not fitted with SBT (Segregated Ballast Tank) capacity according to MARPOL 73/78, and ballast water has to be carried in cargo oil tanks on ballast voyages.
IG operation panel
9.10.2 Inerting of empty gas-free tanks Start up the inert gas plant according to instructions. Close all tank hatches and check proper function of O 2 analyser. Open the lids or hatch covers on standpipes or purge pipes on tanks to be inerted. Open fan discharge valve (6) and main isolating valve (10) (see Fig. 1 on page 19 of this chapter) If the gas pressure control valve (7) is on ³automatic´, there is now a risk of ³overload'´ of the scrubber due to a limited pressure drop in pipes and tanks. The ³overload´ of the scrubber may result in ³carry over´ of water or high-temperature trip of the plant. Because of the above, it is therefore sometimes necessary to control the pressure manually to reduce the gas flow during the first part of the inerting, until a minimum gas backpressure is established in the tanks. When this minimum gas pressure for stable operation is established, the inert gas plant can be operated automatically at full capacity. The oxygen content in the tanks should be checked frequently, at least every half hour. The inerting should be continued until the gas leaving the tank has had an O 2 content of less than 8% for a minimum of 30 minutes. This is because of the possibility of local air pockets in the tanks. When all tanks are satisfactorily inerted, the tanks should be put under a slight overpressure, normally 300-600 mm W.G., and the plant closed down according to i nstructions and the tank isolating valves closed.
Fig. 1 indicates the plant in operation Figure 1 Condition: Inerting of tanks filled with air
9.10.3 Inerting during loading and simultaneous discharge of ballast water The vessel is supposed to arrive at the loading port with all cargo tanks inerted. If the ship is fitted with a central gas vent outlet, all tanks to be l oaded are connected to the vent system. In case only local P/V valves are fitted, the valves are checked and adjusted for evacuation of gas through the high-speed valves. It should be checked that all tank hatches are closed and possible float level indicators are operable.
9.10.4 Discharge of ballast
Discharge of ballast can be done either before or during the loading. During discharge of ballast, before loading is commenced, the inerting procedure is the same as during discharge of cargo. See below. At simultaneous loading of cargo and discharge of ballast, the loading capacity is normally higher than the ballast water discharge capacity. This means that the inert gas volume available in the usage space above the oil level in the tanks being loaded is more than sufficient for the inert gas needed in the ballast tanks during discharge. By simply connecting the ballast tank to the cargo tanks with the inert gas lines on deck, inert gas will flow from the cargo tanks to t he ballast tanks. See Fig. 2. The inert gas system can be operated if required, but will normally not be required to deliver any gas to the deck lines. If by chance the ballast discharge rate is higher than the loading rate, the inert gas system must be in operation and the deck pressure adjusted sufficiently high to give a positive outflow of inert gas through the ventilation mast (or the individual P/V valves on the tanks). This is to avoid air being sucked into the tank system by a possible under pressure in the ballast tanks.
