NOMENCLATURE
wetted surface area of the tank, m2 C tank capacity, m3 F environmental factor stored L latent heat of vaporization of stored liquid at relieving pressure and temperature, kJ/kg M molecular weight of the stored liquid, kg/kmol emptying rate, m3/hr Q e tank emptying filling rate, m3/hr Q f tank filling relieving vapor, vapor, °K T temperature of relieving corrected for V corr venting rate of free air corrected stored liquids other than hexane, std m3/hr V hex venting rate of free air for reference liquid, hexane, std m3/hr V f emergency venting rate, std m3/hr V ib inbreathing, std m 3/hr V ob outbreathing, std m3/hr
Environmental Manager
A
Understanding Attmospheric A Storage Stor age Tank Tankss Several sources of information are available to help the engineer specify storage tanks for particular applications. A sound basis for understanding the various considerations is given here Siddhartha Mukherjee, Mukherjee, Lurgi India Co. Ltd.
L
iquids are accumulated and stored in tanks of various shapes and sizes throughout the chemical process industries (CPI). A small number of companies have made the design, fabrication and erection of these vessels their specialty. Initially, however, it is the process engineer who sizes and specifies tanks according to a specific application’s needs. In order to develop a data sheet for tank specifications, it is important for a process engineer to have a basic understanding of tanks and the related requirements requirements.. Tanks are used to store many kinds of liquids. The focus here is on hydrocarbons, which deserve particular care because of their flammable or combustible properties. Hydrocarbon liquids can be classified per Table 1 [ 1]. The National Fire Protection Association 74
TABLE 1.
CLASSIFICATION OF LIQUIDS
Classification
Type of Liquid
Characteristics
Class IA Liquid
Flammable Liquid
Liquids that have a flash point below 22.8°C, and boiling point below 37.8°C
Class IB Liquid
Flammable Liquid
Liquids that have a flash point below 22.8°C, and boiling point at or above 37.8°C
Class IC Liquid
Flammable Liquid
Liquids that have a flash point at or above 22.8°C but below 37.8°C
Class II Liquid
Combustible Liquid
Liquids that have a flash point above 37.8°C and below 60°C
Class IIIA Liquid
Combustible Liquid
Liquids that have a flash point at or above 60°C and below 93°C
Class IIIB Liquid
Combustible Liquid
Liquids that have a flash point at or above 93°C
(NFPA) has published several volumes on recommended practices for fire protection, wherein guidelines pertaining to the storage of hydrocarbon liquids are included.
Tank classifications 2], atmo According to the NFPA [ 2 spheric storage tanks are defined as those tanks that are designed to operate at pressures between atmospheric and 6.9 kPa gage, as measured at the top of the tank. Such tanks are built in two basic designs — the cone-roof design where the roof remains fixed, and the floating-roof design where the roof floats on top of the liquid and rises and falls with the liquid level. Fixed roof design. Fixed-roof tanks consist of a cylindrical shell with a permanently welded roof that can be flat, conical or dome-shaped. Such
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tanks are used to store materials with a true vapor pressure of less than 10.3 3]. kPa absolute [ 3 External-floatin External -floating g roof design. In floating-roof storage tanks, the roof is made to rest on the stored liquid and is free to move with the level of the liquid. These tanks reduce evaporation losses and control breathing losses while filling. They are preferred for storage of petroleum products with a true vapor pressure of 10.3 to 76.5 kPa absolute 3]. There are principally three differ[ 3 ent types of external floating roofs and an internal floating-roof tank. Figure 1 illustrates each of them. • The pan-type roof (Figure 1.a) is a single-deck roof and has the following characteristic characteristics: s: - Full Full contact contact with liquid liquid surface surface - Has a deck, hence any leak through the deck will cause it to sink
FIGURE 1. Types of floating-roof tanks include a) pan b) pontoon c) double-deck and d) internal
- Has no buoyancy other than that provided by the deck - Rain or snow may cause deformation - Is the least expensive of the floating roofs • The pontoon-type roof (Figure 1.b) is a significant improvement over the pan roof. It has the following characteristics: - Increased buoyancy and stability - Pontoons occupy about 20 – 40% of roof area • The double-deck roof (Figure 1.c) comprises upper and lower decks separated by bulkheads and trusses. These roofs have the following characteristics: - The space between the decks is separated into liquid-tight compartments - Superior loading capacity - Recommended for tank diameters below 12 m and above 60 m Internal-floating roof design. These tanks (Figure 1.d) have an inside floating deck above which, there is a permanently attached roof. Such tanks are preferred in areas of heavy snowfall where accumulation of snow or water on the floating roof may affect buoyancy. In such tanks, the vapor space is normally blanketed with an inert gas.
Physical criteria Tank capacity. Three different types of tank capacity are defined — nominal, gross and net capacity.
FIGURE 2. A typical P&ID for a tank that is blanketed with an inert-gas is shown here. LI represents level indication, and PC, pressure control
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Environmental Manager For fixed-roof tanks, the nominal justifiable to go for larger diameters capacity is the geometric volume from in preference to height. As the tank the bottom of the tank up to the curb height increases, wall thickness plays angle, which is a metallic angle that is a more important role. Higher tanks welded along the periphery at the top also put a greater load on the soil. of the cylindrical portion of the tank. If the pressure becomes more than In the case of floating-roof tanks, the the soil-allowable bearing pressure, nominal capacity is defined as the pile-supported foundations become volume from the underside of the roof necessary and are expensive. This deck up to the maximum floating posi- concern is particularly applicable for tion of the roof. poor soils. In general, tanks that are The gross capacity (sometimes re- higher than 15 m are not commonly ferred to as the total capacity) is the used in industry. volume from the bottom of the tank up Tank-blanketing requirements to its maximum, safe filling height. The net capacity is the volume of In many instances, the vapor space of the tank contents between the low- tanks is blanketed with an inert gas. liquid level (LLL) and the high-liquid This may be needed when the liquid’s level (HLL). vapors are harmful to health or when Tank dimensions. Various factors contact with air could lead to the forcan affect the choice of tank dimen- mation of hazardous compounds or sions. In general, tank heights do product degradation. not exceed one and a half times the To achieve an inert atmosphere in diameter. In cases where availabil- a tank, a blanketing valve senses the ity of land is not a constraint, it is pressure in the vapor space of the tank
and controls the flow of inert gas (usually nitrogen) into the vapor space to maintain the tank pressure within the desired limits. Blanketing pressures are typically in the range of 200–250 mm H2O. When liquid is moved out of a tank or if the temperature decreases, a tank can experience vacuum conditions. In this case, the blanketing valve pro vides primary vacuum relief to the tank by opening to allow gas flow, and then resealing when the pressure has increased sufficiently. Secondary vacuum relief is provided by pressure/ vacuum vents (described later in the section on Pressure/vacuum vents). Figure 2 illustrates a typical process and instrumentation diagram (P&ID) for a blanketed tank that contains a hydrocarbon mixture.
Tank venting Several conditions that subject a tank to venting include the following:
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FIGURE 3. An external floating-roof tank may include these types of accessories
• Inbreathing due to liquid movement out of the tank • Inbreathing due to contraction or condensation of vapors caused by a decrease in the atmospheric temperature (also called thermal inbreathing) • Outbreathing due to liquid movement into the tank • Outbreathing due to expansion or vaporization of vapors caused by an increase in the atmospheric temperature (also called thermal outbreathing) • Outbreathing resulting from external fire Inbreathing. According to the American Petroleum Institute (API) [ 4], the venting capacity for maximum liquid movement out of a tank should be equivalent to 0.159 std m 3 /hr of air for each 0.159 m3 /hr of the maximum emptying rate of liquids. This holds for liquids of any flash point. In other words, for an emptying rate of Q e m 3 / hr, the venting capacity should be Q e std m3 /hr. There are also requirements for thermal inbreathing. The API [ 4] furnishes these requirements as a function of tank capacity in the form of tables. This information can also be expressed as an equation, wherein the thermal venting is expressed as a function of tank capacity. The total venting capacity can be expressed as a sum of liquid movement and thermal inbreathing. For tanks up to 3,500 m3 capacity:
V ib = Q e + 0.178C
For tanks larger than 3,500 m 3 capacity: V ib = Q e + 3.20C0.651
where Q e represents venting capacity due to liquid movement and the second component represents that due to thermal inbreathing. C is the tank capacity. The total venting requirement due to both liquid movement as well as thermal venting is needed to ensure that the venting arrangement is designed for the worst-case scenario. Outbreathing. Whereas venting due to inbreathing is independent of flash point, the requirements for outbreathing differ with flash point. For liquids with a flash point above 37.8°C or a normal boiling point above 149°C, the required venting capacity for maximum liquid movement into a tank should be equivalent to 0.170 std m3 /hr of air for each 0.159 m 3 /hr of maximum filling rate. In other words, for a filling rate of Q f m3 /hr, the venting capacity should be 1.069 Q f std m3 /hr.
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Environmental Manager There are also requirements for thermal outbreathing. The total venting capacity, expressed as a sum of liquid movement and thermal outbreathing can be expressed as follows.
For tanks larger than 3,500 m 3 capacity:
For tanks up to 3,500 m3 capacity:
For these liquids with a high flash point, the thermal outbreathing is roughly 60% of the thermal inbreathing requirement. The reason for this is that
V ob = 1.069Q f + 0.107C
V ob = 1.069Q f + 1.92C0.651
the roof and shell temperatures cannot rise as rapidly as they can fall, for example, during a sudden rain shower. Liquids with a lower flash point, below 37.8°C, or a normal boiling point below 149°C, have the following guidelines. The requirement of venting capacity for maximum liquid movement into a tank should be equivalent to 0.340 std m3 /hr of air for each 0.159 m3 /hr of maximum filling rate. In other words, for a filling rate of Q f m3 /hr, the venting capacity should be 2.138Q f std m3 /hr. Of course, there are additional requirements for thermal outbreathing. The total venting capacity, expressed as a sum of liquid movement and thermal outbreathing, can be expressed by the following equations. For tanks up to 3,500 m3 capacity: V ob = 2.138Q f + 0.178C
and for tanks larger than 3,500 m 3 capacity: V ob = 2.138Q f + 3.20C0.651 Emergency Venting on Fire Exposure. When storage tanks are exposed to fire, the venting rate may exceed the inbreathing or outbreathing rate that results from a combination of thermal effects and liquid movement. For tanks subjected to fire exposure, the required venting capacities are given in Table 2 [ 4], for the reference liquid, hexane. The environmental factor, F , is taken as 1.0 for bare-metal tanks. For insulated tanks, F varies between 0.025 and 0.30, depending upon the insulation conductance. The details are furnished in API 2000 [ 4]. The values in Table 2 are for hexane. For other liquids, the following corrected venting rate is applied. V corr = (3098 · V hex )/( M 0.5 · L)
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where M is the molecular weight of the stored liquid and L is the latent heat of vaporization of the stored liquid. Emergency vents can be in the form of a gauge hatch that permits the cover to lift under high venting loads, or a manhole cover that lifts when exposed to high venting loads (Figure 2). Open vents. Tanks that store harmless or non-toxic liquids, such as firewater or service water, are vented to the atmosphere. These tanks operate
at atmospheric pressure and the venting is called open venting. While being filled, the tank breathes out through the vent. When liquid is pumped out, the tank breathes in through the vent. To prevent rain or snow from entering, the vent pipe is usually provided with a weather hood, or alternatively, the pipe itself is shaped in the form of a goose neck. According to API 2000 [ 4], open vents without flame arrestors may be used for venting under the following circumstances: • For storage of petroleum or petroleum products with a flash point of 37.8°C or above • For tanks holding petroleum or petroleum products at a temperature below that of the flash point • For storage of any product in tanks with a capacity of less than 9.46 m 3 Flame arrestors need to be used with open venting of tanks that store petroleum or petroleum products that have a flash point below 37.8°C. Pressure/vacuum vents. Pressure/ vacuum vents are usually employed to protect blanketed tanks. In situations where the blanketing valve fails and gets stuck in the open position, the tank can be pressurized by the continuous inflow of inert gas. A pressure vent will open to protect the tank from rupture. Conversely, in situations where a tank is being emptied and the blanketing valve fails, the tank can reach vacuum conditions. A vacuum valve will open, thus protecting the tank from collapse. Pressure and vacuum vents may be weight- or spring-loaded. Springs are generally used at set pressures above 7 kPa gage or at vacuum below –7 kPa gage. The pressure setting of the vent is kept slightly above the tank blanketing pressure but below the maximum pressure the tank can withstand. Similarly, the vacuum setting is kept higher than the normal operating vacuum, but at a vacuum level that is below the maximum vacuum that the tank can withstand. Because these vents are designed to remain closed until they must open in order to protect the tanks, another ad vantage is that evaporation losses and fugitive emissions can be minimized by pressure and vacuum vents. This is achieved by preventing the release
TABLE 2.
EMERGENCY VENTING
Wetted surface area, m2
Design pressure, kPa gage
Emergency venting rate, std m 3/hr
Area < 18
All
V f
= 58,791(A·F/L )(T/M )0.5
18 < area < 93
All
V f
= 208,888(A0.566F/L )(T/M )0.5
93 < area < 260
All
V f
= 587,386(A0.338F/L )(T/M )0.5
Area > 260
> 6.9
V f
= 40,248(A0.82F/L )(T/M )0.5
Area > 260
< 6.9
V f
= 3,847,884(F/L )(T/M )0.5
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Environmental Manager
FIGURE 4. An internal floating-roof tank would typically include accessories as shown here
of vapors that would otherwise occur during minor variations in temperature, pressure or level. Please visit us at ACHEMA Hall 8.0, L33-L35 Circle 56 on p. 93 or go to adlinks.che.com/5828-56
Instrumentation Providing proper instrumentation in tanks is an important consideration, especially for large tanks that are subjected to frequent filling and emptying. The following guidelines may be useful for selecting tank instrumentation. Level. At least two level instruments should be included — one with local indication and the other in the control room. In many cases, two level instruments with both local and remote indications are used. High and low level alarms are recommended. Temperature. Both local indication and remote indication with an alarm in case of high storage temperatures should be included. Pressure and flow. In blanketed tanks, it is advisable to install a flow and a pressure indicator with alarms to warn of problems in the blanketing gas line.
Structural accessories Ladders and others. Auxiliary items such as ladders, staircases, handrails and walkways, are particularly important for tank maintenance. Generally, tanks less than 6 m in height are provided with a ladder that is not caged. Tanks that are taller than 6 m require a spiral stairway [5]. Float80
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ing-roof tanks are normally provided with a ladder that automatically adjusts so that access to the roof is always provided. This type of ladder is illustrated in Figure 3 for a typical external-floating tank. Roof Drain. According to API Standard 650 [5], primary roof drains should be of the hose, jointed or siphon type. A check valve should be provided near the roof end of the hose to pre vent backflow of stored product in case of a leak (Figure 3). Manholes. Shell manholes are normally 600 mm in diameter. The minimum number of required manholes is dependent on the tank diameter. Typical guidelines are as follows: • Tanks with a diameter of less than 12 m: 1 manhole •Tanks with diameters of 12-44 m: 2 manholes, spaced 180 deg. apart •Tanks with diameters of 45-59 m: 3 manholes, spaced 120 deg. apart •Tanks with a diameter over 60 m: 4 manholes, spaced 90 deg. apart Guidelines for roof manholes are: •For tanks less than 12 m in diameter, one roof manhole is enough •For tanks larger than 12 m, two roof manholes are recommended Bottom drains. Bottom drains, also referred to as water draw-off sumps, should be provided in all tanks for draining water. They are also needed for tanks that store petroleum products in order to empty the tanks for cleaning. API Standard 650 [ 5] provides di-
TABLE 3.
MINIMUM TANK SPACING1
Floating-roof tanks
Fixed or horizontal tanks Class I or II liquids
Class IIIA liquids
1/6 sum of adjacent tank diameters but not less than 0.9 m
1/6 sum of adjacent tank diameters but not less than 0.9 m
All Tanks not over 45 m in diameter: 1/6 sum of adjacent tank diameters but not less than 0.9 m Tanks larger than 45 m in diameter: If remote impounding is provided
1/6 sum of adjacent tank diameters
1/4 sum of adjacent tank diameters
1/6 sum of adjacent tank diameters
If diking is provided
1/4 sum of adjacent tank diameters
1/3 sum of adjacent tank diameters
1/4 sum of adjacent tank diameters
1. Reprinted with permission from NFPA 30, Flammable and Combustible Liquids Code, Copyright ©2003, National Fire Protection Association, Quincy, MA 02169. This reprinted material is not the complete and official position of the National Fire Protection Association on the r eferenced subject which is represented only by the standard in its entirety.
mensional details of such drains. Figures 3 and 4 show typical structural accessories for both external and internal floating-roof tanks.
Spill control Facilities should be provided in the tankage area to prevent accidental
discharge of any Class I, II or IIIA liquid from endangering adjoining property. National Fire Codes [ 6] describe the requirements in detail. There are two types of facilities that can be provided. Remote impounding. In this case, the adjoining property is protected by
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drainage of the discharge to a remote impounding area. The following guidelines should be followed: • A slope of not less than 1% should be provided for at least 15 m towards the impounding area • The impounding area should have a capacity that can, at minimum, hold the contents of the largest tank that may drain into it • Where it is not practical to have a 100% capacity remote impounding, partial remote impounding should be provided. The remainder of the impounding volume should be pro vided by diking as discussed below Impounding by diking. In cases where the adjoining property is protected by providing diked enclosures with roads all around the enclosure, the following guidelines should be followed: • The diked enclosure should be able to contain the complete contents of the largest tank in the tank farm in case of any emergency. The capac-
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TABLE 4. 01 02 03
Item number Number required Designation
04
Type
05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 42 43 44 45 46 47
Type of roof Nomina l ca pacity m3 Inside diameter mm Product stored Corrosive components Hazardous cla ss Operating temperature °C Operating pressure kPa(g) Flash point Liquid density at operating temperature Viscosity at operating temperature Inert gas blanketing required Corrosion allowance Roof Bottoms Filling rate nor/max Emptying rate nor/max Construction Roof shape Bottom shape Bottom sump Agitator Insulation
TANK DATA SHEET - AN EXAMPLE T - 101 Two Heavy Naphtha Tank Floating Roof Type
Net working capacity Cylindrical height
Single Deck Pontoon Type 461 385 7000 14000 Heavy Naphtha None
m3 mm
Design temperature (min/max) °C Design pressure kPa(g) °C kg/m3 cP Inert gas medium Shell mm Internals mm m3 /h m3 /hr
40 Atmospheric 15 755-770 0.49 - 0.51 No 1 2
2 20/22 50/55 refer type flat yes (as per API 620) No
type (roof) thickness type (shell) thickness
mm mm
none none
Material (tank) Material (internals) Nozzle Table Item
5/70 Full of Water
nil nil CS CS
No. required
size, mm
Rating
Designation
Notes : 1. 2. 3. 4.
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83
Environmental Manager ity of the diked area should be calculated after deducting the volume of the tanks (other than the largest tank) below the height of the dike • The height of the dike wall should be a minimum of 1 m and not more than 1.8 m from that of the internal grade • The minimum distance between the tank shell and the base of the inte-
rior dike wall should be 1.5 m
Layout and design NFPA provides detailed guidelines for layout and installation of aboveground storage tanks. Tanks storing Class I, II or IIIA stable liquids should be separated in accordance with Table 3 [ 6]. An exception to this is the case of crude-petroleum storage tanks
things change...
with individual capacities that do not exceed 476 m3 and are in production facilities in isolated locations. In such cases, the tanks need not be separated by more than 0.9 m. After the process engineer has considered the basic tank requirements, a tank data sheet can be generated. A typical tank data sheet, such as that shown in Table 4 (p. 83), illustrates all process features of a tank including the dimensions, type, capacity, operating and design conditions, physical properties of the liquid stored, materials of construction, corrosion allowances and insulation. In addition, it contains the details of all nozzles in the tank, their designations, sizes and ratings. A process sketch should also be included as part of the data sheet. The process engineer should also specify instrumentation needs, such as pressure, level and temperature sensors. ■ Edited by Dorothy Lozowski
References
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1. National Fire Protection Association, “Flammable and Combustible Liquids Code,” Vol. 2, p. 30-10, Quincy, Mass., 1996 2. National Fire Protection Association, “Flammable and Combustible Liquids Code,” Vol. 2, p. 30-8, Quincy, Mass., 1996 3. Amrouche, Y., et.al., “General Rules for Aboveground Storage Tank Design and Operation,” Chem. Eng. Prog., pp. 54-58, December 2002. 4. American Petroleum Institute, “Venting Atmospheric and Low-Pressure Storage Tanks,” API Standard 2000, October 1992. 5. American Petroleum Institute. “Welded Steel Tanks for Oil Storage,” API Standard 650, 10th Edition, November 1998. 6. National Fire Protection Association, “Flammable and Combustible Liquids Code,” Vol. 2, p. 30-15, Quincy, Mass., 1996
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Author Siddhartha Mukherjee is deputy general manager-process at Lurgi India Company Ltd. (A-30 Mohan Cooperative Industrial Estate, Mathura Road, New Delhi 110 044, India. Phone: +91-11-4259-5050; Fax: +91-11-4259-5051; Email: siddhartha_mukherjee@lurgi .de). For the past six years, he has been involved as a lead engineer in the design, precommissioning and commissioning of chemical and petrochemical plants in India and elsewhere. He has also been involved in inorganic and oleochemistry while at Lurgi. Prior to this, Mukherjee worked as an environmental engineer with the Development Consultants Ltd. (Calcutta), doing various environmental assessment projects in volving thermal power plants. Mukherjee earned his B.Tech. and Ph.D. chemical engineering degrees from the Indian Institute of Technology, Kharagpur. He holds lifetime memberships in India’s Institution of Engineers and the Indian Institute of Chemical Engineers.
