Fluids/Solids Handling
General Rules for Aboveground Storage Tank Design and Operation Yacine Amrouche, Chaitali DavÈ, Kamal Gursahani, Rosabella Lee and Lisa Montemayor, KBR
Various codes and regulations dictate the specification and construction of these tanks, helping to ensure optimum design and safe operation.
V
ertical, aboveground atmospheric-pressure storage tanks are commonly used in processing facilities. By definition, an atmospheric tank has a design pressure less than 2.5 psig (1). Atmospheric tanks can be equipped with a fixed roof or a floating roof. A vertical, fixed-roof tank consists of a cylindrical metal shell with a permanently attached roof that can be flat, conical or domeshaped, among other styles. Fixed-roof tanks are used to store materials with a true vapor pressure (TVP) less than 1.5 psia. (TVP, a measure of volatility, is the equilibrium partial pressure for a liquid at 100°F.) These tanks are less expensive to construct than those with floating roofs, and are generally considered the minimum acceptable type for storing chemicals, organics and other liquids. There are two types of floating roof tanks: • External floating roof (EFR). The roof floats directly on the surface of the stored liquid (called a contact deck). The deck has a seal system attached to the roof perimeter, closing off the annular space between the roof and the tank wall. These tanks store materials with TVPs from 1.5–11 psia. • Internal floating roof (IFR) tanks have an inside floating deck, which is either a contact deck or one that rests on pontoons, and a fixed roof. IFR tanks are used where there can be heavy accumulations of snow or rainwater on the floating roof. Such accumulations affect the operating buoyancy of the roof. In these cases, the vapor space above the liquid is purged with an inert gas.
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Design of storage tanks Various factors play a role in the selection and design of a tank: Process considerations — One of the first steps in selecting or designing a tank is to determine its capacity. The total capacity is the sum of the inactive (nonworking) capacity, actual or net working capacity, and the overfill protection capacity (figure). The inactive working (or non-working) capacity is the volume below the bottom invert of the outlet nozzle, which is normally a minimum of 10 in. above the bottom seam to avoid weld interference (2). The net working capacity is the volume between the low liquid level (LLL) and the high liquid level (HLL). For an in-process tank, the net working capacity is calculated by multiplying the required retention time of the liquid by its flowrate. For large, off-site storage tanks, the net working capacity is determined by performing an economic analysis (3), including items such as the savings in bulk transportation costs, the size and frequency of shipments, and the risks of a plant shutdown. In some cases, the required net working capacity may be divided up into multiple tanks, if the size of a single tank is physically unrealistic, or if separate tanks are needed for other reasons, such as dedicated service or rundown. The overfill protection capacity of a tank is that between the HLL and the design liquid level. The design liquid level is set higher than the normal operating liquid level to provide a safety margin for upsets. The overfill section is filled with vapor under normal operating conditions.
The bottom, shell and roof of storage tanks consist of steel plates that are usually lap-welded togethInert Gas FC er. To calculate plate dimensions, Flare/ Atmosphere designers normally refer to industry codes, such as those of the Design Liquid Level Overflow American Petroleum Institute Liquid Line (API) (4). Overfill Protection Capacity Process Storage tanks must have ladHigh Liquid Level LC Inlet ders to provide access to their top. Per API 650, tanks 20 ft or less in Normal Liquid Level height must be furnished with a Net Working Capacity ladder without a cage. Tanks taller than 20 ft require a spiral stairway. A landing platform at Low Liquid Level Process TC the top of the ladder can lead to Outlet LC Non-working Capacity Cooling/ walkways extending to the center Heating of the roof. Roofs and shells are Utilities Sump provided with manholes that are Optional 2-ft in dia. Details on such requirements are in API 650. Most storage tanks construct■ Figure 1. An aboveground storage tank can have internal coils for heating or cooling the liquid. ed in petroleum refining and petrochemical plants are made to conform to one of the Other process design considerations include specifying API standards. These standards cover design, constructhe temperature and pressure for the tank, and determining tion, inspection, erection, testing and maintenance rethe need for heaters, chillers or phase-separation equipment. quirements. They lay down certain minimum requireMechanical design — This involves specifying the maments for API certification. The key API codes for storterials of construction, determining the dimensions of the age tank design are as follows: tank and the plates used to build it, and sizing and position• “Field Welded Tanks for Storage of Production Liqing the nozzles and accessories. uids,” API Specification 12D — covers vertical, cylindriMild-quality carbon steel (A-36, A-328) is the most cal, aboveground, welded steel tanks in nominal capacities widely used material for storage tanks. For corrosive serof 500–10,000 bbl in standard sizes for production service. vices, a suitable corrosion allowance is added to the thickStandard capacity, dimensions and design pressures of API ness of the structure. If this is uneconomical, or if product 12D tanks are shown in Table 1 (3). contamination due to corrosion cannot be tolerated, then the tank material is upgraded to stainless steel or a high • “Shop Welded Tanks for Storage of Production Liqalloy. Alternatively, carbon steel tanks can be lined with uids,” API Specification 12F — covers vertical, cylindrical, corrosion-resistant materials such as rubber, plastic or ceaboveground, shop-welded steel tanks in nominal capacities ramic tile. Tanks can also be insulated for temperature control, personnel protection, energy conservation, or to preTable 1. Standard capacities, dimensions and design pressures for API 12D tanks (4). vent external condensation. For these instances, materials used are fiberglass, Nominal Outside Dia., Height, Design Pressure, Design Vacuum, oz./in.2 Capacity, ft-in. ft oz./in.2 mineral wool, expanded polystyrene or bbl polyurethane. The wind and seismic loadings, 500 15-6 16 8 1/2 750 15-6 24 8 1/2 available space and soil- bearing 500 21-6 8 6 1/2 strength determine the optimal height1,000 21-6 16 6 1/2 to-diameter ratio. Reduced heights and 1,500 29-9 24 6 1/2 wider shapes are preferred in windy or 1,000 20-9 8 4 1/2 seismically active areas, or where soil2,000 29-9 16 4 1/2 bearing capacity is limited. As available 3,000 29-9 24 4 1/2 plot space decreases and soil-bearing 5,000 38-8 24 3 1/2 strength increases, tanks are designed to 10,000 55-0 24 3 1/2 be taller with smaller diameters.
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of 90–500 bbl in standard sizes for production service. • “Large, Field Welded, Low-Pressure Storage Tanks,” API Standard 620 — covers vertical, cylindrical, aboveground, field-welded steel tanks for oil storage with maximum operating temperatures not greater than 200°F and pressures in the vapor space less than 2.5 psig. • “Large, Field Welded, Storage Tanks,” API Standard 650 — covers vertical, cylindrical, aboveground, fieldwelded steel tanks for oil storage with maximum operating temperatures not greater than 250°F and pressures in the vapor space less than 1.5 psig. Although API standards cover many aspects of storage tank design and operation, they are not all-inclusive. There are several other organizations that publish standards on tank design, fabrication, installation, inspection, and repair that supplement the API standards. These include the American Society of Mechanical Engineers (ASME; www.asme.org); American Society for Testing and Materials (ASTM; www.astm.org); American Water Works Association (AWWA; www.awwa.org); Building Officials and Code Administrators International (BOCA; www.bocai.org); (NACE International; www.nace.org); National Fire Protection Association (NFPA; www.nfpa.org); Petroleum Equipment Institute (PEI; www.pei.org); Steel Tank Institute (STI; www.steeltank.com) Underwriters Laboratories (UL; ulstandardsinfonet.ul.com); and the International Fire Code Institute (Uniform Fire Code; www.ifci.com).
Environmental requirements Storage tanks are considered a source of air emissions due to losses of vapor (5). Emissions from tanks must be addressed in obtaining the air permit. Volatile organic compounds (VOCs) are the major pollutants of concern for air emissions. In addition, specific organics that are toxic or hazardous are also regulated, e.g., benzene. Adequate control and proper management and maintenance are necessary to prevent releases of tank contents. In preparing an application for an air-quality operating permit, a review of all applicable regulations must be completed. Environmental regulations often dictate the type of emissions-control device that must be used in a particular application. Minimum emission-control requirements depend upon the material stored, when the tank was constructed or modified, its capacity, the TVP of the compound at storage conditions, and the location of the facility. Ref. 5 lists some of the national regulatory codes and standards used for the design of storage tanks and control of air emissions. Among these is the “New Source Performance Standards (NSPS), Standards for Performance for Storage Vessels for Petroleum Liquids,” from the U.S. Environmental Protection Agency’s regulation 40 CFR, Part 60, Subparts K, Ka and Kb. This standard sets rules for the systems to control emissions. Emissions-control devices include internal and external floating roofs, seals, vents to
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flares, vapor recovery systems (such as a thermal oxidizer or scrubber) and disposal systems, such as pressure or vacuum vents. Table 2 lists examples of the different types of requirements and their basis for applicability, taken from 40 CFR, Part 60. Information for the permit includes properties of material stored, operating conditions, TVP, tank physical characteristics, tank construction and rim-seal system, roof type, fittings, deck characteristics, estimated emissions, and chemical identification. EPA has guidelines, “Compilation of Air Pollutant Emission Factors,” API-42, that present models for estimating air emissions for organic-liquid storage tanks, and include emissions estimation equations developed by API. An EPA-developed program called TANKS Version 4.09 calculates tank emissions based on API 42 – Chapter 12 methodology. The software is available at www.epa.gov/ttn/chief/software/tanks/index.html. Vent control measures are included in the operating air permit as permit conditions. Some examples of possible operating permit conditions include (5): • For storage and loading of VOCs — An internal floating deck or equivalent control must be installed in all tanks. The floating roof must have one of the following closure devices between the wall and the edge of the deck: (1) a liquid-mounted seal; (2) two continuous seals mounted one above the other; or (3) a mechanical shoe seal. Installation of an equivalent control system requires review and approval. (A shoe seal is a type of rim seal that closes the space between the floating roof rim and the tank shell.) • For any tank equipped with a floating roof, the holder of the permit has to follow the tests and procedures to verify the seal integrity, as given in 40 CFR 60.113b. There are reporting and recordkeeping requirements for the dates that the seals are inspected, their integrity, and any corrective actions taken. • Uninsulated tanks exposed to the sun have to be painted white or made of aluminum.
Structural requirements Tank type and size, the soil conditions at the site, tank loading and tank settlement are critical factors for the design of the tank foundation. Examples of foundation types include earth or crushed stone, concrete slabs, slabs supported by piles and concrete ring-walls. Earth or crushed stone foundations are simply rings of material that support the tank walls. These foundations are typically used in locations with in-situ soil conditions, and can only be used when anchor bolts are not required. A concrete slab set under the entire surface area of the tank is used for tanks less than 15 ft in dia. If soil conditions are poor or the tank needs insulation, piles may needed. A concrete ring-wall is constructed by pouring a concrete mixture around the tank to support it. Ring-wall foundations are an economical way to support tanks, are typically used for large tanks and can withstand uplift
Table 2.Typical regulatory requirements for storage tanks (5). Subpart
Materials Stored
Tanks Modified or Construction Date
Tank Size, gal
True Vapor Pressure, psia
40 CFR, Part 60 Subpart K
Petroleum liquids
After March 8, 1974, and prior to May 19, 1978
> 40,000
> 1.5 but < 11.1
Floating roof, or vapor recovery system (VRS), or equivalent
> 40,000
> 11.1
VRS, or equivalent
After June 11, 1973, and prior to May 18, 1978
> 65,000
> 1.5 but < 11.1
Floating roof, or VRS, or equivalent
> 65,000
> 11.1
VRS, or equivalent
After May 19, 1978
> 40,000
> 1.5 but < 11.1
External floating roof (EFR) with two seals, or internal floating roof (IFR), or VRS with 95% reduction, or equivalent
> 40,000
> 11.1
VRS with 95% reduction
< 20,000
Any
20,000 but < 40,000
< 2.2
40,000
< 0.5
20,000 but < 40,000
> 4.0 but < 11.1
40,000
> 0.75 but < 11.1
20,000
11.1
40 CFR, Part 60 Subpart Ka
40 CFR, Part 60 Subpart Kb
Petroleum liquids
Volatile organic liquids
After July 23, 1984
forces from the tank. Most of the tanks used in chemical plants are greater than 15 ft in dia. and commonly have ring-wall foundations. Estimates of the vertical and horizontal loads of the tank are required for foundation design. Vertical loads to be considered include the empty weight, live load, operating weight, test weight and internal pressure. The live load on the roof is typically 25 lb/ft2, based on API codes (620 and 650). The operating weight is the dead weight plus the weight of the fluid, with corrections made for specific gravities greater than 1.0. The test weight consists of the dead weight of the tank plus the weight of the tank full of water. The tank is subjected to an internal pressure during operating or test conditions. Even a tank that has no liquid in it can still be under pressure. For example, a tank that held a volatile compound can still have vapor in it after being drained. Heat from the sun can pressurize the vapor. Horizontal forces include the wind and any seismic loads. Tank settlement is a common problem with compressible soils. Long-term settling of the foundation often occurs at the edge and center, due to operating conditions. In a ring-wall design, the pressure on the bottom of the ringwall and tank must be equalized to prevent differential settlement of the structure. Cryogenic tanks require cable heating systems to avoid
Control Requirements
Exempt from Subpart Kb
IFR with liquid-mounted seal or with mechanical shoe seal, or with vapor-mounted seal and rim-mounted secondary seal, or EFR with two seals, or VRS with 95% reduction or equivalent VRS with 95% reduction or equivalent
frost heave, or can be put on columns to allow air circulation.
Additional considerations Other items that need to be considered for the foundation are leak detection systems, corrosivity, cathodic protection, and secondary containment. The engineer must consider the environmental and safety implications of leakage into the containment space below the tank floor. For an earth or concrete ring-wall, leak-detection is normally accomplished by providing a flexible membrane liner at grade elevation with a drainpipe under the tank, which drains to the perimeter of the tank. For a concrete slab, leak detection can be achieved similarly or by placing radial grooves in the top of the slab that extend to the perimeter of the tank. When a leak occurs, one or more grooves will contain the tank liquid. Cathodic protection can be used to control electrochemical corrosion. This method uses direct current from an external source to oppose the discharge current from the metal surface, thereby preventing corrosion. Further, metal tanks that store flammable liquids are grounded as a protection against lightning or static electricity. Secondary containment is often required to prevent liquid from a leaking tank seeping into the ground and/or groundwater. This can be achieved by either building dikes
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with liners made of high-density polyethylene (HDPE), or by adding concrete walls and slabs, along with a leak detection system. Curb and dike containment are covered by many regulations that govern the volume, area, height and spacing between multiple tanks and process units. Area sumps may also be required to contain possible leakage. Provisions must be made for removing water or debris from the sumps.
Operation and control Pressure control — The design of a tank must take into account both normal operations and certain upset conditions. Normal operations are filling, emptying and storing. When filling a tank, the displaced vapor must be vented, typically to an emission-control device (or to atmosphere, if allowed by environmental regulations). When withdrawing liquid, the vacuum that is created must be counter-balanced by the infusion of an inert gas, such as nitrogen, through a breathing valve. Vapor “surplus” or “deficit” can also occur in an idle tank as a result of ambient temperature changes or chemical reactions taking place within the liquid inventory. The venting of excess vapor or the infusion of an inert gas for all normal operating conditions is carried out automatically, typically through self-regulating valves. Level control — Level-measuring devices are based on differential pressure, or sonic, capacitance, displacer velocity or liquid-conductivity measurements. Sonar or radar level measurements have recently gained popularity. These devices are usually mounted on the roof of a tank. They send out a signal, which is reflected off the liquid level. The time it takes for the reflected signal to be received is used to measure the liquid height. A major advantage of these instruments is that they can be used with corrosive liquids.
Literature Cited 1. Mead, J., “The Encyclopedia of Chemical Process Equipment,” Reinhold Publishing, New York, pp. 941–956 (1964). 2. Burk, H. S., et. al., “Conceptual Design of Refinery Tankage,” Chem. Eng., 88 (17), pp. 107–110 (Aug. 24, 1981). 3. Newton, P., et al., “Liquid Storage in the CPI,” Chem. Eng. (Deskbook), 85 (8), pp. 9–15 (April 3, 1978). 4. “Welded Steel Tanks for Oil Storage,” 10th ed., Standard 650, American Petroleum Institute (API), Washington, DC (1998). 5. “Technical Guidance Package for Chemical Sources: Storage Tanks,” Texas Natural Resources Conservation Commission (TNRCC), Air Permits Div. (Feb. 2001). Available at http://www.tnrcc.state.tx.us/.
Acknowledgment The authors would like to thank Ahmed Allawi, Benson Pair and the KBR Publications Committee for their guidance and support in writing this article.
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The level is then adjusted by closing or opening the appropriate valves. When precise level control is not required, the liquid level is maintained between the HLL and the LLL. Automatic emergency cut-offs are applied when the liquid level is at the overfill level to avoid overflow, or when it is below the LLL to avoid cavitation of a pump. Temperature control — A thermocouple, which is mounted below the LLL of the tank, provides a continuous readout of the temperature. Multiple measurement points are sometimes required to ensure representative temperature readings when the tank is large, there are different feeds at different temperatures, or there is a heating coil. The tank temperature can be maintained by adjusting the flowrate of a cooling or heating medium in an internal coil. Upsets and safety — Typical upsets include overpressure, overflow, boil-over, over-temperature, water ingress, floating-roof failure, unexpected phase separation, lightning, static-charge buildup, steam coil failure and fires. Adequate monitoring can help to ensure safety during upsets and other incidents. Control and prevention of such situations include the use of: sprays, deluge or foam systems; pressure-, temperature-, level- and fire-monitoring devices; pressure-relief systems; and ensuring proper CEP preventative maintenance.
YACINE AMROUCHE is a process engineer at KBR (601 Jefferson Ave., Houston, TX 77002; Phone: (713) 753-7028; Fax: (713) 753-6097; E-mail:
[email protected]). He is a junior-level engineer with two years of experience in process engineering and is a member of KBR’s young professional network, IMPACT. Amrouche holds a BS in chemical engineering from the Univ. of Sussex, U.K., with a specialization in polymer science. CHAITALI DAVE` is an environmental engineer at KBR (Phone: (713) 7533572; Fax: (713) 753-3123; E-mail:
[email protected]). She is a junior-level engineer with four years of experience in environmental engineering and is a member of KBR’s young professional network, IMPACT. Dave’ holds a BS in chemical engineering from the Univ. of South Florida and is a member of the Environmental Div. of AIChE. KAMAL GURSAHANI is a process engineer at KBR (Phone: (281) 492-5787; Fax: (281) 492-5832; E-mail:
[email protected]). He is a junior-level engineer with one year of experience and is a member of KBR’s young professional network, IMPACT. Gursahani holds a BS in chemical engineering from Bombay Univ. and an MS in chemical engineering from the Univ. of Wisconsin – Madison. ROSABELLA LEE is a process engineer at KBR (Phone: (713) 753-2238; Fax: (713) 753-5353; E-mail:
[email protected]). She is a juniorlevel engineer with four years of experience and is a member of KBR’s young professional network, IMPACT. Lee holds a BS degree in chemical engineering and mathematics from the Univ. of Houston. LISA MONTEMAYOR is a civil engineer at KBR (Phone: (713) 753-5355; Fax: (713) 753-5897; E-mail:
[email protected]). She is a junior level engineer with four years of experience in civil engineering and is a member of KBR’s young professional network, IMPACT. Montemayor holds a BS in civil engineering from Texas A&M Univ.
