54 www.cepmagazine.org December 2002 CEP

Fluids/Solids Handling

ertical, aboveground atmospheric-pressurestorage tanks are commonly used in pro-cessing facilities. By definition, an atmo-spheric tank has a design pressure less than

2.5 psig (1). Atmospheric tanks can be equipped witha fixed roof or a floating roof. A vertical, fixed-rooftank consists of a cylindrical metal shell with a perma-nently attached roof that can be flat, conical or dome-shaped, among other styles. Fixed-roof tanks are usedto store materials with a true vapor pressure (TVP)less than 1.5 psia. (TVP, a measure of volatility, is theequilibrium partial pressure for a liquid at 100°F.)These tanks are less expensive to construct than thosewith floating roofs, and are generally considered theminimum acceptable type for storing chemicals, or-ganics and other liquids.

There are two types of floating roof tanks:• External floating roof (EFR). The roof floats di-

rectly on the surface of the stored liquid (called a con-tact deck). The deck has a seal system attached to theroof perimeter, closing off the annular space betweenthe roof and the tank wall. These tanks store materialswith TVPs from 1.5–11 psia.

• Internal floating roof (IFR) tanks have an insidefloating deck, which is either a contact deck or one thatrests on pontoons, and a fixed roof. IFR tanks are usedwhere there can be heavy accumulations of snow or rain-water on the floating roof. Such accumulations affect theoperating buoyancy of the roof. In these cases, the vaporspace above the liquid is purged with an inert gas.

Design of storage tanksVarious factors play a role in the selection and de-

sign of a tank:Process considerations — One of the first steps in

selecting or designing a tank is to determine its capaci-ty. The total capacity is the sum of the inactive (non-working) capacity, actual or net working capacity, andthe overfill protection capacity (figure). The inactiveworking (or non-working) capacity is the volume belowthe bottom invert of the outlet nozzle, which is normallya minimum of 10 in. above the bottom seam to avoidweld interference (2). The net working capacity is thevolume between the low liquid level (LLL) and the highliquid level (HLL). For an in-process tank, the networking capacity is calculated by multiplying the re-quired retention time of the liquid by its flowrate. Forlarge, off-site storage tanks, the net working capacity isdetermined by performing an economic analysis (3), in-cluding items such as the savings in bulk transportationcosts, the size and frequency of shipments, and the risksof a plant shutdown. In some cases, the required networking capacity may be divided up into multipletanks, if the size of a single tank is physically unrealis-tic, or if separate tanks are needed for other reasons,such as dedicated service or rundown. The overfill pro-tection capacity of a tank is that between the HLL andthe design liquid level. The design liquid level is sethigher than the normal operating liquid level to providea safety margin for upsets. The overfill section is filledwith vapor under normal operating conditions.

Various codes and regulations dictate thespecification and construction of these tanks,helping to ensure optimum design and safeoperation.

General Rules for AAbboovveeggrroouunndd SSttoorraaggee

Tank Design and Operation

VV

Yacine Amrouche, Chaitali DavÈ,Kamal Gursahani, RosabellaLee and Lisa Montemayor,KBR

CEP December 2002 www.cepmagazine.org 55

Other process design considerations include specifyingthe temperature and pressure for the tank, and determiningthe need for heaters, chillers or phase-separation equipment.

Mechanical design — This involves specifying the ma-terials of construction, determining the dimensions of thetank and the plates used to build it, and sizing and position-ing the nozzles and accessories.

Mild-quality carbon steel (A-36, A-328) is the mostwidely used material for storage tanks. For corrosive ser-vices, a suitable corrosion allowance is added to the thick-ness of the structure. If this is uneconomical, or if productcontamination due to corrosion cannot be tolerated, thenthe tank material is upgraded to stainless steel or a highalloy. Alternatively, carbon steel tanks can be lined withcorrosion-resistant materials such as rubber, plastic or ce-ramic tile. Tanks can also be insulatedfor temperature control, personnel pro-tection, energy conservation, or to pre-vent external condensation. For theseinstances, materials used are fiberglass,mineral wool, expanded polystyrene orpolyurethane.

The wind and seismic loadings,available space and soil- bearingstrength determine the optimal height-to-diameter ratio. Reduced heights andwider shapes are preferred in windy orseismically active areas, or where soil-bearing capacity is limited. As availableplot space decreases and soil-bearingstrength increases, tanks are designed tobe taller with smaller diameters.

The bottom, shell and roof ofstorage tanks consist of steel platesthat are usually lap-welded togeth-er. To calculate plate dimensions,designers normally refer to indus-try codes, such as those of theAmerican Petroleum Institute(API) (4).

Storage tanks must have lad-ders to provide access to their top.Per API 650, tanks 20 ft or less inheight must be furnished with aladder without a cage. Tankstaller than 20 ft require a spiralstairway. A landing platform atthe top of the ladder can lead towalkways extending to the centerof the roof. Roofs and shells areprovided with manholes that are2-ft in dia. Details on such re-quirements are in API 650.

Most storage tanks construct-ed in petroleum refining and

petrochemical plants are made to conform to one of theAPI standards. These standards cover design, construc-tion, inspection, erection, testing and maintenance re-quirements. They lay down certain minimum require-ments for API certification. The key API codes for stor-age tank design are as follows:

• “Field Welded Tanks for Storage of Production Liq-uids,” API Specification 12D — covers vertical, cylindri-cal, aboveground, welded steel tanks in nominal capacitiesof 500–10,000 bbl in standard sizes for production service.Standard capacity, dimensions and design pressures of API12D tanks are shown in Table 1 (3).

• “Shop Welded Tanks for Storage of Production Liq-uids,” API Specification 12F — covers vertical, cylindrical,aboveground, shop-welded steel tanks in nominal capacities

Nominal Outside Dia., Height, Design Pressure, Design Vacuum,Capacity, ft-in. ft oz./in.2 oz./in.2bbl

500 15-6 16 8 1/2750 15-6 24 8 1/2500 21-6 8 6 1/2

1,000 21-6 16 6 1/21,500 29-9 24 6 1/21,000 20-9 8 4 1/2

2,000 29-9 16 4 1/23,000 29-9 24 4 1/25,000 38-8 24 3 1/2

10,000 55-0 24 3 1/2

Table 1. Standard capacities, dimensions and designpressures for API 12D tanks (4).

SumpOptional

ProcessOutlet

Non-working Capacity

Net Working Capacity

Overfill Protection Capacity

OverflowLiquid Line

Flare/Atmosphere

Inert Gas

Low Liquid Level

Normal Liquid Level

Cooling/HeatingUtilities

High Liquid Level

Design Liquid Level

ProcessInlet

FC

LC

LCTC

■ Figure 1. An aboveground storage tank can have internal coils for heating or cooling the liquid.

of 90–500 bbl in standard sizes for production service.• “Large, Field Welded, Low-Pressure Storage Tanks,”

API Standard 620 — covers vertical, cylindrical, above-ground, field-welded steel tanks for oil storage with maxi-mum operating temperatures not greater than 200°F andpressures in the vapor space less than 2.5 psig.

• “Large, Field Welded, Storage Tanks,” API Standard650 — covers vertical, cylindrical, aboveground, field-welded steel tanks for oil storage with maximum operatingtemperatures not greater than 250°F and pressures in thevapor space less than 1.5 psig.

Although API standards cover many aspects of storagetank design and operation, they are not all-inclusive. Thereare several other organizations that publish standards on tankdesign, fabrication, installation, inspection, and repair thatsupplement the API standards.

These include the American Society of Mechanical Engi-neers (ASME; www.asme.org); American Society for Test-ing and Materials (ASTM; www.astm.org); American WaterWorks Association (AWWA; www.awwa.org); Building Of-ficials and Code Administrators International (BOCA;www.bocai.org); (NACE International; www.nace.org); Na-tional Fire Protection Association (NFPA; www.nfpa.org);Petroleum Equipment Institute (PEI; www.pei.org); SteelTank Institute (STI; www.steeltank.com) Underwriters Lab-oratories (UL; ulstandardsinfonet.ul.com); and the Interna-tional Fire Code Institute (Uniform Fire Code;www.ifci.com).

Environmental requirementsStorage tanks are considered a source of air emissions

due to losses of vapor (5). Emissions from tanks must beaddressed in obtaining the air permit. Volatile organic com-pounds (VOCs) are the major pollutants of concern for airemissions. In addition, specific organics that are toxic orhazardous are also regulated, e.g., benzene. Adequate con-trol and proper management and maintenance are neces-sary to prevent releases of tank contents.

In preparing an application for an air-quality operatingpermit, a review of all applicable regulations must be com-pleted. Environmental regulations often dictate the type ofemissions-control device that must be used in a particularapplication. Minimum emission-control requirements de-pend upon the material stored, when the tank was con-structed or modified, its capacity, the TVP of the com-pound at storage conditions, and the location of the facility.Ref. 5 lists some of the national regulatory codes and stan-dards used for the design of storage tanks and control of airemissions. Among these is the “New Source PerformanceStandards (NSPS), Standards for Performance for StorageVessels for Petroleum Liquids,” from the U.S. Environ-mental Protection Agency’s regulation 40 CFR, Part 60,Subparts K, Ka and Kb. This standard sets rules for thesystems to control emissions. Emissions-control devicesinclude internal and external floating roofs, seals, vents to

flares, vapor recovery systems (such as a thermal oxidizeror scrubber) and disposal systems, such as pressure or vac-uum vents. Table 2 lists examples of the different types ofrequirements and their basis for applicability, taken from40 CFR, Part 60.

Information for the permit includes properties of materi-al stored, operating conditions, TVP, tank physical charac-teristics, tank construction and rim-seal system, roof type,fittings, deck characteristics, estimated emissions, andchemical identification. EPA has guidelines, “Compilationof Air Pollutant Emission Factors,” API-42, that presentmodels for estimating air emissions for organic-liquid stor-age tanks, and include emissions estimation equations de-veloped by API. An EPA-developed program calledTANKS Version 4.09 calculates tank emissions based onAPI 42 – Chapter 12 methodology. The software is avail-able at www.epa.gov/ttn/chief/software/tanks/index.html.

Vent control measures are included in the operating airpermit as permit conditions. Some examples of possibleoperating permit conditions include (5):

• For storage and loading of VOCs — An internal float-ing deck or equivalent control must be installed in alltanks. The floating roof must have one of the followingclosure devices between the wall and the edge of the deck:(1) a liquid-mounted seal; (2) two continuous seals mount-ed one above the other; or (3) a mechanical shoe seal. In-stallation of an equivalent control system requires reviewand approval. (A shoe seal is a type of rim seal that closesthe space between the floating roof rim and the tank shell.)

• For any tank equipped with a floating roof, the holderof the permit has to follow the tests and procedures to veri-fy the seal integrity, as given in 40 CFR 60.113b. There arereporting and recordkeeping requirements for the dates thatthe seals are inspected, their integrity, and any correctiveactions taken.

• Uninsulated tanks exposed to the sun have to be paint-ed white or made of aluminum.

Structural requirementsTank type and size, the soil conditions at the site, tank

loading and tank settlement are critical factors for the de-sign of the tank foundation. Examples of foundation typesinclude earth or crushed stone, concrete slabs, slabs sup-ported by piles and concrete ring-walls.

Earth or crushed stone foundations are simply rings ofmaterial that support the tank walls. These foundations aretypically used in locations with in-situ soil conditions, andcan only be used when anchor bolts are not required. Aconcrete slab set under the entire surface area of the tank isused for tanks less than 15 ft in dia. If soil conditions arepoor or the tank needs insulation, piles may needed.

A concrete ring-wall is constructed by pouring a con-crete mixture around the tank to support it. Ring-wallfoundations are an economical way to support tanks, aretypically used for large tanks and can withstand uplift

Fluids/Solids Handling

56 www.cepmagazine.org December 2002 CEP

forces from the tank. Most of the tanks used in chemicalplants are greater than 15 ft in dia. and commonly havering-wall foundations.

Estimates of the vertical and horizontal loads of the tankare required for foundation design. Vertical loads to beconsidered include the empty weight, live load, operatingweight, test weight and internal pressure. The live load onthe roof is typically 25 lb/ft2, based on API codes (620 and650). The operating weight is the dead weight plus theweight of the fluid, with corrections made for specificgravities greater than 1.0. The test weight consists of thedead weight of the tank plus the weight of the tank full ofwater. The tank is subjected to an internal pressure duringoperating or test conditions.

Even a tank that has no liquid in it can still be underpressure. For example, a tank that held a volatile com-pound can still have vapor in it after being drained. Heatfrom the sun can pressurize the vapor. Horizontal forces in-clude the wind and any seismic loads.

Tank settlement is a common problem with compress-ible soils. Long-term settling of the foundation often oc-curs at the edge and center, due to operating conditions. Ina ring-wall design, the pressure on the bottom of the ring-wall and tank must be equalized to prevent differential set-tlement of the structure.

Cryogenic tanks require cable heating systems to avoid

frost heave, or can be put on columns to allow air circula-tion.

Additional considerationsOther items that need to be considered for the founda-

tion are leak detection systems, corrosivity, cathodic pro-tection, and secondary containment. The engineer mustconsider the environmental and safety implications of leak-age into the containment space below the tank floor. For anearth or concrete ring-wall, leak-detection is normally ac-complished by providing a flexible membrane liner atgrade elevation with a drainpipe under the tank, whichdrains to the perimeter of the tank. For a concrete slab, leakdetection can be achieved similarly or by placing radialgrooves in the top of the slab that extend to the perimeterof the tank. When a leak occurs, one or more grooves willcontain the tank liquid.

Cathodic protection can be used to control electrochem-ical corrosion. This method uses direct current from an ex-ternal source to oppose the discharge current from themetal surface, thereby preventing corrosion. Further, metaltanks that store flammable liquids are grounded as a pro-tection against lightning or static electricity.

Secondary containment is often required to prevent liq-uid from a leaking tank seeping into the ground and/orgroundwater. This can be achieved by either building dikes

CEP December 2002 www.cepmagazine.org 57

Table 2.Typical regulatory requirements for storage tanks (5).

Subpart Materials Tanks Modified or Tank Size, True Vapor Control Stored Construction Date gal Pressure, psia Requirements

40 CFR, Petroleum After March 8, 1974,Part 60 liquids and prior toSubpart May 19, 1978K

After June 11, 1973,and prior to

May 18, 1978

40 CFR, Petroleum After May 19, 1978Part 60 liquidsSubpart Ka

40 CFR, Volatile After July 23, 1984Part 60 organicSubpart liquidsKb

> 40,000 > 1.5 but Floating roof, or vapor recovery < 11.1 system (VRS), or equivalent

> 40,000 > 11.1 VRS, or equivalent

> 65,000 > 1.5 but Floating roof, or VRS, < 11.1 or equivalent

> 65,000 > 11.1 VRS, or equivalent

> 40,000 > 1.5 but External floating roof (EFR) with< 11.1 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 < 2.2 Exempt from Subpart Kb < 40,000

40,000 < 0.5

20,000 but > 4.0 but IFR with liquid-mounted seal or < 40,000 < 11.1 with mechanical shoe seal, or

with vapor-mounted seal and 40,000 > 0.75 but rim-mounted secondary seal, or

< 11.1 EFR with two seals, or VRS with 95% reduction or equivalent

20,000 11.1 VRS with 95% reduction or equivalent

with liners made of high-density polyethylene (HDPE), orby adding concrete walls and slabs, along with a leak de-tection system. Curb and dike containment are covered bymany regulations that govern the volume, area, height andspacing between multiple tanks and process units. Areasumps may also be required to contain possible leakage.Provisions must be made for removing water or debrisfrom the sumps.

Operation and controlPressure control — The design of a tank must take into

account both normal operations and certain upset condi-tions. 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 withdraw-ing liquid, the vacuum that is created must be counter-bal-anced by the infusion of an inert gas, such as nitrogen,through a breathing valve.

Vapor “surplus” or “deficit” can also occur in an idletank as a result of ambient temperature changes or chemi-cal reactions taking place within the liquid inventory. Theventing of excess vapor or the infusion of an inert gas forall normal operating conditions is carried out automatical-ly, typically through self-regulating valves.

Level control — Level-measuring devices are based ondifferential pressure, or sonic, capacitance, displacer velocityor liquid-conductivity measurements. Sonar or radar levelmeasurements have recently gained popularity. These de-vices are usually mounted on the roof of a tank. They sendout a signal, which is reflected off the liquid level. The timeit takes for the reflected signal to be received is used to mea-sure the liquid height. A major advantage of these instru-ments is that they can be used with corrosive liquids.

The level is then adjusted by closing or opening the ap-propriate valves. When precise level control is not re-quired, the liquid level is maintained between the HLL andthe LLL. Automatic emergency cut-offs are applied whenthe liquid level is at the overfill level to avoid overflow, orwhen it is below the LLL to avoid cavitation of a pump.

Temperature control — A thermocouple, which ismounted below the LLL of the tank, provides a continu-ous readout of the temperature. Multiple measurementpoints are sometimes required to ensure representativetemperature readings when the tank is large, there aredifferent feeds at different temperatures, or there is aheating coil. The tank temperature can be maintained byadjusting the flowrate of a cooling or heating medium inan internal coil.

Upsets and safety — Typical upsets include overpres-sure, overflow, boil-over, over-temperature, water ingress,floating-roof failure, unexpected phase separation, light-ning, static-charge buildup, steam coil failure and fires.

Adequate monitoring can help to ensure safety duringupsets and other incidents. Control and prevention of suchsituations include the use of: sprays, deluge or foam sys-tems; pressure-, temperature-, level- and fire-monitoringdevices; pressure-relief systems; and ensuring properpreventative maintenance. CEP

Fluids/Solids Handling

58 www.cepmagazine.org December 2002 CEP

YYAACCIINNEE AAMMRROOUUCCHHEE is a process engineer at KBR (601 Jefferson Ave.,Houston, TX 77002; Phone: (713) 753-7028; Fax: (713) 753-6097; E-mail:yacine.amrouche@halliburton.com). He is a junior-level engineer with twoyears of experience in process engineering and is a member of KBR’syoung professional network, IMPACT. Amrouche holds a BS in chemicalengineering from the Univ. of Sussex, U.K., with a specialization inpolymer science.

CHAITALI DAVE` is an environmental engineer at KBR (Phone: (713) 753-3572; Fax: (713) 753-3123; E-mail: chaitali.dave@halliburton.com). She isa junior-level engineer with four years of experience in environmentalengineering and is a member of KBR’s young professional network,IMPACT. Dave’ holds a BS in chemical engineering from the Univ. of SouthFlorida 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: kamal.gursahani@halliburton.com). He is ajunior-level engineer with one year of experience and is a member ofKBR’s young professional network, IMPACT. Gursahani holds a BS inchemical engineering from Bombay Univ. and an MS in chemicalengineering from the Univ. of Wisconsin – Madison.

ROSABELLA LEE is a process engineer at KBR (Phone: (713) 753-2238; Fax:(713) 753-5353; E-mail: rosabella.lee@halliburton.com). She is a junior-level engineer with four years of experience and is a member of KBR’syoung professional network, IMPACT. Lee holds a BS degree in chemicalengineering 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: lisa.montemayor@halliburton.com).

She is a junior level engineer with four years of experience in civilengineering and is a member of KBR’s young professional network,IMPACT. Montemayor holds a BS in civil engineering from Texas A&M Univ.

Literature Cited1. 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. (Desk-

book), 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 athttp://www.tnrcc.state.tx.us/.

AcknowledgmentThe authors would like to thank Ahmed Allawi, Benson Pair and the

KBR Publications Committee for their guidance and support inwriting this article.