Improve Selection and Sizing of Storage Tanks

8/2/2019 Improve Selection and Sizing of Storage Tanks

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S T O R A G E T A N K S

Im prove se lection and s iz ingof sto rage tanksGu id elin es h elp e ng in ee rs s ele ct th e b es t ta nk d es ig nfo r p lan t applic a tionsA. HEYDARI GORJI and H. KALAl JARI, SAZEH Consultants, Tehran, Iran

S torage tanks are used to receive and hold raw materials forfurther processing into end-products. Within the plant,the feedstocks and raw materials undergo several processessteps by various operating units. Because the flow of feedstocksand other raw materials is not exactly instantaneous, tanks areneeded to hold various product streams. Finally, storage tanksare used to hold and store finished products until distribution.Storage tanks are classified in different ways. Tanks are clas-

sified depending on the nature of the product to be stored(atmospheric, low pressure and medium pressure), operatingtemperature (ambient and low temperature) as well as the typeof construction (aboveground or underground and double wall).A significant factor to consider when selecting storage tank iseffects from vapor pressure created by the stored product at theambient temperatures.Atmospheric and low-pressure tanks. Six basic tankdesigns are used for liquid storage vessels:fixed roof (vertical andhorizontal), external floating roof, domed external (or covered)floating roof, internal floating roof, variable vapor space andmedium pressure. A brief description of each tank is providedhere:Fixed roof. The fixed-roof tank when compared to an open-

top tank, which is used mainly in water and wastewater treat-ment, contains product vapors with reduced potential for fire.Since the liquid surface is exposed, the tank-vapor space resultssignificant product evaporation occurs.Fixed-roof tanks are generally used to store products that

do not readily vaporize at ambient or stored temperature con-ditions. For instance, fixed-roof tanks can be used to handlenon-volatile products such as gasoil, lubricating oils, asphaltand fuel oil.Regarding emission control, fixed-roof tanks without vapor-

recovery facilities are restricted to materials with a true vaporpressure (TVP) ofless than 1.5 psia at the tank operating tem-perature. This type of tank consists of a cylindrical steel shellwith a permanently fixed roof, which may vary in design fromcone or dome shaped to flat. Each type can be further sub-divided into non-pressure, low-pressure and high-pressure fixed-roof tanks: Non-pressure fixed-roof tanks are used for storage at atmo-spheric pressure and are provided with open vents. Low-pressure and high-pressure fixed-roof tanks are used

for storage at a low- and high-internal pressure or vacuum

TABLE 1. Max imum design c on dit io n fo rc on e- a nd d om e -ro of ta nk sType o f tanks Des ign pressu re /vacuum , m bar (gauge)Non-pressure 7.5/2.5Low-pressure 20/6.0High-pressure 50/6.0

respectively. They are provided with pressure/vacuum reliefvalves (breather valves) that should be set to be fully open at thedesign pressures.Table 1 summarizes the maximum design conditions for each

type of fixed-roof tanks. Atmospheric tanks should not be usedto store liquids at temperatures at or above its boiling point.Losses from fixed-roof tanks are caused by changes in tempera-ture, pressure and liquid level. Fixed-roof tanks should be eitherfreely vented or equipped with a pressure/vacuum relief valve.Floating roof. For this tank type, the roof is not fixed to theshell, but "floats" on the liquid; so, no vapor space is created.Because the floating roof rests directly on the liquid surface, itsignificantly diminishes stock evaporative lossesand reduces thehazards associated with handling a large, possibly combustibletank vapor space.Floating-roof tanks are utilized when the TVP of the stored

liquid at operating temperature is less than 11.1 psia. For TVPsgreater than 11.1 psia, a medium-pressure storage tank or vapor-recovery system ismandatory. There is a preference for floating-roof over fixed-roof tanks as the sizeof the tank can be increased,as the vapor pressure of the stored liquid increases, and the flashpoint is below the storage temperature. Floating-roof tanks arepreferred to store products susceptible to high electric chargedue to friction such as light products, kerosine and crude oil.External floating-roof tanks. This tank type is designed

to work at atmospheric pressure. Although normally recom-mended only for diameters of 15 m and greater, this does notexclude applying this design for smaller-diameter tanks underspecial circumstances. At minimum, the diameter of an externalfloating-roof tank should be equal to its height to enable usinga normal rolling ladder for roof access.A typical external floating-roof tank consists of an open-

topped cylindrical steel shell equipped with a roof that floats onthe surface of the stored liquid. The floating roof consists of adeck, fittings and rim-seal system.

H Y DR OC AR BO N P RO CE SS IN G O CT OB ER 2 00 6 I 95


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S TO RA GE TA NK S

T AB LE 2. Selec tio n o f ver tica l ta nks3-12.5 15-20 22.5-39 4 2 - 7 2

P ro du c t c at eg or y T an k d ia me te r, m

F la sh p o in t l es sthan 21( H i gh - and low -p ressu recone roof

H ig h - a nd l ow - p re s su re co nea nd dome r oo f/ ex te rn al a ndi nt ernal f loa ti ng roof

Ex te rna l f loa ti ng roo f{preferably)/ Iow-pressurec on e a nd d om e r oo f

Ex te rna l f loa ti ng roof

F lash po in t 21O( an dh ig he r b ut le ssth an 5 5 (

L ow -pressure cone roof Ex ternal floating roofow -p re ssur e co ne a nd domeroof /e x te rna l and i nt erna lf loa ti ng roof

Ex te rna l and i nt ernal f loa ti ngroof {preferably)/ Iow-pressurec on e a nd d om e r oo f

F la s h p oin t 5 5 (a nd h ig h er

Non -pressu re cone anddome r oo f

Non -pressu re cone anddome r oo f

Non -pressu re cone roo f *on -p ressure cone roof* M ax im u m d ia m et er f or n on -p re ss ur e c on e r oo f is 6 0 m .

Floating decks are of two general types: pontoon and double-deck. Typical pontoon external floating-roof tanks are shown inFig. 1.With all types of external floating-roof tanks, the roof fallsand rises with the liquid level within the tank. External floatingdecks are equipped with a rim-seal system, which is attached tothe deck perimeter and contacts the tank wall.The purpose of thefloating roof and rim-seal system is to reduce evaporative loss ofthe stored liquid.Internal floating-roof tanks. This tank design was devel-

oped in the mid-1950s to provide protection of the floating rooffrom elements, including lightning strikes. The tank vapor space,which islocated above the floating roof and below the fixed roof,includes circulation vents to allow natural ventilation, thus reduc-ing accumulation of product vapors and mitigating the formationof a combustible mixture (Fig. 2).Such tanks can be used when: Snow loading on a floating roof may be a problem sincesnow

or water on the floating roof will affect the operating buoyancy Contamination by rainwater of the liquid stored in a floatingroof tank is unacceptable Environmental or vapor loss problem with fixed-roof tanks

are evident Contact of the stored liquid with air should be avoided.Internal floating roof can be installed in existing tank as well

as new tanks.96 I O CT OB ER 2006 H YD RO CA RB ON P RO CE SS IN G

There are two basic types of internal floating-roof tanks: Tanks in which the fixed roof is supported by vertical col-umns within the tank Tanks with a self-supporting fixed roof and no internal sup-

port columns.Installing a floating roof minimizes evaporative losses of thestored liquid. Evaporative lossesfrom floating roofs may developfrom the deck fittings, non-welded deck seams and annular spacebetween the deck and tank wall. In addition, these tanks are freelyvented by circulation vents at the top of the fixed roof The ventsminimize the accumulation of organic vapors in the tank vaporspace in concentrations approaching the flammable range. Aninternal floating-roof tank that is not freely vented isconsidereda pressure tank.Domed external floating-roof tanks. Domed external (or

covered) floating-roof tanks have a heavier deck used in the exter-nal floating-roof tanks aswell asa fixed roof at the top of the shell,such as an internal floating-roof tank. Domed external floating-roof tanks usually result from retrofitting an external floating-roof tank with a fixed roof The function of the fixed roof in thedomed external floating-roof tank is not to act as a vapor barrier.However, it should block the wind. Like the internal floating-rooftanks, these tanks are freelyvented by circulation vents at the topof the fixed roof. A typical domed external floating-roof tank isshown in Fig. 3.


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Medium-pressure tank. Spherical-shaped storage tanksare used to store products with high vapor pressure in very largecapacity (Fig. 4). Spherical storage tanks can be 10 times largerthan bullet tanks.Bul let t anks are long cylindrical (usually horizontal) pressure

S TO R A G E T A N K S

vessels most frequently used for liquid petroleum gas (LPG).The working pressure of these tanks can be from 1 to 70 barg orgreater.These tanks often have hemispherical head. High-pressurematerial such as propane and ammonia can be stored in thesetanks at ambient temperature.


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Sphere, hemispheroid and spheroid are rarely used in petro-leum and petrochemical industries.

Selection of vertical t anks . Table 2 summarizes the types andranges of tanks recommended to store different classes of liquidproducts. Some important factors such as evaporation losses,pumping losses,climatic conditions, air pollution, soil conditionsand local regulations shall also be considered when selecting themost suitable type(s) and sizets).Sizing and design procedure. The presented procedureincludes a method to calculate the diameter and height of a fixed-roof and external and internal floating-roof tanks, minimumliquid level and maximum liquid level in the atmospheric andlow-pressure tanks.For floating-roof tanks, tank capacity dependson the type of floating roof, roof seal, roof cover and internal tankconstruction that may affect these guidelines.Nominal capacity of the tank. Nominal tank capacityassumes that the flat-bottom tank is filledto the top of the shell plateand iscalculatedby summing the net capacityof the tank (network-ing capacity), dead capacity of the tank and liquid volume pumpedout of the tank in three minutes (between LLL and LLLL).The later item is important from an operational point of

view, because the operator will switch off the pump three min-utes after hearing the alarm or interlock shutdown system willswitch off the pump.

1tD2VN=(Hl+H2+ht)-4-+V;+~ (1 )where:

VN = Nominal capacity of the tankD = Diameter of the tankHI = Tank dead heightH2 = Vapor space height or maximum safeworking levelV I = Net working capacity of the tankV2 = liquid volume pumped out of the tank in three minutes.

Diameter and height of the tank. The following steps listthe assumptions and operations to find the diameter and heightof the tank:1.Assume the first larger value of net working capacity (Vj),

from Table 3 (considering as V~ VI)2. Read the related diameter to V from Table 3. Consider

that the diameter of a floating roof shall at least be equal to itsheight.3. Calculate the nominal tank capacity (VN) from Eq. 1 using

this diameter.4. Compare calculated nominal tank capacity (VN)with V : If VN:5 V ; then select the related diameter and height ofV from Table 3 as diameter and height of the tank. If VN>.V; then pick the next greater value for Vfrom theTable 3 and go to Step 2 until VN:5 V.

Tank-height calculation for a fixed-roof tank. To findthe tank for a fixed-roof tank, follow these steps:

Bottom dead height. Tank dead height isthe distance betweentank outlet nozzle (connected to pump suction) and tank floor.Consider the height needed to prevent vortexing in the connec-tion point of the outlet nozzle with the tank on the outlet line[This height isassumed equal to outlet line diameter (dl).]


In this regard, tank dead height (LLLL) iscalculated by:3H =h+-d1 2 1 (2)

where: h = dl+ 5 in.Please note that the minimum sizeofHI is 24 in. (seeFig. 5).Vapor space height of the tank can be calculated by:

3H2 =-d2 (3 )2

where: r l z = overflow line diameter.The minimum value of H2 is 18 in. Note: If the level switchhigh-high is considered in a fixed-roof tank, then the overflownozzle is not installed.Floating roof. To find the tank for a floating-roof tank, followthese steps:

Bottom dead height. The same procedure used for the fixed-roof tank should be applied. However, designers should considerthat the roof must be supported approximately 1.2 m from thebottom for external floating roof and 1.6 m of the bottom forinternal floating-roof tank, to keep the roof floating; therefore:

H YD RO CA RB ON P RO CE SS IN G O C TO B ER 20 06 I 99


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oo TABLE 3 . N om in a l c a pa c it ie s o f s ta n da rd v er ti ca l c yli nd r ic a l t a nk s, m 3oaIDm::III-


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External floating-roof tank:Internal floating-roof tank:

HI 2: : 1.2mHI 2: : 1.6m.

Maximum safe working level. Maximum safeworking level isa level in which any additional product will overflow and contactor damage will occur. This level is taken approximately 0.6 ~0.8m below the top curb angle.External floating-roof tank:Internal floating-roof tank:

H2 2: : 0.8mH22:: 0.6 m forD < 15H22::0.8m forD 2: : 15.

Low-liquid level inthe tanks. Low-liquid level (LLL) in fixedand floating-roof tanks is the summation of tank dead head (HI)and height of the liquid in three-minute suction (hb), which canbe calculated as below:

( 4 )

or hn = HLL - (HI + hw), whichever is greaterwhere:Q= Pump discharge flowrate, m3/hr


D =Tank diameter, mh b = Liquid height above LLLL in three-minute suction, m.High-liquid level inthe tanks. After calculating H2, high-liquid level (HLL) height from tank bottom for fixed- and float-

ing-roof tanks can be calculated as:( 5 )

h, is the height between high-high liquid level (HHLL) andHLL and calculated as:

h = O.Olh =(0.04 V ; )t W nD' ( 6 )

h, shall be at least 100mm.Note: Some internal parts such as heating coils inside tanks

may affect the height and liquid levels.Overflow-liquid line sizing. The overflow line inside diam-

eter is determined as:1

d =(0.81 NW2)42 Mp ( 7 )

where:W = Vesselinflow, kg/secap = Pressure drop (pa), [as experience, 996 Pa can be used]N = Friction loss (for simple overflow = 1.5)P =Density, kg/m3

d z =Overflow internal diameter, m.

liTERATURE CITED1 American Petroleum Institute, "Welded Steel Tanks for Oil Storage," lOth ed.,Standard 650, Washington, DC, 1998.

2 Mead, J ., The Ency c loped ia o /Chem i cal P r oc es sEqu i pmen t , Reinhold Publishing,New York, 1964, pp. 941-956.3 Burk, H. S., et. a 1 . , "Conceptual Design of Refinery Tankage," ChemicalEnginming, No. 88, Vol. 17, Aug. 24, 1981, pp. 107-110.

4 Iranian Petroleum Standard, "Process design of l iquid and g a s transfer andstorage: December 1997.

ACKNOWLEDGMENTThe authors would like to thank the Board of Directors and Process Division

Director of Sazeh Consul rants Company for their support.

Aliakbar Heydari Gorji is a s en io r p ro ce ss e ng in ee r a tS A lE H C on su lta nts in T eh ra n, Ira n. H e is a P h D s tu de nt o f c he mi-c al e ng in ee rin g a nd h old s B S a nd M S d eg re es in c he mic al e ng i-n ee rin g, a ll f ro m t he Am ir ka bir U niv er sity o f T ec h no lo gy ( Te h ra nPo ly t echn ic ) . H i s a r ea o f sp e c ia li za t io n ha s i nc lu d e d ga s se par a ti on

w ith m em bra ne s a nd h is in du stria l e xp erie nc e h as fo cu se d o n s im ula tio n, b as ica nd d eta il d es ig n o f g as a nd p etro ch em ic al p la nts . H e c an b e re ac he d a t e -m ail:a .heydar [email protected] r .

Hamid Reza Kalat Jari is a s en io r p ro ce ss e ng in ee r in th ep ro ce ss d ep artm en t o f S A ZE H C on su lta nts in T eh ra n, Ira n. M r.K ala t J a ri h a s b e en w ith S A lE H P ro ce ss d ep ar tm e nt fo r fo ur y ea rs .P re vio us ly , h e w o rk ed fo r fo ur y ea rs w ith T ota l F in a E lf a nd e ig hty ea rs w ith N a tio na l Ira nia n G as C om pa ny a s a p ro ce ss e ng in ee r

a nd p ro ce ss c on tro l e ng in ee r. H e h old s a B S d eg re e in c he mic al e ng in ee rin g fro mth e S h a rif U niv er sity o f T ec h no lo gy a nd a n MS d eg re e in c h em ic al e ng in ee rin g fr omT a rb ia t Mo da rr es U n iv e rs it y. H e c a n b e r ea c h ed a t e -ma il: h .ka la t jar [email protected] r .

H YD RO CA RB ON P RO CE SS IN G O CT OB ER 20 06 110 1
mailto:[email protected]:[email protected]:[email protected]:[email protected].

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Improve Selection and Sizing of Storage Tanks