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Engineering layout of fuel tanks in a tank farm

Angan Sengupta * , A.K. Gupta 1, I.M. MishraDepartment of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 24766, Uttarakhand, India

a r t i c l e i n f o

Article history:Received 17 December 2009Received in revised form21 May 2010Accepted 22 June 2010

Keywords:Tank farmWind effectCross-windSafe separation distanceModi ed point source model

a b s t r a c t

The present paper deals with the location of tanks in a tank farm, in chemical and allied industries.Ideally the tanks are so placed and installed that in case of re, the neighbouring tanks could remain safe.

The safe distance of separation among the tanks is calculated in no wind condition, as well as, in thepresence of wind. The paper uses the methods available in literature and modi es the point sourcemodel to include the effect of wind vector on the ame height during the calculation of safe inter-tankdistance. It is found that for wind velocity > 4 m/s, the modi ed point source model provides appropriateinter-tank distance. However, for no wind and with wind velocity < 4 m/s, the Shokri e Beyler ’ s methodprovides safe inter-tank distance.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

Tank farm is a synonym of an oil depot, a facility for storage of liquid chemicals, such as oils, gasoline, diesel, aviation turbine fuel,solvents, petrochemicals, etc. The tank farm is a piece of land onwhich a number of fuel oil or chemical storage tanks are located orsited together. The storage tanks may also be used to store baseblending components, solvents, additives, acids, caustic, chemicals,or nished products. They may also be used as blending vessels.Storage tanks are expensive to build and they require periodicmaintenance to keep them in proper condition when storingvolatile and ammable liquids. It is, therefore, necessary that theybe properly sized and utilized to maximize the return oninvestment.

The safety aspects, apart from periodic maintenance, areextremely important. The recent incident at Jaipur oil depot of Indian Oil Corporation Ltd. where 12 tankers containing 10 5 kl of diesel and gasoline caught re and the re continued for a weekresulting in several fatalities and about 200 injuries besides damage

to property worth 3 103 million INR( w 75 million USD) is a case inpoint. This re caused very serious environmental pollution around Jaipur and its adjoining areas. This incident has underlined theimportance of proper layout with safe separation distance toprevent such hazardous episodes.

The tank farms usually have a number of tanks of equal heights.However, they may be of varying capacities. Equal height adds toaesthetics and lends them economic credence in the constructionof access structures, easy movements of operators from one tank toanother and of re service men during emergency episodes. Thetanks may be laid out in square pitch having safe inter-spacebetween them. The inter-tank optimum separation distance iscrucial for safe operations, piping design, and maintenance accessand emergency/accident control and mitigation measures. Thedesired inter-tank distance depends largely on the materials/chemicals to be stored and the capacity of the tanks.

The tanks in re neries are generally constructed from steel orpolyethylene or ber glass and are either having xed roofs and/or

oating roofs for storing liquids. Non-steel constructions lower costconsiderably, and make them preferred choice for storing corrosiveand reactive chemicals.

Various regulatory and professional bodies like AmericanPetroleum Institute (API), National Fire Protection Association(NFPA), and Environmental Protection Agency (EPA) have sug-gested standards for such tank layouts in a tank farm. The layout of tanks as distinct from their spacing, should always take intoconsideration the accessibility needed for re- ghting and thepotential value of a storage tank farm in providing a buffer area

* Corresponding author. Tel.: þ91 9012390134; þ91 9433948999.E-mail addresses: angan.sengupta@gmail.com (A. Sengupta), akgd30@

indiatimes.com , akgd30@gmail.com (A.K. Gupta), imishfch@iitr.ernet.in(I.M. Mishra).

1 Scientist and former Head, Fire Research Laboratory, Central Building ResearchInstitute, Roorkee, India.

Contents lists available at ScienceDirect

Journal of Loss Prevention in the Process Industries

j o u rn a l h o mep ag e : www.e l sev i e r. co m/ l o ca t e / j l p

0950-4230/$ e see front matter 2010 Elsevier Ltd. All rights reserved.

doi: 10.1016/j.jlp.2010.06.016

Journal of Loss Prevention in the Process Industries 24 (2011) 568 e 574

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between process plant and public roads, houses, etc., for environ-mental reasons. The location of tanks relative to process units mustbe such as to ensure maximum safety from possible accidents.Fig. 1a, b shows schematic representation of tanks in a tank farmlaid out in square and triangular pitch.

Primary requirements for the layout of re nery tanks farms aresummarizedas follows( Digrado& Throp,1995;Long& Gardner, 2004 ):

1. Inter-tank spacing and separation distances between tank andboundary line and tank and other facilities are of fundamentalimportance.

2. Suitable roadways should be provided for approach to tanksites by mobile re- ghting equipment and personnel.

3. The re water system should be laid out to provide adequatere protection to all parts of the storage area and the transfer

facilities.4. Bunding anddrainingof thearea surrounding the tanks shouldbe

such that a spillage from any tank can be controlled to minimizesubsequent damage to the tank and its contents. They should alsominimize the possibility of other tanks being involved.

5. Tank farms should preferably not be located at higher levelsthan process units in the same catchment area.

6. Storage tanks holding ammable liquids should be installed insuch a way that any spill will not ow towards a process area orany other source of ignition.

A key safety consideration for tank farm siting, spacing, andlocation is the separation of non-compatible materials by the use of an internal bund or dike wall within the tank farm. Providing bundor dike checks the ow of the spilled oil to the neighbouring areas.Thus in case of re engul ng the tank farm, the reis con ned to itsorigin. The bunds, however, need to be designed to have suf cient

strength to withstand the pressure that may be created in the eventof an oil spillage and the capacity to store the spilled liquid.

Somemethods are available in literature for modeling inter-tanksafe distance in a tank farm. However, these methods do not takeinto account the effect of cross-wind which is important from respread point of view. The present paper deals with the assessmentof various methods to estimate the safe separation distancebetween two storage tanks in a tank farm, from the re spreadpoint of view. A modi cation is proposed to the point source modelto incorporate the effect of wind velocity on the ame tilt and respread. The estimated separation distance is also compared withthe values given by various regulatory bodies and prescribed bystandards and reported in the present paper.

2. Models to estimate the safe inter-tank spacing

Following methods are generally used for the determination of safe inter-tank spacing:

(a) Point source model.(b) Shokri e Beyler ’ s method.

(c) Mudan’

s method.

Using these methods, the heat ux at various distances,between a tank on re and the adjacent (target) tank is calculated.The distance at which the heat ux becomes equal to 4.732 kW/m 2

(1500 BTU/h/ft 2) (Daniel, Crowl, & Louvar, 2002; Lees, 1995; SFPEHandbook of Fire Protection Engineering, 1995 ) is considered tobe the safe inter-tank distance. No material is expected to ignitewith a heat ux lower than 4.732 kW/m 2.

2.1. Point source model

In this model, it is customary to model the ame by a pointsource located at the center of the real ame in order to predict the

thermal radiation

eld of

ames. The point source model ( SFPEHandbook of Fire Protection Engineering, 1995 ) is the simplestcon gurational model of a radiant source. Fig. 2 shows the sche-matic diagram of two tanks for using point source model. Thecritical value of incident heat ux, de ned as the minimum value of the heat ux which can ignite the fuel in the target tank is given as

_q00r ¼ Q r cos q

4p R2 kW =m 2 (1)

Fig. 1. (a) Top view of a tank farm square pitch layout. (b) Top view of a tank farm

triangular pitch layout.

R

D

H

H f /2

L

H f

Fig. 2. Schematic diagram of a tank on

re for point source model.

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(Heskestad, 1984 ) in the case of gasoline. In the case of LNG, ameheight as calculated by Heskestad method provides a lower valuethan that by Moorhouse ( Mudan, 1984 ).

Table 2 shows the calculated results obtained using the abovemethods. Figs. 4 and 5 show the incident heat ux at variousdistances and the safe distance between the tanks when a tank ison re under no wind condition, for gasoline and LNG, respectively.It is clear that under no wind condition, heat ux decreases with anincrease in distance of separation between the tanks. While Shok-rie Beyler method gives the highest value of safe distance, pointsource and Mudan ’ s methods give very close values, but lower thanthat given by Shokri e Beyler method. It may be seen from Table 2 ,that the values of the safe distances as proposed by differentstandards are widely different; the most conservative and thehighest estimate being that given by EPA, whilethe lowest is that of NFPA (NFPA-30, 2001 , Chap. 4). The value recommended by API isnear to that estimated by point source model and the Mudan ’ smethod for LNG re. For gasoline, the values obtained by pointsource model and Shokri e Beyler method are nearer to API value.

From Table 2 , it is observed that LNG storage tanks require largerseparation distance than that for gasoline, in the absence of windeffect. This is obvious since LNG ( ash point ¼ 148.89 C) is more

ammable than gasoline ( ash point ¼ 42.7 C).

Table 2Comparison of values of safe separation distancewith variousstandards and models.

Regulatory bodies Rim to rim distancebetween tanks (for class e I fuel) (m)

NFPA 3.33EPA 30.48BN-DG-C01J plant

layout-storage tanks10 e 15

API 15Point source model Gasoline: 13.6LNG: 14.6

Shokri e Beyler method Gasoline: 16.25LNG: 20.35

Mudan ’ s method Gasoline: 12.5LNG: 15

1 4

. 6 ,

4 . 7

2 6 1

2 0 . 3 5

, 4 . 7 2

7

1 5 , 4

. 7 3 0 4

0

2

4

6

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12

14

16

18

20

22

24

26

0 5 10 15 20 25 30

Distance from Rim (m)

R a d i a t i v e

H e a t F l u x

( k W / m ^ 2 )

Point Source: ; Shokri-Beyler: ; Mudan’s Method:

Fig. 5. Radiative heat ux versus distance of separation from rim in no-wind conditionfor LNG.

0

2

4

6

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54

56

0 5 10 15 20 25 30

Distance from Rim (m)

R a d

i a t i v e H e a t F

l u x

( k W / m ^ 2 )

Wi nd Spe ed ( m/s ): 4 ; 6 ; 8 ; 10 ; 12

Fig. 6. Radiative heat ux versus distance of separation from rim in cross-wind

condition for gasoline, using modi ed point source model.

1 3 . 6

, 4 . 7 2

2

1 2 . 5 ,

4 . 7 3

1

1 6 . 2 5 ,

4 . 7 2

9

0

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0 5 10 15 20 25 30

Distance from Rim (m)

R a d i a t i v e

h e a t F l u x

( k W / m ^ 2 )

Point Source: ; Shokr i-Beyler : ; Mudan’s Method:

Fig. 4. Radiative heat ux versus distance of separation from rim in no-wind condition

for gasoline.

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Figs. 6 and 7 show the variation of incident radiative heat uxwith distance, while incorporating the wind effect for gasoline andLNG, respectively. Table 3 gives the values of safe distance ascalculated by modi ed point source model, incorporating the effectof wind on ame.

It is seen from Table 3 that the height of the ame decreaseswith an increase in the wind speed while the tilt angle with respectto vertical axis increases with an increase in wind speed. As a resultof the decrease of ame height, it is expected to have a decrease inthe safe distance between the two tanks. But due to tilting of the

ame, the safe distance is increasing up to a wind speed of 8 m/s.Thereafter, it starts decreasing, due to attening of the ame.

Thus from Figs. 6 and 7 , it is clear that the wind speed of 8 m/s isthe critical speed, for which we require maximum safe distance of separation. Beyond this speed of the wind the ame becomesalmost at thus radiating less heat at a particular location.

The square pitch layout of the tanks in the tank farm requiresmore area per tank, than that required for the triangular pitch.However, the tank farm layout in square pitch is better for passagein between the tanks and for smooth maintenance and controlwork.

4. Conclusions

The layout of a tank farm for storage of volatile and ammablesubstances is very important. Although some methods are availablefor determining the minimum distance of separation of one tankfrom the other, these methods do not take into account the effect of wind on the ame height and re spread. The point source modelhas been modi ed to incorporate the effect of wind velocity on the

ame height and re spread.Shokri and Beyler method gives a higher safe distance in

comparisonto other methods under no windcondition.The radiativeheat ux increases under windy condition, the safe distance of separation between the tanks will, therefore, also increase. Themodi ed point source model can be used for determining thisincreased safe distance. However, for wind speed below 4 m/s, thesafe distance of separation estimated by Shokri e Beyler ’ s methodmay be maintained between the tanks. For wind speed above 8 m/s,safe distance of separation decreases, as the ame gets almost at-tened at thiscondition. Thus, modi ed point sourcemodel, proposedin this paper should be used for tank farm layout, when the windspeed, in general, remains above 4 m/s. It is found that square pitchmay be used for tank farm layout from safety point of view.

Nomenclatures

c p: speci c heat of fuel (kJ/kg/K).D: diameter of the pool or source tank (m).DT : target tank diameter (m).E : emissive power (kW/m 2).F 12: view factor. g : acceleration due to gravity (9.81 m/s 2).h0: difference in height of the two tanks (m) ¼H 1 H 2.D H c : heat of combustion of fuel (kJ/kg).H f : ame length/height (m).h g : convective heat transfer coef cient of fuel (W/m 2/K).H R: H f h0 (m).D H V : heat of vaporization (kJ/kg).H 1: height of source tank (m).H 2: height of target tank (m).K g : thermal conductivity of fuel (W/m/K).L: inter-tank distance measured from the center of source tankto the edge of target tank (m).Lcorr : corrected distance to obtain q (m).

_m00: mass burning rate per unit pool area (kg/m 2 s).P w: partial pressure of water vapour in air (Pa).Q : total heat produced by the re (kW).Q E

00: incident radiative heat ux from external source (such assun) (kW/m 2).Q r : total radiative energy output from the re (kW)._q00r c

: critical value of incident heat ux (taken as 4.732 kW/m 2).

R: hypotenuse from ame center to target tank top edge (m).RN : regression rate (mm/min).u : wind speed (m/s).u*: dimensionless wind speed as given in equation (17) . x: path length (m) ¼(hypotenuse of re elevation fromground) (radius of re).l : fraction of total heat which is radiated.h: ef ciency of combustion.f : ame tilt angle from vertical axis ( ).q: angle between the normal to the target and the line of sightfrom the target to the point source location.r : density of fuel (kg/m 3).r a: air density (kg/m 3).r v: fuel vapour density (kg/m 3).s

: transmissivity of air.

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0 5 10 15 20 25

Distance from Rim (m)

R a d i a t i v e

H e a t F

l u x ( k W / m ^ 2 )

Wind Speed (m/s):4 ; 6 ; 8 ; 10

Fig. 7. Radiative heat ux versus distance of separation from rim in cross-windcondition for LNG, using modi ed point source model.

Table 3Variation of safe distance with wind speed (modi ed point source model).

Fuel Wind speed (m/s) Flame height (m) Tilt angle ( ) Safe distance (m)

Gasoline 4 9.53 60.62 12.236 8.76 66.38 14.428 8.24 69.70 18.52

10 7.86 71.92 17.5912 7.51 73.54 12.71

LNG 4 10.63 54.17 13.556 9.76 61.45 13.608 9.19 65.55 19.36

10 8.77 71.92 14.25

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