Fire Protection System_storage Tank

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7 Tank fittings and ancillary equipmentfor ambignt tempemturc tanks

Figure 7.33 Vertical ladder

Cowlesy of Royal Vopak

7.10 Fire protection systemsAs one can readily understand, the planning for the preventionoffire, especially in petrochemical installations, is high on theirmanagements' priolity list, as the consequences of an infernocan have disastrous results, not only to the installation but tothe surrounding area and environment.

The subject is well-documented in the National Fire ProtectionAssociation, Institute of Petroleum and British Standard Codes.References 7. 3lo 7.6 provide useful information on this impor-tant issue.

For the purposes ofthis Section the protection of storage tanksby the use offoam and waterwill be considered.

7.10.1 Foam systems

The foam methods considered to be the most widely used andregarded to give an acceptable overall level of protection arereferred to in this Section. The design guidelines are to befound in References 7.3 to 7.6.

The foam fire fighting system works by introducing a foam mak-ing concentrate into the fire fighting water main. This producesa solution, which is fed to a foam generator, and the resultingfoam is directed to the fire.

For fixed roof, floating roof and Internal floating roof storagetanks there are three principal foam systems available andtheseare;base injection, top foam pouring andfoam cannons.

These systems are categorised in Figure 7.34.

7.10.1.1 Base injection

Base injection systems (also known as sub-surface foam injec-tion systems) are suitable for use on fixed roof tanks containingliquid hydrocarbons with the exception of Class 1A hydrocar-bon liquids or alcohols, esters, ketones, aldehydes, anhy-

drides, or other products requiring the use of alcohol-resistantfoams. In operation, specialised equipment designed to oper-ate against a back pressure introduces aspirated foam at a pre-

2OO STORAGE TANKS & EOUIPMENT

' Fo* enffi aE plac€d 6xt mat lo th6 bntr in .fth a position th*, tn lha €wnt of6 tir€, toam c€n bs spEygd on to th€ tank tom s safe di.tanc€. This hdnod is norr€comh€.tdBd as lhs pdma.y fqn of prcl€ction tof tank os ,8 m in diam€t€f

Figu.e 7.34 Pdncipal foam systems

determined application rate at the base of the tank, above thebottom water layer. The foam rises through the stored productto form an extinguishing blanketat the surface. The rising foamcauses rotational currents, which carry cold product to theburning surface, which can aid extinction.The concept of base injection only became possible with thedevelopment of fluoro protein type foam concentrates, whichhave high resistance to product contamination and good fluid-ity. Additionallythe finished foam must have excellent burnbackresistance (the ability of a foam blanket to resist direct flameand heat impingement) and stability.

The system requirements are:a) A pressurised supply of fresh or sea waterb) Suitable foam concentrate induction equipment to pro-

duce a 3% solution of foam concentratec) Foam concentrate storage facilitiesd) H;gh back pressure foam generators (HBpGs)

e) Non-return valve

f) Bursting disc (where a non-return valve is not consideredsufficiently secure to prevent leakage of product back

along the foam line)g) lsolation gate valve on the tank (normally lefr open)h) Suitable interconnecting pipe work and valving

Systems may be fullyfixed with all components permanently in-stalled, or alternatively semi-fixed, using portable HBpGs forconnection to suitable tank inlets or product lines.

The number and diameteroffoam inlets willdeoend on the tankdiameter and the type of stored product.

Figure 7.35 may be used as a guide for the number of inleb.The minimum foam application rate is 4.1 litresimin/m, (0.1gpmift2) and this rate will decide the size of the foam inleb.Inlets must be positioned above anywaterlayer in the iank and

mayterminate flush with the tank wallor be fitted with stubs pro-truding into the tank. The latter may discharge horizontally ormay be angled vertically. Discharge downwards should beavoided, particularly if the foam can enter a water bottom or im-

Fla3h loint r >37.0' cuD o 24 1 1

>24 10 36 2 1

>36 o 42 3 2

>42|o 4A 2

>48 lo 54 5 2

>54 io 60 6 3

>60 ona addltionEl inlet 465m' oI 6xp&ed pmdud 697mr df €xpos€d produd

Figure 7.35 Number and diameleroffoam inlels



pinge on the base of the tank. Where more than one inlet is re-quired, they should be spaced equally around the tank shell, us-ing either separate inlets, or alternatively a single inlet feedinginto an internal manifold with outlet oiges towards the tankcircumference.

Correct design will take into account pressure losses in the fol-lowrng areas:

a) Friction loss in pipe work, fittings and valves

b) The maximum static head of the stored product

c) Pressure loss through the foam induction equipment andfoam generators

Features of the base injection system include:

a) Rapid response with minimum demand on resources, wa-ter supply, foam compound and manpower

b) Desig n application rates of foam are achieved with 'l 00%of the foam reaching the surface of the stored product.

c) High resistance of the system componenis to damageduring tank explosion or fire.

d) Circulation of cold product dissipates hot product layersnear the burning surface and aids extinction.

Aschematic ofa base injection system is shown in Figure 7.36.

The selection of HBPGS and foam concentrate requirementsare by reference to data produced by the manufacturers of theproprietary equipment and foam concentrates.

7.10.1.2 Top foam pourers

Top foam pouring systems are used to protect fixed roof tanksand fixed rooftanks fitted with internal covers. ln each case thesystems are designed on the basis that the fire risk comprisesthe total surface area of the stored product.

The sysiem operates by introducing a foam concentfate into afire water feed line outside the tank bund area. This line is led toa foam generator, foam box and pourer all of which aremounted in line at the top of the tank shell. When inliiated, thefoam solution is propelled to the tank where the foam generaior

aerates the solution and delivers the resulting foam thfough abursting disc in the foam box. A pourer unit immediately insidethe tank shell and connected to the foam box, directs the foamdown the shell to form a blanket which extinguishes the burningprooucl

The system requirements are:

a) A pressurised supply of fresh or seawater

b) Suitable foam concentrate induction equipment to pro-duce the required percentage offoam concentrate in wa-ter

c) Foam concentrate storage facilities

d) Foam generator (immediately under the foam box)

e) Foam box with bursting disc (this prevents tank vapours

Wte t9tr1 9j3 tescaping via the foam pipework)

fl Foam oourer

Normally each ofthe fixed tank shell units are supplied by indi-vidual lines from a safe area outside the tank bund but they canbe supplied by one line to the tank which splits at a manifold tofeed each unit.

The number offoam inlets is as shown in Figure 7.35 and this,together with a minimum foam application rate of 4.1litres/min/m'? (0.1 gpm/ft2) willdetermine the size of the foam in-lets. The foam solution flow ihrough each inlet should be simi-lar. By dividing the total minimum foam solution application rateby the minimum number of inlets required, the flow rate perpourer unit is established.

Certain low boiling point flammable stored products, gasoholsand high viscosity heated liquids may require higher or, in cer-iain circumstances, lower application rates than that statedhere. These should, in all instances, be determined by test.

Design noteslf two or more inlets are required they should deliverthe foam atthe same rate to the surface of the tank and that they are ar-ranged at equal spacing around the shell.

All pipe work, valving and riser systems should be designed togive

approximaiely equal flow rates from eachpourel

Tests have shown thatfoam willtraveleffectively across at least30 m of exposed burning product surface. Thus on very largetanks, it may be necessary to increase the number of pourerunits above the minimum recommended number.

The foam inlets to the tank should be 300 mm above the maxi-mum designed product storage level.

Cautionary note

ln the event of an exploslon in a tank causing ruptures at theroof{o-shell joint and distortion in the upper shell plating, if thisis in the area of any of the foam units, these units may be ren-dered ineffective.

Protection of bitumen storage tanksFor fixed protection on bitumen tanks the only suitable systemsare inert gas or steam injection into the vapour space. Watermust not be used as this is likely to result in a hazardous, un-controllable froth-over or a steam explosion owing to the vapori-sation ofthe water at the high storage temperatures used for bi-tumen. For further information refer to Reference 7.6.

lllustrations and examples of top foam pourers are shown inFigures 7.37 to 7.39.

7.10.1.3 Rimseal foam pourers

The basis ofthis system has already been described in Chapter6, Section 6.5.6.

The concept of a rimseal protection system is based on the as-sumption that, in the event of a fire, the fire will be contained in

BURSTING DISC

FOAM BLANKET

GATE

Figure 7.36 Base lnjeclion sysiem schematicCounesy of Angus Fire

STORAGE TANKS & EQUIPMENT 201



I l3 ltl 9:3 4 flf@yg Jp : 9 blent temperaturc tanks

Figure 7.37 Top foam pourer schematic

Couftesy of Angus Fie

the seal area between the foam dam and the tank shell and thesystem design is based on treating only this annular area. Thismeans that if a fire should occur it must be detected earlv andtackled rapidly before the roof becomes damaged and ailowsthe fire to spread - often to the extent of engulfing the entiresurface area. Should a situation arise in which th-e flre doesspread to the whole exposed surface area then a rimseal oro_tection mechanism alone (as dictated by design of the system)is unlikely to achieve extinguishment. lf this ii perceivjd as apossibility, ihen consideration should be given to a top pouringsystem designed to provide total coverage ofthe roof area. -The minimum recommended foam solution application rate fornmseal systems is 12.2 litreslminl m2.

The minimum number of rimseal foam pourers is dictated bvthe height of the foam dam and is as follows:. For a 300 mm high foam dam the maximum spacing be_

tween foam pourers should be j2.2 m.

. For 600 mm high foam dams this can be increased to amaximum of 24.4 m.

7.10,1.4 Foam cannonsFixer and trailer-mounted foam cannons are suitable for pro_

tecting all types ofvertical storage tanks and though subject toperformance limitations they can be used as the primary pro_tection system to protect tanks up to 1g m in diameter.'l-iow_ever, they are often better suited and more commonly installedas €rther a secondary fixed foam system or to tackle spill fireswith the added benefit of being able to be used for tank coolinq.A foam cannon in operation is shown in Figure 7.40.The single most important considerataon when proposing foamcannons as the primary system is that, to be effective, ex_panded finished foam must first be delivered to the seat of thefire. As, in most systems, the foam cannons will be close toground level, the foam produced willfirst be required to reachup and over the tank shell. This requirement may prove difficultto achieve because of:

a) The height of the tankb) The distance between a tank and the cannon positionc) The prevailjng weather conditionsd) The fire updraught

e) The high probability that a partial rupture of a fixed rooftank may only leave a small aperture through which theexpanded foam can be targeted

Afurther problem exists in that expanded foam is applied force-fully to the surface of the burning product, which leads to in_creased contamination of the foam. The effects of this mav bereduced by directing the foam stream onto the inside of the iankshell and allowing it to run down onto the su rface ofthe product.However, in a live fire situation this may prove impossible

toachreve.

System deslgn criteriaIn all primary protection systems using foam cannons it is as-sumed that all the calculated foam solution requirement actu-ally reaches the area to be protected. As has alreadV been ex-plained. to achieve the minimum foam solution reouired.consideration must also be given to the potential foam solutionlosses that will occur due to access and windage problems.Enough equipment must therefore be available to ensure thatunder all conditions the minimum application rate is beinqachieved. This will. in most circumstances, result in consider_able over-capacity in terms ofequipment resource. This is oftenof the order of 2:1

The minimum specific design requirements can be summa_flsed as:

a) The mjnimum foam solution application rate should be 6.5

Fgurc 7.38 Top foam pourer unil

Courtesy of Angus Fire

Figure 7.39 Foam pourcr and water detuge pipework (al cenlre oftank)

202 STORAGE TANKS & EQUIPMENT



Figure 7.40 Afoam cannon in operation - 15,000 tiire/min offoam sotr.jlionCourtesy of Angus Firc

litres/min/m2 for all types of foam concentrates on iankscontaining liquid hydrocarbons.

b) The minimum foam solution application rate may have tobe increasedto tackle specjalrisks i.e. gasohols, Class 1Ahydrocarbons, etc.

c) Greater minlmum foam solution application rates mayalso be required for hot fuels afrer a prolonged pre_burn.

d) Foam cannons should not be considered as primary pro_tection mechanisms on vertical fixed roof storaqe tanksover 18 m diameter.

e) The minimum foam solution discharge duration timeshould be:

- Crude petroleum and hydrocarbons wjth flash oointsbelow 37.8'C - 65 mins.

- Hydrocarbons with flash points between 37.g.C and93.3'C - 50 mins.

7.11 Water cooling systemsThe. individual tank design, layout and piping system for anyparticular installation will be a function both ofthe phvsicalfac_tors like terrain. site elevation, drainage, etc. and oi the govern-ing Standards regarding permissible tank spacings and posi_tion within the installation.

Despite taking all reasonable precautions as demanded bvthese considerations, a fire in an individual storaqe tank wiilgenerate signlficant radiated heat, which can damioe and/orignite adjacent tanks which would not otherwise be d]recflv in-volved. A deep-seated fire in even the smallest diameter iankcan create major problems unless cooling wateris applied to itsclose neighbours.

Tank cooling is therefore recommended as essential to com_plete the protection ofa particular installation and the followinoguidelines are given in the part 19 of the lp Code. (Referenci7.5).

Tanks within two tank diameters distance downwind of a tankfire, or one tank diameterdistance in other directions, should beprotected by application of water spray at minimum recom-mended rate of 2 litres/min/m2.

7.11.1 Special case - Floating roof tanks

With rimsealfires in floating roofianks, the shellwhich is heatedfrom the fire may be cooled with waterwhilst attempts are madeto achieve and maintain an effectivefoam blanket, and to avoidre-ignition from hot surfaces. The recommended application

7 Tank fittings and ancillary equipment far ambEr: :e-a+-2.--i -a-. :

rate of water is 10 litres/min/m2 of vertical tank surface - :e--tact with the fire.

For the calculation of water requirements, the area sholro €assumed to be that based on a nominal half of the veftcaheight ofthe tank. Water should not be applied to the tank roo.but foam may be used at a rate of 6.5 litres/min/m2. based ol.tank cross-sectional area.

This rate may reduce to 4 litres/min/m2 for tanks equipped withfixed foam pourers.

7.11,2 Tank cooling methods

The methods by which tanks may be cooled can be summa-rised as follows:

7.11.2.1 Water spray and deluge sprinkler systemsThis is the most efficient method ofdelivering water, evenly dis-tributed and at the correct application rate, to the outside roofand shell of the storage tank.

There are two principal ways of accomplishing this:1) Using concentric rings of piping supported about 300 mm

above the roof. These rings are fitted with spray nozzles,which give an overlapping spraypattern to coveithewholeroof with water The shell is similarly protected, usuallywith one spray ring atthe top ofand about 600 mm clearoftheshell. Spray nozzles fitted to this ring and angled downslightly are arranged to spraywateroverthe whole cjrcum_

&ry..

-*.'-* )J\@aJ/.-'\\i/\\\w-

Figure 7.41 Walerdeluge system with conicatdiffuser

Figurc 7.42 Delail of sptash. plate

STORAGE TANKS & EQUIPMENT 203



7 Tank fiftings and ancillary equipment for ambient tempenlurc tanks

Figure 7.43 Roof deluge system using a coronetCourtesy of McTay

ference and run down the shell.2) The deluge system consisb of a single water main being

led to the crown ofthe iank roof where the water is directedvertically on to the roof and ls evenly spread overthe roofby a conicalnozzle atthe end ofthe ouflet pipe or by a cor-onet attached to the roof plating, (shown schematicaly inFigures 7.41 and 7.43).

As the waterstreams down the roof it is directed on to the shellby splash plates fitted to the curb angle at the pedphery of the

shell. These plates are angled so that as the water hits them it isdirected against and runs down the shell. See Figure 2.42.These systems can be fed from a waterdeluge valve, which isautomatically triggered, by some form of electric, pneumatic or

hydraulic detection system

7.11.2.2 Fixed and trailer-mounted water cannonsBoth static and oscillating water cannons are a cost-effectivemeans of delivering water to cool slorage tanks and the num-ber, capacity, position and deployment will ultimately dependupon individual site requiremenb. However, access problemsand local water supply considerations must be taken into ac-count when @nsidering their introduction.

7.12 References7 .1 Reinforcement of Manholes, R. T. Rose, British Wetding

Joumal, October 1961.

7.2 Tank Calibration, Sect'on 1, The Institute of petroleum,Petroleum Measurement Manual, part ll.

7 .3 NFPA 1 1 &andard for Low -, Medium -, and High - Ex-pansion Foam, 2002 Edition.

7.4 NFPA 30 Flammable and Combustibte Liquids Code.

7 -S BS 5306 Seclion 6.7: 1988 Specification follow axpan-sion Foam systems.

7.6 lP Model Code of Safe Practice: part 19, Fire precau-

tions at Petroleum Refineries and Bulk Storage lnstalla-tions.

7.7 Bitumen, lnstitute of Petroleum Code of Safe practice,Paft 11.

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Fire Protection System_storage Tank

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