Storage Tank Protection With High Flow · PDF filefires involving large storage tanks, it...

CHEMGUARD INC. 204 S. 6th Avenue • Mansfield, Texas, USA 76063 • (817) 473-9964 • FAX (817) 473-0606

Rev: 04012001 Page XV - 9

TECHNIQUE IN EXTINGUISHING LARGETANK FIRES

Large storage tank fires are very complexevents and satisfactory extinguishment requiresmethodical planning and the effective use ofresources. At this time, existing codes andstandards do not provide guidelines for usinghigh flow foam monitors for large tank fires. Theexisting codes and standards do howeverprovide good recommendations for fixed fireprotection systems. Full surface fires involvinglarge diameter tanks have occurred around theworld. Historically, extinguishment of such fireshas not been totally successful. However, withthe introduction of large capacity foammonitors, new varieties of foam concentratesand improvements in application techniquesthere has been some degree of success inachieving extinguishment.

The largest fully involved tank fire that has beensuccessfully extinguished was 150 ft. (46meter). The extinguishment was carried outwith a large capacity monitor/cannon applyingnon-aspirated foam "over-the-top" onto theburning surface. It is believed that present firefighting technology is capable of extinguishingfully involved tank fires up to 197 ft. (60 meters)in diameter. In theory it may be technicallyfeasible to extinguish tank fires in excess of 200ft. (61 meters) using the "over-the- top" methodutilizing very large capacity mobile monitorswith improved types of foam concentrates. Thelogistics for mounting such massive operationsmust be fully considered.

APPLICATION RATE

NFPA 11 - Application Rate for MobileEquipment is often interpreted as 0.16 gpm/ft2.(6.5 L/min./m7).

The code also states that flammable liquidshaving a boiling point of less than 100ºF(37.8ºC) may require higher rates of application.In addition flammable liquids with a wide rangeof boiling points such as crude oil may requireapplication rates of 0.2 gpm/ft2. (8.1 L/min./m2)or more. The application rate stated in the codeis based on the assumption that all the foamsolution reaches the burning surface.

Note: The rates are intended for liquid hydro-carbon fuels. Polar solvent liquids are destruc-tive to regular foams and require the use ofalcohol resistance foams. Chemguard, Inc.should be consulted to determine the recom-mended application rate.

Taking into consideration the above rates andpractical experience gained from full surfacefires involving large storage tanks, it would bemore appropriate to consider 0.25 gpm/ft2. (10.4L/min./m2). For burning crude oil tank a rate of0.32 gpm/ft2. (12.9 L/min./m2) may be moreappropriate.

The elevated application rates ensure a betterchance of foam reaching the burning surface,thus increasing the probability of extin-guishment. Consideration for such high ratesshould take into account fall out from thedelivery system, losses due to strong thermalupdraft, break down of foam as it travelsthrough the flames to reach the burning fueland destruction of the foam due to the hot fueland any hot metal surface.

WATER AND FOAM CONCENTRATE RE-QUIRED FOR FIGHTING LARGE TANKFIRES

Water supply in terms of pressure, flow rate andadequate amount of foam concentrate areamong the most important factors in launchinga successful extinguishing operation. Unless

Storage Tank ProtectionWith High Flow Monitors


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an adequate and uninterrupted supply isguaranteed, an attempt to extinguish a fullyinvolved large tank fire is doomed to failure atthe very onset of the operation.

The amount of water and the flow rate neededto produce foam solution to fight a large tankfire can be found in Table 1.

The quantity of 3% foam concentrate and theflow rate needed to produce foam solution togenerate foam to fight a large tank fire can befound in Table 2.

COOLING INVOLVED TANK AND THE PRO-TECTION OF ADJACENT TANKS FROMRADIATED HEAT SOURCE

With reference to existing guidelines, theamount of water needed to cool the involvedtank shell is estimated by tank size:

100 ft. (30 meter) diameter 750 gpm (3m3/min.)120 ft. (36 meter) diameter 1000 gpm (4m3/min.)160 ft. (48 meter) diameter 1250 gpm (5m3/min.)220 ft. (67 meter) diameter 1500 gpm (6m3/min.)

Cooling water required to protect each adjacenttank not shielded from the tank on fire is 500gpm (2 m3 /min.).

In practice water applied to the shell of a largetank on fire is ineffective in preventing it frombuckling and deforming. In the late stages ofextinguishment, cooling water applied on thearea above the liquid level would help the foamstay in contact with the tank shell. The coolingstreams should be stopped when foam attackhas started to conserve water and toconcentrate on extinguishment.

The need for protecting adjacent tanks can bestbe illustrated with information and datapublished in a recent study done on large tankfires. Although not yet fully validated itnevertheless provides valuable information forpre-fire planning purposes.

The time required to create an escalation condi-tion in an adjacent tank depends upon anumber of factors including: tank size,distance/separation, type construction, initial

boiling point of flammable liquid in the tanks,water cooling, tank design, wind speed anddirection.

For example, a full surface fire involving a 164ft. (50m) diameter open top, floating roofnaphtha tank fire could be expected to fullyinvolve a neighboring identical tank inapproximately 1.5 hours under the followingconditions.

- 4 m/sec.(14 km/hr.) wind towardsneighboring tank

- intertank separation of 0.5 diameter (82 ft.)(25 m)

- neighboring tank having pontoon roof andinadequate water spray protection

Altering any of the above conditions canchange the time for ignition of the adjacenttank:

Base Case = 1.5 hrs.

Change of conditions:

Calm (no wind condition) = 2.8 hrs.Intertank separation increased to 1.0 D (50 m)

= 3.0 hrs.

Intertank separation increased to 2.0 D (100 m)= 17.0 hrs.

Water protection on side facing exposure= 2.8 hrs.

Double deck roof on exposed tank= 1.5 hrs.

Water protection on side facing exposure +double deck roof = 24.0+hrs.Tank diameters only 30 m but with 0.5 Dseparation = 0.5 hrsNeighboring tank contains kerosene, notnaphtha

= 22.0 hrs.

Some conclusions drawn from the results are:

• Escalation is likely for unprotected tanks ofvolatile material with normal separation un-less the original fire is extinguished quickly.


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• Calm conditions only delay the escalationpotential.

• Increased separation alone only delays theescalation potential.

• Water spray protection or roof insulationalone does prevent escalation.

• Water spray and roof insulation together areeffective.

• Smaller diameter tanks at normal separationare at greater risk of escalation than largerdiameter tank.

• Lower volatility fuels provide more responsetime for fire fighter.

Cooling of adjacent tanks is best achieved withfixed systems that are designed to provideeffective water film coverage of all exposedmetal surfaces. A cooling water rate of 0.05gpm/ ft2. (2.0 L/min./m2) is sufficient to absorb90% of incoming radiant heat. Any increase inthe cooling water rate does not increase thecooling effect significantly. The figure of 10.2L/min./m2 by NFPA 15 relates mainly to theprotection of pressurized vessels such as LPGtanks subject to direct flame impingement.

OVER THE-TOP-APPLICATION TECHNIQUEWITH LARGE CAPACITY FOAM MONITORS

A present concept in extinguishing large tankfires is to employ large capacity non-aspiratedfoam monitors to apply foam "over-the-top" ofthe involved tank onto the burning fuel surface.Although they are normally known as non-aspirated monitors, these monitors are capableof producing foam with an expansion ratio ofabout 3.1 to 4.5 when used with alcoholresistance type foam concentrates.

Chemguard has large capacity foam monitorscurrently available with capacities ranging from2,000 to 4,000 gpm (7,570 L/min.). The equip-ment operates at inlet pressure between 100 to130 psig (690 to 890 kPa) and have a range ofabout 250 to 300 feet (61-99 meter).

AR-AFFF type foam concentrate is preferredand it should be transported in bulk totes ortrailers having large capacities. The logistics fortransporting foam in 5 gallon pails or 55 gallondrums to the fire scene should not beconsidered, for obvious reasons.

Large diameter hose should be used to supplythe flow required for large volume foam attack.The use of 5" (125 mm) diameter hose ispreferred due to low frictional loss and ease ofutilization. It must be remembered that it isextremely difficult to move the hose once it ischarged with water. For quick estimation,provide one 5" (125 mm.) hose line for every1,000 gpm (3.8 m 3 /min.) flow requirement. Atthis flow rate the friction loss is 8.0 psig (55kPa) for every 100 feet (30.5 m). Table 3provides information on friction loss of somelarge diameter hoses.

The “over-the-top” foam technique attacks theburning tank with either a very large capacitymonitor that meets the required application rateor combines several monitors to form a massstream discharging with the wind to concentrateon a selected landing zone within the tank.

This extremely high “local applicationrate/density” promotes survivability of the foamjourney through the fire to establish a footholdon a relatively small area of the burningsurface. Once the foam blanket at the landingzone is established it can then be expanded bymaking adjustments to the mass stream. Theadded advantage of large volume application ina small area may help to reduce “local fueltemperature” and the associated actual vaporpressure which in turn can help in lowering thefire severity. These factors requireconsideration because as the fuel temperatureapproaches the boiling point of water, it isdifficult for the foam to survive. As fueltemperature increases the true vaportemperature will increase to overcome theeffectiveness of the foam blanket.

Large volume foam attack should be launchedas quickly as possible; however, it must bestressed that application must not be carriedout until all equipment and logistic support arein place. The longer a tank is allowed to burn,


Rev: 04012001 Page XV - 12

the greater the danger of escalation. The fueltemperature increases making it more difficultto extinguish; and the exposed tank shelldeforms (normally the exposed steel curlsinwards to create nooks and crevices) making itdifficult for foam to cover all the burningsurface. In the case of crude oil, the possibilityof having a “boilover” increases with time.

The ability to deal with large tank fires dependson methodical pre-fire plan, regular training andexercises. The most important factor, however,rests on minimizing the risk of having a fullyinvolved large tank fire through goodengineering design, effective management andmaintenance programs.


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STORAGE TANK PROTECTION SUMMARY

Fixed-Roof (Cone) Tanks Pontoon or Double-Deck Floating Roof Tanks

NumberRequired

Not applicable Mechanical Shoe Seal.1 - For each 130 ft. (39.6 m) of tank circumference (no foamdam required)Tube Seal - Over 6 in. (15.2 cm) from top of seal to top ofpuntoon with foam outlets under metal weather shield orsecondary seal.1 - For each 60 ft. (18.3 m) of tank circumference (no foamdam required)Tube Seal - Less than 6 in. (15.2 cm) from top of seal to topof pontoon with foam outlets under metal weather shield orsecondary seal.1 - For each 60 ft. (18.3 m) of tank circumference (foamdam at lease 12 in. (30.5 cm) high required).

HydrocarbonApplicationRates

Not Applicable. 0.30 gpm. (1.14 L/min.) per sq. ft. (sq. m) of annular ringarea with foam dam or with foam application under metalweather seal or secondary seal. 0.50 gpm (1.9 L/min.) persq. ft.. (sq. m for all other applications).

DischargeTimes

Not Applicable. 20 min. - with foam dam or under metal weather shield orsecondary seal.F

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PolarSolvents

Not Applicable. Not covered by NFPA 11.

Size ofTank

Monitors for tanks up to 60 ft. (18.3 m) indiameter.Hand hoselines for tanks less than 30 ft. (9.2 m)in diameter and less than 20 ft. (6.1 m) high.

Monitors not recommended.

Handlines are suitable for extinguishment of rim fires inopen-top floating roof tanks.


0.16 gpm/ft2.[(6.5 L/min.)/(m2)]

0.16 gpm/ft2.(6.5 L/min./m2)For rim fires in open-top floating roof tanks.

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DischargeTimes

Flash point below 100ºF (37.8ºC) 65 min.Flash point 100ºF - 140ºF 50 min.Crude Oil 65 min.

Use same times as for open-top floating roof tank rim fires.

NumberRequired

Same as table for foam chambers. Not Recommended.


Minimum 0.1 gpm/ft.2. [(4.1 L/min.)/m2] of liquidsurface.Maximum 0.2 gpm/ft2. [(8.2 L/min.)/m2]Foam velocity from outlet shall not exceed 10 ft.per sec. (3.05 m per sec.) for Class 1B liquids or20 ft. per sec. (6.1 m per sec.) for all otherliquids.

Not Recommended.

DischargeTimes

Flash point 100ºF (37.8ºC) 30 min.To 140ºF (194.4ºC)Flash point below 100ºF (37.8ºC) 55 min.Crude Petroleum 55 min.

Not Recommended.Su

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PolarSolvents

Not Recommended Not Recommended

For S1 units: 1 gpm/ft2. = 40.746 (L/min.)/m2; 1 ft. = 0.305 m; 1 ft2. = 0.0929 m2; 1 in. = 0.0245 m; ºC = ºF - 32/1.8.


Rev: 04012001 Page XV - 29

STORAGE TANK PROTECTION SUMMARY

Fixed-Roof (Cone) Tanks andPan-Type Floating Roof Tanks

Pontoon or Double-Deck FloatingRoof Tanks, (Open-Top or

Covered) Annular Seal Area

NumberOf FoamOutletsRequired

Up to 80 ft. (2.44 m) dia. 1 Foam Chamber81 to 120 ft. (24.7 - 36.6 m) dia. 2 Foam Chambers121 to 140 ft. (36.9 - 42.7 m) dia. 3 Foam Chambers141 to 160 ft. (43 - 48.8m) dia. 4 Foam Chambers161 to 180 ft. (49 - 54.9 m) dia. 5 Foam Chambers181 to 200 ft. (55.2 - 61 m) dia. 6 Foam ChambersOver 210 ft. (61.2 m) 1 additional for each 5,000 sq. ft.

1 for each 40 ft. (12.2 m) of circumferencewith a 12-inch (30.5 cm) high foam dam.

1 for each 80 ft. (24.4 m) of circumferencewith a 24-inch (61 cm) high foam dam.


0.10 gpm (0.38 L/min.) per sq. ft. (sq. m) of liquid surface. 0.30 gpm. (1.14 L/min.) per sq. ft. (sq. m) ofannular ring area between tank wall and foamdam.

PolarSolventRates

See Manufacturer's Approval Report. Not covered by NFPA 11.

HydrocarbonApplicationTimes

Type I Type II

Flash Pt. 100ºF - 140ºF (37.8ºC - 194.4ºC 20 min. 30 min.Flash Pt. Below 100ºF (37.8ºC) 30 min. 55 min.Crude Petroleum 30 min. 55 min.

20 min.

To

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PolarSolvents

Type I 30 min.Type II 55 min.

Not covered by NFPA 11.


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SURFACE APPLICATION

Determining Discharge Time and Application Rate (Cone Roof Tanks)

Discharge time and application rates are determined according to the type of fuel contained in the storage tank beingprotected. The following are minimum discharge rates recommended by Chemguard.

Foam Chambers Monitors/Hand Hose Lines As Primary Protection -OR- As Primary Protection

Foam Application Rate Discharge Application Rate DischargeFuel Protected Concentrate gpm/ft2 (Lpm/m2) Time gpm/ft2 (Lpm/m2) Time

HydrocarbonFlash point AR-AFFF .10 (4.1) 30 min. .16 (6.5) 50 min.between Fluoroprotein .10 (4.1) 30 min. .16 (6.5) 50 min.100ºF and AFFF .10 (4.1) 30 min. .16 (6.5) 50 min.200ºF (38ºC and 93ºC)

HydrocarbonFlash point AR-AFFF .10 (4.1) 55 min. .16 (6.5) 65 min.below 100ºF Fluoroprotein .10 (4.1) 55 min. .16 (6.5) 65 min.(38ºC) or AFFF .10 (4.1) 55 min. .16 (6.5) 65 min.liquid heated above flash point

Crude Petroleum AR-AFFF .10 (4.1) 55 min. .16 (6.5) 65 min.Fluoroprotein .10 (4.1) 55 min. .16 (6.5) 65 min.AFFF .10 (4.1) 55 min. .16 (6.5) 65 min.

AlcoholsMethanol

U.G. .10 (4.1) 55 min. .16 (6.5) 65 min.3/6 .10 (4.1) 55 min. .16 (6.5) 65 min.

EthanolU.G. .10 (4.1) 55 min. .16 (6.5) 65 min.3/6 .10 (4.1) 55 min. .16 (6.5) 65 min.

IsopropanolU.G. .15 (6.1) 55 min. .16 (6.5) 65 min.3/6 .15 (5.7) 55 min. .16 (6.5) 65 min.

KetonesMethyl Ethyl Ketone

U.G. .10 (4.1) 55 min. .16 (6.5) 65 min.3/6 .10 (4.1) 55 min. .16 (6.5) 65 min.

AcetoneU.G. .15 (6.1) 55 min. .24 (9.8) 65 min.3/6 .15 (6.1) 55 min. .24 (9.8) 65 min.

AldehydesU.G. .17 (6.5) 55 min. .16 (6.5) 65 min.3/6 .17 (6.5) 55 min. .16 (6.5) 65 min.

EstersU.G. .10 (4.1) 55 min. .16 (6.5) 65 min.3/6 .10 (4.1) 55 min. .16 (6.5) 65 min.

EthersU.G. .15 (6.1) 55 min. .24 (9.8) 65 min.3/6 .15 (6.1) 55 min. .24 (9.8) 65 min.

U.G. Ultraguard 3% AR-AFFF3/6 3% - 6% AR-AFFF @ 6% Proportionion

Storage Tank Protection With High Flow · PDF filefires involving large storage tanks, it...

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