fixed or semi-fixed foam fire protection systems for storage tanks - NIFV

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The amount of water and the flow rate needed toproduce 3% foam solution to generate foam tofight a large tank fire can be found in Table 1.The quantity of 3% foam concentrate and the flowrate needed to produce 3% 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 FROM RADI-ATED HEAT SOURCEtank fire could be expected to fully involve aneighboring identical tank in approximately 1.5hours under the following conditions.- 4 m/sec.(14 km/hr.) wind towards neighboringtank- intertank separation of 0.5 diameter(82 ft.) (25 m)- neighboring tank having pontoon roof andinadequate water spray protectionAltering any of the above conditions can changethe time for ignition of the adjacent tank:With reference to existing guidelines, the amountof water needed to cool the involved tank shell isestimated by tank size:Base CaseChange of conditions:= 1.5 hrs.100 ft. (30 meter) diameter 750 gpm (3m 3 /min.)120 ft. (36 meter) diameter 1000 gpm (4m 3 /min.)160 ft. (48 meter) diameter 1250 gpm (5m 3 /min.)220 ft. (67 meter) diameter 1500 gpm (6m 3 /min.)Cooling water required to protect each adjacenttank not shielded from the tank on fire is 500 gpm(2 m 3 /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 the areaabove the liquid level would help the foam stay incontact with the tank shell. The cooling streamsshould be stopped when foam attack has startedto conserve water and to concentrate on extinguishment.The need for protecting adjacent tanks can bestbe illustrated with information and data publishedin a recent study done on large tank fires.Although not yet fully validated it neverthelessprovides valuable information for pre-fireplanningpurposes.The time required to create an escalation conditionin an adjacent tank depends upon a numberof factors including: tank size, separation, type,initial boiling point of flammable liquid in the tanks,water cooling, tank design, wind speed anddirection.For example, a full surface fire involving a 164 ft.(50m) diameter open top, floating roof naphthaCalm (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, not naphtha= 22.0 hrs.Some conclusions drawn from the results are:−−−−−escalation is likely for unprotected tanks ofvolatile material with normal separation unlessthe original fire is extinguished quicklycalm conditions only delay the escalationpotentialincreased separation alone only delays theescalation potentialwater spray protection or roof insulation alonedoes prevent escalationwater 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 provide effectivewater film coverage of all exposed metalsurfaces. A cooling water rate of 0.05 gpm/ft 2 .(2.0 L/min./m 2 ) is sufficient to absorb 90% ofincoming radiant heat. Any increase in thecooling water rate does not increase the coolingeffect significantly. The figure of 10.2 L/min./m 2by NFPA 15 relates mainly to the protection ofpressurized vessels such as LPG tanks subject todirect flame impingement.OVER THE-TOP-APPLICATION TECHNIQUEWITH LARGE CAPACITY FOAM MONITORSA present concept in extinguishing large tank firesis to employ Large Capacity Non-aspirated FoamMonitors to apply foam "over-the-top" of theinvolved tank onto the burning fuel surface.Although they are normally known as nonaspiratedmonitors, these monitors are capable ofproducing foam with an expansion ratio of about3.1 to 4.5 when used with alcohol resistance typefoam concentrates.Chemguard has large capacity foam monitorscurrently available have capacities ranging from2,000 to 4,000 gpm (7,570 L/min.). The equipmentoperates 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 preferred andit should be transported in bulk totes or trailershaving large capacities. The logistics for transportingfoam in 5 gallon pails or 55 gallon drumsto the fire scene should not be considered, forobvious reasons.Large diameter hose should be used to supply theflow required for large volume foam attack. Theuse of 5" (125 mm) diameter hose is preferreddue to low frictional loss and is relatively easy touse. It must be remembered that it is extremelydifficult to move the hose once it is charged withwater. For quick estimation, provide one 5" (125mm.) hose line for every 1,000 gpm (3.8 m 3 /min.)flow requirement. At this flow rate the friction lossis 8.0 psig (55 kPa) for every 100 feet (30.5 m).Table 3 provides information on friction loss ofsome large diameter hoses.The “over-the-top” foam technique attacks theburning tank with either a very large capacitymonitor that meets the required application rate orcombines several monitors to form a MassStream discharging with the wind to concentrateon a selected landing zone within the tank.This extremely high “local application rate/density”promotes survivability of the foam journey throughthe fire to establish a foothold on a relatively smallarea of the burning surface. Once the foamblanket at the landing zone is established it canthen be expanded by making adjustments to theMass Stream. The added advantage of largevolume application in a small area may help toreduce “local fuel temperature” and the associatedactual vapor presssure which in turn can help inlowering the fire severity. These factors requireconsideration because as the fuel temperatureapproaches the boiling point of water, it is difficultfor the foam to survive. As fuel temperatureincreases the true vapor temperature will increaseto overcome the effectiveness of the foamblanket.Large volume foam attack should be launched asquickly as possible; however, it must be stressedthat application must not be carried out until allequipment and logistic support are in place. Thelonger a tank is allowed to burn, the danger ofescalation becomes greater, the fuel temperatureincreases making it more difficult to extinguish,the exposed tank shell deforms (normally theexposed steel curls inwards to create nooks andcrevices) making it difficult for foam to cover allthe burning surface. In the case of crude oil, thepossiblility of having a “boilover” increases withtime.The ability to deal with large tank fires depends onmethodical pre-fire plan, regular training andexervises. The most important factor, however,rests on minimizing the risk of having a fullyinvolved large tank fire through good engineeringdesign, effective management and maintenanceprograms.STPrv995
Page 1 and 2: FIXED OR SEMI-FIXEDFOAM FIRE PROTEC
Page 3 and 4: SUB-SURFACE BASE INJECTIONThe sub-s
Page 5 and 6: Discharge device - Foam Chamber, Qt
Page 7 and 8: Foam dams are to be at least 1 ft.
Page 9: STORAGE TANKPROTECTION WITH HIGHFLO
Page 13 and 14: TABLE 1WATER FLOW RATE TO PRODUCE 3
Page 15 and 16: TABLE 3FRICTION LOSS PER 100 FEET /
Page 17 and 18: FIG. 10SEMI-FIXED FOAM SYSTEM WITH
Page 19 and 20: TYPICAL OUTLETS FOR SUB-SURFACE INJ
Page 21 and 22: EXPANDED FOAM VELOCITY vs PIPE SIZE
Page 23 and 24: 50FOR SI UNITS1gpm=3.785 l / min.1p
Page 25 and 26: InternalFloatingRoofFOAM CHAMBER TO
Page 27 and 28: EXAMPLES OF TYPICAL TOP AND BELOW S
Page 29 and 30: DikeMonitor(Typical)WaterSupplyTANK
Page 31 and 32: TYPICAL ARRANGEMENT FOR SUB-SURFACE
Page 33 and 34: STORAGE TANK PROTECTION SUMMARYFixe

fixed or semi-fixed foam fire protection systems for storage tanks - NIFV

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