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

SUB-SURFACE BASE INJECTIONThe sub-surface method of fire protectionproduces foam with a "High Back Pressure FoamMaker" located outside the storage tank. Thissystem delivers the expanded foam mass throughpiping into the base of the tank. The pipe may bean existing product line or can be a dedicated fireprotection foam line. The expanded foam enteringthe tank through a discharge outlet is injected intothe flammable liquid. The discharge outlet mustbe a minimum of 1 ft. above any water that maybe present at the base of the tank. The foam willbe destroyed if injected into the water layer. Wheninjected into the fuel, the foam will rise through thefuel and form a vapor tight foam blanket on thefuel surface.Advantages of Sub-surface• The rising foam can cause the fuel in the tankto circulate which can assist in cooling the fuelat the surface.• If there is an explosion and fire that coulddamage the top of the tank, the sub-surfaceinjection system is not likely to suffer damage.• The discharging foam is more efficientlydirected to the fuel surface without anyinterruption from the thermal updraft of thefire.Disadvantages of Sub-surface• CANNOT be used in storage tanks containingpolar solvent type fuels or products thatrequire the use of AR-AFFF type foamconcentrates.• Not Recommended for use in either FloatingRoof or Internal Floating Roof type tanks.• Caution must be used so that the maximumfoam inlet velocity is not exceeded; otherwise,excessive fuel pickup by the foam as it entersthe tank will be experienced.• Not to be used for protection of Class 1Ahydrocarbon liquids.HIGH BACK PRESSURE FOAM MAKERThe HBPFM device is mounted in the foam lineused to aspirate the foam solution before it isdischarged into the storage tank base. It willtypically give an expansion ratio of between 2 -1and 4 - 1. The device is capable of dischargingagainst considerable back pressure which can beas high as 40% of the operating pressure. Theback pressure is an accumulation of the headpressure of the fuel inside the storage tank andany friction loss between the foam maker and thetank. A minimum of 100 psi inlet pressure into theHBPFM is normally required to ensure correctoperation. The foam velocity through the piping tothe tank from the HBPFM is very critical. Withflammable liquids, the foam velocity entering thetank should NOT exceed 10 ft. per second andwith combustible liquids the foam velocity shouldNOT exceed 20 ft. per second.The following chart shows the minimum dischargetimes and application rates for Sub-surfaceapplication:Hydrocarbon Minimum MinimumType Fuel Discharge ApplicationTimeRateFlash point between 100 o F and 30 min. 0.10 gpm / ft.140 o F (37.8 o C and 93.3 o C) 4.1 L/min./mFlash point below 100 o F 55 min. 0.10 gpm / ft.(37.8 o C) liquids heated above 4.1 L/min./mtheir flash points.Crude Petroleum 55 min. 0.10 gpm / ft.4.1 L/min./mNOTE: The maximum application rate shall be0.20 gpm / ft. ( 8.1 L/min./m ).FOAM CHAMBERSTYPE II DISCHARGE DEVICEThe Foam Chamber is normally used on coneroof storage tanks. The chamber is bolted orwelded on the outside of the tank shell near theroof joint. A deflector is mounted on the inside ofthe tank so that the discharging foam from thefoam chamber will be diverted back against theinside of the tank wall.The foam chamber is mounted on the cone roofstorage tank wall shell in a vertical position justbelow the roof joint, or approximately 8" to 12"down from the roof joint to the center point of thefoam chamber outlet.In sequence Chemguard Models FC 2.2, 3, 4 and6 foam chambers are to be mounted from approx.8" to approx. 12" down from the roof joint.