Evaluating Foam Characteristics
The characteristics of foam extinguishing agents and the conditions for which each is best suited were discussed at a seminar conducted at the Howard County, Md., facility of The Johns Hopkins University Applied Physics Laboratory.
In evaluating the relative efficiency of fluoroprotein foam and aqueous film-forming foam, two speakers agreed that they preferred to use fluoroprotein foam on fires in petroleum product storage tanks. They also recommended subsurface injection of fluoroprotein foam to extinguish tank fires.
The two speakers were Norman R. Lockwood of the Mobil Oil Corporation and Don N. Meldrum, president pf National Foam System, Inc.
In recommending subsurface foam injection, Lockwood said that a high back pressure foam maker should be used. ‘He added that Mobil buys such foam makers with check valves to prevent the tank back pressure from moving any of the petroleum product back into extinguishing equipment lines. Foam can be applied to the surface of a burning tank while subsurface injection of foam is being accomplished, he remarked.
Why is subsurface injection of foam recommended? Lockwood explained that with tanks 200 or more feet in diameter, there is a very remote possibility of extinguishing a tank fire with foam chambers or foam towers. The foam won’t float across such an expansive surface, he said. Another disadvantage of a portable foam tower, he noted, is that several men are needed to set up a tower.
Lockwood stated his reference for 3 percent fluoroprotein foam for tank fires and remarked that whether you use 3 or 6 percent foam concentrate, “you’ll wind up spending about the same amount of money.” For subsurface injection of foam, he said that at Mobil tank farms, “we generally use our 3 percent at 4 percent” eduction rate.
One of the advantages of both fluoroprotein and aqueous film-forming foam (AFFF), a film Meldrum showed pointed out, is that the bubbles are not saturated with fuel when a foam stream plunges below the surface of burning petroleum products. When fighting tank fires, the height and diameter of a tank can make it impossible to apply the foam gently without roiling the fuel surface. Saturating the foam bubbles, it was explained in a film, slows the extinguishing action of foam application.
Use of AFFF
In explaining why he did not recommend AFFF for fighting tank fires, Meldrum said that with a 15-minute preburn, the extinguishment time for AFFF is triple what it is with a oneminute preburn. He said the AFFF will knock down fire faster than will fluoroprotein if the fire has not been burning too long, but fluoroprotein will provide a better-blanket over the surface of the liquid.
However, he said that AFFF is ideal for shallow gasoline-type fires where a fluid, fast-flowing foam is most effective. He cautioned that the burnback resistance of AFFF is somewhat limited and recommended a delivery of 1 to 10 gpm of AFFF and water solution for every 10 square feet of burning area. The film he showed pointed out that fluoroprotein foam is more effective on gasoline tank fires because it can be applied through subsurface injection. The film also advised that enough foam should be on hand for a one-hour application on a tank fire.
Meldrum stated that his company has recently developed a universal foam concentrate for use on both hydrocarbon and polar solvent fires.
When the question of foam compatibility arose, Lockwood said that foam concentrates of the same percentage can be mixed “without too much trouble” for immediate use. However, both Lockwood and Meldrum recommended avoiding longterm storage of mixed brands of foam concentrates. In talking of AFFF, Meldrum said that the 6 percent concentrates are in the neutral pH range and are compatible.
It also was pointed out that AFFF and fluoroprotein foams are compatible with dry chemical extinguishing agents.
Although AFFF can be applied through ordinary fog nozzles, Meldrum advised using aspirating nozzles designed for the generation of foam. He explained that when AFFF is applied to a fire without being foamed, its burnback resistance is lowered.
For the’application of foam, Meldrum recommended the use of balanced pressure proportioning-systems, which can provide accurate proportioning of foam concentrate over a wide range of water flow—100 to 1000 gpm.
Tests of the use of high expansion, or synthetic detergent, foams, said George B. Geyer of the Federal Aviation Administration, indicate that 500-1 to 800-1 expansion ratios are effective. He reported that high expansion foam is capable of controlling a JP-4 fire at an application rate of 0.04 to 0.1 gpm per square foot. He reported one test fire with such a strong upward convection current that it drew high expansion foam into it.
In talking about aircraft fires, Geyer cited the flexibility of hand-held high expansion foam units in attacking interior fuselage fires. These units provide a 100-1 expansion ratio. He pointed out that when exposed to fire, the aluminum skin of a plane can melt in less than 45 seconds. Furthermore, the biggest problem may not be the exterior fuel fire, but the flammable materials inside an aircraft. Geyer reported that in fighting Class A fires inside an airliner, the “best performance was with water fog.”
At a tank farm fire involving a spill, the ground fire should first be extinguished before attempting to extinguish the tank, Lockwood advised. He stated that tanks should be built on a rise inside a diked area so that any product spill can flow to a drain. Lockwood said he preferred the use of monitors on fires in diked areas.
Floating roof seal fires, he said, can be extinguished with a 1 ⅛ -inch hose line by taking it up the stairs on the side of the tank.
A much more serious problem, Lockwood commented, is presented by a fire in a tank of crude oil because it raises the danger of a boilover. The volatile light ends burn off and the heavy oil then sinks, causing more light ends to surface. After half an hour, it’s questionable whether a burning tank of crude oil can be extinguished, he said.
The moderator for the seminar was Dr. Richard L. Tuve of the Applied Physics Laboratory.