The Combination Dry Chemical and Foam Truck PART II

The Combination Dry Chemical and Foam Truck PART II

In this, the conclusion of this two-part article, performance and capacities are analyzed



The foam liquid proportioning system on the combination dry chemical and foam truck should be sized for extinguishment of the largest cone roof oil storage tank in accordance with NFPA Fire Code 11, “Foam Extinguishing Systems.”

Low-expansion, protein-base air foam liquid of the 3 percent type is preferred, although the 6 percent type is also satisfactory. If the largest cone roof tank exceeds 120 foot diameter and more than one truck is needed, the second vehicle can be a foam truck rather than a combination dry chemical and foam type.

Due to weight considerations as well as truck maneuverability, a 500gallon foam liquid storage tank is recommended. This amount of foam liquid should be sufficient for combined agent attack on process unit fires.

The 500-gpm foam-water monitor uses 15 gpm of 3 percent foam liquid or 30 gpm when proportioning at 6 percent. Thus, the foam liquid supply is sufficient for 33 minute operation at 3 percent or 10 minutes at 6 percent. Assuming an expansion of 8, the amount of foam discharged will provide a 4-inch blanket over 50,000 sq ft in area at 3 percent or over 25,000 square feet at 6 percent.

Adjustable foam monitor

The optimum size monitor, when considering range, foam quantity discharged and foam liquid consumption, appears to be the size rated at 500 gpm. The monitor should be adjustable from straight stream to spray. This design is needed to provide a soft stream for blanketing and securing spill fires when foam is used in combined agent attack with dry chemical.

The foam monitor is demountable so that it can be moved to a forward position for fighting tank fires or large spill fires such as might occur within tank impounding basins. When demounted, two 2 1/2-inch hose lines are used for foam solution supply. The monitor can also be used on water for cooling, or extinguishment of heavy oils. For this purpose, water rather than foam can be used in combination with dry chemical. The monitor’s performance in still air will approximate the values given in Table 1.

Foam hand lines and nozzles

The truck carries 1200 feet of 2 1/2-inch hose and 400 feet of 1 1/2-inch hose for use with foam or adjustable water spray nozzles. The 2 1/2-inch hose can also be used for foam solution supply to portable foam towers or to foam chamber laterals when fighting tank fires.

The performances of the foam nozzles are shown in Table 2.

Two, 2 1/2 adjustable, water fog-to-straight stream nozzles are also available for use when water only is desired.

Also, two hose controls are carried so that two charged hand lines can be positioned and then left unmanned when fire fighting.

Foam proportioning

A pressure differential type piston or diaphragm valve is used to maintain constant pressure for proportioning the correct foam liquid percentage while recirculating excess liquid back to the foam liquid storage tank. A metering valve at each truck outlet can be used to vary the foam liquid percentage from 3 to 6 percent. When the metering valve is shut, the outlet supplies water only.

The figure above is a diagrammatic sketch of this automatic proportioning system.


Combination dry chemical and foam design considerations

When sizing the dry chemical and foam systems, the following factors should be considered:

  1. The type process units and elevations of equipment, and pipeways above grade.
  2. The largest cone roof oil storage tank requiring foam protection.
  3. The fire main residual pressure at the design fire water flow.
  4. Turning radius required and available access for the truck’s operation within the plant.

Each combination truck should be equipped with a 500-gpm, foam-water monitor and a 500-gallon foam liquid

The need for, and size of the truck’s water booster pump is determined by the plant iire main residual pressure at design flow and the foam solution rate needed for the largest cone roof tank. At least 150-psig residual hydrant outlet pressure is needed for good monitor operation if a pump is not provided. A 750-gpm, NFPA rated, water booster pump will provide adequate pressure for monitor operation, even with a negative suction pressure. This size pump will also furnish about 1200 gpm of foam solution at 150-psig discharge with a minimum of 50-psig residual hydrant outlet pressure. Table 3 (right), lists the recommended equipment sizes and estimated truck chassis gross vehicle weight needed.

Preferably, the Truck GVW should not exceed 30,000 pounds to be of reasonable dimensions and turning radius for manueverability within plant areas. If the 750-gpm water pump is not furnished, the weight saving will be about 2000 pounds. When fighting tank fires, additional foam liquid would be brought to the fire scene by trailer.

Foam compatibility with dry chemical

Although all dry chemical powders, even the foam-compatible type, cause some foam bubble breakdown, this effect is not a problem as indicated in numerous fire tests. Underwriters’ Laboratories Research Bulletin No. 54 entitled “The Compatibility Relationship Between Mechanical Foam and Dry-Chemical Fire-Extinguishing Agents” discusses this problem in detail.

A paper presented at the May 1962 National Fire Protection Association Meeting by Donald N. Meldrum entitled “Research on Combined Use of Foam and Dry Chemical” contains valuable data regrading combined agent attack.

In summary, when using combined agent attack, the foam application rate on flammable liquids must be increased to 0.50 gpm per square foot instead of the recognized 0.10 gpm per square foot to establish a foam blanket and overcome the dry chemical breakdown effect.

Applying foam to process area fires when using the 500-gpm monitor presents no problem in reaching the higher application rate.

Furthermore, fire tests indicate that less foam liquid and powder will be used than would be required if either agent is used alone to extinguish a fire. Fire tests also indicate less foam breakdown when 3 percent foam liquid is proportioned at 6 percent. Even at this higher percentage, the 500-gallon supply should be adequate for fighting process area fires. When fighting large tank fires, foam with no dry chemical is used to blanket and extinguish the large area involved. In this case, the 0.1 gpm per square foot foam application rate at 3 percent foam liquid proportioning would be used. Future protein foam research or possibly future use of “light water” foam may result in no foam breakdown with combined agent attack.

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