FOAM-WATER SPRINKLER SYSTEMS: WHAT THE FIREFIGHTER NEEDS TO KNOW
Recent fire losses in fully sprinklered flammable liquids warehousing and processing facilities have led the fire protection professionals to rethink their approach to designing sprinkler protection for these kinds of facilities. Firefighters have known for years that water alone is not effective for extinguishing fires in bulk storage of flammable liquids and hence have used foam. Yet for some reason, when those same flammable liquids were packaged in smaller containers and stored indoors, water was the chosen extinguishing agent. Unfortunately, it has taken some major fires in fully protected buildings to prove that water alone is not sufficient. Quite simply, the fires tend to intensify faster than the sprinkler system can control them. In the past few years, the fire protection industry has been adopting the widespread use of foam in what traditionally would have been water sprinkler systems. These new systems are known as foam-water sptrihkler systems.
Except for the addition of the foamproportioning equipment, the configuration and operation of the sprinkler system have not changed. However, the manner in which firefighters operate in conjunction with these systems is slightly different from that used with ordinary sprinkler systems. As with any hazard, preplanning is the key to a successful and safe operation.
PREPLANNING THE OPERATION
The first step in preplanning operations with a foam-water sprinkler system is to learn how the system operates. NFPA-16, Standard for Deluge Foam Water Sprinkler Systems and Foam Water Spray Systems, and NFPA16A, Recommended Practice for Closed Head Foam Water Sprinkler Systems, discuss the need for system operating procedures to be readily available. It is desirable to have the instructions posted adjacent to the foam-proportioning system. These instructions should be clear and concise with regard to exactly what should happen automatically and what, if anything, firefighters must do to activate the system. The instructions also should explain how to recognize a problem and the action necessary to correct it. Responding personnel should familiarize themselves with the location and operation of the foam equipment. Also, the preplan should include the location(s) of the fkx»r drainage system and the slope of the floor. This information will provide insight relative to the direction in which spills can be expected to travel, information that will aid in determining the direction of the fire attack. The drainage system also may consist of a holding basin outside the building.
Primarily there are two types of foam-proportioning systems the firefighter can expect to encounter: bladder tanks and pump-type, balancedpressure systems. Technically, bladder tanks also work on a balancedpressure principle, but for our purposes this article will differentiate between the two systems.
Bladder tanks. This system consists of a steel pressure vessel with an internal, reinforced, rubber bladder that contains the foam concentrate (see Figure 1). When the system is activated, water from the system pressurizes the outside of the bladder within the tank, which forces the foam concentrate to the proportioned where it is mixed with water and becomes a foam solution. (The process is similar to forcing toothpaste out of the tube by squeezing it.)
Pump system. In this balancedpressure system, a pump draws the foam concentrate from an atmospheric storage tank and then delivers it to the proportioner so it can be mixed with water. A pressure-balancing valve matches foam pressure to water pressure, ensuring correct proportioning (see Figure 2).
The presence of a foam-water sprinkler system indicates that potentially hazardous materials are stored in that facility. The time to identify the materials expected to be involved in an incident is during the preplanning process, when any needed adjustments in response and standard operating procedures (SOPs) should be made.
Arriving companies can expect operating conditions to be much the same as those in buildings with ordinary sprinkler systems—smoke and heat will tend to bank down to floor level and make operations difficult. Since flammable and combustible liquids may be involved, smoke and heat generation most likely will be more intense than they would be when ordinary7 combustibles are involved. As is the case with any sprinklered building, a responding engine company usually is assigned the task of “backing up” the sprinkler system. NFPA-13E, Recommendations for Fire Department Operations in Properties Protected by Sprinkler and Standpipe Systems, recommends charging the supply line to 150 psi. Department SOPs also may recommend a particular discharge pressure, without considering the possible impact it may have on the foam proportioning system. Increasing water pressure beyond the design pressure of the system can result in failure of the fixed proportioning system.
Pump-type, balanced-pressure systems have a foam pump that can withstand a given pressure. If the water pressure is too great for the foam pump, foam will not be injected into the system. Although bladdertank systems are less susceptible to the effects of increased water pressure, the tanks generally have a pressure limitation of 175 psi. Each tank is required to have a permanently attached label listing its maximum operating pressure. This label should be checked during preplanning, and under no circumstances should the maximum operating pressure of the tank be exceeded.
Determining the pressure for “backing up” the system is fairly simple. Each sprinkler system riser within a building is designed for an optimum flow and pressure that should be posted on a sign permanently attached to the riser (see Figure 3). SOPs should be revised to reflect backing up the system only to the design operating pressure posted on the riser. The old adage of “more water” does not apply when operating with foam systems. The foam concentrate supply for foam-water sprinkler systems usually is sized for 10 to 20 minutes of operating time for the highest flow riser. Increasing the system pressure will increase the flow through the sprinkler heads and prematurely deplete the foam concentrate. It is unlikely that the fire department will be able to replenish the system before the initial foam concentrate supply is depleted.
The normal assignment of sending a firefighter to the sprinkler riser should be carried out as soon as safely practical. If possible, another firefighter should be sent to the foamproportioning equipment to check for proper operation. When it is determined that the incident is sufficiently under control, an order to stop the sprinkler flow should be given. Avoid discharging water onto the foam blanket after the foam concentrate supply has been depleted. A prolonged discharge of water will weaken the foam blanket. The foam-proportioning system should not be reset until the incident is concluded, in case the sprinkler system flow has to be restarted.
When the incident commander determines that conditions warrant entering the building for overhaul and final extinguishment, the firefighters must be equipped with foam hoselines, which should come from the fire department’s own foam-proportioning equipment. Avoid using foam hose stations within the facility unless prior fire department testing has proven that they will provide correctly proportioned foam at sufficient nozzle pressure. Fixed-foam proportioning equipment designed to operate with the sprinkler system may not work at the lesser flows expected from handlines. The foam concentrate should be an alcohol-resistant type, proportioned properly for alcoholtype fuels unless positive identification of the products involved indicates otherwise. As is true of any foam operation, the attack should not begin until sufficient foam is on hand to carry out the complete operation. When the nozzle team enters the building, exercise care to avoid disturbing the existing foam blanket, which may allow vapors to escape and cause a possible reignition.
Foam-water sprinkler systems can make the difference between quick extinguishment and a long, exhaustive, defensive operation. The building owner makes a substantial financial investment in fire protection when installing such a system. Firefighters owe it to the building owner and themselves to make the investment in training and preplanning so that when the need arises, the incident can be brought to a safe and successful conclusion.