Many fire protection system designs assume some level of public fire department intervention. This might be implicit, as with the sprinkler system hose stream allowance (as discussed in earlier articles) so that the fire department can complete extinguishment. It may also be explicit, such as the case of semi-fixed systems.
Semi-fixed systems have pre-installed piping and discharge devices or hose outlets, but the extinguishing agent is usually supplied by a fire department vehicle. This is typically done to reduce system cost and to take advantage of the investment in mobile fire apparatus. Examples of semi-fixed systems are dry standpipes such as those found in automobile parking garages, semi-fixed foam systems as described in NFPA 11 (Figure 1), and semi-fixed CO2 systems supplied by CO2 fire fighting vehicles.
In order to be able to rely on such an arrangement, it is extremely important that assumptions be tested and validated regularly. Personnel, contractual expectations and apparatus change, often to the point that the original assumptions are no longer valid.
As an example, a jet fuel supplier at an airport installed an NFPA 11 semi-fixed foam system at its fuel tank farm. The expectation is that an industrial foam pumper will be used to supply these systems. These foam pumpers are purpose-built for this task. System designers may assume that airport fire fighting and rescue (ARFF) vehicles might have the same capabilities simply because industrial foam pumpers and ARFF vehicles are often called “foam trucks.” This is not true at all.
The assistant chief of fire operations at this airport wanted to test the assumptions. A new 80-foot diameter cone roof jet fuel tank was being commissioned and the contractors wanted to test the new semi-fixed foam system. It was a perfect and rare opportunity to accomplish both tasks.
The first screening test that must be passed is whether the ARFF vehicle can discharge foam through its 2½-inch (65 mm) hose outlets. Many modern ARFF vehicles can do this but many 1980s vintage - especially those operated by the U.S. Air Force - cannot. These older vehicles are still in service at smaller airports. The fleet at this airport was able to discharge foam this way.
The model of ARFF vehicle used at this airport was not equipped with a “structural kit.” The structural kit allows the ARFF vehicle to operate somewhat similar to an industrial foam pumper. This means that “crash mode” needs to be used. In the crash mode, some of the vehicles are set to operate at a fast-idle when discharging through the 2½-inch hose outlets and some are set to throttle up to full speed once agent flow was sensed.
At full speed, discharge pressure is 240 psi (16.5 bar). It was felt that it would be too difficult to reduce full pressure to the 150 psi (10.3 bar) maximum specified semi-fixed system pressure (the required pressure for this tank’s system was lower). This is because pressure needs to be reduced by partially closing the quarter-turn discharge valves. Besides the risk of over pressurizing system piping, operating at higher than design pressure consumes foam supplies faster than anticipated.
The vehicles that operated at fast-idle when flowing from the 2.5-inch outlets allowed for enough flow and pressure to supply the semi-fixed system. It is important to note that at reduced speed, the flow and pressure are both reduced. The flow and pressure available at other than design speed can be calculated using the pump affinity laws. These laws are described in the NFPA Fire Protection Handbook. Any fire protection engineer who deals with pumps is able to assist with this calculation. In this case, the flow was sufficient (505 gpm, 1900 l/min) at reduced speed. This might not be true for larger tanks. The reduced flow needs to be compared to system’s required flow. The system’s required flow must be determined from the installer, or it can be calculated using the same type of calculations used to determine sprinkler system demand. A fire protection engineer can assist with these calculations.
In discussing this test with another airport using the same vehicles, I was told that all of their vehicles went to full speed upon sensing flow from the 2.5-inch outlets. It is essential to determine how each of the vehicle performs under this circumstance.
Another difference between ARFF vehicles and industrial foam pumpers is that ARFF vehicles are designed for fast and short duration operations at aircraft fires. They do not carry enough foam or water for the longer operations needed at tank fires. In this operation, water resupply was easily accomplished from an airport fire hydrant. Water resupply rates for larger tanks may not be achievable because the resupply hose feeds through a pipe into the tank, and then into the pump. The refill pipe is not necessarily sized with full flow rates in mind. The hydrant flow, pressure, and fill hose diameter might also be limiting factors. Foam would have been resupplied via foam concentrate tanker although the foam tanker was not part of this test. Many airports do not have foam tankers and can not ensure a continuous foam supply for the duration needed.
Finally, ARFF (aircraft rescue and fire fighting) vehicles carry AFFF (aqueous film-forming foam) or FFFP (film-forming fluoroprotein) in the UK as opposed to the alcohol-resistant AFFF or FFFP carried by industrial foam pumpers. Alcohol-resistant foam is widely thought to be more effective on tank fires, even if only hydrocarbons (such as Jet A) are involved.
The test was successful due to taking the time to carefully plan under non-emergency conditions.
The author would like to thank Assistant Fire Chief Steven Woodworth of the Atlanta Fire Rescue Department for his assistance in the preparation of this article. Figure 1 is courtesy of Garry Bennett of XL Global Asset Protection Services.
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