(Wed 25th Jun 2008)
Industrial Fire Journal / June 2008:
According to David Owen of Firemain Engineering, a fire engineering company based in St Helens UK, the Hi Ex Foam system is used for special applications such as aircraft hangars and warehouses. The company recently designed, supplied and commissioned the low expansion foam systems for Virgin Atlantic's Airbus A380 hangar at Heathrow. Its High Expansion systems were also installed by Argus Fire at RAF Fairford.
Owen explains the NFPA 409 has been the standard of choice in hangar protection for many years. However, the continued concern over firewater run-off with any foam system has prompted the use of high expansion foam. "Although, Hi Ex uses a great deal less water in operation, it's important to understand the special parameters of the alternative design standards such as NFPA and the USAF military Engineering Technical Letter (ETL)."
The water supply is generated by the sprinkler pumpset fed from a dedicated water tank. The hangar is protected by two types of fire protection system, the compulsory sprinklers at roof level, as well as the high expansion foam system. In order for the high-expansion foam system to work, some of the water from the sprinkler tank is diverted through a horizontal bladder tank, containing the foam concentrate and mixed into a solution via a proportioner. It then flows to the Hi Ex generators mounted at roof level. There, the water and foam mixture is aspirated and discharged onto the risk. Firemain Engineering's foam solution's rate is 2.75% and because less concentrate is required for Hi Ex, the bladder tank size is smaller in comparison to low expansion foam deluge systems. The expansion ratio is 1,000:1 achieving excellent quality Hi Ex foam with low water content. The standard calls for 90 per cent coverage of the floor area within one minute and coverage to a depth of one metre within four minutes. Therefore the generators have to be mounted in specific locations to achieve the most efficient coverage.
Owen explains: "The design requires a large amount of planning and detailed measuring. The plane itself is the key obstacle because topside of the plane should never be flooded. The idea is to get around the bottom of the aircraft to deal with spill fires at floor level. The assets are worth tens of millions each.
"Anyone trying to undermine the standard is strongly advised not to as Defence Estates have put much time and effort into developing it," says Owen. When looking at civil aircraft hangars, NFPA 409 ‚Äì the general aircraft hangar standard ‚Äì gives the option to use oscillating monitors, which spray the foam under the wings. It also gives the option to use a foam water deluge system at the roof, which is low expansion foam. The ETL is much more precise because it asks for high expansion foam and roof sprinklers. The British military falls under the Defence Estates, which covers the RAF and the United States Airforce airfields. Although some of these are seen as American bases, the policy of the US is to use the standards of the host country.
Normally, application rates for foam systems are considered in LPM (Litres Per Minute) per square meter, which is not relevant for high expansion systems, because they aim for total coverage to a depth of one metre within four minutes.
Owen continues: "During the commissioning stage, we set our one-metre posts over the hangar floor to measure the depth and we take photographs. We carry out full discharge tests on the site itself. Our system can handle any type of high expansion foam. We are the UK Distributor's for Ansul equipment and foam, by combining their application experience with ours we can provide the most appropriate solution. We have a full understanding of foam applications and can design according to world standards, but also look at one-off jobs as well."
He adds that there are several benefits attached to the Hi-Ex foam system supplied by Firemain Engineering. "This system uses a lot less water than conventional low expansion foam deluge systems. Smaller bore pipe work means there is less load on the roof structure of the hangar but most importantly it means reduced fire water run-off," concludes Owen.
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