Sprinkler systems in defence facilities The Swedish Fortifications Agency, which owns and manages Sweden’s governmental properties intended for defense purposes, has a responsibility to ensure good resource utilisation and high economic efficiency. To this end, writes Magnus Arvidson, Fire Protection Engineer, with RISE, the agency undertook a cost-benefit analysis which investigated the design, reliability, performance, and cost of a fire sprinkler system for typical underground fortification facilities.
The aim was to study how a sprinkler system for a typical fortification facility underground could be designed, based on relevant risk classes in Swedish sprinkler rules (SS-EN 12845). Then, the installation and operating costs for two fictitious facilities; a small facility with a net area of 1000 m2 and a large facility with a 5000 m2 net area were calculated. The input on the reliability and efficiency of sprinklers were based on a literature review. The installation and operating costs were also calculated for two different water mist fire protection systems, a commercial low- and high pressure system, respectively. For these systems, however, there is no good basis for assessing reliability, so it was assumed to be equivalent to a traditional sprinkler system. It was also assumed that the performance efficiency at least corresponds to a traditional sprinkler system, which is part of the accepted fire testing methods. Another assumption was that the annual cost for inspection, testing and maintenance was similar for the studied systems. With this information as a basis, a costbenefit analysis was made.
Estimated installation cost
It was assumed most occupancies in a typical facility, such as
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accommodation, kitchen, dining rooms and medical rooms could be regarded as OH1 per SS-EN 12845 and for occupancies such as workshops, a design according to OH2 is reasonable. As opposed to stores and other spaces with temporary or permanent storage of materials which require a design according to risk class OH3. The water mist fire protection systems in the study were approved by FM Approvals for Hazard Category 1 (HC-1). The design and installation recommendations of these systems were judged to be acceptable for OH2 occupancies per SS-EN 12845. For storage spaces, the assessment was made that water mist fire protection systems are applicable if a storage room has a maximum floor area of 50 m2; partly because it limits the number of automatic nozzles that can activate and partly because an enclosed room contributes to the efficiency of water mist. The estimated installation cost for a traditional fire sprinkler system in the smaller type of facility is about SEK 1,3 million and about SEK 3,3 million for the larger facility whereas the installation cost for a high-
pressure water mist system is higher than that of a traditional sprinkler system for the smaller type of facility but comparable for the larger facility. A low-pressure water mist system seems to be the least expensive option for both types of facilities, probably because the system, unlike a traditional sprinkler system, requires smaller pipe sizes, smaller water pumps and a smaller water tank and, unlike a high-pressure system, uses normal steel pipes and less expensive centrifugal pumps.
Reduced property damage
In general, it can be stated that the greatest benefit of a sprinkler system is that the property damage is reduced. The expected benefit for personal injury is around a few percent of the total benefit of the sprinkler system. This is of course due to the fact that the risk of death and other injuries in a fire is small in these types of facilities as people can usually reach safety. The real estate is always affected by a fire however, the reduction in property loss was assumed to be 75%, based on the results of the literature review.
