The LWF Blog

Fire Engineering Design and Risk Assessment – Sprinkler System Installations – Part 25

December 14, 2017 3:59 pm

In LWF’s Fire Engineering blog series for Architects and others in the building design industry, we have been looking at the use of sprinklers as part of active fire protection. In Part 24, the types of sprinkler system were considered, including wet, alternate and dry installations. Part 25 will continue from that point and cover tail-end alternate and tail-end dry systems and pre-action installations.


While it is most common for outside temperatures to affect a sprinkler system as an entirety, there are situations where only a part of a wet installation may be affected by low temperatures. As a solution to this issue, a standard wet installation can be adapted by the inclusion of a smaller alternate or dry sprinkler system as an extension to the main. Such systems are known as tail-end alternate or tail-end dry systems. This means that while the majority of the building will be covered by a wet installation already primed with water and with a very low reaction time, a part will operate as a dry or alternate wet and dry system when temperatures are in danger of causing freezing in that part of the building.


A pre-action installation is a particular type of dry installation which relies on the detection of the fire from a fire detection system separate to the sprinkler system. Pre-action systems work by pre-arming in the event of a fire being detected. They can be categorised into two types of system – Type A and Type B, but in both cases, a fire detection system is required and both systems and the integration must comply with the relevant standards to ensure it is of efficient and operational quality.


Type A Systems comprise pipework which is fed through a pre-action valve. Upon activation of the fire detection system, water is released. When the system is in its passive state, the pipes are charged with low-pressure condensed air. Any damage to a sprinkler head or pipework would allow the air to escape and this raises an alarm, but does not release the water. For the water to be released it is required that both the fire detection system and the sprinkler head have indicated a fire is in progress.


This additional step which is not found in the majority of sprinkler systems means that a Type A system is most suited to premises where any accidental operation of the sprinkler system would be the cause of expensive damage or considerable disruption.


However, the added precaution of a two-step fire detection process means that reliability is reduced and Type A pre-action sprinklers are not suited to many applications. They should only be considered where there is no other viable choice and where there are high hazard risks.


A Type B system is similar to a Type A system in that it relies on the use of a separate fire detection system. The detection system is used to charge the sprinkler system with water at an early stage of fire development, prior to activation of the sprinkler heads. Such systems are most appropriate where the area covered by the sprinkler system is large and where high hazards are involved. If the detection system failed, the sprinkler system would operate as a standard alternate or dry sprinkler system.


In Part 26, LWF will look at Recycling installations and Deluge installations. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.


Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.


While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.



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