Many laypeople think of fire sprinkler systems (well, maybe they don’t think about them too often!), and may not realise that there are quite a few different types. Each type of automatic fire sprinkler system has different advantages, disadvantages, design considerations, system components, maintenance requirements and more. We summarise below as an overview only, for passing interest and for non-design specialists, the various types.
Fire Sprinkler System standard are governed largely but not exclusively by the following:
In the UK, mainly BS EN 12845 (commercial systems) and BS9251 (domestic and residential systems) and the LPCB Rules, but also sometimes either of the below depending on insurer and/or building owner requirements.
Based in the USA but adopted more or less globally, there are various standards issues by FM Global, and NFPA (National Fire Protection Association) mainly the NFPA 13 series.
Wet pipe
In this type of sprinkler system, all pipes are permanently filled with water, ready to be emitted from a sprinkler head as soon as it opens. A sprinkler head will open and allow water to escape when the temperature in its vicinity is hot enough to cause the glass bulb to burst. This is the most reliable, cost-effective and common type of sprinkler system and should be considered first. But if there is a risk of the water inside the pipes freezing, then a different type of system should be chosen because an ice plug in a sprinkler system will prevent water moving through the pipes freely. Unlike some sprinkler activations you may see on film or TV, only the head/s subject to the high temperature required will activate, there is no multiple or total activation of all the sprinkler heads in a building. Wet pipe sprinkler systems can be mains-fed if flow permits (perhaps with a pressure-boosting pump/s) or tank-fed (which will require fire pumps). This type of system is almost always used in a domestic or residential setting, because the risk of freezing is relatively low.
Dry Pipe
These sprinkler systems are similar to wet-pipe systems in that a sprinkler head is opened as a reaction to high temperatures in the vicinity; however, as the pipes are initially filled with air and not water, it will take longer for the water to emit from the head since air pressure has to be lost from the pipework and the water must travel down the pipe from the alarm valve to the activated sprinkler head. Since regulations state that water must be emitted within a certain amount of time, dry-pipe systems are limited in their size. There are several variations of this type of system: certain systems may only be ‘dry’ for parts of the year when there is a risk of water freezing in the pipes (known as ‘alternate’ systems, as they alternate from being wet to dry, generally over the winter); certain systems may only be ‘dry’ in certain areas of the building, for instance car parks or loading canopies where the risk of freezing is higher, but will be ‘wet’ elsewhere in warmer parts of the building (known as ‘tail-end’ dry systems). Because water will not flow to the sprinkler head until air pressure has dropped sufficiently, it is important not to over-pressurise systems and use the correct size of compressor; some systems also incorporate ‘accelerators’ which increase the rate of air pressure loss in the event of a sprinkler activation. Dry systems are more complex than wet systems, because the correct air pressure has to be maintained at all times in order to keep the alarm valve closed against water entry; a drop in pressure will allow water to enter the system which can be a problem if unplanned due to pipework damage or air compressor failure for example.
Preaction
This type of fire sprinkler system is the most complex, since it initially behaves like a ‘dry’ system (pipework filled with lightly pressurised air rather than water), but then may become like a ‘wet’ system (pipework fills with water) so that if and when the sprinkler heads activate then the water is available immediately. There are three types of Preaction sprinkler system:
- A non-interlock system will replace the air with water on the operation of a detection device (e.g. heat detector, manual call point) OR the activation of a sprinkler head. In the first case, no water will be emitted unless and until the sprinkler head also activates whereas in the second case water will be emitted (even if the activation of the sprinkler head is accidental, for instance it has been damaged by a forklift).
- A single-interlock system will replace the air with water on the operation of a detection device…then, if a sprinkler head then activates, water will be emitted. In this case, the accidental breakage of a fire sprinkler head will not result in the (accidental) emission of water which then may not be detected for some time, for instance when maintenance is carried out or on the operation of a detection device. There may be a risk of water freezing if accidental damage occurs and remains undetected.
- A double-interlock system will replace air with water in the system only if TWO events take place – the operation of a detection device AND the activation of a sprinkler head. In this case, although similar to the above, this system behaves more like a ‘dry’ system because the initial operation only of a detection device will NOT have caused the pipes to fill with water. The restrictions in size of dry systems therefor also apply to double-interlock preaction systems.
Preaction alarm valves may vary slightly, but in general the sprinkler head activation will work on the valve in a similar way to the dry valve (with a drop in air pressure allowing one part of the valve to open), and the ‘detection device’ activation will work through a release panel which in turn will operate a solenoid valve to release the second part of the interlocking valve.
Deluge
This type of system is similar to a single-interlock Preaction system, because detection devices are used (e.g. smoke or heat detectors, or manual call points); however, the sprinkler heads used on these systems are all open – so, in the event of a deluge activation, water is emitted from ALL sprinkler heads in the area and not just that/those which have been activated by heat. These systems are generally used in areas of high hazard, for instance flammable chemical plants/delivery areas or aircraft hangars. Extra care needs to be taken to avoid accidental operation of deluge systems, because water damage can be significant.
Understanding the above, you can see that fire sprinkler systems are not necessarily simple, and choosing the correct system depends on many risk assessment and design considerations. SEP is a supply-only business, so we are able to supply various specialised components for all systems; however, we always recommend that you take advice from a specialist fire sprinkler designer/installer. Sources might include bafsa and RedBook Live or please contact us and we can help.