A pre-action sprinkler system consists of a sprinkler system and an electrical detection system with water held back by the pre-action valve and pressurized air in the sprinkler pipework. In the event of the operation of the electrical detection system the pre-action valve operates and allows water into the sprinkler pipework.
'Pre-action systems' differ from the normal wet or alternate systems in that operation is initiated by an automatic detection system linked to the sprinkler system, and not by the sprinkler itself. Discharge of water can only occur, however, when the sprinkler head or heads operate. The operation of the detection system merely allows water to flow from the alarm valve level into the sprinkler distribution network. Thus a system which is normally 'dry' is given the speed of response of a 'wet' system. A typical pre-action system is shown in the demonstration.
When the detection device senses a fire, it opens the main valve, allowing water to flow through the pipes before the sprinklers are set off. When the heat activates the sprinklers, water flows through immediately, as in a wet-pipe system.
In some instances, the preaction system may be set up with a double interlock in which pressurized air or nitrogen is added to system piping. The purpose of this feature is two-fold: first to monitor piping for leaks and second to hold water from system piping in the event of inadvertent detector operation. The most common application for this system type is in freezer warehouses.
Pre-action systems are usually employed in areas that are at risk for serious water damage due to damaged sprinklers and/or piping.
They operate faster than dry systems, but tend to be significantly more expensive