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Pre Action Systems
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.
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