Pre-action fire sprinkler systems employ the basic concept of a dry pipe system in that water is not normally contained within the pipes. The difference, however, is that water is held from piping by an electrically operated valve, known as a pre-action valve. Valve operation is controlled by independent flame, heat, or smoke detection.
Two separate events must happen to initiate sprinkler discharge. First, the detection system must identify a developing fire and then open the pre-action valve. This allows water to flow into system piping, which effectively creates a wet pipe sprinkler system. Second, individual sprinkler heads must release to permit water flow onto the fire.
In some instances, the pre-action 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.
Advantages of using pre-action fire sprinkler systems include:
- The dual action required for water release – The pre-action valve must operate and sprinkler heads must fuse. This feature provides an added level of protection against inadvertent discharge. For this reason, pre-action systems are frequently employed in water sensitive environments such as archival vaults, fine art storage rooms, rare book libraries and computer centers.
Disadvantages of using pre-action fire sprinkler systems include:
- Higher installation and maintenance costs – Pre-action systems are more complex with several additional components, notably a fire detection system. This adds to the overall system cost.
- Modification difficulties – As with dry-pipe systems, pre-action sprinkler systems have specific size limitations which may impact future system modifications. In addition, system modifications must incorporate changes to the fire detection and control system to ensure proper operation.
- Potential decreased reliability – The higher level of complexity associated with pre-action systems creates an increased chance that something may not work when needed. Regular maintenance is essential to ensure reliability.