PRE-ACTION SYSTEM
A pre-action system is a sprinkler system with a second lock on the door. The pipework above the pre-action valve holds supervised air or nitrogen rather than water, and the valve only opens when an independent detection system in the same space tells it to. Water reaches a sprinkler head, and the floor below it, only once the system is satisfied a fire is real.
That matters wherever water leaving the pipe by accident — a damaged head, a struck pipe, a contractor’s drill — would do more damage than a small fire: data centres and server rooms, telecom and control rooms, archives and record stores, libraries, museums and galleries, clean rooms and process areas, and freezer and cold store spaces where a wet system would freeze.
The supervisory air pressure is monitored continuously, so a leak or a physically damaged head raises a trouble alarm at the panel long before it can become a discharge.
TYPES OF PRE-ACTION SYSTEM
- SINGLE INTERLOCK
- DOUBLE INTERLOCK
- NON-INTERLOCK
SINGLE INTERLOCK
The pre-action valve opens on a signal from the detection system alone. Water then fills the pipework and stands at the closed sprinkler heads, and the system behaves from that point exactly like a wet system — water flows as soon as a head operates.
Because detection responds before a sprinkler does, the pipe is normally charged and ready by the time the first head opens, so there is no delay in getting water on the fire. This is the common arrangement where the concern is a broken pipe rather than a false detection.
DOUBLE INTERLOCK
Water is admitted only when the detection system operates and a sprinkler head has opened, the second condition being sensed as a loss of supervisory air pressure. Either event on its own does nothing.
This is the arrangement for freezer rooms and cold stores, where ice in a charged pipe would be a hazard in itself, and for high-value spaces where even the filled pipe is considered a risk. The trade-off is a slightly longer time to water, since the pipework fills after the head has already opened.
NON-INTERLOCK
Water is admitted when either the detection system operates or a sprinkler head opens. It is the most responsive of the three and the least restrictive, used where the system is wanted mainly for the supervision and the early warning that detection gives, rather than to prevent water entering the pipework.
INSTALLATION AND COMMISSIONING
STARCO designs the system to the hazard classification of the space, installs the pre-action valve assembly, the supervised pipework and the detection that releases it, integrates the release panel with the building fire alarm system, and carries out the trip, air pressure and flow testing required for Civil Defence approval. Pre-action valves and their air supervision are inspected under the same preventive maintenance contract as the rest of the sprinkler installation.