In a pre-action system the water supply is held back by a pre-action valve. This valve is connected to a supplemental detection system. Water will not enter the pipe until the detection system is activated. Once activated, the valve is released and allows water into the sprinkler piping. Water will not come from the system, until sufficient heat causes the individual sprinkler head to activate (after the pre-action valve activates, the system functions the same as a wet-pipe system). This type of operation is known as a standard or single-interlock system.
The air pressure on the pre-action valve is constantly monitored. If the pressure changes (due to leak in pipe or other issue) an alarm will sound, however, the system will not activate under this condition. The valve will remain closed, preventing water running into the system until the detection system is activated.
Another type of pre-action system known as a double-interlock system will only operate when both the supplemental detection system and a sprinkler head is activated.
These systems are commonly found in high value areas such as computer rooms, communications centers, and museums.
These systems are to be installed per the requirements of NFPA 13 and NFPA 72. The following inspections are required for system acceptance:
1. Standard hydrostatic test at 200psi for 2 hours.
2. Air pressure leakage test at 40psi for 24 hours.
3. Signage and labeling of all controls and valves to be posted.
4. Alarms monitoring the supervisory air pressure shall be tested.
5. Flow switch and water flow alarm are to be tested.
6. Operational test shall be performed, requiring the pre-action valve to trip.
7. The supplemental detection system shall be tested without operating the pre-action valve.
8. Full fire alarm system function test to be conducted.
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