Which system uses closed sprinklers but needs a separate detection system to operate before water is admitted to the piping?
- Deluge system
- Preaction system
- Wet pipe system
- Antifreeze system
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B. Preaction system
In a preaction system the piping is normally dry and the valve is opened by detection, after which individual sprinklers must still fuse to discharge. It is used where accidental discharge would be costly, such as computer rooms and archives.
Reference: NFPA 13 system types: preaction
How can a fairly low air pressure hold a dry pipe valve closed against a much higher water pressure?
- The air acts on a larger clapper area than the water does
- The air is compressed by a spring acting on the clapper
- The water pressure is reduced by a regulator at the valve
- The air is heavier than the water held below the clapper
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A. The air acts on a larger clapper area than the water does
A differential dry pipe valve has a clapper with a large air-side seat and a small water-side seat. Force is pressure times area, so modest air pressure on the large area balances higher water pressure on the small one.
Reference: NFPA 13: differential dry pipe valves
What does a quick-opening device (accelerator) do on a dry pipe system?
- Speeds up the refilling of the system with air
- Raises the water pressure at the sprinklers
- Speeds up the tripping of the dry pipe valve
- Drains condensate from the low points
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C. Speeds up the tripping of the dry pipe valve
On a large system it takes time for air to bleed down to the trip point through one open sprinkler. An accelerator senses the pressure drop and directs air to the valve's intermediate chamber so it trips sooner.
Reference: NFPA 13: dry pipe quick-opening devices