Dry-Pipe Trip Test Failure in an Unheated Warehouse
An 18,000 sq ft unheated warehouse has a dry-pipe system protecting 2,600 sq ft of loading-dock storage and a canopy. The system has a 4-inch riser, 120 ft of 3-inch main, eight 1.25-inch branch lines with 24 pendent heads, and a compressor maintaining 35 psi air pressure. During the combined trip test, the inspector's test valve was fully opened and water reached the test outlet in 82 seconds. The outside temperature was 34 degrees F. The contractor's fix is to raise the air pressure to 45 psi and add a larger compressor so the valve trips faster. The building official notes that the system also has a drum drip at the low point that was full of rusty water, and that the dry valve priming line is fed from a 1-inch domestic line. The project is in the final inspection phase, and the contractor wants the certificate of occupancy released.
1. Evaluate the 82-second trip time and the contractor's proposed fix. What should be required before acceptance?
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The dry-pipe system must deliver water to the test outlet within 60 seconds. The test measured 82 seconds, so it fails. Raising air pressure from 35 to 45 psi makes the problem worse: the compressor must push more air out of the test outlet before water arrives, and the trip time normally increases. A larger compressor only shortens recovery time, not trip time. The likely causes are excessive air pressure, an undersized test or drain outlet, trapped water, a restricted priming line, or a dry valve that is slow to trip. The standard remedies are to reduce air pressure to the minimum recommended by the valve manufacturer, install a listed quick-opening device, enlarge the test outlet if permitted, and split the system into smaller zones if the volume is excessive. After any change, repeat the trip test at the normal air pressure and record the time. The certificate of occupancy should not be released until the trip test passes.
2. What is the significance of the drum drip full of rusty water and the 1-inch domestic priming line?
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The drum drip full of rusty water shows that compressor condensate is accumulating at the low point. In freezing weather, that water can block the drain or freeze in the branch, preventing the system from draining and adding to trip time. The drum drip and all auxiliary drains must be drained after each trip test and before freezing weather, and the compressor should have an air dryer or aftercooler. The 1-inch domestic priming line is also a concern: it must be a reliable, supervised source and must not be subject to freezing or shutdown. If the priming line is taken from a domestic line that can be valved off, the dry valve may not trip. The contractor should provide a dedicated, supervised priming water supply and verify the priming water pressure.
3. Does the system capacity affect the acceptance decision? Show the check.
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Check the system capacity as well as the trip time. The 4-inch riser and 3-inch main plus branch lines are roughly 110 to 120 gallons, well under the 750-gallon dry-pipe capacity limit, so volume is not the reason for rejection. That does not excuse the 82-second trip. The capacity check is still required because a system over the limit must be subdivided. Also verify that the dry valve is listed for the air pressure and that the compressor can refill the system within the required time after a trip. The contractor must correct the trip time, drains, and priming line before the AHJ accepts the system.