Dry-Pipe System Fails Water Delivery Time
An unheated 60,000 sq ft warehouse has a dry-pipe system with a 6 in riser, a system capacity of 750 gallons, and a dry-pipe valve equipped with a 10 cfm air compressor. The system is maintained at 40 psi air pressure. During the annual trip test, the valve trips at 26 psi, but water reaches the inspector’s test valve in 75 seconds. The contractor says the system passes because the valve tripped and water arrived. The owner wants to avoid adding a quick-opening device or a larger compressor. The local code enforces NFPA 13 and NFPA 25. The system serves ordinary hazard Group 2 storage. The air compressor is sized to restore pressure, but the contractor has not documented the refill time. The dry-pipe valve is listed for 20–40 psi air pressure.
1. Determine whether the 75-second water delivery time is acceptable. Cite the applicable limit and explain the effect of the 40 psi air pressure.
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The 75-second delivery time is not acceptable. NFPA 13 limits water delivery time to the inspector’s test valve to 60 seconds for dry-pipe systems with a capacity of 750 gallons or less. This system is exactly at 750 gallons, so the 60-second limit applies, and 75 seconds exceeds it. The valve tripping is necessary but not sufficient; the test measures how quickly water reaches the most remote point. The 40 psi air pressure is within the valve’s listed range, but it is at the high end and increases the volume of air that must be vented before water can travel. Reducing air pressure to the manufacturer’s minimum that still keeps the valve closed can shorten delivery time, but it may not be enough by itself. The contractor must also consider a quick-opening device or increased compressor capacity, and then repeat the trip test to document compliance.
2. List acceptable corrections and explain how you would verify that the air compressor is adequate.
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Acceptable corrections include adding a listed quick-opening device, reducing air pressure to the valve manufacturer’s minimum, increasing compressor capacity, or splitting the system to reduce capacity below the threshold. The most reliable single correction is usually a quick-opening device because it accelerates the venting of air without depending on a large pressure change. To verify the compressor, calculate the air volume needed to restore normal pressure after a trip and compare it with the compressor’s rated delivery. NFPA 13 requires the compressor to restore normal air pressure within 30 minutes after the system trips. A 10 cfm compressor on a 750-gallon system may be marginal, especially if the system has leaks or a high initial pressure. After any change, perform another trip test and record the water delivery time. Also check for trapped sections, incorrect pipe slope, and obstructions that could delay water.