Standpipe Pressure-Reducing Valve Settings
A 14-story, 180 ft high office building has a Class I manual standpipe with 2½ in hose valves. The water supply is a fire pump rated 1,000 gpm at 100 psi, taking suction from a city main. A hydraulic calculation shows that with the pump at 150% flow, the pressure at the lowest hose valve on the first floor would be 210 psi; at the top floor, the residual pressure during a 250 gpm flow is 70 psi. The building has pressure-reducing valves at each hose valve. During a flow test, the PRV at the top floor delivers 250 gpm at 65 psi outlet, and the PRV at the first floor delivers 250 gpm at 130 psi outlet. The contractor proposes removing the first-floor PRV to reduce cost and adjusting the top-floor PRV to a lower setting to save water. The local code enforces NFPA 14.
1. Evaluate the contractor’s proposal to remove the first-floor PRV.
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Removing the first-floor PRV is not acceptable. NFPA 14 limits the maximum pressure at any hose valve to 175 psi unless a specific approved arrangement permits otherwise. The hydraulic calculation shows 210 psi at the first-floor hose valve with the pump at 150% flow, which exceeds that limit. Without the PRV, the hose valve, hose, and nozzle could be subjected to excessive pressure, creating an unsafe condition for firefighters and possible damage to the valve. The existing first-floor PRV outlet pressure of 130 psi is acceptable because it is below 175 psi and still provides adequate operating pressure. The PRV must remain, be listed for standpipe service, and be set so that the outlet pressure does not exceed 175 psi under all expected flow conditions. Removing it to save cost would create a code violation and a safety hazard.
2. Evaluate the top-floor PRV outlet pressure of 65 psi at 250 gpm. What must be done?
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The top-floor outlet pressure is not acceptable. For a Class I standpipe, NFPA 14 requires a residual pressure of 100 psi at the topmost hose outlet when flowing 250 gpm. A 65 psi outlet is below that minimum and would not provide an adequate hose stream. The cause could be a PRV set too low, an undersized supply, or an inaccurate hydraulic calculation. First verify the pump curve and the actual supply pressure at the top floor without the PRV. Then adjust or replace the PRV so it delivers at least 100 psi at 250 gpm while still protecting the lower floors. If the supply cannot provide 100 psi at the top, the pump or piping must be upgraded; simply lowering the PRV setting further would worsen the problem. After adjustment, flow-test the top and first-floor outlets to confirm both the minimum and maximum pressure limits are met.