Adding a Standpipe Demand to an Existing Combined System
A 6-story building has an existing combined sprinkler/standpipe system fed by a 500 gpm fire pump. The building is being renovated: the sprinkler demand is recalculated at 700 gpm at 80 psi, and a new Class I standpipe is added with a 500 gpm demand at the top. The engineer proposes to keep the 500 gpm pump and "let the city main make up the difference." The city main test shows 65 psi static and 40 psi at 1,500 gpm. The pump is 15 years old and its curve shows 500 gpm at 95 psi, 250 gpm at 105 psi, and churn at 115 psi. The contractor asks whether the existing pump can be retained with a new controller.
1. How should the combined sprinkler-plus-standpipe demand be determined, and does the 500 gpm pump suffice?
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For a combined system, the demand is not the simple sum of sprinkler and standpipe flows in all cases; the code allows the sprinkler demand plus the standpipe demand, but the standpipe portion is typically taken at the required flow for the most remote outlet (often 500 gpm for Class I at the top). Here the sprinkler demand is 700 gpm at 80 psi and the standpipe adds 500 gpm, so the combined flow is at least 1,200 gpm at the required pressure. A 500 gpm pump cannot deliver that; the city main at 1,500 gpm is down to 40 psi, which is below the 80 psi needed at the base. Retaining the 500 gpm pump is not defensible - the pump must be upsized or a second pump added, and the combined supply curve must be verified against the combined demand.
2. What does the pump's churn-to-rated relationship tell you, and what verification is required before approval?
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Churn at 115 psi vs. rated 95 psi is about 121% - within the typical 140% limit, so the pump itself is not over-pressurizing. But the pump's rated point (500 gpm at 95 psi) is below the sprinkler demand (700 gpm at 80 psi) and far below the combined demand. The required verification is a hydraulic calculation plotting the existing pump curve plus the city supply curve against the combined sprinkler + standpipe demand, with the code safety margin applied. The operating point must sit above the demand curve at the required pressure. Given the city residual of 40 psi at 1,500 gpm and the undersized pump, the calculation will fail; the engineer should propose a larger pump or a second pump and resubmit.