Fire Pump Sizing Against a City Main With a Steep Curve
A new 5-story office building (Light Hazard, wet system) is supplied by a city main tested at 70 psi static and 52 psi residual at 1,000 gpm. The sprinkler demand is calculated at 850 gpm at 95 psi at the base of the riser, plus a 250 gpm hose allowance. The engineer selects a 750 gpm fire pump rated at 100 psi churn, 90 psi at 750 gpm, and 65 psi at 150 gpm overcapacity. The pump is set to start automatically on pressure drop. The contractor notes the pump's rated flow (750 gpm) is less than the sprinkler demand plus hose (1,100 gpm) and asks whether that is a problem.
1. Is a 750 gpm pump acceptable when the system demand plus hose is 1,100 gpm? Explain the rating logic.
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The pump does not have to equal the total demand including hose; it must be rated at or above the sprinkler demand (850 gpm here, so 750 gpm is undersized for the sprinkler demand alone and should be at least 1,000 gpm to cover 850 gpm with margin). The hose allowance is handled by the supply curve, not by pump rating alone. The correct check: at the pump's rated flow, the pump plus city supply must meet or exceed the system demand curve at the required pressure. A 750 gpm pump rated below the 850 gpm sprinkler demand is a red flag and should be upsized. The engineer should select a pump whose rated flow covers the sprinkler demand and verify the combined supply curve meets demand at the required pressure with the code safety margin.
2. Explain how the churn, rated, and overcapacity points combine with the city curve, and what the 65 psi at 150% tells you.
Read the worked answer
The pump curve must be relatively flat: churn pressure no more than about 140% of rated, and at 150% of rated flow the pressure should be at least 65% of rated. Here churn is 100 psi (about 111% of the 90 psi rated point - acceptable), and 65 psi at 150% is about 72% of rated - acceptable. But the real test is adding the city curve: at 1,100 gpm the city residual will have dropped well below 52 psi (extrapolating the 70/52 at 1,000 gpm curve). Combined with the pump at overcapacity, the net pressure at the base of the riser may fall short of the 95 psi demand. The engineer must plot pump + city supply vs. demand and confirm the operating point sits above demand with margin; the pump alone passing its curve tests does not prove the system works.