Existing Fire Pump and a New Mezzanine Demand That Lands Off the Curve
A distribution center has an existing 1,000 gpm fire pump rated at 90 psi serving an ESFR system. The tested curve reads: churn 102 psi at 0 gpm, 90 psi at 1,000 gpm, and 58.5 psi at 1,500 gpm. A 30,000 ft² mezzanine is being added, and the new hydraulic calculation puts the combined demand at 1,150 gpm at 96 psi at the pump discharge flange. Suction from the city main is 26 psi static and 18 psi at 1,150 gpm. The owner's engineer proposes to let the pump run out a little further on its curve rather than replace it, and points out that the pump passed its acceptance test when it was installed. The building department has asked for your review of the fire pump before it will approve the mezzanine permit.
1. Read the curve: what pressure does this pump actually produce at 1,150 gpm, and does it satisfy the demand?
Read the worked answer
Interpolate between the rated point (1,000 gpm at 90 psi) and the 150 percent point (1,500 gpm at 58.5 psi). The drop is 31.5 psi across 500 gpm, or 0.063 psi per gpm. At 1,150 gpm:
90 − (150 × 0.063) = 90 − 9.45 ≈ 80.5 psi
Treat that as an estimate only - read the manufacturer's plotted curve, since most fire pump curves are not perfectly straight and the true value may be a psi or two lower.
Demand at the discharge flange is 96 psi at 1,150 gpm. The pump delivers about 80 psi. Shortfall is roughly 15 psi. The system fails.
The engineer's argument is backwards. On a centrifugal curve, running out further means less pressure, not more. The demand point at 1,150 gpm is already past the point where this pump can hold its rated 90 psi, and it is about 50 psi lower than what the demand requires once the actual curve is honored. A past acceptance test proves only that the pump met its rated point on the day it was tested; it says nothing about a demand 150 gpm beyond rating.
One caution on the arithmetic: the 18 psi suction at 1,150 gpm is already reflected in the 96 psi demand figure. Do not add it again. What the curve represents is net pump pressure (discharge minus suction), so compare like with like.
2. If the owner replaces the pump, how would you size it and what acceptance criteria govern the selection?
Read the worked answer
Sizing. Choose a rated flow at or above the system demand of 1,150 gpm - so a 1,250 gpm or 1,500 gpm rating - so the demand point lands on the flat part of the curve rather than out near the runout. For pressure, the pump only has to add the difference between the demand and the suction: at 1,150 gpm the demand is 96 psi and the suction is 18 psi, so the required net pump pressure is about 78 psi at 1,150 gpm. A 1,250 gpm pump rated near 85–90 psi typically lands close to that, but the actual curve at 1,150 gpm governs, not the rating.
Criteria to check:
- 150 percent rule: at 150 percent of rated flow the pump must still deliver at least 65 percent of rated pressure. Note the existing pump sits exactly at that floor (58.5/90 = 65 percent), which is legal but leaves nothing to spare.
- Churn versus component rating: churn pressure plus maximum static suction must not exceed the 175 psi rating of the piping and components downstream. Here 102 psi churn plus 26 psi static suction is about 128 psi, so there is room - but do the check explicitly.
- Driver sizing: the driver must carry the brake horsepower at 150 percent of rated flow, using the applicable service factor.
- Suction: confirm NPSH available exceeds NPSH required across the full flow range with the city main at its worst case, not just today's static.
Before recommending replacement, price the cheaper alternative: upsizing the friction-heavy portions of the new mezzanine mains can recover several psi and may pull the demand back onto the existing curve. If it recovers the full 15 psi with margin, the existing pump stays.