Diesel Fire Pump and Suction Tank Sizing
A new 1,000 gpm diesel fire pump is rated at 60 psi with a churn pressure of 78 psi. It takes suction from a 90,000-gallon ground-level concrete tank through 15 ft of 8-inch suction pipe with two long-radius elbows and a vortex plate. The sprinkler demand is 1,150 gpm at 52 psi at the pump discharge flange. The tank is filled by a 2-inch line that delivers 150 gpm. The contractor selected the 90,000-gallon tank by multiplying 1,000 gpm by 90 minutes and says the exact match is acceptable because the pump is rated at 1,000 gpm. The AHJ is concerned about the pump operating beyond its rated flow, the tank net usable volume, and the suction arrangement. A preliminary acceptance test showed the pump running at 1,050 gpm with 54 psi discharge and no cavitation.
1. Evaluate the 90,000-gallon tank sizing. What minimum gross capacity would you require?
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1,000 gpm times 90 minutes = 90,000 gallons. That is the theoretical net fire-flow volume. The proposed tank is 90,000 gallons gross, so it has no margin for vortexing, sediment, ice, or the unusable volume below the vortex plate. The net usable volume will be less than 90,000 gallons, so the tank does not meet the duration requirement. Also, the demand is 1,150 gpm, and if the pump is allowed to run beyond rated flow, the drawdown rate is higher. The tank should be sized on the net usable volume at the actual maximum flow for the required duration, with a margin. A practical minimum is 95,000 to 100,000 gallons gross, subject to the manufacturer vortex plate and submergence requirements. The exact-match argument is not defensible.
2. Evaluate the churn pressure and the pump operating point against the 1,150 gpm demand.
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Churn pressure 78 psi is 130 percent of the 60 psi rating. For a centrifugal fire pump, churn pressure is generally limited to 140 percent of rated pressure, so 130 percent is acceptable, but the shutoff pressure must not exceed the working pressure of the system components. The pump is running at 1,050 gpm and 54 psi, which is 105 percent of rated flow; that is within the usual 150 percent overload check, but the sprinkler demand is 1,150 gpm at 52 psi, and the pump cannot be assumed to deliver 1,150 gpm at 52 psi just because it did 1,050 gpm at 54 psi. The contractor must plot the pump curve and the system demand curve. If the demand point falls beyond the pump curve, the pump is undersized or the demand must be reduced. The 54 psi at 1,050 gpm is not proof of the 1,150 gpm point.
3. Evaluate the suction arrangement and the 2-inch refill line.
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An 8-inch suction at 1,000 gpm has a velocity of about 6.4 ft/s, well below the usual 15 ft/s limit, so the pipe size is not the problem. The concern is the arrangement: 15 ft of suction pipe with two long-radius elbows and a vortex plate can create swirl and air entrainment if the submergence is inadequate. Check the vortex plate size, the minimum submergence, and the net positive suction head available against the pump NPSH required, especially at 1,150 gpm. The 2-inch refill line at 150 gpm takes 10 hours to refill 90,000 gallons; that is not a fire-flow issue, but the AHJ may require a refill rate that restores the tank in a reasonable time. The contractor should submit suction and NPSH calculations and confirm the tank usable volume.