Fire Pumps and Water Storage
Read the pump curve
A fire pump is selected for a rated flow and pressure, and its curve must satisfy two checkpoints: at 150 percent of rated flow the pump must still deliver at least 65 percent of rated head, and at shutoff (churn) it must not exceed 140 percent of rated head. Those two numbers are the most frequently tested facts in this domain. The pump must be listed for fire service, have a controller matched to the driver, and be arranged so that it starts automatically on pressure drop.
Sequencing with the jockey pump
A small pressure-maintenance (jockey) pump holds system pressure and prevents the main pump from starting on every small fluctuation. Typical settings: jockey starts first as pressure drops, then the fire pump starts at a lower pressure, and the jockey stops at a pressure above the fire pump's churn point so it does not fight the main pump. Test headers, relief valves, and suction piping must be sized so the pump can reach 150 percent of rated flow without cavitation.
Water storage
Stored water must cover the design demand for the required duration, which varies with hazard - commonly 30 minutes for light hazard, 60 to 90 for ordinary, and 90 to 120 for extra hazard, and longer where the code requires. The supply must also provide the hose allowance and any domestic demand that would occur simultaneously.
Traps
- Reversing the 150 percent and 140 percent rules.
- Setting the jockey pump to start below the main pump.
- Ignoring churn pressure when sizing relief valves and PRVs.
Worked example
A 1,000 gpm, 100 psi pump must deliver at least 1,500 gpm at 65 psi or better, and must not exceed 140 psi at shutoff. If churn measures 150 psi, the curve or the relief setting is wrong.