Fire Pumps and Water Supply: Demand, Curves, and Acceptance
Core concepts
This domain is about proving the water supply can meet the sprinkler demand. Start with the supply curve (static pressure, residual pressure at a given flow, and the shape of the curve) and overlay the system demand point. The pump must add enough pressure to move the demand point onto or below the available supply.
Know the pump curve: rated flow, rated pressure, shutoff (churn) pressure, and 150% overload flow. Standard rules of thumb: shutoff should not exceed about 140% of rated pressure, and at 150% of rated flow the pump should still deliver at least 65% of rated pressure. Understand net positive suction head and why suction conditions matter, plus the roles of the jockey pump, main pump, and controller in maintaining pressure without nuisance cycling.
Traps
- Selecting a pump on static pressure alone; questions often hide a steep supply curve that collapses at the required flow.
- Confusing churn (zero flow) pressure with rated pressure.
- Forgetting that the pump must be tested at 150% of rated flow during acceptance.
- Ignoring suction-side losses, which reduce available NPSH and can cause cavitation.
- Mixing up the jockey pump setpoint (higher) with the main pump start setpoint (lower).
Worked example
A supply gives 60 psi static and 45 psi residual at 500 gpm. The sprinkler demand is 500 gpm at 70 psi. The pump must add at least 70 − 45 = 25 psi at 500 gpm, plus a safety margin. Verify the chosen pump's curve delivers that pressure at 500 gpm and check its 150% point against the rule of thumb before accepting it.