Water Supply Analysis, Fire Pumps, and Standpipe Systems
This domain tests whether you can prove the supply side of the demand: available flow at the required pressure, how a fire pump changes the curve, and how standpipe requirements add to the system demand.
Water supply analysis
A flow test records static pressure, residual pressure, and flow. The available flow at a chosen residual pressure follows the 1.85 exponent relationship: Q₂ = Q₁ × [(Pstatic − P₂)/(Pstatic − P₁)]^0.54. Plot the supply curve against the system demand curve. The supply must exceed the demand at the required residual pressure, including hose allowance. A supply that is adequate at 20 psi but not at the sprinkler demand pressure is not adequate.
Fire pumps
A fire pump adds pressure but does not create water. Its rated point is 100 percent flow at 100 percent rated head; at 150 percent flow it must produce at least 65 percent of rated head. Churn is no-flow operation. The pump must have a adequate suction supply, and the driver and controller must be listed for fire service. A pump that is oversized can overpressure the system; a pump that is undersized cannot meet the demand.
Standpipes
NFPA 14 defines Class I (2.5-in. outlets for fire department use), Class II (1.5-in. outlets for occupant use), and Class III (both). Systems may be automatic or manual, wet or dry. Minimum residual pressures are commonly 100 psi at 2.5-in. outlets and 65 psi at 1.5-in. outlets, with a maximum outlet pressure of 175 psi unless approved otherwise. Standpipe demand is added to the sprinkler demand at the supply point.
Worked example
A flow test gives 70 psi static and 50 psi residual at 1,000 gpm. Estimate available flow at 20 psi: Q = 1,000 × [(70 − 20)/(70 − 50)]^0.54 = 1,000 × (2.5)^0.54 = 1,640 gpm. Compare 1,640 gpm at 20 psi with the combined sprinkler plus standpipe demand. If the demand requires more pressure than 20 psi, evaluate at that pressure instead.
Traps that cost marks
Adding pump pressure to static pressure incorrectly. Treating churn pressure as the design point. Forgetting that standpipe demand is additive. Using a linear interpolation instead of the 1.85 curve. Ignoring the 175 psi maximum outlet pressure.