How to check standpipe flow and pressure
NFPA 14 sets the performance a standpipe system has to deliver at the hydraulically most remote hose connection. For a Class I or Class III system with 2 1/2 in. outlets that means a minimum residual pressure of 100 psi at a minimum flow of 500 gpm through the most remote riser, with 250 gpm added for each additional riser. Class II systems serving 1 1/2 in. hose stations are held to 100 gpm at 65 psi instead.
Discharge from a hose nozzle follows the classic fire flow formula Q = 29.83 × C × d² × √P, with Q in gpm, d the tip diameter in inches, P the nozzle pressure in psi and C the discharge coefficient. A 1 1/8 in. smooth bore tip at 100 psi delivers roughly 366 gpm, which is why hand lines are rarely flowed at full standpipe pressure. The same formula with C values of 0.90, 0.80 or 0.70 is what fire flow tests use at hydrant outlets.
Total system demand is capped at 1,000 gpm in fully sprinklered buildings and 1,250 gpm where sprinklers are absent, and the water supply has to sustain that demand for 30 minutes. Required pump head adds the static lift to the topmost outlet, the pipe and hose friction losses, and the 100 psi residual expressed as head, which is about 231 ft. Where outlet pressure would exceed 175 psi, pressure regulating devices are required. Confirm the final hydraulic calculation and device selection with a licensed fire protection engineer.
Frequently asked questions
That is the 100 psi minimum residual pressure converted to feet of head at 2.31 ft per psi. Without it the remote outlet cannot reach the pressure NFPA 14 requires.
Lower floors on tall risers routinely exceed it. NFPA 14 requires a pressure regulating device at those hose connections so the nozzle reaction stays controllable.