Class I Standpipe Flow Test: A PRV That Is Fine Static and Short Under Flow
You are witnessing the acceptance flow test on a new 12-story Class I standpipe in a high-rise. Each floor landing has a field-adjustable pressure reducing valve set to 100 psi, and the system is supplied by the city main at 68 psi through a fire pump rated 750 gpm at 110 psi. At the topmost 2½ in. outlet the static pressure reads 105 psi. You then flow that outlet at 250 gpm using a playpipe with a 1⅛ in. smooth bore tip, coefficient 0.97, and a pitot reading of 46.6 psi. The residual pressure at the same outlet gauge is 62 psi while flowing. The contractor says the PRV is "set at 100 psi, so it is fine," and points out that the static check never showed a problem. A 1½ in. hose station is also installed on the top floor downstream of the same PRV.
1. Verify the flow and evaluate the result against what a Class I standpipe must deliver at the topmost outlet.
Read the worked answer
First confirm the flow: Q = 29.83 × C × d² × √P = 29.83 × 0.97 × (1.125)² × √46.6, which gives about 250 gpm. The test is therefore flowing the intended amount, so the pressure reading is meaningful.
A Class I system must deliver 100 psi residual at the hydraulically most remote 2½ in. outlet while flowing 250 gpm, unless the design and AHJ specifically allow otherwise. The measured residual is 62 psi, about 38 psi short. The 105 psi static reading proves nothing about performance: a pressure reducing valve is set and must be verified under flow, because its outlet pressure falls as flow increases through its own port and trim. The contractor has demonstrated only that the valve holds pressure with no demand. The acceptance test fails, and the system should not be signed off until the residual at the topmost outlet meets the design requirement.
2. How would you determine whether the shortfall is the PRV, the pump, or the piping design?
Read the worked answer
Measure pressure on both sides of the PRV while the outlet is flowing at 250 gpm. That one reading splits the diagnosis:
- If upstream of the PRV is high, say 140 psi or more, while downstream is 62 psi, the PRV is dropping far more than it should. Causes include an undersized valve for 250 gpm, a partially clogged integral strainer, a valve installed backward, a direct-acting valve where a pilot-operated or properly sized valve is needed, or a valve adjusted statically and never verified under flow.
- If upstream of the PRV also sags toward 70 psi, the PRV is not the problem; the pump, the riser size, or the hydraulic calculation is.
Also check the gauge itself, confirm the hose valve has no separate restricting device, and compare the measured pump discharge with the rated curve at this flow. Document both gauge readings; a single downstream number cannot support a conclusion about which component is at fault.
3. What remediation and re-testing would you require, and what do you tell the contractor about static-only verification?
Read the worked answer
The PRV must be re-selected or replaced with a listed standpipe PRV sized for the design flow at that location, then adjusted under flow according to the manufacturer's instructions. If the measurements show the supply or riser is the constraint, the fix is hydraulic - larger piping, a different pump selection, or a revised calculation submitted to the AHJ - not a valve adjustment.
After correction, repeat the flow test at the topmost 2½ in. outlet at 250 gpm and record the residual. Because the 1½ in. station shares the same PRV, verify it too at its own required flow and pressure, since a valve changed to satisfy the 2½ in. outlet can leave the smaller station over- or under-pressured. Tell the contractor plainly that a static set point is a starting adjustment, not proof of performance; PRVs must be set and verified with water flowing, and the acceptance record must show the flowing pressure at the required flow.