Static Pressure Is Not Available Pressure
A 180,000 ft² warehouse is protected as Ordinary Hazard Group 2 with a wet system: 0.20 gpm/ft² over a 1,500 ft² remote area plus a 250 gpm hose allowance. The layout is complete, and the calculated demand at the base of the riser is 550 gpm at 68 psi, with the highest sprinkler 45 ft above that point. The only flow test available was run at a hydrant on the same 8 in main, 500 ft of pipe upstream of the riser: static 68 psi, residual 52 psi at 1,100 gpm. The contractor's estimator says, "Static pressure is 68 psi and our demand is 68 psi, so we are right at the line and the system passes." The building official requires a 10 psi safety margin between available and required pressure at the base of the riser.
1. Estimate the pressure available at the base of the riser at 550 gpm and compare it with the demand. State your assumptions.
Read the worked answer
The static reading is not the available pressure; the pressure that matters is the residual at the demand flow.
- Characterize the supply from the test: pressure drop at the test point = 68 − 52 = 16 psi at 1,100 gpm. Using Q = K√ΔP, K = 1,100 ÷ √16 = 275.
- At 550 gpm: ΔP = (550 ÷ 275)² = 4.0 psi, so residual at the test hydrant = 68 − 4.0 = 64 psi.
- Allow for the 500 ft of 8 in main between the hydrant and the riser. Using Hazen-Williams (C = 140) at 550 gpm, the loss is roughly 1.1 psi (fittings and valves will add a little more).
- Available at the base of the riser ≈ 64 − 1.1 = about 63 psi.
Demand is 68 psi. The system is short by roughly 5 psi, and about 15 psi short of the official's 10 psi margin. Assumptions to state: the flow test curves the supply as Q = K√ΔP (a screening approximation), the test is representative of the current supply, and the main loss is estimated rather than calculated.
2. The contractor proposes replacing the 500 ft of 8 in main with 10 in pipe to solve the problem. Evaluate whether that is the right fix.
Read the worked answer
It is not. Run the numbers before agreeing to a costly main replacement.
Hazen-Williams loss varies roughly as d^(−4.87). Going from 8 in to 10 in multiplies the friction by (8 ÷ 10)^4.87 ≈ 0.34. So the loss drops from about 1.1 psi to about 0.4 psi - a saving of less than 1 psi. The system is about 5 psi short of demand and 15 psi short of the required margin, so the upsize does essentially nothing.
The shortfall lives in the city main and the service tap, not in the 500 ft of pipe. Realistic options: (a) confirm with a flow test taken at the actual tap, flowing in the direction of supply; (b) add a fire pump sized to make up the 15 psi at 550 gpm (and check suction pressure at that flow); (c) add storage plus a pump if the city main cannot be relied on; (d) connect to a stronger main if one is available. The density cannot be reduced - 0.20 gpm/ft² is the code minimum for OH2.
Recommend the pump or a verified tap capacity, and note the decision on the plan review comment so it is not relitigated in the field.
3. A second flow test at a hydrant across the street shows static 85 psi and residual 70 psi at 900 gpm. The contractor wants to design from that test instead. May you use it?
Read the worked answer
No - not as the design basis. A flow test is only representative of the supply feeding the system if it is taken on the same main that serves the system, with flow in the direction the system will draw, and the results are then corrected for the pipe between the test point and the riser. A hydrant across the street can be fed by a different main, a different valve alignment, or a different path through the grid, so its 85 psi static says nothing reliable about the pressure at this riser.
Appropriate steps:
- Use the test on the serving main as the basis, corrected for the 500 ft of 8 in main.
- Have the contractor request a fresh test at a hydrant downstream of the tap, flowing the hydrants further downstream, and confirm the test is recent and reflects current seasonal and industrial demand.
- Use the across-the-street test only as a cross-check to see whether the grid has excess capacity worth chasing - not as a substitute.
If the current test is old or was taken in a high-demand season, note that in the record; supply capacity can only be improved, never assumed.