Flow Test Marginal After Private Main and Seasonal Low
An existing 6-story office building has a wet-pipe system with a required demand at the base of the riser of 850 gpm at 62 psi, including hose allowance. The city conducted a flow test at a hydrant 600 ft from the building: static pressure 72 psi, residual pressure 50 psi at 1,050 gpm. Water enters the building through a private 8-inch main with a calculated friction loss of 18 psi at 850 gpm, then passes through a backflow preventer with a 7 psi loss at that flow. The riser base is 8 ft above the hydrant. The test was run in June; the water department warns that seasonal low pressure can reduce static pressure by about 8% in August. The owner wants to know whether the system is adequate without a fire pump and what to do next.
1. Calculate the available pressure at the base of the riser at 850 gpm and compare it with the demand.
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Use the standard flow-test relationship, Q ∝ (ΔP)^0.54, to estimate the hydrant pressure at 850 gpm. The drop from static at the test flow is 72 − 50 = 22 psi. At 850/1,050 = 0.8095 of the test flow, the pressure drop is 22 × (0.8095)^(1/0.54) ≈ 22 × 0.676 ≈ 14.9 psi. So the hydrant pressure at 850 gpm is about 72 − 14.9 = 57.1 psi. Subtract the private main loss (18 psi), backflow preventer loss (7 psi), and elevation (8 ft ≈ 3.5 psi): 57.1 − 18 − 7 − 3.5 ≈ 28.6 psi at the riser base. The demand is 62 psi, so the supply is short by about 33 psi at 850 gpm. Even before seasonal adjustment, the system is not adequate without a pump or a demand reduction.
2. How should the seasonal low-pressure warning affect the conclusion, and what must be verified before selecting a fire pump?
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The June test is not the controlling condition if August static pressure can drop by about 8%. Recalculate the supply curve using the lower static pressure and a correspondingly adjusted residual point; do not simply subtract 8% from the 28.6 psi result, because the flow-test curve changes. The seasonal case will likely be worse than the June case, so the pump selection should be based on the lowest expected supply. Before selecting a pump, verify the flow test location and hydrant coefficient, the actual private main friction loss, the backflow preventer model and approved loss at 850 gpm, the elevation datum, and whether the demand includes an appropriate safety margin. Also confirm the pump suction conditions, available city pressure at maximum day demand, and any standpipe or hose-stream requirements. If the shortfall remains about 33 psi plus a safety margin, a fire pump sized for the required flow and pressure is a reasonable solution; reducing the demand through hydraulic redesign may be another.
3. What acceptance and documentation steps follow if a fire pump is installed?
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The pump installation should be designed with the pump curve, suction supply, relief valve, sensing lines, and controller coordinated with the existing system. At acceptance, run a flow test at churn, 100% rated flow, and 150% rated flow, and record suction and discharge pressures, flow, voltage, and current. Compare the field curve to the certified curve and verify that the pump meets the system demand point. Confirm that the controller, alarms, and automatic transfer switch, if any, function correctly, and that the relief valve does not discharge excessively at churn. Update the hydraulic nameplate and as-built drawings, provide operating and maintenance manuals, and train the owner’s staff. The ITM program should then include weekly no-flow/churn checks where required, annual flow tests, and documentation of any deficiency. Notify the AHJ before final acceptance and retain test records.