Town Main Capacity and Fire Pump Suction
A proposed eight-storey office building has a wet-pipe sprinkler system with a design demand of 2,600 L/min at 4.2 bar at the base of the riser. The only supply is a 150 mm town main. A flow test gives 4.5 bar static and 3.1 bar residual at 2,000 L/min. The contractor proposes a 2,600 L/min fire pump taking suction directly from the town main, with no storage tank, arguing that the pump will 'boost' the residual pressure. The building has no other fire water source. The AHJ requires a reliable supply for the pump at 150% of rated flow. The contractor's pump curve shows a rated point of 2,600 L/min at 3.4 bar and a churn pressure of 4.5 bar. You are asked to review the suction arrangement before the pump order is placed.
1. Compute the town main pressure at 2,600 L/min and at 3,900 L/min. Use the 1.85 exponent and show the calculation.
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Use the standard flow-test relationship:
P = P_residual - (Q/Q_test)^1.85 x (P_static - P_residual)
At 2,600 L/min: P = 3.1 - (2,600/2,000)^1.85 x (4.5 - 3.1) P = 3.1 - (1.30)^1.85 x 1.4 (1.30)^1.85 = 1.62 P = 3.1 - 2.27 = 0.83 bar
At 3,900 L/min (150% of 2,600 L/min): P = 3.1 - (3,900/2,000)^1.85 x 1.4 (1.95)^1.85 = 3.44 P = 3.1 - 4.82 = -1.72 bar
The result is a negative pressure, which is physically impossible for the town main. The main cannot supply 3,900 L/min. At 2,600 L/min the main still has only about 0.83 bar residual, which is a thin margin and is not the required 150% flow condition.
2. Can the fire pump take suction directly from the main? Explain using the 150% flow requirement.
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No. A fire pump must have a suction supply capable of delivering at least 150% of its rated flow without causing a negative pressure at the pump suction. The town main cannot do that.
At 3,900 L/min the flow-test calculation gives a negative pressure, meaning the main would be drawn below atmospheric pressure. The pump would cavitate, lose performance, and may be damaged. Even at the rated flow of 2,600 L/min, the main residual is only about 0.83 bar, leaving little margin for suction losses, and the pump would be operating near the limit of the supply.
The contractor has confused 'boosting pressure' with 'creating water.' A fire pump adds pressure; it does not add flow. The main must be able to pass the required flow. The correct arrangement is a dedicated break tank or other reliable suction supply sized for the pump demand and duration, with the pump taking suction from the tank. The town main may be used to refill the tank if the refill rate and reliability are acceptable.
3. What sizing and acceptance information should be required for the break tank and pump?
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The tank should be sized for the pump rated flow multiplied by the required duration from the adopted building code or AHJ, plus any domestic or standby demand that cannot be isolated. The design should show the tank capacity, refill rate, overflow, vortex plate, access, and low-water protection.
The pump submittal should show the rated flow and head, churn pressure, 100% and 150% performance points, suction and discharge gauge locations, relief valve, and controller. Acceptance should include a field flow test at churn, 100%, and 150% of rated flow, with suction pressure recorded, and verification that the tank refill and low-water alarm work. The AHJ should see the hydraulic calculation using the actual pump curve, not the rated point alone. If the pump cannot meet 150% without negative suction pressure, the installation should not be accepted.