Water supply pressure failure in a 5-storey building
A 5-storey residential building is connected to the city water main at a guaranteed pressure of 0.18 MPa at the connection point. The top floor fixtures are 14 m above the connection. The internal network is a dead-end scheme with a booster pump installed at the basement inlet, rated for 0.3 MPa head. After commissioning, residents on the top two floors report weak flow in the morning and evening peaks, and the pump cycles on and off rapidly. A pressure gauge at the top floor shows 0.05 MPa at peak. The building has no storage tank. The contractor proposes increasing the pump head to 0.45 MPa.
1. Diagnose why the top floors have weak flow and why the pump cycles.
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The guaranteed main pressure is 0.18 MPa, which corresponds to about 18 m of water column. The top-floor fixtures are 14 m above the connection, so the static residual available at the top floor before losses is only about 4 m, roughly 0.04 MPa. Friction and local losses in the riser and fittings consume most of that, which is consistent with the measured 0.05 MPa at peak.
The pump cycling is a separate symptom. A booster pump without a storage tank and without a properly sized pneumatic vessel responds to small pressure changes at the inlet. At peak draw-off the inlet pressure drops, the pump starts, the flow demand briefly exceeds the pump capacity, pressure falls again, and the pump stops and restarts. Rapid cycling damages the motor and does not solve the pressure deficit.
Increasing the pump head alone does not fix the root cause: the dead-end scheme and the absence of storage mean the top floors depend entirely on main pressure plus pump head, and the pump may cavitate if the inlet pressure is insufficient.
2. Recommend a technically sound solution and explain why it is preferable to raising the pump head.
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The sound solution is a storage-and-boost scheme: a storage tank or a break tank at the basement or an intermediate floor, filled from the main, plus a booster set with a pneumatic vessel and controls that maintain pressure independently of main fluctuations. This decouples the internal network from the variable main pressure and eliminates cycling.
Alternatively, a zone scheme with a rooftop or intermediate tank can serve the top floors by gravity. The choice depends on the required pressure at fixtures, the building height, and the space available.
Raising the pump head to 0.45 MPa is inferior because it increases pressure on the lower floors, raising the risk of excessive pressure at the lowest fixtures and of noise and water hammer, while the top floors still suffer whenever the main pressure falls. It also does not address cycling. The design should be recalculated from the guaranteed main pressure, the building height, and the peak demand, with the pump selected to operate in its efficient range against a controlled suction condition.