High-Rise Domestic Booster With Pressure Fluctuation Complaints
A 14-story residential tower is served by a duplex constant-speed booster pump set drawing from a 2,000-gallon break tank fed by the municipal main. Static inlet pressure at the tank fill is 42 psi. The design calls for 55 psi minimum at the top floor fixture, which sits 138 ft above the pump discharge. Friction loss through the riser, meter, and fittings at design flow of 180 gpm is calculated at 24 psi. The pump set has a 20 psi pressure differential between cut-in and cut-out. Tenants on floors 9 through 14 report pressure swings and intermittent low flow during morning peak. A downstream pressure-reducing valve on the lower zone is set at 50 psi. The contractor proposes simply raising the pump cut-out pressure by 15 psi.
1. Determine whether the proposed pump adjustment alone can satisfy the top-floor minimum, and identify what else must be evaluated.
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Compute the required discharge pressure at the pump: 55 psi residual at the top fixture + 24 psi friction + static head of 138 ft (138 / 2.31 = 59.7 psi) = 138.7 psi. This is the minimum pump discharge at design flow. The 20 psi cut-in/cut-out band means the low end of the cycle must still deliver 138.7 psi; the cut-in setpoint governs, not the cut-out. Raising cut-out by 15 psi raises the whole band only if cut-in is adjusted too - the contractor's proposal as stated does not guarantee the low end. Also verify the break tank cannot run dry at 180 gpm peak draw against the fill rate, confirm the PRV on the lower zone is not starving upper floors by shifting flow, and check that fixtures on the top floor are not served through an undersized branch. The pressure swing symptom points to a constant-speed pump cycling against a variable demand, so a variable-speed control or a properly sized hydropneumatic tank should be evaluated rather than a setpoint bump.
2. The break tank is fed through a 1.5-inch fill line with an inlet pressure of 42 psi. Assess whether tank refill capacity is adequate during peak demand.
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At 42 psi inlet, a 1.5-inch line with a typical atmospheric discharge into a tank will deliver roughly 80 to 100 gpm depending on the valve and fittings - well below the 180 gpm peak draw. That means the tank is being drawn down faster than it refills during morning peak, so the pump may lose suction or the level control may cycle the pumps off. The correct approach is to size the fill line and fill valve for the peak demand plus a margin, or to provide a storage volume sized for the peak draw period. Confirm the actual fill valve capacity (a pressure-reducing fill valve is typically rated in gpm at a given inlet pressure), verify the overflow is sized for the maximum possible fill rate, and check that the tank has an air gap or approved backflow protection at the fill connection. A larger fill line or a storage buffer sized to the peak duration is the fix, not a pump setpoint change.