Pump Pressure Drop on the Second Floor
A two-story training building has a booster pump rated at 80 psi discharge feeding a standpipe. On the first floor, a pressure gauge at the outlet reads 78 psi. On the second floor, 24 feet above the first-floor outlet, a gauge on the same standpipe reads 67 psi. The crew suspects a leak or a partially closed valve between floors. A note in the maintenance log says the pump was serviced last month and the discharge pressure was set to 80 psi. No fittings were changed. The standpipe is filled with water. The crew has a calibrated gauge, a valve wrench, and a way to isolate each floor.
1. Before tearing into the pipe, should the crew treat the 11 psi difference as evidence of a leak? Explain using the expected pressure change from elevation.
Read the worked answer
No. Water pressure changes with elevation at about 0.433 psi per foot of rise (1 psi per 2.31 ft).
- Elevation difference: 24 ft
- Expected drop: 24 ft x 0.433 psi/ft ≈ 10.4 psi
The gauge on the second floor reads 67 psi against 78 psi below, a drop of 11 psi. That is within about 0.6 psi of the elevation-expected drop, which is well inside gauge and rounding error.
So the reading is consistent with a static column of water, not a leak. A leak would typically show a larger or growing discrepancy, and often a flow indication or wet area. The crew should first verify the gauges read the same when both are at the same elevation, then check for a leak only if the discrepancy exceeds what elevation explains.
2. If the second-floor gauge instead read 60 psi, what would that suggest, and what is the first isolation test you would run?
Read the worked answer
A reading of 60 psi is about 18 psi below the first floor, while elevation explains only about 10.4 psi. The extra ~7.6 psi loss points to a restriction or leak between the floors.
First test: isolate the section between the two floors using the valves at each floor, then pressurize that isolated segment and watch the gauge. A steady pressure that holds indicates a closed valve or blockage (no flow path); a pressure that bleeds down indicates a leak. If the segment holds pressure but the second-floor gauge still reads low when the isolation valve is open, suspect a partially closed valve or obstruction rather than a leak.