The Overloaded Standpipe
A three-story office building has a standpipe system fed by a fire department connection (FDC) with two 2.5-inch inlets. A pumper is supplying the FDC at 150 psi at the intake. The building's rated standpipe demand is 500 gpm at 65 psi at the topmost outlet, three floors up (about 35 ft of elevation). Two 1.75-inch handlines are operating from a second-floor outlet, each flowing 150 gpm. The pump operator notices the intake gauge holds steady but the discharge gauge reads lower than expected, and a crew reports weak streams on the second floor. A second pumper is available at a hydrant 300 ft away.
1. Identify the most likely cause of the weak streams and state whether adding a second pumper at the FDC is the right first move.
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Likely cause: The system is being asked for more pressure than the supply is delivering at the elevation, or there is a partial obstruction/air in the line. The intake gauge is steady, so the hydrant supply is not the immediate problem; the discharge gauge dropping while demand stays constant points to insufficient pressure at the outlet (elevation loss plus friction loss in the standpipe and hose). Rough check: 35 ft of elevation costs about 0.434 psi/ft × 35 ≈ 15 psi of the outlet pressure just to overcome height, and the rated outlet pressure is 65 psi, so the pumper must overcome both.
First move: Do not assume a second pumper fixes it. Confirm the FDC connections are tight and unobstructed, verify the pump is in pressure mode and not cavitating, and check that both inlets are being used. A second pumper helps only if the limit is the supply volume/pressure from the first pumper - which the steady intake gauge argues against. Diagnose before adding apparatus.
2. The crew wants to add a third handline from the same outlet. Using the numbers given, explain the pressure consequence and give a defensible recommendation.
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Each 1.75-inch line at 150 gpm is already drawing on the same standpipe. Adding a third line raises total flow from 300 to 450 gpm through the same standpipe riser. Friction loss rises roughly with the square of flow: tripling flow through a fixed pipe raises friction loss by about nine times, and even going from 300 to 450 gpm (a 1.5× increase) raises it by roughly (1.5)² = 2.25× in that section. That extra loss comes straight out of the pressure reaching the nozzles, so streams get weaker, not stronger.
Recommendation: Hold at two lines until supply and pressure are verified. If more flow is genuinely needed, secure a second water source and a second pumper feeding the FDC (both inlets) rather than stacking lines on an already-marginal system. Document the gauge readings so the decision is traceable.
3. The building engineer says the standpipe was 'just tested last year,' so obstruction is impossible. How should the operator weigh that claim?
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Treat it as useful but not conclusive. A passing test last year does not rule out: a partially closed valve, debris or a dropped cap in an inlet, a hose coupling gasket blocking flow, air in the line, or a change in the building's plumbing since the test. The operator's job is to weigh the physical evidence (steady intake, falling discharge, weak streams) against the claim. The evidence points to a pressure/flow problem at the outlet, and the cheapest checks - valve position, coupling integrity, both inlets used, pump mode - come before accepting or rejecting the engineer's assurance. Respect the engineer's information but verify with gauges, not assurances.