Relay Pumping: Discharge Pressure Is Right, Suction Is Failing
A relay is set up from a pond to an attack pumper 1,400 ft away. The source pumper drafts through 20 ft of 4-inch hard suction with a 12-ft lift. It discharges through 1,400 ft of 5-inch LDH to the attack pumper, which is 60 ft higher. The target flow is 1,000 gpm. The source pumper is running at 1,250 rpm and 165 psi engine pressure. The attack pumper's intake gauge reads 10 psi and fluctuates; the source pumper is making a cavitation rumble and its intake gauge is dropping. The attack pumper operator radios that he cannot hold 1,000 gpm. The relay has one source pumper, one attack pumper, and no booster pumper. A second draft site is 400 ft away but requires 300 ft of additional LDH.
1. Calculate the source pumper discharge pressure needed at 1,000 gpm, using 8 psi per 100 ft of 5-inch friction loss and 20 psi residual at the attack pumper. Compare with the 165 psi being run and interpret the result.
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Friction loss: 1,400 ft / 100 = 14 lengths; 14 x 8 psi = 112 psi. Elevation: 60 ft x 0.434 psi/ft = 26 psi. Residual at the attack pumper: 20 psi. Required source discharge pressure = 112 + 26 + 20 = 158 psi. The source pumper is running 165 psi, only 7 psi above the calculated requirement, so discharge pressure is not the main problem. The interpretation is important: the source pumper is not failing because it lacks engine pressure; it is failing on the suction side, and adding throttle will only worsen the cavitation. The attack pumper's fluctuating 10-psi intake confirms that the relay is not receiving the calculated pressure, because the source pump cannot produce its rated flow while cavitating. The correct response is to fix the suction condition or reduce flow, not to increase the source pump pressure.
2. Diagnose the cavitation and list corrective actions in priority order.
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At 1,000 gpm, 4-inch suction hose carries about 2.23 cfs through an area of about 0.0873 ft2, or roughly 25 ft/s, well above the usual 15 ft/s suction-velocity limit, and the pump is also lifting 12 ft. That combination gives insufficient net positive suction head, so the pump cavitates before it can deliver 1,000 gpm. Corrective actions in priority order: 1) reduce flow immediately to about 700-800 gpm and tell the IC the relay cannot safely support 1,000 gpm; 2) change to 6-inch suction or use two 4-inch suction lines if available; 3) reposition the source pumper closer to the water to reduce lift and suction length; 4) add a booster pumper at the pond or use the second draft site with additional LDH; 5) continue monitoring the intake gauge and pump sound. Do not try to solve cavitation by increasing engine speed, because that increases the demand for water the suction cannot supply.
3. What relay communication and monitoring plan do you establish?
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Before charging the line, I would conduct a radio check between source and attack pumpers and assign a dedicated tactical channel. I would agree on a start and stop signal, and on a standard pressure report: source discharge pressure, source intake condition, and attack intake pressure. The attack pumper operator should report any intake fluctuation immediately rather than waiting for a scheduled check. I would set the source relief valve or pressure governor to protect the discharge line, assign a gauge watcher at the source pump, and post a spotter at any hose crossing. I would schedule pressure reports every 5 minutes and after any throttle change, and I would have a planned shutdown sequence: attack pumper notifies source, source reduces throttle slowly, then the line is drained or left charged as directed. Clear communication is what turns a marginal relay into a controlled one instead of a burst line or a pump run dry.