300 Level

301 - Chapter 3

Advanced Pump Topics – Suction Cavitation.

If you run a high head pump at too low a head height, with too little restriction, too little work to do, your pump may cavitate, which can lead to premature pump failure in a matter of months or even weeks. Pondwater contains dissolved air, just like champagne contains dissolved CO2. Although no bubbles are visible inside the unopened, pressurized bottle, when the pressure is released by popping the cork, the gas comes out of suspension, creating those thousands of bubbles that ‘the bubbly’ is nicknamed for. There is plenty of air dissolved in pondwater, enough to keep the fish and plants that extract it alive and well. That dissolved air can come out of suspension in exactly the same way that champagne bubbles, when the ambient pressure drops, which can happen at the inlet of a high head pump operated below its minimum head range. These pumps, which create pressures high enough to drive water dozens of feet into the air, create correspondingly low pressures at the pump inlet. The drop in pressure to near vacuum draws air bubbles out of the pondwater where they are immediately sucked into the volute. If there’s enough of a pressure drop, the water in the volute foams like just-opened champagne. This causes two problems for the pump. First, the motor is spinning the impeller in a foamy froth instead of a solid column of water, and the greatly lowered resistance causes the motor to overspeed. Just like a parked car with the engine racing, floored, with a brick on the accelerator, without enough work to do the pump motor can burn itself out in a matter of days or weeks. The other problem occurs at the other side of the volute, where the impeller creates those pressures that send water up, up and away. The bubbles formed in the near-vacuum at the inlet almost instantaneously find themselves under high pressure, and IMPLODE SUPERSONICALLY, damaging volute and impeller. Though the bubbles are small, some even microscopic, the energy released will start to eat away at the impeller, eventually removing the vanes as if they were sandblasted! The solution is simple: just restrict the flow out of the pump with a valve until there’s enough backpressure to bring the pump back up to its Best Efficiency range. If you’re not sure how far to shut the valve, just use your ears. Cavitation sounds like gravel getting chewed up in the pump. If you slowly close a valve on the output side of the pump until the noise lessens and finally stops, you will have found the minimum operating head of the pump, and the pump will thank you for it by lasting as long as it was designed to.