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How to Detect Cavitation

Pump Cavitation on an Impeller
Excerpt from the September 2021 Pumps & Systems Article by Asma Motlani

Learn the symptoms of cavitation and solutions to correct it.

Pumps are at the heart of most industrial processes and the second most common machine in the world (after the electric motor). Because they are so common, pumps are often overlooked as a potential source of improved productivity or a cause of excess costs if not operated properly.

As with any machine, some problems can occur. A common problem in pumping systems is pump cavitation. Pump cavitation causes a number of issues including excess noise, vibration and energy usage, not to mention serious damage to the pump itself.

What causes cavitation?

Imagine pinching a water hose and the water moves faster coming out but it is aerated and spread out. That is what happens with pump cavitation. It is a physical phenomenon that occurs when the pressure of the liquid incoming becomes lower than the vapor pressure of the liquid. A reduction in pressure is typically caused by increased speed of the fluid. When fluid pressure on the trailing side of the impeller blade (opposite the pump intake) falls below the vaporization point of the fluid, vapor bubbles begin to form. As the bubbles/cavities travel to the discharge side of the pump, moving to a high pressure area, the cavities implode. The imploding or collapsing of these bubbles trigger intense shockwaves inside the pump, causing damage to the impeller, vibration and excess noise.

These shock waves can cause mechanical damage to the impeller and pump, only increasing in severity over time and leading to potential pump failure. Factors that impact the degree of compression for the vapor bubbles are the speed and shape of the impeller.

When dealing with liquids of higher temperatures, the risk of this occurrence is increased due to increased vapor pressure.

What’s the difference between deadhead & cavitation?

Pump cavitation and dead head are similar concepts but far from the same.

When a pump operates with no flow through the pump due to a closed discharge valve or line blockage, a dead head has occurred. The pump recirculates the same water, causing water temperature to continually rise. If the pump continues to run in a dead-headed condition for too long, excessive heating can damage expensive seals and reduce the life of the pump.

Dead heading in a centrifugal pump can lead to explosions due to the energy being put into the liquid in the pump. Hydraulic overpressure and possible chemical reactions in the pump can also be caused by the overexertion of pressure. The same results can be caused by running the pump dry for an extended period, which can lead to cavitation.

Dead head means the outflow valve is open so the pump continues to circulate the same liquid over and over, which can damage the pump motor because the liquid can get too hot. The low power protection will work to detect this condition and trip to protect the motor.

Cavitation is similar in that the low power protection will detect it and trip and protect the motor as well. But this condition means that the pump is running dry and no liquid is there to pump. That too will cause the motor to overheat.

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