What is cavitation in my pressure washer?

Monday, 4 November 2019  |  Ben Marriott

In 1754 Swiss mathematician Leonhard Euler postulated that cavitation was a physical phenomenon which occurs when the oxygen suspended in a liquid implodes, in our application this can be felt as vibration in the machines hoses and lance.  

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Rapid changes in areas of low pressure (suction hose) cause the formation of small vapour filled cavities when subjected to high pressure these bubbles collapse creating a shock wave or vibration which causes surface fatigue to metal components. This damage is specifically evident on a pumps intake valves.

Cavitation is perhaps one of the biggest problems faced by the high pressure pump, most issues can be avoided with regular checks and preventative maintenance.

WHEN DOES CAVITATION OCCUR?

When pressure drops, the fluid tends to expand in volume. This is the basic principle of any pumping system, so a fluid placed in two environments at different pressures tends by its very nature to expand to compensate for this Δ, moving from the higher-pressure environment to the lower-pressure environment. In practice, the Δ pressure generates a downward movement of the fluid, then a rise, and in short, pumping.

However, when the pressure is lowered beyond a certain threshold, the fluid expands so much that it passes from the liquid to the gaseous state, in the form of bubbles (real cavities) containing vapour. This can have very negative consequences on both the pump and the machine.

In a pumping system, the causes of cavitation can vary: 

  • bottlenecks in the suction pipe

  • excessive length of the pipe itself

  • excessive difference in level between the supply tank and the pump, i.e. greater than the pump's head capacity. Head means the ability of a pump to raise a certain volume of liquid to a certain height. In other words, it is the maximum lifting height difference that can be achieved with a given pump. This is generally 3m for a pressure washer pump.

In case of cavitation, the vapour bubbles rise from the suction pipe towards the diaphragm pump, where they undergo a drastic increase in pressure (in practice they are crushed violently) and this causes a violent implosion, recognisable by a characteristic noise, i.e. as if gravel was passing through the pump. 

These violent shocks lead to a significant erosion of the components of the pump, which can cause valve rupture, diaphragm tearing, and even real head fractures.

CAN CAVITATION BE AVOIDED?

Yes, to prevent cavitation it is necessary to ensure that the negative pressure on the suction line never exceeds the limits set by the pump. This is achieved:

  • by avoiding sucking in water from excessive depths (maximum 3 metres);

  • by avoiding bottlenecks on the suction line: the pipes must be reinforced (spiralled) to prevent the walls from sticking to each other, blocking the passage towards the pump;

  • by cleaning the suction filters daily;

  • by reducing to a minimum the curvatures of the circuit and the use of fewer fittings on the suction line;

  • by using pipes with dimensions equal to or greater than the diameter of the suction connection of the pump. It is better to avoid using smaller pipes.

  • By avoiding the use of reducers, elbow fittings and quick release couplings.

  • By checking with a vacuum gauge that the negative pressure on the suction line of the installed pump never exceeds -0.3 bar.

Regular maintenance is always highly recommended to avoid problems and malfunctions, and to extend the life of your pumps.

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