The theory behind cavitation
Cavitation is a common mode of wear in hydraulic pumps and control valves. The damage to components more often than not results in severe losses of service life times and flow efficiencies. Understanding the causes behind this phenomenon is the first step in developing solutions to combat its harmful effects.
When the local static pressure in a fluid (inlet pressure) falls below the fluids vapour pressure (a characteristic property) at that particular temperature, vapour bubbles form in the fluid/hydraulic oil. Now, when we talk about bubbles, it is difficult to imagine that something so small can do incredible amounts of damage to hydraulic components. The vapour bubbles themselves pose no threat when floating around in the fluid. When the bubbles make it to the high pressure side of the pump/valve, however, they condense instantaneously and the bubbles collapse and produce hydraulic micro-jets which act similar to shockwaves. These micro-jets impinge on the metal surfaces, thereby destroying the material bonds (cavitation damage), and often result in (1) The creation of loud rolling noises (2) Vibration of the pump (3) The delivery of less flow and (4) Pitted erosion (as shown below).
Common causes of cavitation and how to counteract them
The common causes of cavitation that allow the fluid to have a low static pressure can be grouped into three categories i.e. inlet inadequacies, fluid properties, and pump position.
- Inlet inadequacies: These are often the case in systems when there is a restriction to the flow of the hydraulic fluid. Be sure to regularly look out for clogged strainers/filters, too many bends in the inlet line, and/or a collapsed inlet hose. Additionally, during the design and installation stage of your system, ensure the inlet pump lines are correctly sized and not too small.
- Fluid properties: If your fluid is in a state that allows it to vaporise easily, the risk of cavitation is increased. This means that there is a presence of water particles and/or the hydraulic fluid is too viscous to be easily forced into the pump as a result of low temperature. Ensure that you use a good quality hydraulic oil, keep it isolated from external wet sources and, if necessary, heat it up before use as you would with warming up your rig’s engine.
- Pump position: If your pump is too far away from the reservoir or too far above your reservoir you will have a low suction pressure allowing the formation of cavitation bubbles to occur. Make sure to use a pump with good filling characteristics or with a flooded suction. Alternatively, your design could incorporate a supercharged inlet.