When a partial vacuum develops in the pump intake line, the decrease in absolute pressure results in the formation of gas and/or vapor bubbles within the oil. When these bubbles are exposed to elevated pressures at the pump outlet they implode violently. When bubbles collapse in proximity to a metal surface, erosion occurs. Cavitation erosion contaminates the hydraulic oil and damages critical surfaces.
When a partial vacuum develops at the pump inlet, the mechanical forces induced by the vacuum itself can cause catastrophic failure. In vane pump designs, the hydraulic motor vanes must extend from their retracted position in the rotor during inlet. As this happens, fluid from the pump inlet fills the void in the rotor created by the extending vane. If excessive vacuum exists at the pump inlet – it will act at the base of the vane. This causes the vanes to lose contact with the cam ring during inlet, and they are then hammered back onto the cam ring as pressurized fluid acts on the base of the vane during outlet (figure 1). The impact damages the vane tips and cam ring, leading rapidly to catastrophic failure.
The intolerable noise is referring to is symptomatic of cavitation bubble collapse and the vanes being hammered against the cam ring. Both of these conditions are intensified by increasing system pressure.
The solution to the problem is simple: replace the suction filter or better still, discard it completely. If suction filtration must be installed, follow these precautions to prevent pump damage:
- A filter located outside of the reservoir is preferable to a suction strainer. The inconvenience of servicing a filter located inside the reservoir is a common reason why suction strainers go unserviced – until after the pump fails.
- If a suction strainer is installed, opt for 250 microns rather than the more common 150 microns.
- The filter should be grossly oversized for the pump’s flow rate to ensure that pressure drop is minimized, even under the most adverse conditions.
- Regardless of the type of filter employed, it must incorporate a bypass valve to prevent the element from creating a pressure drop that exceeds the safe vacuum limit of the pump.
- A gauge or transducer should be installed downstream of the filter to enable continuous monitoring of absolute pressure at the pump inlet.
This article comes from hydraulic edit released