Cavitation and Its Control
Posted on September 30, 2012
This form of material degradation and the factors that affect its control involve both mechanical and material parameters. The material damage mechanism consists of high speed, repetitive interaction between a material surface and a liquid such that bubbles alternately form and implode. This creates major, concentrated impact forces at the material – liquid interface. The eventual result is the formation of material pits – that can vary greatly in size – in the solid surface. Depending on the material and liquid interacting, the growth of pitting can be accelerated by corrosion but this is not necessary for the primary mechanical damage to occur. The general mechanism can affect both glass and ceramic materials and not just metallic surfaces. In all cases the process is vibratory with impact forces and compressive stresses so it has been recognized as one type of wear fatigue.
Cavitation occurs most frequently and most severely on the lower-pressure areas of equipment where bubble formation is favored. It can occur on marine propellers or on turbine blades but it is most frequently a potential problem on impellers in centrifugal pumps. In pumps the damage is not confined to pitting on material surfaces. The vibrations generated lower pumping efficiency and often can damage bearings, wear rings and seals in pumps. When cavitation is occurring in an operating pump there is a distinctive increase in noise level above that of normal operation.
Control methods for metallic-alloy pumps:
The most basic rule to prevent this damage is to assure that the Net Positive Suction Head Available (NPSHA), for the given pumping system, is greater than the NPSH Required (NPSHR) for the particular pump being used at a given rotational speed. Potentially there are several ways improve a given situation.
If possible, do any of the following: increase the pressure on the liquid source, increase the height of the liquid source relative to the elevation of the pump, lower the pump’s elevation relative to the height of the liquid source.
Minimize frictional flow losses and turbulence on the suction size of the pump via flow path changes and/or use of larger diameter inlet piping. These will increase NPSHA.
If possible, lower the inlet temperature of the liquid being pumped so as to lower its vapor pressure and thus increase the NPSHA.
If possible, reduce the rate of pumping by restricting flow on the discharge side of the pump – this will decrease NPSHR. Note – never restrict the flow rate into the pump.
Changing metallic material properties can provide significant resistance to cavitation. Helpful actions include use of a different material or heat treatment to obtain greater surface hardness, specifying a smooth surface finish and, if corrosion is expected to be an issue, specifying a more resistant alloy.
Finally, if other methods are impractical or insufficient, inspect a pump’s impeller during periodic maintenance outages, keep a careful record of any progressive damage and, if indicated, complete a replacement during a planned outage before deep pitting occurs.
