Suppose the droplet enters the inlet of the pump at a pressure slightly above the vapor pressure. Next, the droplet happens to run into the leading edge of the impeller blade, cutting the droplet into two pieces. Half of the drop travels into the impeller channel on the positive-pressure side, where the pressure increases quickly. The other half-drop travels into the low-pressure side, where the pressure is reduced below the vaporization pressure.
When the droplet travels into a low-pressure zone, it vaporizes. Vaporization is a change from liquid state to vapor (a compressible state). As it vaporizes, it expands to meet the pressure requirements of thermodynamic laws.
The vapor continues to travel through the impeller channel, constantly increasing with pressure, until the pressure is above the vaporization pressure. At this instant, the vapor collapses into a solid droplet. During the vapor collapse into a solid, there is a high amount of shock energy propagated from the implosion. Damage to the impeller and other pump parts are common. The damage usually occurs closest to the implosion.
Cavitation is not air traveling through the pump. If air is mixed with liquid and traveling through the pump, it is called two-phase flow. The introduction of air or two-phase flow will reduce pump performance where the air will be compressed and worked on. Centrifugal pumps work ideally on incompressible liquids. Do not equate vaporized liquid with air because they are not the same.
Protecting Pumps from CavitationIf a pump and system has cavitation, introducing a small amount of air into the inlet will reduce or eliminate the cavitation damage by absorbing the shock within the air at the time of transformation phase from vapor to liquid.
Cavitation is accompanied by a random gravely noise within the pump. The shock waves that tear the metal within the pump can selectively pit the metal, leaving the hard elements behind and attacking the softer elements in the alloy. Minor cavitation damage usually is seen at the low-pressure side of the inlet blades. Severe cavitation damages the impeller outer diameter and sometimes the volute.
One thing you can count on: Wherever the cavitation implosion occurs, there will be damage. High frequency vibrations from cavitation also damage seals and bearings.
You can avoid cavitation by following these simple steps:
- Reduce inlet restrictions. Use long-radius elbows and tees. Keep the inlet piping as large as practical. Reduce inlet pipe lengths.
- Do not allow the pump to run out on the end of the flow curve. Introduce some restriction to effectively throttle the pump with the outlet.
- Know your suction pressure by adding a gauge in the inlet before the pump. Do not assume that the system has high suction pressure just because the suction is flooded. Many static-flooded systems are actually pulling a lift when flow is established at the operating condition.
- Calculate the NPSH available from the system conditions and compare it with NPSH required from the pump data curve. A margin of 3' minimum difference -- where the available is 3' greater than required -- is a good standard.
Pumps are tested for cavitation performance. The result is a value called net positive suction head (NPSHr). NPSHr is a calculated value from pump test data where the pump is tested with high suction pressure conditions, and then the pump is tested with the suction conditions reduced to where a reduction of 3 percent differential pressure is attained.
This data is used to calculate the NPSHr from other conditions from the liquid temperature, vapor pressure and barometric pressure at the time of test. The final value for NPSHr is actually a small amount of cavitation where only a 3 percent reduction of pressure differential performance occurs.
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