Page 229 - Intro Predictive Maintenance

220 An Introduction to Predictive Maintenance

Cavitation

Cavitation in a centrifugal pump, which has a significant, negative effect on perfor-
mance, is the most common failure mode. Cavitation not only degrades a pump’s per-
formance but also greatly accelerates the wear rate of its internal components. There
are three causes of cavitation in centrifugal pumps: change of phase, entrained air or
gas, and turbulent flow.
Change of Phase. The formation or collapse of vapor bubbles in either the suction
piping or inside the pump is one cause of cavitation. This failure mode normally occurs
in applications, such as boiler feed, where the incoming liquid is at a temperature near
its saturation point. In this situation, a slight change in suction pressure can cause the
liquid to flash into its gaseous state. In the boiler-feed example, the water flashes into
steam. The reverse process also can occur. A slight increase in suction pressure can
force the entrained vapor to change phase to a liquid.
Cavitation caused by phase change seriously damages the pump’s internal compo-
nents. Visual evidence of operation with phase-change cavitation is an impeller surface
finish like an orange peel. Prolonged operation causes small pits or holes on both the
impeller shroud and vanes.

Entrained Air/Gas. Pumps are designed to handle gas-free liquids. If a centrifugal
pump’s suction supply contains any appreciable quantity of gas, the pump will cavi-
tate. In the example of cavitation caused by entrainment, the liquid is reasonably
stable, unlike with the change of phase described in the preceding section. Neverthe-
less, the entrained gas has a negative effect on pump performance. Although this form
of cavitation does not seriously affect the pump’s internal components, it severely
restricts its output and efficiency.

The primary causes of cavitation resulting from entrained gas include two-phase
suction supply, inadequate available net positive suction head (NPSH A), and leakage
in the suction-supply system. In some applications, the incoming liquid may contain
moderate to high concentrations of air or gas. This may result from aeration or mixing
of the liquid before reaching the pump or inadequate liquid levels in the supply reser-
voir. Regardless of the reason, the pump is forced to handle two-phase flow, which
was not intended in its design.

Turbulent Flow. The effects of turbulent flow (not a true form of cavitation) on pump
performance are almost identical to those described for entrained air or gas in the
preceding section. Pumps are not designed to handle incoming liquids that do not
have stable, laminar flow patterns. Therefore, if the flow is unstable, or turbulent, the
symptoms are the same as for cavitation.

Symptoms
Noise (e.g., like a can of marbles being shaken) is one indication that a centrifugal
pump is cavitating. Other indications are fluctuations of the pressure gauges, flowrate,
and motor current, as well as changes in the vibration profile.