Page 253 - Root Cause Failure Analysis

Pumps 241

Cavitation due to phase change seriously damages the pump’s internal components.
Visual evidence of operation with phase-change cavitation is an impeller surface fin-
ish like an orange peel. Prolonged operation causes small pits or holes on both the
impeller shroud and vanes.

Entrained Air or Gas Pumps are designed to handle gas-free liquids. If a centrifu-
gal pump’s suction supply contains any appreciable quantity of gas, the pump will
cavitate. In the example of cavitation due to entrainment, the liquid is reasonably sta-
ble, unlike with the change of phase described in the preceding section. Nevertheless,
the entrained gas has a negative effect on pump performance. While this form of cavi-
tation does not seriously affect the pump’s internal components, it severely restricts
its output and efficiency.

The primary causes of cavitation due to entrained gas include two-phase suction sup-
ply, inadequate available net positive suction head (NPSH,), and leakage in the suc-
tion-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 the liquid
prior to reaching the pump or inadequate liquid levels in the supply reservoir. Regard-
less of the reason, the pump is forced to handle two-phase flow, which was not
intended in its design.

nrbulent 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 have
no stable, laminar flow pattern. 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, flow
rate, and motor current, as well as changes in the vibration profile.

Solutions
Several design or operational changes may be necessary to stop centrifugal-pump
cavitation. Increasing the available net positive suction head (NPSH,) above that
required (NPSHR) is one way to stop it. The NPSH required to prevent cavitation is
determined through testing by the pump manufacturer. It depends on several factors,
including type of impeller inlet, impeller design, impeller rotational speed, pump flow
rate, and the type of liquid being pumped. The manufacturer typically supplies curves
of NPSH, as a function of flow rate for a particular liquid (usually water) in the
pump’s manual.
One way to increase the NPSH, is to increase the pump’s suction pressure. If a pump is
fed from an enclosed tank, suction pressure can be increased by either raising the level
of the liquid in the tank or increasing the pressure in the gas space above the liquid.