Cavitation is the localized formation and collapse of vapor or bubbles in a liquid.
In a centrifugal pump vapor is formed or released when a liquid moves from a
higher to a lower pressure region and collapses when the liquid moves to a
region of higher pressure. The pressure fluctuation in the pump can also be
induced by flow separation, or non-uniform flow velocities that may result in the
formation of localized low pressure points.

The repeated formation and collapse of vapor will result in increased vibration
and noise levels, impairs the pump performance and, over time depending on its
severity, will cause bearing and mechanical seal failures and damage to the
pump impeller and casing. With this occurrence the pump is said to be cavitating.

(That flow separation or non-uniform velocities can reduce the pressure in
certain areas is similar to the effect of air flowing against the leading edge of an
airplane wing where flow separation and difference in air velocities reduce the
pressure at the top of the wing causing the plane to be lifted.)

Cavitation is usually caused by insufficient NPSHA. When there is insufficient
positive pressure to suppress them, vapor forms quickly when the liquid passes
through a localized low pressure point in the impeller and casing. Cavitation is
also likely to occur when the liquid has a high amount of entrained, or dissolved,
gas. In such service a higher NPSH margin must be maintained for vapor
suppression.

Cavitation can occur at both the suction and discharge sides of an impeller.
Most suction side cavitation is caused by insufficient NPSHA, whereas most
discharge side cavitation is caused by discharge flow recirculation when the
pump operates at low flow whereby a higher amount of liquid recirculates back
to suction. The flow recirculation creates localized low pressure points causing
the formation of cavities at the discharge side of the impeller. This can cause
what is referred to as discharge cavitation.
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Cavitation
Impeller cavitation-erosion damage

Q - We have a single stage, double suction, horizontal pump in cooling tower
service. The impeller has had cavitation-erosion damage at the inlet vanes so
severe that we have to replace the carbon steel impeller every six months. The
vendor advised that the pump NPSHR cannot be reduced because its suction
specific speed is already too high. We cannot increase the NPSHA and
replacing the pump with one of slower RPM, or with a vertical canned-type, is not
an option at this time. How can we prolong the impeller life beyond six months?

A - The cavitation-erosion damage is localized at the impeller inlet vanes and
does not show at the impeller outlet, or at the volute. We suspect the damage is
partly caused by suction flow recirculation possibly due to an oversized impeller
eye area which is typical of high suction specific speed impeller. Upgrading the
impeller material from carbon steel to the more cavitation-resistant ASTM A-743,
Grade CA6NM, may help prolong the useful life of the impeller. Or, consider
overlaying the carbon steel impeller with Stellite #6 material at the inlet vanes.

Cavitation noise and erosion damage

Q - We have five, single stage, double suction, axial-split, horizontal pumps in
raw water service. When we run three pumps in parallel they operate smoothly.
But when we run four units, to get slightly more flow and head, they make loud
cavitation noise.
We disassembled one pump and found that:
The impeller high pressure vane tips had moderate erosion damage on its
outboard side.
The volute lips also had erosion damage - about 1 square inch and 3/16 inch
deep.
The damage was aligned with the damage on the impeller. What was the likely
cause of the damage, and how can we prevent it in the future?

A – Since the erosion damage is on one side of the double suction impeller, the
problem was likely caused by unbalanced flow and the cavitation-like noise was
likely caused by internal flow recirculation, rather than by classical cavitation.

In classical cavitation, caused by insufficient NPSHA, the noise dissipates as the
flow rate is reduced, and the damage is likely to be on the low pressure side of
the impeller. These are exactly the opposite of what the pumps were doing. In an
unbalanced flow, one side of the double suction impeller is getting more flow
whereas the other side is starving causing some discharge flow recirculation to
occur.

The recirculation becomes more severe when more pumps operate because
each unit pumps at lower flow rate. In double suction pumps the inlet flow on
both sides of the impeller should be the same making them more sensitive to
improper suction piping compared to single suction pumps. To prevent
unbalanced flow make sure that their suction nozzles are piped properly. Do not
run the pumps below their recommended minimum stable flow. Install a minimum
flow by-pass line with ARV, if needed. Find out if it were doable to re-rate the
pumps for higher flow rate and head so that only three pumps would be needed.
Or, re-rate the pumps for lower flow rate to avoid low flow recirculation when four
units are in operation. Check that the impeller outlet width is properly centered
in the volute.

Symptoms of Flow Recirculation versus Inadequate NPSHA

The following are some typical symptoms of cavitation caused by internal flow
recirculation:

  • Cavitation damage in the high pressure side of impeller vanes
  • Random crackling noise at pump suction side
  • Noise is reduced at higher flow rate
  • Vibration is at (or less than) 1x RPM, at vane pass frequency, or at odd
    frequencies

The following are some typical symptoms of cavitation caused by inadequate
NPSHA:

  • Cavitation damage in the low pressure side of impeller vanes
  • Steady crackling noise at pump suction side
  • Noise is reduced at lower flow rate
  • Vibration is at odd frequencies



    Photo shows extensive
    cavitation and erosion
    damage on volute throat
    and lip area of a pump
    casing due to severe
    internal discharge flow
    recirculation.
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