Efficient Plant October 2017 - 51

VIBRATION ANALYSIS

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Cavitation is just
one of several flowrelated pump-vibration problems. To
keep the problems
from damaging your
pumping systems,
it's important to be
able to distinguish
between the causes.

Pump 'Cavitation'
Might Not Be
THE WORD "CAVITATION" should be quite familiar to
those who work around pumps. As Stan Riddle of VibrAlign
Inc. (vibralign.com, Richmond, VA) points out, it's often used
as a catch-all term for any flow-related pump vibration. It's
also frequently reported by vibration analysts when:
pump noise level is high
pump vibration is erratic, with lots of noise
in the spectrum.
Riddle cautions, however, that of the several flow-related
pump-vibration problems, only one is truly cavitation. All
of those problems, though, share similar vibration characteristics that can lead to inefficient pumping systems and,
if severe, damaged pumps. Given the fact that different
solutions are required for dealing with them, it's important
to determine if a pump is truly cavitating or experiencing
another issue.
In a recent blog post on VibrAlign's website (Aug. 2, 2017),
Riddle discussed the various causes of pump noise and how
to distinguish them-and the problems they reflect-from
each other.
- Jane Alexander, Managing Editor

For more information on vibration and other plant-floor issues, visit
VibrAlign's "Long Live the Machine" blog at vibralign.com/blog.

OCTOBER 2017

CAVITATION

True cavitation occurs when "cavities," i.e., low-pressure vapor bubbles,
form in pumped liquid on the unit's suction side. This can be a result of:
clogged suction strainers or plugging on the suction side
poor piping design
a pump that's running too far to the left of its curve-a condition
becoming more common with the increasing popularity of variable-frequency drives (VFDs).
When the bubbles get into the impeller section, the pressure balances
and they implode, often onto the impeller surface. This can damage the
impeller and housing. Cavitation is often very loud, typically sounding like
gravel going through the pump.

AERATION, OR AIR ENTRAPMENT
Air entrapment happens when bubbles appear in a pumped liquid before
it reaches the impeller. This can happen when:
the liquid falls into an open tank system, or is aerated near the pump
inlet, due to the liquid level being lower than the top of the pipe inlet
de-foaming agents aren't or can't be used
liquid is near its boiling point, as in a condensate pump.
While air entrapment isn't always as serious (or as loud) as cavitation,
left unchecked, it can damage an impeller.

RECIRCULATION
Recirculation is most often caused by low flow. Two types can occur:
Suction recirculation is reversal of flow at the impeller eye, causing
low-pressure bubbles to form, very much like cavitation. This type of
recirculation is just as damaging to the suction side of the impeller.
Discharge recirculation is reversal of flow at the discharge side of the
pump and can also damage the pressure side of the impeller.

DETERMINING THE ACTUAL CAUSE
Vibration analysis is good at detecting flow-related pump problems but,
by itself, may not be capable of distinguishing, if at all, the actual cause of
a noisy pump. A method for determining the type of problem is to slowly
throttle the discharge valve closed. As the valve is closing:
if noise and vibration improve, expect cavitation
if noise and vibration stay the same, expect air entrapment
if noise and vibration worsen, expect recirculation.
Throttling the discharge valve may seem like a straightforward fix, but
it is merely a diagnostic technique. If any of the above issues are occurring, you have either an operational or design problem-which must be
addressed by engineering. EP
EFFICIENTPLANTMAG.COM |

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Table of Contents for the Digital Edition of Efficient Plant October 2017