Efficient Plant October 2017 - 51

VIBRATION ANALYSIS

+
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 |

51


http://www.vibralign.com http://www.vibralign.com/blog http://www.EFFICIENTPLANTMAG.COM

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

Editorial
Implementations
Profitable Reliability Control On Your Horizon
Experts Provide IIoT Insights
Integrating Automation Into Shade Manufacturing
Fluid-Flow Challenges
Looking For Failure In All The Wrong Places
Closer Human/Robot Collaboration
Note These Causes Of Motor Failure
Reliable Pumping Supplement: Food & Beverage Processing
On The Floor
Uptime
IIoT
Pump 'Cavitation' Might Not Be
Never Too Late For Basic Navigation
Audit or Assess? Understand The Difference
A Perfect Pair: Barcodes and Your CMMS
Lubrication Strategies
Products
Ad Index
Showcase
Efficiency Insight
Efficient Plant October 2017 - 1
Efficient Plant October 2017 - Cover1
Efficient Plant October 2017 - Cover2
Efficient Plant October 2017 - 1
Efficient Plant October 2017 - 2
Efficient Plant October 2017 - 3
Efficient Plant October 2017 - 4
Efficient Plant October 2017 - 5
Efficient Plant October 2017 - 6
Efficient Plant October 2017 - 7
Efficient Plant October 2017 - Editorial
Efficient Plant October 2017 - 9
Efficient Plant October 2017 - Implementations
Efficient Plant October 2017 - 11
Efficient Plant October 2017 - Profitable Reliability Control On Your Horizon
Efficient Plant October 2017 - 13
Efficient Plant October 2017 - 14
Efficient Plant October 2017 - 15
Efficient Plant October 2017 - Experts Provide IIoT Insights
Efficient Plant October 2017 - Integrating Automation Into Shade Manufacturing
Efficient Plant October 2017 - 21
Efficient Plant October 2017 - 22
Efficient Plant October 2017 - 23
Efficient Plant October 2017 - 17
Efficient Plant October 2017 - 18
Efficient Plant October 2017 - 19
Efficient Plant October 2017 - Integrating Automation Into Shade Manufacturing
Efficient Plant October 2017 - 21
Efficient Plant October 2017 - 22
Efficient Plant October 2017 - 23
Efficient Plant October 2017 - 24
Efficient Plant October 2017 - Fluid-Flow Challenges
Efficient Plant October 2017 - 26
Efficient Plant October 2017 - 27
Efficient Plant October 2017 - 28
Efficient Plant October 2017 - Looking For Failure In All The Wrong Places
Efficient Plant October 2017 - 30
Efficient Plant October 2017 - 31
Efficient Plant October 2017 - 32
Efficient Plant October 2017 - 33
Efficient Plant October 2017 - Closer Human/Robot Collaboration
Efficient Plant October 2017 - 35
Efficient Plant October 2017 - Note These Causes Of Motor Failure
Efficient Plant October 2017 - 37
Efficient Plant October 2017 - 38
Efficient Plant October 2017 - Reliable Pumping Supplement: Food & Beverage Processing
Efficient Plant October 2017 - 40
Efficient Plant October 2017 - 41
Efficient Plant October 2017 - 42
Efficient Plant October 2017 - 43
Efficient Plant October 2017 - 44
Efficient Plant October 2017 - 45
Efficient Plant October 2017 - 46
Efficient Plant October 2017 - On The Floor
Efficient Plant October 2017 - 48
Efficient Plant October 2017 - Uptime
Efficient Plant October 2017 - IIoT
Efficient Plant October 2017 - Pump 'Cavitation' Might Not Be
Efficient Plant October 2017 - Never Too Late For Basic Navigation
Efficient Plant October 2017 - Audit or Assess? Understand The Difference
Efficient Plant October 2017 - A Perfect Pair: Barcodes and Your CMMS
Efficient Plant October 2017 - Lubrication Strategies
Efficient Plant October 2017 - 56
Efficient Plant October 2017 - 57
Efficient Plant October 2017 - 58
Efficient Plant October 2017 - 59
Efficient Plant October 2017 - Products
Efficient Plant October 2017 - 61
Efficient Plant October 2017 - Ad Index
Efficient Plant October 2017 - Showcase
Efficient Plant October 2017 - Efficiency Insight
Efficient Plant October 2017 - Cover3
Efficient Plant October 2017 - Cover4
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