Which is Which ? Cavitation
or Recirculation?
By John C. Robertson PE/CPE
How many times have we heard sounds emanating from pump
casings like ball bearings rattling around inside a tin
can? The first thing that comes to mind is cavitation.
We know what it sounds like and we see the damage it can
do to impellers, but do we know what to do about it? In
many cases ? no! It has been classified as ?the nature
of the beast? and other non-technical terms but it is allowed
to continue until the inevitable happens when the impeller
disintegrates. That is when maintenance can be expensive.
On one site that I visited, there were four impellers
in the warehouse just for one pump because they could not
understand why the working impeller failed through severe
pitting. This had been going on for nearly three years
since the pump went into operation. They thought the rattling
noise was due to the fluid contents being pumped. As the
suction pressure head could not be increased, the fluid
temperature was lowered out of its saturation temperature
range and the noises stopped. This was a simple solution
to a complex problem. But, in some cases where cavitational
noises are heard and the above solution is not effective,
impeller damage is still taking place. What is the problem?
A cavitation sound-alike problem was causing havoc in
one pump on an oil platform about 120 miles offshore in
the Gulf of Mexico . Because of higher production outputs,
some of the existing pumps were ?redesigned? to produce
more. On this particular pump, a new motor was installed
which increased the power to10 horsepower above the original
design specification requirements. The original thought
being that more power increased output. Unfortunately,
the design change did not consider any increase in output
piping dimensions to cope with the greater increase in
fluid flow. Due to this oversight, severe backpressure
in the discharge area of the pump's impeller and volute
resulted and the pressure energy that was created, converted
to heat energy. The subsequent flash-off created, what
the operators considered to be, cavitation. From that point
on for the next two years, they had unsuccessfully tried
to eliminate cavitation. What they were actually experiencing
was recirculation and not cavitation.
Simply put, cavitation is usually confined to the low
pressure or suction side of the impeller whereas; recirculation
is found in the impeller's exit area. The visual results
of damage are the same in each condition but they are confined
to the areas discussed. The implosions of the vapor bubbles
have a devastating effect on the surface of surrounding
metals; one expert estimated that the force of the implosion
was in the area of (14.7 lbs/sq. ins) 10 . This is a highly
destructive condition in which pump impellers will fail.
If vibration analysis, or ultrasonic programs are being
used to support a predictive maintenance program, both
cavitation and recirculation problems can be detected early
enough to avoid problems. The following spectrum visually
shows the difference between these two phenomenons.
Cavitation
The area of cavitation is within the eye of the impeller,
therefore, there are no vane pass frequencies. Cavitation
can cause axial surging, which, if excessive, will cause
damage to both bearings and seals. The spectra produced
have a very broad frequency range with low amplitude. The
highest amplitudes are most often found in the suction
area of the pump and are usually highest in the axial direction.
It is also significant to note that in cavitation the peaks
are non-synchronous, and there is also a lot of broadband
noise.
Recirculation
Note the difference between cavitation and
re-circulation.
It is very noticeable that there is now a clear peak at
the impeller vane pass frequency. The cure for
re-circulation
is to operate the pump at or close to its rated capacity
for the operating conditions and to avoid excessive throttling
on the discharge side. Re-circulation can also be identified
by washout of the impeller exit faces. This usually shows
up as a polished, sharp edge on the impeller exit face.
From experience, it is essential that those people, who
are assigned to troubleshooting activities on pumps and
other machinery, fully understand the basic theory of how
their charges work. An experienced operator should be able
to recognize and control such conditions as cavitation
and re-circulation without having to bring in an engineer
to determine the problem. If this basic knowledge were
introduced during the initial training process, the amount
of money spent on maintenance would be minimal. The personnel
operating and maintaining the equipment would be much happier
too.
If there are any questions about this subject or others,
please feel free to call.
John C. Robertson PE/CPE
AMSCO Inc.
105 Goldenrain Way
Simpsonville , S.C. 29680
Tel#: (864) 967-7183
Mobil Tel#: (864) 350-5280
E-mail: RobertsonJC1@cs.com
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