Multiple PdM Technologies
and Out-of-the-Box Thinking Solve Pump Problem
In the chemical industry,
sealless, self-contained pumps are a staple in our
process designs. Although these assets are typically
not large
physically, they are expensive. The three-horsepower
pump in this case study costs approximately $7,200
for the rotor and stator assembly. Our plant has many
of
these
assets, so monitoring these pumps has become an important
part of our predictive maintenance strategy.
In this specific case,
a call was received to troubleshoot
a pump that was "kicking out" on thermal load
protection. Initial observations indicated process
conditions seemed
normal. Valve positions were in the correct locations,
pressure was within historical range, and the flow
transmitter was indicating expected flow. Experience
has taught us there are other tests we can run to check
for proper flow conditions in these pumps. One is the
temperature gradient throughout the pump, and the other
is monitoring run speed through vibration.
This style of pump
uses process fluid to lubricate the sleeve bearings for
obtaining hydraulic stability of the rotor. The infrared
image below indicates the pump does have bearing flush
fluid flow to the rear bearing housing. The cooler colors
at the rear bearing housing is evidence of this flow.
Even with the appearance of proper fluid flow through
the pump, there appeared to be excessive heat buildup
in the stator and rotor assembly, as evidenced by the
white hotspot.
The next test was
to take vibration data. The vibration plot below shows
the signature indicating the likelihood of a rotor
problem. There were multiple harmonics of run speed that
were surrounded by 2x slip frequency. This signature
can be driven by several different faults, including
broken/cracked rotor bars, shorted rings and shorted
rotor laminations. At this point a recommendation was
made to schedule a change out of this
pump. The spectrum also supports the fact that the pump
was running under acceptable flow conditions. The optimum
run speed for this class of pump is 3,450 RPM.
The rotor
and stator windings are covered by a thin stainless
steel layer, therefore the efficiency is lower than typical
two-pole motors. This pump was running at 3466 RPM,
well within this pump's best efficiency point. We would
have to wait for the teardown to attempt to find the specific
driver for this vibration signature. A unique and troublesome
problem in doing an autopsy on these pumps is the rotor
has a thick
stainless
steel covering 2 to 3 ml thick.
We needed to have our machine shop carefully take off
this covering in order to expose the rotor
bars and laminations for inspection.
The rotor
minus the covering shows the clear indication
of the cause of the spectral signature. The right side of
the rotor shows a clean separation between the rotor bars
and laminations. The left side illustrates a breech of this
separation between the rotor bars and laminations, as well
as discoloration most likely caused from heat buildup.
In
this case, a new rotor was installed in the existing stator
and the pump was back to running under normal operating conditions.
Replacement of the rotor costs approximately $3,000 of
the $7,200 total. I am confident that if the pump continued
to run this way undetected, we would also have eventually
lost the stator. And by the nature of the design of
these
pumps, a catastrophic failure of the stator has a potential
for atmospheric release of the process fluid.
Conclusion: This
case is not a typical failure mode for this class of pump.
Generally, we are tracking the subsynchronous energy
representing oil whirl/whip to assess bearing wear. This
is critical for these pumps; the tolerance between rotor
and stator is approximately 5 to 7 ml. Once you have rotor
to stator contact, the integrity of the pump/motor is compromised
and
it must be scrapped. A bearing rebuild kit costs $400
as opposed to scrapping a $7,200 pump.
This case also illustrates
two additional powerful attributes that vibration analysis
brings to troubleshooting and tracking equipment issues.
No. 1:
not only does vibration analysis have the ability to predict
impending mechanical failures, it also has the ability
to
rule out hypothetical failures during the troubleshooting
discussions. Specifically in this case, the run speed was
an indicator the pump was running under good flow conditions.
No. 2: using vibration analysis on specific assets can
be a leading indicator of changing process conditions that
can
be the root cause to machine failures, or even product
quality
issues. Vibration analysis has been able to indicate process
changes by showing changes in the pump flow characteristics,
and then fixing the problem prior to machine failures.
Coupling
vibration with additional technologies, and thinking outside
the box when troubleshooting can make an analyst an invaluable
asset to our plant customers. And in today’s economy, becoming
invaluable is more important than ever.
Submitted by Dan Warren, Condition-based Monitoring
Specialist, Dow Corning
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