Case Histories: Applying vibration and related test and analysis methods to solve problems by Eric...
30
Case Histories: Applying vibration and related test and analysis methods to solve problems by Eric J. Olson – Vice President Mechanical Solutions, Inc. [email protected]973-326-9920 ext. 125 www.mechsol.com Whippany NJ January 22, 2014
Case Histories: Applying vibration and related test and analysis methods to solve problems by Eric J. Olson – Vice President Mechanical Solutions, Inc
Case Histories: Applying vibration and related test and
analysis methods to solve problems by Eric J. Olson Vice President
Mechanical Solutions, Inc. [email protected] 973-326-9920 ext. 125
www.mechsol.com Whippany NJ January 22, 2014
Slide 2
1.Morton Effect Oscillating Vibration Phenomenon on a Double
Suction Feed Water Pump 2.Excessive Vibration Amplitude at the
Inboard Bearing of a Reactor Feedwater Pump
Slide 3
Case Study 1: Morton Effect Oscillating Vibration Phenomenon on
a Double Suction Feed Water Pump 3
Slide 4
Background February 2011 one of the single-stage double-suction
pumps in feed water service, driven by a steam turbine, indicated a
step change in 1x vibration accompanied by a significant change in
performance. The 1x vibration level increased from 1.8 to 6.5 mils
pk-pk. Simultaneously, flow rate decreased from 22400 to 21600 gpm.
This issue was a result of a broken impeller vane causing imbalance
and rubbing. April 2012, after a major outage, the same pump began
increasing shaft vibration at the inboard and outboard end. The
pump was completely refurbished during the outage. The vibration
amplitude was not constant. The overall amplitude was fluctuating
with a period between 10 to 60 seconds. The overall vibration was
mostly at 1x rpm.
Slide 5
Background
Slide 6
Vibration Testing Prox probe outboard bearing housing Vibration
amplitude was not constant (prox probes). The overall amplitude was
fluctuating with a period between 10 to 60 seconds
Vibration Testing Average Speed RPM fluctuations 4845rpm to
4888 rpm (350 second sample)
Slide 9
Vibration Testing 0 to 344 seconds
Slide 10
Vibration Testing The steady average mechanical imbalance
vector combined with a secondary non-constant phase imbalance
vector (0 to 344 seconds)
Slide 11
Morton Effect: the circles are the result of a static imbalance
vector (i.e. the steady average mechanical imbalance) combined with
a secondary non-constant phase imbalance vector where the secondary
vector cycles its phase from 0 360 degrees. The secondary (cycling)
imbalance vector is produced from a thermal distortion of the shaft
in which a localized high temperature area of the shaft travels the
full circumference with every vibration swell. The imbalance
vectors behaving in this way is what causes the vibration level
cyclic trends such as those experienced by the pump. This
phenomenon is typically caused by either a light rub or a near rub
(boundary lubrication at the pinch point) in a synchronously
whirling shaft leading to a hot spot on the heavy side of the shaft
that leads to thermal bow of the shaft, changing the heavy side
location. Such a condition can occur in either a bearing or a seal,
but in the case of a bearing the vibration cycles are typically
longer (on the order of 5 to 10 minutes) than observed in the
present case. Vibration Testing
Slide 12
Morton Effect is dependent on the thermal state, structural
internal clearances, and fluid dynamics within the seal cavities of
the pump. However, Morton Effect may be affected by changes in pump
speed, seal water injection temperature, and flow rate. Plant
personnel tried different ways to eliminate this phenomenon and
detected a strong influence on the condensate seal water
temperature (seasonal) or by changing the seal water injection
flow. However, the results were not always consistent and sometime
without any effect. Vibration Testing
Slide 13
Morton Effect not detected after slight reduction of seal water
injection flow and increment of drain temperature. June 2012
Slide 14
Vibration Testing End of Morton Effect after reducing seal
water injection flow and drain temperature. August 2012
Slide 15
Vibration Testing Start of Morton Effect after slight increment
of seal water injection flow and increment of drain temperature.
August 2012
Slide 16
Vibration Testing No effect on Morton Effect after seal water
injection flow increment and reduction of drain temperature. August
2012
Slide 17
ODS Animation @1x rpm (72.3 Hz) Operating Deflection Shape
(ODS)Testing Key Symptoms: Erratic vibration at 1x but not classic
imbalance. Sensitive to seal drain temperature. What was the root
cause? Used an ODS test to help answer
Slide 18
After a thorough investigation between site personnel and MSI,
it was concluded that the root cause of the Morton Effect was due
to a casing distortion that was altering the internal clearances
between the shaft and the long seal sleeve. The torque applied to
the hold-down bolts to the pedestals at the outboard and inboard
end of the pump was approximately 4,400 ft-lb. The required torque,
based on the OEM manual, is 1,800 ft-lb at the inboard end and 550
ft-lb at the outboard end. This would allow the casing to grow
axially while it reaches the thermal equilibrium. Root Cause,
Conclusions, and Solution
Slide 19
Case Study 2: Excessive Vibration Amplitude at the Inboard
Bearing of a Reactor Feedwater Pump 19
Slide 20
Background In August 2013, the Bravo single-stage
double-suction pump in feed water service, driven by a steam
turbine, indicated elevated vibration amplitude (over 0.8 in/s RMS
@ 1x rpm) when the pump was operating at its maximum speed of 3930
rpm (100% load, 65.5Hz). The plant decided to operate the pump at
90% load (3510 rpm, 58.5HZ) to maintain the 1x rpm vibration near
0.60 in/s RMS. Others were recommending that the pump needed to be
shut down. Questions: Can the pump continue to operate? If needed
is there a fix that can be implemented with the pump operating? If
needed, what is a long term fix?
Slide 21
How Natural Frequencies Affect Vibration Typical FFT *FRF plot
from impact test Forces causing the problem. Will be amplified by a
natural frequency *FRF is a Frequency Response Function. Used FFT
on log scale to determine resonance was a likely issue. gs of
vibration per pound of force
Slide 22
Modal Impact Testing FRF plot before rebuild Frequency Response
Function (FRF) Plot Resulting from Modal Test Cold Non-Operating
Condition Before Rebuild Measured at West End Pump Foot Mid-Span in
the Horizontal Direction (Horizontal Casing Mode at ~57.3 Hz, 90%
of load) Log scale vibration acceleration (gs) per pound of force
Frequency 0.000282 gs of vibration per pound of force at 57.5
Hz
Slide 23
ODS Animation @1x rpm (58.6 Hz) Baseline Conditions Operating
Deflection Shape before rebuild 90% load OBB 1x rpm OB and IB
probes 1.8 and 5.4 mils pk-pk
Slide 24
1.The high vibration root cause was predicted to be a soft or
sprung foot along most or all of the foot length along with pumps
west side (no issue on east side). 2.The looseness and/ or
excessive flexibility at the foot-to-pedestal-top-pad west side
locations enabled the casing to swing in an elongated circular arc,
moving on the order of 5 plus mils p-p, primarily in the horizontal
direction, with largest motion at the bearing housings. 3.Although
the pump or driver were not in immediate jeopardy, the casing
vibration was strong enough that the bearing cavity walls are
repetitively striking the shaft, resulting in a very brief impact
and rub within the bearing, approximately once per revolution over
a small arc (maybe 30 degrees or so) which will lead to premature
bearing wear. 4.The large casing motion can happen when welds of
any gussets and cross-members internal to the pedestal develop
fatigue cracks and/ or gradually corrode and break free. If there
was not a soft foot, this situation could be tolerated with no
problem until the next planned outage, but the soft foot allows a
strong (on a relative basis) contortion of the pedestal,
facilitating greater casing vibration. Conclusions at this point -
1
Slide 25
5.Fix the flexible foot and the extra vibration caused by this
condition will be minimized to acceptable levels. 6.The reason for
the sudden appearance of the soft foot is unknown. It is possible
that action by piping loads during load swings contributed to this
condition. A fatigue crack loosening the entire top plate of the
pedestal or the foot gussets on the west side is another
possibility. Recommendations: 1.The foot bolts should be tightened.
This was done immediately to limit further increase in vibration
and gradual damage to the pump IB bearing. 2.Tightening of the foot
bolts will shift the casing natural frequency from 57.3 HZ (about
90% of running speed) upwards, and the natural frequency may still
be in running speed range (max rpm 3930rpm, 65.5Hz). This may not
necessarily be a problem depending on damping and system stiffness.
Conclusions at this point - 2
Slide 26
Modal Impact Testing - FRF after rebuild/foot bolt torquing
Frequency Response Function (FRF) Plot Resulting from Modal Test
Cold Non-Operating Condition After Rebuild Measured at West End
Pump Foot Mid-Span in the Horizontal Direction (Horizontal Casing
Mode at ~67.5 Hz, 100%) Log scale vibration acceleration (gs) per
pound of force 0.00057 gs of vibration per pound of force at 67.5
Hz
Slide 27
ODS Animation @1x rpm (65.5 Hz) After Modifications Operating
Deflection Shape after rebuild at 100% where the NF is now located
Turbine Bearing 1x rpm OB and IB probes 2.1 and 0.7 mils pk-pk
Slide 28
1.The pump foot west side soft foot condition appears to be
corrected upon examination of the ODS 1x animations. This was
largely due to the installation of the dowel pins on the pump feet.
As a result, the pump structural natural frequency (which involves
significant horizontal relative motion) had changed from ~57.3 to
67.5 Hz due to the general increase of the system stiffness which
also allows the pedestal better restrain the pump casing motion.
2.The overall vibration levels as measured on the pump casing are
generally low, on the order of ~0.1 in/s rms. Seismic spectra also
indicate that the pump is not significantly exciting the 67.5 Hz
structural natural frequency at the 65.5 Hz (3930 rpm) running
speed. ODS animations at the 1x running speed confirms that the
casing motion is significantly low relative to the rotor vertical
indications and the IB turbine bearing. Results after second round
of tests - 1
Slide 29
3.Vibration levels at 1x rpm on the OB and IB probes changed
respectively from 1.8 and 5.4 mils pk-pk before the rework to 2.1
and 0.7 mils pk-pk after the rework. 4.Furthermore, an examination
of the proximity probe spectrum after the rework for the IB probe
does not indicate any serious detrimental condition involving the
coupling such as severe misalignment or imbalance. Results - after
second round of tests -2
Slide 30
1.The root cause of the soft or sprung foot on the pumps west
side should be investigated during the next outage opportunity to
avoid a chronic problem. 2.Plant personnel should check for
pedestal weld cracks and any fatigue damage to supports / cross
members in the pedestal structure. Such damage may contribute to
pedestal flexibility and would certainly be beneficial to correct
if present. Final Recommendations