PTC Software and Capabilities

Case Histories:Applying vibration and related test and analysis methods to solve problemsby Eric J. Olson Vice PresidentMechanical Solutions, Inc.ejo@mechsol.com973-326-9920 ext. 125 www.mechsol.comWhippany NJ

January 22, 2014

1Centrifugal Pump Components and Construction Pump Hydraulic Design, Specific Speed, and Affinity Laws Head-Capacity and System Curves Operating Point Impact on Vibration. Cavitation Detection and Damage Measurement MethodsFFT Spectra Interpretation, and Vibration Measurement Units and Formats (displacement, velocity, acceleration, RMS, peak, etc.) Pump Styles/Services Versus Typical ProblemsThrust Balancing and Related IssuesTypical Vibration Problems OverviewDamaging Forces, Failure Modes, and Instrumentation TypesMechanical Imbalance, Resonance, and Phase AngleMisalignment, Looseness, and RubbingPiping Issues and Acoustic Natural Frequency Problems and SolutionsVane Pass and Its Impact on Pump DynamicsHigh-Harmonics of Running Speed ProblemsThe Impact of Off-Design Operation on Vibration and Life Recirculation, Stall, Rotating Stall, Hydraulic Forces, and Pressure PulsationsSub-synchronous FFT Interpretation Recirculation, Stall, Bearing Instability, RubPressure Pulsations, Dynamic Pressure Measurements and Vibration/Pulsation Introduction to Rotordynamics Analysis to Support Problem Solving Bearings, Seals, and Gyroscopics; Lomakin Effect; Campbell Diagrams; Critical Speed Map; and Lateral and Torsional Rotordynamic Issues Impacting Vibration and ReliabilityBest Measurement Locations and SpecificationsNatural Frequencies, Resonance, and Mode Shapes. What Natural Frequency Shifts Can Mean.How to Characterize and Solve a Resonance Problem and How to Determine the Degree of Danger FFT Spectra, Phase Angle, Water Fall, Impact Test, ODS Test.Experimental Modal Analysis (Impact) Testing on Operational and Non-Operational EquipmentHow to Use Operational Deflection Shape (ODS) Testing to Solve Problems.Case Histories: Vertical Pumps, Charging/Safety Injection, Feed, RHR, Circ. Pumps, Condensate, and Others.

Morton Effect Oscillating Vibration Phenomenon on a Double Suction Feed Water PumpExcessive Vibration Amplitude at the Inboard Bearing of a Reactor Feedwater Pump

Case Study 1:Morton Effect Oscillating Vibration Phenomenon on a Double Suction Feed Water Pump33BackgroundFebruary 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.Background

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 secondsVibration Testing Prox probe inboard bearing housing

Vibration Testing Average SpeedRPM fluctuations 4845rpm to 4888 rpm (350 second sample)Vibration Testing

0 to 344 seconds

Vibration Testing

The steady average mechanical imbalance vector combined with a secondary non-constant phase imbalance vector (0 to 344 seconds)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 TestingMorton 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

Vibration TestingMorton Effect not detected after slight reduction of seal water injection flow and increment of drain temperature.

June 2012Vibration Testing

End of Morton Effect after reducing seal water injection flow and drain temperature.August 2012Vibration Testing

Start of Morton Effect after slight increment of seal water injection flow and increment of drain temperature.August 2012Vibration TestingNo effect on Morton Effect after seal water injection flow increment and reduction of drain temperature.

August 2012

ODS Animation @1x rpm (72.3 Hz)Operating Deflection Shape (ODS)TestingKey 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

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 SolutionCase Study 2:Excessive Vibration Amplitude at the Inboard Bearing of a Reactor Feedwater Pump1919BackgroundIn 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?

How Natural Frequencies Affect VibrationTypical FFT*FRF plot from impact testForces 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 Modal Impact Testing FRF plot before rebuild Frequency Response Function (FRF) Plot Resulting from Modal TestCold Non-Operating Condition Before RebuildMeasured 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 forceFrequency0.000282 gs of vibration per pound of force at 57.5 HzODS Animation @1x rpm (58.6 Hz)Baseline ConditionsOperating Deflection Shape before rebuild 90% load

OBB

1x rpm OB and IB probes 1.8 and 5.4 mils pk-pk 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).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. 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.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 Fix the flexible foot and the extra vibration caused by this condition will be minimized to acceptable levels.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:The foot bolts should be tightened. This was done immediately to limit further increase in vibration and gradual damage to the pump IB bearing. 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 - 2Modal Impact Testing - FRF after rebuild/foot bolt torquing

Frequency Response Function (FRF) Plot Resulting from Modal TestCold Non-Operating Condition After RebuildMeasured 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 force0.00057 gs of vibration per pound of force at 67.5 HzODS Animation @1x rpm (65.5 Hz)After ModificationsOperating Deflection Shape after rebuild at 100% where the NF is now located

TurbineBearing

1x rpm OB and IB probes 2.1 and 0.7 mils pk-pkThe 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.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

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. 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

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. 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