There are four of these pumps at the facility
– This pump has been modified structurally a couple years ago near the floor at the “seismic damper”
– This was the only pump so modified
– The discharge/recirculation piping was modified a couple years ago on all 4 units
– After piping mods this pump has higher velocity flow rates than all the other identical pumps
– Start of vibration problem coincides with mods

Limitations to testing
– Must be completed during normal quarterly test
– Limited to 30 minutes maximum running time
– Shaft cannot be touched to apply reflective tape for RPM reference 

Vibration spectrum from top of pump motor
- Data captured during low point of overall trend

Vibration spectrum from top of pump motor
- Data captured during high point of overall trend
- Amplitude at about 14 Hz (829 CPM or 0.46 orders) ranges from 0.1 to 0.4 ips (peak)
- Subsynchronous vibration dominates on the motor

Summary of Findings:
– Overall vibration intermittently high/low
– No consistent period associated with changes in amplitude
– Amplitude consistently higher in radial direction perpendicular to discharge axis
– Amplitude highest at top of motor, lowest just above the floor
– Pump is running at or near “shut-off” head creating severe turbulence and broadband low-frequency vibration energy due to output restriction
– The dominant frequency is 0.46XRPM (subsynchronous)
– 0.46XRPM frequency is “whirl” common in this type of pump arrangement (but not as dominant amplitude)
– This is also seen on the other 3 pumps, but not at excessive amplitudes
– Audible sound energy in compartment tracks variation in vibration levels
– High flow velocities create more turbulence in piping and higher vibration levels

Vibration spectrum from top of 2” diameter recirculation pipe
–         Piping in compartment was unsupported
–         Amplitude at 14 Hz (829 CPM or 0.46 orders) is over 2.0 ips (peak)!!

Vibration spectrum from top of 10” diameter discharge pipe
–         Piping in compartment was unsupported
–         No dominant frequency peaks

Piping Vibration Test Conclusions
– International guidelines for nuclear plant piping vibration say piping with amplitudes greater than 1.1 inches/sec (peak) should be operated for only a few minutes at a time.
– Recirculation pipe has levels twice this high!
– Discharge pipe is OK.
– Natural frequency test needed to determine if resonant amplification is happening on pump and piping.

What type natural frequency test can be done (in 2 hours, before end of day)....
– Uncoupled coast-down?
–         Cannot uncouple pump
–         Cannot run unit again
– Temporary brace?
–         Not allowed to temporarily brace structure
–         Cannot run unit again
– Impact response with rubber mallet?
–         Perfect!

Impact response waveform from top of motor
– Horizontal axis
– 90 degrees from pump discharge
– Waveform of response has Impact response pre-impact and is fully decayed

Spectrum from top of motor
– Dominant response indicates resonance
– Natural frequency near 14 Hz is coincident with the “whirl” forcing frequency

Impact response waveform from top of recirculation pipe
– Pipe “rings” in response for 10 seconds!
– Sensor orientation is vertical

Impact response spectrum from top of recirculation pipe
– Dominant response indicates resonance
– Frequency coincides with “whirl” forcing frequency

Is there a method to determine from the acquired data how much amplification is a result of the resonance?
– Calculate the Critical Damping Ratio and the Amplification Factor (Q)
– This involves looking at the rate of decay of the energy in the waveform

Resonance Amplification Factors (Q)
– Tell us by what factor the forcing frequency amplitude is being increased due to resonance
–         Motor/pump assembly = 34!
–         Recirculation pipe = 96!
These are extremely high Q's indicating very high amplification of forcing frequency amplitude
 

Can any other useful information about the resonance be obtained from the acquired data?
– Yes. The MAARS Model 5000 data acquisition system stores data simultaneously on all channels.
–         This means that phase relationships can be examined in the wave data to determine operating deflection shapes (ODS).
–         ODS actually allows us to visualize the unit's deflection at the offending forcing frequency.
 

Amplitudes of Whirl Frequency in mils
–         Phase readings of Whirl frequency in degrees
– Range of less than 30 degrees is considered “in phase”
 

Conclusions
– Moving the natural frequency away from “whirl” will dramatically reduce Overall vibration amplitudes
– Probably will need to add stiffness to raise the natural frequency
– Only after this is done will we be able to assess the true amplitude at “whirl” frequency
– Mod of “Seismic Damper” in the past probably contributed to resonance
 

Recommendations
– Recirculation Pipe
–         Attach pipe brace to adjacent, stiff, 10” diameter discharge pipe to raise the natural frequency of the pipe away from “whirl”
– Motor/pump
–         Do comprehensive Modal Analysis Test
–         Use acquired data with model to determine best location to add bracing
 

Eventual Corrective Actions
– Modal Analysis confirmed initial test analysis.
– Computer model was used to determine bracing method.
– Braces (turnbuckle style) installed just below pump/motor flange.
– First attempt moved resonance to 1X RPM!
– Stiffness of brace increased by increasing torque on turnbuckle braces and was successful (moved resonance above 1X RPM).
– Recirculation pipe braced to 10” discharge pipe.

Test Equipment

– MAARS Model 5000
– 1 pound rubber mallet
– CTC AC102-1A accelerometers
– CTC shielded twisted pair cables.

 Analysis Software

– MAARS Pathfinder
– SpectraPlus
– OpenOffice.Org spreadsheet