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Background:

During an asset assessment survey of a Pump Set we detected an anomaly in the vibration data.

This is a ‘Brook Crompton Parkinson’ motor D112 Frame, 2865 RPM, 50Hz 3 Phase, 415V 4.9A with and integral ‘APE-Lee Howl Limited’ pump end.

Image 1 is of the pump set, this is a circulation pump for a water system.

Image 1:

Pump Set

Analysis:

Using experience, the human sense and the vibration data the conclusion was that there is a flow related issue.

 

Vibration Data:

Figure 1 is the Autocorrelation of the pump end PeakVue data in a circular plot.

Fig 1:

Figure 1.

This data shows the abnormal ‘wobble’ operation of the pump and that for each revolution there are three restrictions in the motion.

 

Inspection:

On visual inspection of the pump system it was found that the pump supply valve was closed. This was opened and water was then allowed thought the pump.

 

Follow up Vibration:

Figure 2 is the same PeakVue data as before but now with the pump system in its correct operational state.

Figure 2.

This data now shows the smooth circular motion of the pump.

 

Summary:

This case study brings a few things to mind

  1. The most important part of any program is the person performing the data collection and analysis
  2. When all else fails, leave the air conditioning, and go examine the operating equipment. Go look, touch, feel, smell and listen to the machinery

 

A profitable plant is reliable, safe and a cost-effectively maintained plant

This month’s blog is to promote the thinking that when drive trains are aligned they should be aligned to the bearing tolerances and not the coupling tolerances. In addition how many people receive an alignment report with a soft foot check? We have found that some companies allocate their employees a laser alignment kit tell them what buttons to press and send them in the field. Without proper training and mentoring how will these employees learn correct Precision Alignment? Without correct training they will not know how to fix problems if they don’t understand fully what they are doing.

This month’s blog shows the importance of Precision Alignment including soft foot check and that the users of laser alignment equipment should be properly trained and mentored in Precision Alignment.

This survey was conducted by a great friend of mine and recent VCAT 3 Certified Seasoned Analyst James Pearce. linkedin.com/in/james-pearcevibrationanalysis

 

Background:

We were called to investigate an apparent increase in vibration levels after a high pressure hot water pump was replaced with a new pump end and a reconditioned drive motor. The operator felt that it was not running as smooth as the old pump set.

 

Instrumentation:

For this survey James used the CSI 2140 Dual channel Machinery Health Analyser. Data analysis was carried out using the CSI AMS Machinery Health manager software V5.61.

 

Methodology:

Vibration data including Velocity, Acceleration and bearing condition unit PeakVue was collected from each bearing location as close as possible to the source. Where applicable additional data including high resolution vibration data was collected.

 

Executive summary:

There are elevated directional Velocity vibration levels when running at 2680 RPM (Low speed). This is due to a coincidence of a system natural frequency being excited by a motor Soft Foot condition.

 

Maintenance Recommendations:

  • Check/inspect condition of the foundation, looking for looseness and any deterioration in the base plate.
  • Perform precision alignment that must start with a soft food check and soft foot elimination. Followed by precision laser alignment.
  • If these actions do not resolve the issue then stiffening of the base may allow for improved precision alignment and may move the system resonance out of the running speed range.

 

Analysis Summary:

  • There are increased directional Velocity vibration levels at the motor when running at 2680 RPM.
  • After reviewing the vibration data it was decided to perform further checks and the motor holding down bolts was loosened one time when in operation, this is to check for distortion of the motor rotor to stator air gap. During this test it was and found that the Velocity amplitude reduced. The amplitude reduced to its lowest level when the motor non-drive end foot bolt (bolt closest to pump #1) was loosened (see figure 1&2). This indicates there is a soft foot issue.
  • In addition an overall vibration coast down test & resonance bump test was performed. This data confirmed a natural frequency at 2X 2680RPM (see figure 3).

 

HPHW Pump #2 Motor Non-Drive End

The motor has elevated directional Velocity vibration levels. By loosening one motor fixing foot bolt at a time, the Velocity amplitude reduced. The amplitude reduced to its lowest level when the motor non-drive end foot bolt (bolt closest to pump #1) was loosened.

Figure 1 compares the Velocity spectra when running at 2680RPM, for the one order levels, as found state (4.332mm/sec RMS) & where the amplitude decreased the most after the motor foot bolt was loosened (2.651mm/sec RMS).

Fig 1:

 

HPHW Pump #2 Motor Non-Drive End

Figure 2 is a photo of the motor indicating which foot bolt was loosened which resulted in the best decrease in amplitude.

Fig 2:

 

HPHW Pump #2 Motor Drive End

Figure 3 is the data from a resonance bump test & overall vibration coast down test, performed at the motor drive end (DE).

The top plot bump test result indicates a system natural frequency that will coincide with twice the running speed (when running at the low speed) and amplify the vibration levels.

The bottom plot amplitude peak from the coast down test also confirms this condition with a peak at 5336 RPM, twice the running speed at the low speed setting.

Fig 3:

This is one I recently finished and thought it would be a great one to share so people know what can be achieved.

 

Background:

We had three pump sets suffering from elevated vibration levels when operated in different combinations. Conventional vibration analysis was performed and this indicated a structural resonant condition.

The pump motors are mounted on a false floor:

and the pump barrels are below the floor:

The pump with the worst motion was on pump 3, the one far away from the edge of the drop. Also this pump has the least structural support under the floor. When ran in certain combinations pump 3 would be excited very badly.

 

The cost effective solution.

I designed a vibration dynamic absorber.

Dynamic Absorbers are often overlooked and not used, they can be seen as a band aid or a last option for some vibration problems. Whereas in some cases they can be the only cost efficient option, and they are very effective.

The Vibration Dynamic Absorber is a unique bespoke item, maintenance free, that is designed to absorb unwanted energy. It is tuned to have the same resonant frequency as the structure to set up an out of phase signal reducing the signal generated by the structure.

 

How did I design these?

For this one it was more of a ‘gut feel’. I looked at the motor and then drew out a design that wouldn’t look out of place when mounted, and that had some adjustment to it when fitted as theory doesn’t always pan out in real life. Then from this I worked backwards to get the correct material dimensions/configuration so it was resonant at the target frequency. I also made some weight configurations so I could cover my target range.

I will be going back in 6 months to see how it fairs. I did consider a round bar and weight but thought that with the rectangular bar you have more control on what way it will be resonant. As once you have performed phase analysis on the motor you then know what way it is moving and can mount the absorber accordingly.

Image of Pump 2 Vibration Dynamic Absorber:

Image of Pump 3 Vibration Dynamic Absorber:

Pump 2 Live Motion Video

 

Pump 3 Live Motion Video

 

Pump 3 Slow Motion Video

 

What am I covering?

On pump 3 I am covering the one problem frequency, 1 Order, but the two arms are of different lengths in terms of the length from the point of pivot (clamping) to the mass. Also the arms are of different dimensions with different mass at the end so they could be tuned to the same frequency.

I also did find that the sweet spot was not necessary the point of higher deflection of the absorber and that the three motors all reacted differently.

 

Final Review of actual vs theory:

I have had time to review the final theoretical tuning of the three pumps to actual results. They are all different and no one motor is the same, they all have their own personalities dynamically wise.

Pump 3 had the highest overall vibration, one dominant frequency at 1 Order on pump 3 and this was successfully reduced.

Pump 1 and pump 2 had two frequencies in the data. And both of the vibration dynamic absorbers were tuned to the lower frequency not the one order.

 

Table of final overall levels:

Before After % reduction
Pump 1 Motor NDE (Top) 4.314 2.854 33.84%
Motor DE (Coupling end) 2.092 1.617 22.71%
Pump 2 Motor NDE (Top) 9.95 6.959 30.06%
Motor DE (Coupling end) 4.05 3.012 25.63%
Pump 3 Motor NDE (Top) 27.02 7.59 71.91%
Motor DE (Coupling end) 10.73 5.113 52.35%

 

Pump 1:

Pump 1 actually showed the text book results. The theoretical calculations for the tuned damper was for the lower frequency not the running speed (1520 CPM yes they are on soft start VFD). It actually split the frequency – text book……….beauty!!

 

I have more questions and theories now, this is pretty exciting stuff. Hopefully I can keep this going on other pumps.

G,day all, here is another interesting job I got called to

 

Background:

This pump and motor had a history of reliability failures from bearings, shaft shearing and pipework flanges leaking. This was a pair of pumps on separate base frames but secured to the same concrete floor with a pipework common outlet.

I performed vibration analysis with phase analysis and diagnosed a foundation and structural problems as the root cause.

 

Vibration Data:

Pump Vibration Data:

Figure 1 shows the overall Velocity vibration trend from our first visit and second visit. This is gathered at the motor DE.

From this trend you can see a marked increase in the velocity vibration levels from 6.304 mm/s RMS and 8.388 mm/s RMS.

Fig 1:

 

Fig 2 compares the acceleration time waveforms from the motor drive end bearings

From this comparison you can see the lower levels of pump A (Blue Plot) and the very high impacting from pump B (Green Plot)

Fig 2:

 

Fig 3 is the vibration data from the motor drive end bearing

There is high impacting form the motor bearing and damage to the inner and outer raceway

Fig 3:

 

Pump B – Motor Bearing Inspection

Failure Mode:

From inspection the failure mode as per ISO 15243:2004 is 5.3.3.3 False Brinelling.

False Brinelling occurs in the contact area due to micromovements and/or resilience of the elastic contact under cyclic vibrations. Depending on the intensity of the vibrations, lubrication conditions and load, a combination of corrosion and wear can occur, forming shallow depressions in the raceway. In the case of a stationary bearing, the depressions appear at rolling element pitch.

In many cases, it is possible to discern rust at the bottom of the depressions. This is caused by oxidation of the detached particles, which have a large area in relation to their volume, as a result of their exposure to air.

Key Points are:

  • rolling element / raceway contact areas
  • micromovements / elastic deformation
  • vibrations
  • corrosion/wear & shiny or reddish depressions
  • when stationary: at rolling element pitch
  • when rotating: parallel “flutes”

 

Findings:

  1. Depressions appearing at rolling element pitch indicating damage while the pump was in standby stationary bearing (Image 1)
  2. Indications of oxidation of the detached particles, which have a large area in relation to their volume, as a result of their exposure to air.

 

Bearing Inspection: Motor Drive End Bearing – FAG X-lite NU319E.TVP2

Image 1 is the outer raceway, and displays depressions appearing at rolling element pitch which indicates damage to the bearing when the motor was stationary

Image 1:

 

Image 2 is a close up of the depression at rolling element pitch on the outer raceway, this is from the load side of the bearings and also shows the roll over.

Image 2:

 

Image 3 is a microscopic image of a depression on the outer raceway.

Image 3:

 

Image 4 is an image from the inner raceway, this also displays the depressions at the pitch of the rolling elements.

Image 4:

 

Image 5 is a microscopic image of a rolling element.

Image 5:

 

 

Motion Amplification

Even with this vibration data the client was not convinced so I had to use another technology to show the client how the structural and base was causing them their reliability headache.

 

This first video shows how the pipe work was moving, this was the cause of the stress and strain to the flange joints that led to the leaks

 

This second video is of the base plate, this showed the true motion of the pump and how these failures were being induced.

Vibration Dynamic Absorbers:

Who has given these a try?

Dynamic Absorbers are often overlooked and not used, they can be seen as a band aid or a last option for some vibration problems. Whereas in some cases they can be the only cost efficient option, and they are very effective.

The Vibration Dynamic Absorber is a unique bespoke item, maintenance free, that is designed to absorb unwanted energy.

It is tuned to have the same resonant frequency as the structure to set up an out of phase signal reducing the signal generated by the structure.

Knowing the forcing frequency of interest, the material dimensions and parameters including Density (S.I. Units), Young’s Modulus (S.I. Units) and Area Moment of Inertia (si) we can design a component that is tuned to the problem frequency.

Example of a Vibration Dynamic Absorber in action on a horizontally mounted motor.

Example of a Vibration Dynamic Absorber in action on a vertically mounted motor in slow motion.

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