Due to positive feedback from our previous 5 case studies series this is our bonus case study brought to you with the Reliability Training Institute. If you have missed any of the previous case studies, they can be found on my LinkedIn profile, Seasoned Analyst blog and on the RMS Website.
Please like, follow, and share for more content.
This case study is a lot longer at over 34 minutes. Here we cover a 2-year project period from initial root cause diagnosis to designing, trialling, and implementing an innovative cost-effective solution.
A profitable plant is reliable, safe and a cost-effectively maintained plant.
The Seasoned Analyst
CBM Conference 2019 UK
I presented at my first Conference, and it was the first CBM Conference in the UK. It was well worth attending as I got to discuss various challenges in the condition monitoring and reliability sector, and the most common discussion point was buying from senior management and how to continually highlight the benefits of a condition monitoring program.
In addition there were vendors there demonstrating the latest advancements in condition motioning technology from vibration online and walk around, thermal imaging safety, ultrasound new high sampling rates time waveform analysis to motor electrical condition monitoring.
My Presentation was on a case study solving a reliability issue at a pumping station by designing and using a Dynamic Vibration Absorber. You can download the presentation below.
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.
