Reliable Flexible Couplings

05.12.2017

Simon is a condition monitoring specialist from a local oil refinery. He contacted my office for advice about predicting flexible coupling failures. Currently, they perform basic vibration analysis on their pumps and motors using an overall meter.  They have some success predicting bearing failures but the same cannot be said for couplings. Several unexpected failures shut them down this year. 

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Figure 1 - Flexible Coupling and Airborne Ultrasound Sensor

Within the facility, 58 pump systems considered “A Critical” were identified, meaning if they go down, the plant goes down. I suggested ultrasound as a fast, safe, and affordable solution. Specifically, the SDT270DU offered the best value. Not only could Simon use it to monitor couplings with ultrasound, it could also take vibration measurements, thus eliminating the need for Simon to carry two data collectors.

Flexible couplings cannot be monitored with vibration analysis for two reasons. First, there is no suitable contact point for an accelerometer. Second, the primary defect symptoms, friction and impacting, are best measured with ultrasound. By placing an airborne sensor near to the coupling Simon can quickly trend an evolving defect. The SDT270DU gives Simon the choice to either spot check for defects – good – or integrate all 58 couplings into his established bearing routes – best. 

I explained to Simon how several clients already trend couplings using the Flexible Wand. The SDT270 collects a STATIC ultrasound measurement that gives four indicators of condition. The first two – Overall RMS and Max RMS – indicate the level of friction produced by the defect. When these indicators rise, maintenance may consider corrective alignment during a planned shutdown. The second two – Peak and Crest Factor – identify the emergence of impacting. Together, all four indicators establish a lifecycle trend for each coupling. 

Once impacting appears, the Peak indicator increases in step with Overall RMS. Since Crest Factor (CF) is a ratio between RMS and Peak, a rising CF indicates that the window for simple maintenance has narrowed. At this stage inspectors collect a DYNAMIC measurement. The DYNAMIC measurement gives a visual representation of friction and impacting severity using the time view. For both STATIC and DYNAMIC measurements it is important to define the signal acquisition time. 

User defined signal acquisition time, available exclusively on SDT instruments, is a luxury that lends ultrasound technicians the highest level of precision. Without the ability to set the sample time, inspectors must guess when to pull the measurement trigger, and question the validity of their data. Simon explained that all 58 pumps turn at speeds above 1800 RPM. Accordingly, he should set his SDT270’s signal acquisition time to between one and three seconds. One to three seconds at 1800 RPM samples the coupling for 30-90 revolutions. 

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Figure 3 - SDT270DU: ultrasound and vibration input

Shaft couplings are guarded for safety

Any ultrasound inspector working around rotating equipment must be required to demonstrate an understanding of company safety policies. Safety considerations are engineered into SDT sensors. The Flexible Wand’s 10mm diameter sensor is designed to access the coupling with the safety guard in place (see figure 2). The 21” long sensor sports a comfortable, ergonomic grip that allows an inspector to collect danger-free data. 

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Figure 2 - A Flexible Wand from SDT detects friction and impacting from a flexible coupling

Simon seemed convinced, but wanted to hear more. Since this solution was already working well at a nearby paper mill, I introduced Simon to the plant manager, Sunil, and invited them both to lunch. Sunil and Simon connected on so many common reliability issues that afternoon. He confirmed the affordability of this solution was based on coupling failures alone but went on to explain how their mill was rolling out ultrasound for acoustic lubrication, steam trap monitoring, electrical inspection, and air leak management. Simon and Sunil continued their conversation well into the afternoon. They agreed that ultrasound, with its 8 primary applications for reliability, represented a fast, safe, and affordable technology with the potential to revolutionize reliability culture. I sat back, happily watching two impassioned specialists strategize about reliability culture. I love my job.  

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Figure 4 - Measuring ultrasound without user defined acquisition time results in missed data points

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Figure 5 - User defined acquisition time samples the complete signal resulting in more reliable, meaningful data

Allan_risestra

Allan Rienstra, SDT International,

allan(at)sdthearmore.com

www.sdtultrasound.com

 

About the Author

Allan Rienstra is the director of business development for SDT International, a Brussels-based manufacturer of ultrasound solutions comprised of hardware, software, training and consulting. A 25-year commitment to ultrasound applications has seen Rienstra play a leading role in the deployment of ultrasound-based solutions in 5 continents. He is the co-author of “Hear More, A Guide to Using Ultrasound for Leak Detection and Condition Monitoring.” Rienstra lives in Cobourg, Ontario, Canada with his wife and two sons. 

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