Vibration monitoring and analysis keep machinery rotating

Predictive maintenance is synonymous with reliability as it’s one of the tools reliability engineers depend on to create cost savings that give plants a competitive edge. Predictive maintenance technicians use many technologies that can provide plants with the information necessary to identify faults. When used as intended, this can and will provide process, quality, and equipment reliability. A few of these technologies are vibration analysis, thermography, and ultrasonics.

Vibration analysis is a large component of predictive maintenance programs for plants with many pieces of rotating equipment. Anyone in industry has heard the stories of the "old way" of vibration testing. Individuals that have been in industry more than 30 years have probably seen some of the old ways for testing if vibration on a piece of equipment was excessive. The old ways only indicated the vibration was excessive and nothing else. How many of you have used or heard about the nickel test? How many of you have seen the Starrett gauge mounted in a spring-loaded body that would measure the amount of movement on a bearing cap?

Many electronic vibration instruments with software are available that not only indicate if vibration is excessive but allow analysis by breaking down the vibration into frequencies so faults can be readily identified by an experienced analyst. Vibration readings can be in displacement (mils), velocity (in./s, ips), and acceleration (g), and into acceleration’s high-frequency data. All the vibration measurements used for identifying various fault possibilities permit a more accurate diagnosis and can lead the repair crew in the right direction to complete that repair. This can decrease repair turnaround time and will increase plant and maintenance production. In many plants, a very high percentage of rotating equipment faults can be identified by simply using vibration spectrum analysis. Once a suspected fault is identified, it’s ideal to use supporting technology to confirm the diagnosis before reporting.

Donald Jones is predictive systems specialist at Citizens Energy Group in Indianapolis, Indiana. Contact him at [email protected].

Route-based vibration programs monitor rotating equipment condition on a weekly, bi-weekly, monthly or even quarterly schedule. Identifying condition status by trending vibration overall amplitudes to include specific frequencies will allow the analyst to provide plant personnel with an early warning of developing faults. A good program will also test the repaired equipment to verify acceptable vibration condition and report if that equipment will have continued availability and reliability. I prefer the extra benefits of a walk-around vibration program. The person that collects the vibration data should also use natural senses — vision, sound, smell, and feel. Using the human senses can provide valuable information to consider when analyzing. A complete program, using all the available technologies and human senses, increases plant and rotating equipment reliability.

Given all the vibration technology and analytical skills of the analyst, it will be worth nothing if the identification and explanation of found faults can’t be presented in an understandable report. But, on the other hand, even though vibration technologies have been around for more than 40 years, you can still find some people who are still skeptical of the information from the vibration guy. To some, it’s like the diagnosis of the fault came from a crystal ball. A vibration analyst has the tools that, when used properly, can see faults developing at a very early stage. Care must be taken to know your personnel and when a report should be provided. Too early of a call for repairs and no discernable faults may be seen with the naked eye; the analyst will be questioned on an alleged bad call. Too late of a call for repair and secondary damage will begin, which decreases the life of the coupled equipment.

I’ve provided predictive services to several different groups of maintenance and operations personnel at several different sites over my almost 30 years in plants. Each has differences on their approaches to the faults or how and when they want a report. To some, if the noise changes, a squeak develops, or the tone changes, they’re on top of it wanting answers. Others have to feel the floor shake before questions are asked and repairs are scheduled. Responding too early to developing faults can waste money and resources. However, if vibration gets to a point that you’re feeling it in the floor, it most likely has generated secondary damage. Reporting can be a balancing act. As in any plant, the scheduling of a repair has to be made so production doesn’t suffer and you want to get the most life from the rotating equipment without generating secondary damage. Finally, buy-in from supervision and management is imperative for any predictive maintenance program to be successful and have longevity.