Ultrasound:  A new method for condition monitoring

Ultrasound:  A new method for condition monitoring

Many failures and repairs that commonly occur in the industrial setting can be prevented with ultrasonic technology, a highly effective non-destructive, predictive maintenance method.

Ultrasonic sensors designed with the right technology and software can be used for condition monitoring and predictive maintenance. This will minimize production downtime, improve quality control and safety, and decrease man-hours by improving troubleshooting capabilities.

Overview of the technology

Lightweight and portable, ultrasonic translators are often used to inspect a wide variety of equipment. Some typical applications include: bearing inspection; testing gears/gearboxes; pumps; motors; steam trap inspection; valve testing; detection/trending of cavitation; compressor valve analysis; leak detection in pressure and vacuum systems such as boilers, heat exchangers, condensers, chillers, tanks, pipes, hatches, hydraulic systems, compressed air audits, specialty gas systems and underground leaks; and testing for arcing and corona in electrical apparatus.

What makes airborne ultrasound so effective?

All operating equipment and most leakage problems produce a broad range of sound. The high-frequency ultrasonic components of these sounds are extremely short wave in nature. A shortwave signal tends to be fairly directional. Therefore, it is relatively easy to detect its exact location by separating these signals from background plant and operating equipment noises. In addition, as changes begin to occur in mechanical equipment, the subtle, directional nature of ultrasound allows these potential warning signals to be detected early, before actual failure, often before they are detected by vibration or infrared.

Airborne ultrasound instruments, often referred to as “ultrasonic translators”, provide information three ways:

  • qualitative with the ability to “hear” ultrasounds through a noise isolating headphone;
  • quantitative via intensity (dB) readings on a meter or display panel; and,
  • analytical with the use of spectral analysis software to review recorded sound samples.

Although the ability to gauge intensity and view sonic patterns is important, it is equally important to be able to “hear” the ultrasounds produced by various equipment. That is precisely what makes these instruments so popular. They allow inspectors to confirm a diagnosis on the spot by being able to clearly discriminate among various equipment sounds. This is accomplished in most ultrasonic instruments by an electronic process called “heterodyning” that accurately translates the ultrasounds sensed by the instrument into the audible range where users can hear and recognize them through headphones.

Applications

Generically, applications for ultrasonic translators fall under three basic categories: mechanical inspection, leak detection and electrical inspection.

Mechanical inspection
Mechanical equipment produces a “normal” sound signature while operating effectively.

As components begin to fail, a change in the original sonic signature occurs. This change can be noted as a shift in intensity on a display panel and/or as a qualitative sound change that can be heard through headphones and recorded for further analysis. An ultrasonic translator may be connected to a vibration analyzer, or the sound samples may be reviewed through spectral analysis software on a personal computer.

According to NASA research, “Ultrasonic monitoring of bearings provides the earliest warning of bearing failure. They noted that an increase in amplitude of a monitored ultrasonic frequency of 12 decibels over baseline would indicate the initial (incipient) stages of bearing failure. This change is detected long before it is indicated by changes in vibration or temperature.”

Other opportunities for ultrasonic mechanical inspection include: cavitation in pumps, compressor valve leakage, faulty gears, excessive rubbing and poor connections, to name a few.

Leak detection

The reason ultrasound is so versatile is that it detects the sound of a leak. When a fluid (liquid or gas) leaks, it moves from the high-pressure side of a leak through the leak site to the low-pressure side where it expands rapidly and produces a turbulent flow. This turbulence has strong ultrasonic components. The intensity of the ultrasonic signal falls off rapidly from the source. For this reason, the exact spot of a leak can be located. This can apply to pressure leaks, such as compressed air, and negative pressure (vacuum) leaks, leaks in valves and in steam traps.

Electrical emissions

Some preliminary experimentation has demonstrated that the main harmonic of an electrical emission will be most prevalent in corona. As the condition becomes more severe, there will be fewer and fewer  harmonics observed. As an example, arcing has very few 60/50-cycle components. Mechanical looseness will demonstrate harmonics other than 60/50 Hz with little to no frequency content between peaks.

Lubrication

As the concept of “predictive” lubrication vs. time-based (“preventive”) lubrication has emerged, there are times when it is useful to use spectral analysis combined with sound. Instead of lubricating bearings on a routine, “time-based” schedule, inspectors can routinely test bearings and identify those that need lubrication, leaving the others alone. In this manner, lubrication technicians can be taught how to effectively apply just enough lubricant to prevent over-lubrication.

Conclusion

Airborne ultrasound instruments are becoming an important part of condition monitoring, fugitive emissions and energy conservation programs. Their versatility, ease of use and portability enable managers to effectively plan and implement inspection procedures. By locating leaks, detecting high-voltage electrical emissions and sensing early warning of mechanical failure, these instruments contribute to cost reductions, improved system efficiencies and reduced downtime. For optimum effectiveness, it is recommended that all major technologies – infrared, vibration and ultrasound – be used as part of a comprehensive inspection program.

If properly implemented and used on a regular basis, ultrasound technology can be a fast, cost-effective means of monitoring critical components in plants.

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