Ultrasonic: A New Method for Condition Monitoring

R. P. Naik, Raipur, National Thermal Power Corporation
Tags: ultrasonic inspection

At the National Thermal Power Corporation’s Sipat plant in India, we are relying on only vibration measurement as a basic technique for condition monitoring of bearings. Sometimes infrared temperature detectors are also used, but their use becomes necessary mainly when temperature monitoring becomes important.

A new technique called ultrasonic condition monitoring is now available which can detect even incipient faults in bearings as well as adequacy of lubricant.

IRD measurements are generally not useful for very slow speeds, but ultrasonic detectors can be used even in slow-speed machines such as APH guide / support bearings, etc. This technology can be considered an integrating technology since it can be used with infrared and vibration inspections as well as stand alone to perform a multiplicity of inspection activities. Instruments based on this technology can monitor a wide range of plant operations and yet are simple enough to be used with minimal training for basic, effective inspection routines.

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 leak detection is recommended by entities such as the U.S. Department of Energy as the best method for detecting the location of leaks in order to minimize energy waste and improve plant efficiency. 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.

Consider the following summary from a third-party evaluation team for the integration of ultrasonic technology in a single organization with more than 500 sites:

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:

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.

Condition Monitoring
Condition monitoring and predictive maintenance have traditionally been performed through vibration analysis, infrared and other technologies. Ultrasonic technology is an excellent option, especially for organizations with lower budgets.

Ultrasonic detectors are capable of accurately interpreting the sounds created by under-lubrication, over-lubrication and early signs of wear. The right ultrasonic technology is a fast and effective means of determining such conditions in moving, mechanical components such as bearings, gearboxes, motors, compressors, etc.

Ultrasound is produced by friction, impact, turbulence and electrical discharge. Friction and impact are the by-products of mechanical equipment. For example, a roller bearing will produce friction as the shaft and balls roll around the center. If there is too much friction, however, problems begin to occur on the equipment due to imbalance, or the bearing might seize, thereby shutting down equipment altogether.

Proper lubrication of critical bearings is important at all times. A properly lubricated bearing will produce a smooth rolling ultrasound, detectable by an ultrasonic receiver whose microphone can be placed in contact with the housing.

If the bearing is over-lubricated, very little ultrasound can be heard through the headset. If the bearing is under-lubricated, the intensity of the bearing will increase dramatically, and other sounds may be produced such as fluttering or scratchiness. Indications of an under-lubricated bearing will appear in ultrasound even before infrared can detect heat increases and well in advance of vibration analysis.

In addition, once a bearing begins to wear, the ultrasonic wave will produce large spikes in the signal caused by flat spots or scratches on the race. The spikes are heard as pops or crackles through the headset.

Once the ultrasound produced by the bearing begins to indicate these characteristics, the replacement of the bearing can be planned during normal production shutdown. The detection of wear is instantaneous. It is not necessary to take readings of the bearing from several points of contact along different axes and send the readings away for analysis.

The use of ultrasound technology for condition monitoring does not need to be complex, however. Software may be used to record the output of the ultrasonic sensor. Once a baseline or benchmark signal of a component is recorded, future recordings may be compared to it in order to determine the wear or proper lubrication of the component over time.

The basic advantages of ultrasound and ultrasonic instruments are:

  1. They are directional and can be easily located.

  2. They provide earliest warning of impending mechanical failure.

  3. Many problems are only detectable in the ultrasonic range.

  4. Audible noise is ignored, increasing the selectivity of the ability to pinpoint. Therefore, they are more accurate at pinpointing problems.

  5. They can be used to locate leaks and potential electric failure conditions.

  6. Instruments can be used in loud, noisy environments.

  7. They support and enhance other predictive maintenance (PdM) technologies or can stand on their own in a maintenance program.

  8. They are instantaneous in inferring diagnosis.

  9. Isolation of faulty components, even internally, is possible.

  10. More versatile – Ultrasound can be used for several applications.

  11. Non-destructive – Ultrasonic instruments do not adversely affect or interfere with the component under test.

  12. Ultrasonic testing can be performed while the equipment is operating.

  13. Maintenance personnel currently using IRD can easily use these equipments.

  14. They can detect even airborne sound waves from the equipment and many motor NDE bearings.

Airborne ultrasound translators are relatively simple to use. They consist of a basic handheld unit with headphones, a display panel, a sensitivity adjustment, and (most often) interchangeable modules that are used in either a scanning (airborne) mode or a contact (structure-borne) mode.

Some instruments have the ability to adjust the frequency response from between 20 to 100 kilohertz (kHz). Ultrasound instruments may be analog or digital. Digital instruments indicate intensities as decibels. Digital instruments generally have onboard data logging with data management software to provide trending information and create alarm groups for equipment needing special attention. Some of the digital instruments also have on-board sound recording, which enables users to grab sound samples and review them on spectral analysis software.

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.

Reciprocating compressor valves are very noisy and produce a lot of extraneous vibration. By isolating the sound with the advantage of the shortwave nature of ultrasound, it is possible to listen to and view the sounds of these noisy valves in real time and to determine a leaking valve. As it opens and closes, there will be a definite, pronounced clicking sound, and sound from any leaking valve will be clearly distinguishable from normal valves. Similarly, many other steam, air and/or H2 leakages also can be located by these instruments.

When a leak occurs, the turbulent flow produces sound pressure waves all along the spectrum from 0 hertz to100 kHz and beyond. Lower frequency sounds travel greater distances and interfere with ambient noise such as running machinery. Also, these sounds have greater energy and can easily reflect off surfaces, minimizing the ability of a low-frequency microphone to accurately locate the leak. High-frequency waves (those far above 40 kHz) do not have sufficient energy to be detectable from reasonable distances. An ultrasonic sensor that is used for the detection and location of leaks should:

Electrical emissions
Some preliminary experimentation has demonstrated that the main harmonic of an electrical emission (60 Hz in the USA, 50 Hz elsewhere) will be most prevalent in corona. As the condition becomes more severe, there will be fewer and fewer 60/50 Hz 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.

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.

When a bearing has exceeded a baseline by 8 dB with no change in acoustic quality, the bearing should be lubricated. While applying lubrication, the technician should stop when the sound level has dropped to the predetermined baseline level. A way to demonstrate this process is to view a sound image while noting changes in amplitude and listening to acoustic properties in real time.

Spectral analysis software
With the introduction of spectral analysis software, a similar type of FFT diagnosis can be performed on a standard PC as long as the PC has a sound card. These programs not only provide the spectral and time series views of sound, but enable users to hear the sound samples simultaneously as they are viewing them on the PC monitor.

Typically, a sound sample is recorded using an MP3 recorder or tape recorder. Some ultrasonic instruments have on-board sound recording, which can be downloaded to a PC via a compact flash card. When played back in real time, the acoustic properties can be analyzed. Based on a known “good” or “normal” condition, an anomaly can be quickly determined.

However, before going for this technology, we must consider the following turnkey implementation program for ultrasonic technology integration:

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 at our plants. The approach to implementation should include the right product, training of personnel, identification of critical components, benchmarking, and the determination to follow through on good intentions.

A lack of training and understanding, irregular monitoring and a lack of commitment to the predictive maintenance program will lead to poor results. However, proper implementation of ultrasonic technology will increase reliability, decrease troubleshooting time, and decrease time spent putting out fires by our operation and maintenance staffs.


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