As maintenance and reliability professionals, we must ensure that the assets that we are responsible for are operating as they were designed and installed to do. The hope is to move more toward predictive rather than reactive maintenance. It has been proved that when plants and facilities have a robust proactive maintenance program, that facility will operate more safely, with less downtime, and with improved product quality.
Maintenance and reliability professionals have tools at their disposal to use to monitor asset health on a routine basis. Similar to the way a doctor may use a stethoscope to listen to a patient’s heartbeat or a thermometer to measure the patient’s temperature, the maintenance and reliability professional uses condition monitoring tools that can help assess the health of a mechanical asset.
How does ultrasound work?
All operating equipment, most leakage problems, and electrical discharges produce a broad range of high-frequency sound. Ultrasonic instruments, sometimes referred to as ultrasound translators, sense and receive these high-frequency sound waves. High-frequency sounds are above the range of normal human hearing. Typically, sound waves sensed by human hearing are between 20 Hertz and 20 kilohertz, with average threshold of normal human hearing around 16.5 kilohertz (kHz). If a maintenance professional uses an ultrasound instrument with frequency tuning capabilities, the lowest frequency that it can be adjusted to is 20kHz. If the instrument being used is on a fixed frequency, it is usually centered around 38kHz. As a result, the ultrasonic instrument is already listening for sounds that are beyond the range of normal human hearing.
By nature, high-frequency sound waves are short-wave signals; therefore, they are very low-energy and tend not to travel very far from their source. This makes it easy for users of ultrasound technology to pinpoint the location of a compressed air leak or a particular mechanical problem. There are three main sources of ultrasound for typical plant/facility maintenance applications:
- Turbulence, such as a compressed air/gas leak to atmosphere, air in leakage (as with a vacuum leak), or a leaking valve or steam trap.
- Friction, created by a bearing lacking lubricant, which helps dampen the stress distribution between the bearings and the contact area. Along the same lines as friction, as a bearing begins to wear and show fatigue, there also are increases in high-frequency sound, even to the point of the user being able to hear and identify bearing fault frequencies such as inner race, outer race, cage, and ball-pass faults.
- Ionization created from electrical anomalies such as corona, tracking, and arcing in energized electrical equipment.
Ultrasound instruments give both qualitative and quantitative information. Qualitative information is given via the headset because of the ability to be able to “hear” what the bearing sounds like. Qualitative information also is given on the ultrasound instrument's display by way of the decibel (dB) level once contact is made with the bearing or once an air leak is detected. Some ultrasound instruments have the ability to view the FFT or time wave form of what the inspector hears in real time as the data is being collected. The ability to analyze and view FFT and Time Wave Form data of recorded ultrasound sound files brings about a more diagnostic use for ultrasound. Now, bearing fault frequencies can be identified, or a lack of lubrication condition can be noted. If ultrasound is being used for electrical inspection, corona, tracking, and arcing can be easily identified through FFT or time-wave form analysis of previously recorded ultrasounds.
Bearing inspection with ultrasound
Vibration analysis has long been the instrument of choice to use for bearings and other rotating equipment. More commonly, ultrasound is being used in conjunction with vibration analysis to help technicians confirm the condition of mechanical assets. Because of ultrasound's versatility, if a facility does not have a robust vibration analysis program in place, ultrasound can be implemented to detect early-stage bearing failures as well as other issues.
If the vibration analysis is performed by an outside service provider on a quarterly or monthly basis, ultrasound can be used during the interim. This will help the facility know the condition of some of the more-critical assets before the service provider enters the facility. The service provider’s time thus can be used more efficiently because the plant will know whether there are any eminent problems with the assets that are being monitored by ultrasound. The service provider can then prioritize work based on the ultrasound findings. Another scenario in which ultrasound may be used first over vibration analysis is with the monitoring of slow-speed bearings.