Airborne and structure-borne ultrasound has been around for more than 50 years. In the technology’s early days, the main application was compressed air leak detection. Even today, that’s still the most widely used application for airborne ultrasound.
Over the years, through advancements in ultrasound instrumentation and software, more maintenance and reliability personnel have begun to use ultrasound technology for more than just compressed air leak detection. Three applications in particular have seen a large increase in use: condition monitoring of bearings and rotating equipment, condition-based lubrication using ultrasound, and electrical inspection of energized electrical equipment.
Ultrasonic equipment detects airborne and structure-borne ultrasounds normally inaudible to the human ear and electronically “transposes” them into audible signals that a technician can hear through headphones and view on a display panel as a dB level. On some instruments, incoming sound can also be viewed on a spectral analysis screen that shows either the FFT or the time wave form. With this information, a trained technician can interpret the bearing condition to determine what, if any, corrective action is needed, and the current data can be compared on the spot with the baseline data.
Ultrasound technology has many advantages:
- It can be used in virtually any environment.
- Learning to use ultrasound technology is relatively easy.
- The technology is relatively inexpensive.
- Modern ultrasonic equipment makes it easy to track trends and store historical data.
- Ultrasonic technology has proved itself to be extremely reliable as a predictive maintenance tool, helping organizations save thousands of dollars and hours of productivity.
- There are remote monitoring options for both mechanical and electrical applications.
Airborne and structure-borne ultrasound instruments are an extension of the user’s sense of hearing. Similar to how vibration feels what you can’t feel and infrared cameras see what you can’t see, ultrasound hears what you can’t hear. There are sounds in a typical manufacturing environment (machines running, etc.) that prevent us from hearing other sounds, such as compressed air leaks or electrical discharges such as corona, tracking, or arcing. Ultrasound instruments listen for sounds that are not present in our normal audible range.
Typically, the sounds outside normal human hearing are high-frequency sounds. The high-frequency sounds are detected by the instrument and translated through a process called heterodyning into an audible sound that the inspector hears in the headset. The unit of measurement for sound is a dB level, which is indicated on the display of the ultrasound instrument.
Why remote monitoring with ultrasound?
Remote monitoring with vibration analysis and temperature has been available for many years. For ultrasound, remote monitoring is a fairly new addition to the technology’s repertoire of capabilities. When you’re considering adding ultrasound to your condition monitoring program, your decision will depend ultimately on which assets you would like to monitor. Once you have determined the assets that you would like to monitor, you need to identify the failure modes related to those assets. Understanding how those assets will fail will help you determine which condition monitoring tool can be applied to find those failure modes.
Ultrasound is a proven technology that can detect certain mechanical and electrical faults much sooner than other technologies can. By sensing subtle changes in ultrasonic amplitude, ultrasound is adept at finding early-stage premature bearing faults, as demonstrated by the I-P-F curve.
Ultrasound plays a critical role in helping extend the life of bearings in the I-P interval by condition lubrication of bearings. Studies have shown that the majority of premature bearing failures can be attributed to lubrication errors. Whether it’s over- or underlubrication, using the wrong grease for the wrong application, or lubricant contamination, it all comes back to improper grease application. Ultrasound can prevent over- and underlubrication, thus potentially eliminating a large number of bearing failures.
When a bearing lacks lubrication, there’s an increase in friction. The higher friction also increases the amount of ultrasonic noise the bearing produces; this is indicated by a rise in the decibel (dB) level. When greasing a bearing that needs lubrication, one should see a gradual decrease in the dB level. Once the dB level has fallen back to a normal or baseline level, greasing can cease. If the bearing already has sufficient grease, then the dB level will slowly begin to rise as more grease is applied. That’s because overlubrication also increases friction in the bearing housing, thus producing a higher dB level. The inspector would notice the rise in dBs as grease is applied and would stop greasing.
In the P-F interval, once a failure has begun, ultrasound is excellent at finding it. These are bearing failures that can be detected even before changes in vibration are. If you’re monitoring critical assets, ultrasound and vibration should be used together in an effort to potentially detect multiple failure modes that may be missed when using one technology alone.
Remote monitoring - mechanical inspection
Remote monitoring of bearings and other rotating equipment with ultrasound can be done one of two ways.
The first is by using wired remote access sensors (RAS). The sensors are mounted to the assets when it is safe to do so, and the cables are brought out to a safe area (outside of guarding) where they can be connected directly to a portable handheld ultrasound instrument. The cable lengths for the ultrasound remote access sensors can be made to up to 100 ft (30.48 m).