Remote Monitoring with Ultrasound
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. Today 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 usage: condition monitoring of bearings and rotating equipment, condition-based lubrication, and electrical inspection of energized electrical equipment.
Why Remote Monitoring with Ultrasound?
Remote monitoring with vibration analysis and temperature has been available for many years. For ultrasound, it’s a fairly new addition to the technology’s repertoire of capabilities.
Ultrasound is a proven technology that can detect certain mechanical and electrical faults much sooner than other technologies. By sensing subtle changes in ultrasonic amplitude, ultrasound is adept at finding early stage premature bearing faults. This is demonstrated by the I-P-F Curve.
Ultrasound plays a critical role in helping to 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. Ultrasound can prevent over- and under-lubrication, thus potentially eliminating a large number of bearing failures.
When a bearing lacks lubrication, there is an increase in friction. The higher friction also increases how much ultrasonic noise the bearing produces, indicated by a rise in the decibel (dB) level. While 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 be ceased. If the bearing already has a sufficient amount of grease when greasing, then the dB level will slowly begin to rise as more grease is applied. That’s because over-lubrication also increases friction in the bearing housing, thus producing a higher dB level.
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. If you are monitoring critical assets, ultrasound and vibration should be used together in an effort to potentially detect multiple failure modes that may be missed when only using one technology alone.
Remote Monitoring – Mechanical Inspection
Remote monitoring of bearings and other rotating equipment with ultrasound can be done in one of two ways:
The first is to use wired remote access sensors (RAS), which are mounted to the assets when it is safe to do so, and the cables brought out to a safe area (outside of guarding) where they can be connected directly to a portable hand-held ultrasound instrument. The cable lengths for the ultrasound remote access sensors can be made to lengths of up to 30 meters.
The ultrasound remote access sensors can also be connected to a junction box or switch box. As many as eight ultrasound RAS’s can be connected to one switch box. Similar to the way vibration analysis switch boxes work, the ultrasound sensors are connected to the switch box, along with the hand-held ultrasound instrument. During analysis, the inspector turns the dial to the next point to collect a reading.
Remote monitoring with ultrasound can also be done continuously. For continuous monitoring with ultrasound, UE Systems offers a sensor called the Ultra-Trak 750. This is another stud-mounted sensor, but instead of connecting directly to a hand-held ultrasound instrument, this sensor has a 4-20mA output that allows for easy connection to existing plant process monitoring systems. An audio output also allows for sound recording for further diagnostics or comparing baseline sound files to alarm level sound files. Ideal applications for this sensor could include: high-pressure safety relief valves, robotic applications, early warning of failure for a mechanical asset, flow disruption, and prompting a lubrication PM.
Remote Monitoring – Electrical Inspection
Ultrasound can be used to inspect almost any energized electrical equipment. This equipment may include metal-clad switchgear, transformers, substations, relays, and motor control centre, along with many others. Ultrasound can be used to measure equipment voltages from the low end (110 volts) to well over 12000 volts (12kV).
Traditionally, inspection of energized electrical equipment has been performed using noncontact infrared cameras. However, in recent years, ultrasound has been added to these inspections for various reasons. One of the main reasons has been for safety: An ultrasound inspection of electrical equipment can be done without the need to open the energized cabinet or enclosure. The high frequency sound produced by corona, tracking, and arcing from inside of the enclosure will exit through any of the openings. The inspector will hear the sound via the headset, and know that an anomaly is present. The sound can then be recorded to determine if the condition is corona, tracking, arcing, or some type of mechanical looseness.
Corona, by nature, does not produce significant heat that would be detected by an infrared camera. However, it does produce high frequency sound that can be detected by the ultrasound instrument. If corona discharge continues to occur, it can lead to a more severe problem such as tracking or arcing.
When the sound file of corona is recorded, there are signature characteristics visible in the FFT and Time Wave Form (TWF) that will help to diagnose the condition. For corona, the discharge points only occur at the highest voltage point on the sine wave. This means that the amplitude peaks in the TWF are somewhat equally spaced as the discharges are only at the positive peak of the sine wave. The result will be well-defined 60Hz or 50Hz harmonics.
For tracking and arcing there are also certain characteristics to look for. With tracking, the discharge does not have to take place at the peak of the waveform. Instead, it can happen anywhere on the positive portion of the cycle. The spacing of the peaks in the TWF would be similar, but not uniform. As tracking becomes more severe, there are more discharge events and therefore more non-uniformly spaced narrow peaks.
Arcing has the most non-uniform “look” in the FFT and TWF. Only the bursts of the discharge can be heard and these will be seen as wide peaks in the TWF view.
To inspect electrical equipment that can’t be easily reachable, one can attach a parabolic disc to an Ultrasound instrument and increase the detection range by 4 times, making it possible to detect electrical failures at a distance of approximately 50 meters.
Remote Monitoring with Ultrasound – Conclusions
Remote monitoring with ultrasound is a viable option for maintenance and reliability programs that are already monitoring assets traditionally with hand-held devices, and for programs where ultrasound is currently not a technology that is being used. Because it is very complimentary to vibration analysis for mechanical inspection and infrared thermography for electrical inspection, ultrasound will only enhance condition-monitoring efforts already in use.
Leaks can form practically anywhere in a plant. This includes pressurised systems and systems under a vacuum. Leaks can occur internally through valves and steam traps, in heat exchanger and condenser tubes or to the atmosphere. While it is important to locate potential safety hazards from leaks, the loss of gases through leaks can be very costly.
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