Corona Discharge

Traditional use of infrared cameras reveals "hot spots" that the naked eye otherwise misses.   But corona, arcing, and tracking do not always generate significant increases in temperature.  Ambient high temperatures can also mask them from the camera.  They do however generate distinct noises in the ultrasonic range and are detectable using ultrasonic listening equipment.  Ultrasonic detection systems complete the picture by extending the range of human hearing to listen for ultrasounds produced by faulty insulators, line bushings, transformers, potheads, and arresters.

Analyze Lubrication Needs

All rotating equipment produces ultrasonic or acoustic vibration regardless of the state of lubrication.  By measuring and trending this energy with a digital ultrasonic detector lubricators trend deviations from normal baselines and determine specific lube needs.  Lubrication absorbs energy created by friction between the balls and raceway of a bearing. Acoustic vibration is low when the bearing is properly lubricated but as the lubrication film breaks down this energy proportionally increases even though the bearing may not have any significant wear.  This energy can be measured and trended on the digital readout of the SDT 170.  An increase of 8 to 10 decibels over historical baseline indicates a need for lubrication.  This is confirmed by listening to the bearing's acoustic qualities in the headphones, or by viewing the waveform on a spectrum analyzer.  Bearings lacking lubrication will sound louder, with a rough growl, compared to the relatively smooth whirring noises of a well-greased bearing.  The waveform on an oscilloscope will show inconsistent peaks if the bearing is lacking grease.

Heat Exchangers

Pinpointing leaks in heat-exchanger tubes and tube sheets is an important use for ultrasonic diagnostic equipment.  Labor intensive methods include flooding the tube walls with water and watching for leaks but its time consuming and not always effective.

Ultrasonic detection can be carried out using air pressure or with an ultrasonic transmitter.  When pressurizing the exchanger with air the method used is the same as if detecting a compressed air leak.  The hissing rush of air at the leak reveals quickly the source of the problem and the directional nature of a quality ultrasonic detector will allow the leak to be pinpointed immediately. 

The other off-line method involves flooding the tube shell with ultrasound from an ultrasonic transmitter.  The low energy ultrasound generated by the transmitter is not strong enough to penetrate the steel walls of the tube but easily pass through hairline cracks.  Using a flexible airborne sensor the integrity of each tube can be quickly verified. 

With this method it is extremely important that the transmitter used works in a bi-sonic mode.  That is to say that the ultrasonic transmitter emits two separate, overlapping frequencies.  Some products may claim to use an alternating sound pattern but in reality do not.  The problem that arises is a phenomenon known as standing waves.  The outgoing wave hits a surface and reflects back to the source.  On its travels it meets more source waves with the same amplitude and frequency.  The resulting sine waves cancel each other out creating silent dead zones.