Ultrasound is defined as high-frequency sound waves that are above the range of human perception. They range from 20 kHz to more than one megahertz. Airborne/structure-borne ultrasound ranges from 20 kHz to 100 kHz.
Airborne ultrasound instruments translate ultrasound signals into an audible range electronically by using heterodyning. As a result, they can then be heard through headphones or seen on a meter or display panel.
When a leak occurs, the fluid moves from the high-pressure to the low-pressure side, where it produces turbulence. This turbulence generates strong ultrasonic components. In addition, the intensity of the ultrasonic signal declines rapidly with distance, which helps isolate the leak's location.
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Detecting leaks is straightforward. A technician uses an ultrasonic translator to listen for distinct rushing sounds. The leak sound is followed to its loudest point. Because ultrasound has short wavelengths, the technician can discriminate between irrelevant background noise and the leak signal. Some instruments include a rubber focusing probe that narrows the reception area and shields against competing ultrasounds, which helps pinpoint small leaks.
Uncovering heat exchanger leaks
Ultrasound inspection is suitable for heat exchangers. Ultrasonic translators can easily detect tube leaks, whether the heat exchanger is under pressure or in a vacuum. In both situations, inspectors listen for a distinct rushing sound that signifies a leak. Heat exchanger heads are usually removed to access tube sheets.
If multiple heat exchangers of the same type require inspection, a contact method may be used. Contact zones are established along the length of the exchanger shell. An inspector touches each zone with a contact probe and compares the sounds to similar zones on other heat exchangers at the same test points. If a leak is present, the acoustic properties are different. It often produces a loud, popping or crackling sound.
Inspectors also can conduct tests when the heat exchanger is off-line. In such cases, the inspector uses a test unique to ultrasound. It incorporates ultrasonic tone generators, which are placed to produce a uniform ultrasound around the tube bundle. The inspector scans both tube ends and listens for an increase in sound level, which indicates a leak or, in some cases, that a tube is thinning.
Airborne ultrasound instruments are an important part of predictive and preventative maintenance. In addition to heat exchangers, they have proven effective for inspecting bearings, seals, gaskets, pipe systems and compressors.
Ensuring product quality
At a 200-acre manufacturing facility, two-man crews were assigned to check two to five heat exchangers per week to ensure that they were not leaking water into the product stream.
Inspectors pressurized the heat exchangers on the shell side, then brushed a soapy water mix over the tube sheet and ends. If bubbles formed, they knew they had a leak. A complete inspection took days. Even then, it was possible for tiny leaks to go undetected.
After adopting ultrasonics, inspection times were cut by two-thirds. Furthermore, ultrasonics significantly improved the technicians' ability to find the small leaks.
Doug Waetjen is with UE Systems, Inc. He can be reached at 800-223-1325.