Reposted from ReliabilityWeb
One method of detecting vacuum leaks is to use airborne ultrasound detection, a technology already widely used for positive ultrasound leak detection in compressed air systems. But finding vacuum leaks is not as straightforward as finding pressure leaks, and often times, the method is abandoned in frustration.
One problem here is the quality of the ultrasonic instrument which can vary significantly from one manufacturer to another. Lesser quality detectors cannot function well in high noise situations. They simply have difficulty differentiating a leak sound from ambient plant noise. Since vacuum pumps already generate a lot of background noise, rarely will an inspector perform vacuum leak inspections in a quiet atmosphere. Another problem is lack of inspector training which really plays a role when searching for vacuum leaks in high noise environments.
Just like positive pressure leaks, vacuum leaks produce a rushing, whooshing ultrasonic signal with peaks around 35-40 kHz. The ultrasound is caused by turbulent flow of air molecules at the leak site. Positive pressure leaks, such as those found in compressed air systems, push the turbulent flow outward making them easily detectable from several feet with a quality ultrasound tool. Vacuum leaks behave quite the opposite, drawing the turbulent flow inward, decreasing the distance of detection as compared with positive pressure leaks. Most of the telltale leak sound is contained within the body which means inspectors must diligently trace an entire installation leaving no stone unturned in the search for ingress.
Read the full story by Allan Rienstra – SDT International and Karl Hoffower – Failure Prevention Associates including details and photos for a Vacuum Leak Inspection on Multiple Effect Evaporator at major Pacific Northwest Pulp & Paper Mill.
Ultrasound by Allan Rienstra - SDT Ultrasound Solutions