A manufacturing company had a pressure decay leak system to check for leaks in compressed air housings. Their detector was able to find leaks as small as 0.02 cc/min. The leak program was designed for recording each housing with a batch/lot number and the corresponding leak data. If the housing reached or surpassed the leak limit, the part would be marked and quarantined. The pressure decay leak detector was a sensitive instrument, but it could not tell the operator where the leak was occurring.
How the pressure decay leak detector worked was by pressurizing the housing to a target pressure. The flow valves would shut, isolating the housing. After the pressure stabilized, the sensitive pressure sensors would pick up any loss in pressure over time. If the leak limit wasn’t reached, a green light would indicate a good leak test. If the limit was reached, a red light would indicate a failed leak test, and the housing would have to be segregated.
The housing design used a head, a bowl, a drain, and a differential pressure gauge. The leak paths were numerous. It could be at the drain, between the drain and the bowl, between the head and bowl, at the differential pressure gauge, and even in the casting of the head. The heads were made from a die-casted aluminum. If the process was not done properly, porosity could occur in the head. The leak detector was sensitive enough to find any voids that would allow air to pass through the head casting. With these many areas of potential leaks, it could be problematic if the reject rate was high.
For the application above, it is important to find where the leaks are occurring in order to create a corrective action. In order to find the leaks, they purchased a model 9061 Ultrasonic Leak Detector from EXAIR. Instead of pressure decay, the Ultrasonic Leak Detector uses sound. Whenever a leak occurs, it will generate an ultrasonic noise. These noises have a range of frequencies from audible to inaudible. The frequencies in the range of 20 Khz to 100 Khz are above human hearing, and the Ultrasonic Leak Detector can pick up these high frequencies, making the inaudible leaks, audible. The model 9061 has three sensitivity ranges and a LED display; so, you can find very small leaks. This unit comes with two attachments. The parabola attachment can locate leaks up to 20 feet (6.1 meters) away. And the tube attachment can define the exact location. With this application, they used the tube attachment to locate the leaks. After retesting the failed housings, they found that 80% of the rejects were from a sealing surface. They were able to replace or repair the o-rings. 10% of the leaks were coming from the drain. 3% of the rejects were leaking at the differential pressure gage. Both the drains and the pressure gages could be replaced with new units. 7% of the housings had a porosity problem in the head of the housing. For these, they were shipped back for evaluation to create a modification for a better casting. The production manager shared with me that an extra vent hole was required to reduce the void. This was a huge savings for the die-caster and manufacturing plant.
EXAIR Ultrasonic Leak Detector is a great tool. It can be used in a variety of applications including compressed air systems, bearing wear, circuit breakers, refrigerant leaks, and gas burners to name few. For the company above, it was a great tool to improve their assembly and testing process for their housings.
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