Case Study by: Alan Bandes, UE Systems, Inc.
A manufacturer of heat exchangers was confronted with a problem. The exchanger had to be delivered without leaks that would create production problems for the customer. The need to find all types of leaks, including low level leaks was demanded.
They typically use hydrostatic inspection which revealed some leaks but the production manager felt the need to investigate further to be sure no low level leaks were present. He considered utilizing helium testing which he felt would find smaller leaks than ultrasound typically can sense. His concern was two-fold, the proximity of the tubes and time it would take to identify the leaks. Typically helium detection is time consuming in that the sensor has to be carefully manipulated around the test area. In addition there is the potential for confusion should the helium from one site drift to the sensor as it is scanning an adjacent site.
His concern with using ultrasound was the limitation of detecting leaks below a rate of 1X10-3 std cc/sec.
Upon consulting with the factory it was determined that the use of a method called “Liquid Leak Amplification” might work. This incorporates the use of a surfactant with low surface tension. The heat exchanger is pressurized and the liquid is applied to sections of the tube sheet. This is similar in nature to the typical “bubble test” with one exception. The fluid used has low surface tension so that a low-flow leak will form a bubble that will burst almost immediately.
This produces a detectable ultrasound. The advantage is the bubbles form quickly and the bubbles do not have to be seen.
Due to the configuration of the tubes extending several inches beyond the tube sheet, it would have been extremely difficult to locate bubbles visually.
The decision was made to try the Liquid Leak Amplification method. The exchanger was pressurized to about 50 PSI and sprayed the Liquid Leak Amplifier over four-foot square sections of the exchanger tube sheet. With the scanning module of the Ultraprobe plugged in, the inspector began to scan along the tube sheet. He identified the leaks by detecting what he describes as a slow “pop-pop” sound occurring about 1-2 seconds apart. While he did not see the bubbles, he was able to confirm the leak by noting that the popping sounds did not occur around any adjacent tubes. It took him only 8 hours to complete the scan of 8,000 tubes. 3 leaks were identified in addition to those he had located previously with the hydrostatic test.
The net result, a leak free exchanger was delivered to a contented customer.