Leak Detecting With Hydrogen
The Kenmore UK operation is one of Europe's leading manufacturers of wire-on-tube evaporators, which are exported all over the world. The wire-on-tube evaporators are produced under tightly controlled conditions. The evaporators are made from refrigeration-quality steel tube, and range in size from one to eight shelves, with a maximum overall evaporator length of about 1.2 meters.
The tubes themselves often take convoluted shapes, and can be round, square or a combination of both. This inevitably leads to joints being introduced to achieve the final shape. The number of joints per unit ranges from two to 11, and can involve steel-to-steel or steel-to-copper welds. Even with the utmost care being taken during the manufacturing process, these joints are a potential source of leaks, and so it is necessary to introduce some form of leak detection to provide quality control at the end of the production line.
Traditionally, leak testing has been carried out by immersing the units in water, filling them with pressurized nitrogen, and then watching for bubbles. While generally effective, this method is relatively cumbersome and messy, and is not always 100 percent effective because bubbles from the smallest leaks (which are invisible to the naked eye) are relatively infrequent - perhaps one every 10 seconds - and can occasionally be missed.
Kenmore engineers were aware that other techniques for sniff testing are available, and looked at a number of alternatives including a chamber test system using helium as the tracer gas. They were eventually led to Sensistor Technologies hydrogen-based units by a customer who was using them.
Initially Kenmore started out with three of the company's earlier 8505 models. Kenmore then invested in the latest H2000 leak testers, which have built-in digital electronics to make the logging and display of results easier.
The Sensistor units are mounted on a control station in conjunction with controllers that allow each operator to log on and specify the unit being tested, as well as storing the test results and pass/fail details. Once the operator has logged on and specified the test, the operator passes the sniffer probe over the joints and everything else proceeds automatically.
Advocates of hydrogen consider it an ideal tracer gas for leak testing in production environments such as Kenmore's plant. It is the lightest element, they said, with higher molecular speed and lower viscosity than any other gas. As a result, it can easily be introduced into the system to be tested, mixes more quickly with air and other gases, and is easily evacuated. More importantly, it has the highest leakage rate of any gas. The normal background concentration of hydrogen is 0.5 parts in 106, in contrast to helium, which is 5 parts in 106, 10 times more common.
Background concentration in air is a limiting factor for any detector. The Sensistor H2000 is sensitive enough to detect background concentration levels of hydrogen. This allows it to identify even the smallest leaks, with a sensitivity of 5 x 107 mbarl/s (millibar-liters per second). The detector uses a semiconductor sensor head mounted on a handle. The sensor is specifically designed for hydrogen, and the presence of other gases does not affect the results. The device does not contain any moving parts.
Les Shaw, quality engineer at the Kenmore plant, said, "The instruments are easy to use, and the operators like them because the test process is clean and dry. In addition, because they are more effective at finding the smaller leaks, we believe the overall attention to detail in our manufacturing process has improved."
For more information, visit www.sensistor.com, or call 978-439-9200.
Publication date: 04/10/2006