1. Basic Introduction of Leak Testing in Investment Casting
Leak testing is the process of checking a system for leaks (a defect). Leak testing can be achieved using various leak detection methods and is employed to test for defects in products and to ensure the proper function and maintenance of industrial systems and pipelines. The choice of the detection method depends on which method is most appropriate for the system at hand.
There are several points on performing leak test:
1. Leak test shall be conducted after passing the hydraulic test. The leak test can be carried out simultaneously with the air pressure test for the pressure vessel required by the design. The test pressure shall be the pressure of the air pressure test.
2. The gas used in the leak test should be dry, clean air, nitrogen or other inert gas.
3. When conducting leak test, safety accessories should be fully installed.
4. During the test, the pressure shall rise slowly, after reaching the specified test pressure, the pressure shall be maintained for no less than 30 minutes, and then the pressure shall be reduced to the design pressure. All welds and joints shall be coated with soapy water for inspection, and no leakage shall be considered as qualified. In case of leakage, hydraulic test and leak test shall be conducted again after repair.
2. Water Leak Testing VS Air Leak Testing
Water leak testing is performed according to the requirements of the product function. Products will be submerged in a certain depth of water, so there will be a corresponding stress around parts, and water droplets are likely to enter the product. In the process of the test, there will be many bubbles from the position of the leaking parts. According to the size of the bubbles, we can roughly determine whether the products meet the requirements.
The leak detector made of compressed air is used for air tightness detection. Compressed air is used to fill into the parts. After the pressure stabilizing test, the leakage rate can be obtained directly through the sensor. Air tightness test replaces water test precisely because of its high detection efficiency and data display, which can realize automatic detection and won’t damage the work piece in the detection process. It can be tested from negative pressure to high pressure, which reduces the cost of waterproof grade test. To find leaks, a low pressure (30-50 PSI) air test is just as effective as high pressure (150 PSI) water test. This is because the viscosity and surface tension of water is greater than that of air. For example, the viscosity of water is about 89 times greater than the viscosity of air. Viscosity is the internal friction of water making it resist the tendency to flow, particularly through a small opening. And, while the surface tension of a water to air surface is 0.005 lb. ft./ft., the air has no surface tension. Both viscosity and surface tension are forces that will prevent water from escaping through even a very small hole, forces that do not prevent air from escaping through the same size hole. Therefore, many times systems show a leak with a high-pressure air test when in actuality no water is leaking.
Airtight test is not so simple as installing the equipment into the tooling and running directly, but the test result can be obtained only after mastering various knowledge. From the design of tooling, pipeline design, leakage detector equipment selection, unit display, test method selection, leakage rate critical value selection, environmental temperature control, airflow control, leakage detector equipment internal temperature control to temperature compensation, all need to be strictly controlled.
3. How does JC Casting Perform Leak Testing?
JC Casting has water leak inspection equipment. Moreover, JC Casting customized special gauges for different products in leak testing. Furthermore, JC Casting applies countering device to set fixed time and movement. Therefore, the steps and time cost are fixed. They are not changed by inspectors. Thus the results are more reliable.