What is the difference between a gas-tightness test and a leakage test?
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The relationship between airtightness testing and leakage testing can be understood as a \"species-subspecies relationship.\" In simple terms, airtightness testing is a type of leakage testing, but leakage testing includes more than just airtightness testing. In engineering fields such as chemical industry and pressure pipelines, the two terms are often used interchangeably in a broad sense; however, from the perspective of standards and regulations (such as TSG D0001 and GB/T 20801), there are clear differences in terms of inclusion relationships and focus. The following are the detailed differences and connections between the two: 1. Core relationship: inclusion and being included. Leak Test: is a general term. It refers generally to all test methods used to detect whether there are leaks in a system. Tightness Test: It is a specific method. It specifically refers to a method that uses gas as the medium, and at the design pressure, checks for leaks by applying soapy water or observing the pressure drop. To use an analogy: the “leakage test” is like “fruit,” while the “airtightness test” is like “apples.” In addition to apples (air-tightness testing), there are also bananas and oranges (helium leak testing, ammonia leak testing, etc.).
3. Why is there such a distinction? This distinction is mainly aimed at addressing different risk levels and detection accuracy requirements. Normal situation (using a gas-tightness test): For ordinary chemical pipelines, as long as there is no obvious hissing sound or soap bubbles, they can be considered qualified. Special cases (using other leak tests): For pipelines transporting highly toxic gases (such as phosgene) or high-purity gases, even trace leaks are not permitted. At this point, soapy water may not be able to detect minor leaks; more precise instruments such as helium mass spectrometry leak detectors must be used for detection. In this case, although it is essentially also a form of \"leak detection,\" we usually refer to it as a \"sensitivity leak test,\" rather than simply calling it a \"air-tightness test.\" 4. Summary: In the preparation work for operating a chemical plant, if the document mentions a \"air-tightness test\", it usually refers to the conventional process of applying pressure with nitrogen or air and using soapy water to detect leaks. If the document states “leakage test,” you need to check the specific regulations; it may involve a airtightness test, or it may require more advanced helium leak detection methods. In one sentence: all airtightness tests are types of leakage tests, but not all leakage tests are simply airtightness tests.
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The specifications divide the leakage tests into airtightness leakage tests and sensitivity leakage tests! Sensitivity leakage tests are further divided into bubble leakage detection – direct pressure method, as well as several other halogen leakage tests with higher sensitivity. Sensitivity leakage tests operate at low pressures and thus present a low risk, while airtightness leakage tests operate at high pressures and carry a higher risk ； Usually, the bubble leakage detection – direct pressurization method is used, with a relatively low test pressure! However, the corresponding construction specifications in GB and SH both refer to airtightness tests (airtightness leakage tests), with relatively high pressures! Bubble leak detection – the direct pressure application method and the airtightness leak test seem to have little difference other than the pressure level; in both cases, bubbles are observed visually to make a judgment! Generally speaking, once a strength test has been conducted, it can be assumed that the safety of the pipeline has been verified; I don’t quite understand why, in standard 20801, the pressure used for the leak detection – direct pressurization method – is so much lower than that used in the strength test.
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