Helium leak test for high-pressure pipelines

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After the pipeline strength testing of high-pressure toxic fluids and flammable fluids is completed, a leak test must also be carried out in accordance with the requirements of the standards. Conducting high-pressure pipeline gas leakage tests on ordinary gases (air) involves very high test pressures (usually the design pressure of the pipeline), which is extremely dangerous. By using helium as the test medium, the high sensitivity of helium leak detection enables **reduction of the leak test pressure**, thereby decreasing the risks associated with pneumatic leak tests while still meeting the stringent sealing requirements of piping systems. 1. Overview: As a type of integrity test, the helium leak test is currently widely used in certain pressure vessels that require high temperatures, high pressures, and excellent sealing properties. However, due to the complexity of pressure pipeline systems and the harsh conditions at the installation site, this test is less frequently employed. In recent years, with the construction of numerous petrochemical projects, higher demands have been placed on the integrity testing of high-pressure piping systems for highly toxic, toxic, and flammable fluids. After the strength test is completed for such pipes, it is generally required that a leak test be conducted as well. For pipelines with a low design pressure, the leak test pressure (usually set at the design pressure) is also low, resulting in a lower risk during the test ； However, if the design pressure of the pipeline is relatively high, then the pneumatic leak test conducted with ordinary media will be carried out at a higher pressure; undoubtedly, this also **increases the risk** of the test. The helium leak test offers us a testing method that ensures the integrity of pipelines while remaining relatively safe, thanks to its high sensitivity and relatively low test pressure (typically 0.35 MPa or the minimum of 25% of the design pressure). Its successful application in important pressure vessels has provided us with ideas for its use in piping systems. The following applications of helium leak testing in the high-pressure liquefied natural gas pipelines of LNG projects along China’s coast serve as good examples, and they provide strong evidence for the future use of helium leak testing in pipelines carrying high-pressure, highly toxic, toxic, or flammable fluids. 2. Helium leak testing
2.1 Principles and methods of helium leak testing
During a helium leak test, a helium mass spectrometer leak detector is connected to a sniffer probe to form a leak detection system. This system detects trace amounts of helium escaping from the pipeline under test. The sniffer probe draws in the helium gas and delivers it to the leak detection system, where it is converted into an electrical signal. The leak detector then displays this electrical signal in the form of light or sound. The helium mass spectrometer leak detector allows the detection sensitivity to be adjusted to meet the requirements of different sealing levels in tube systems. The commonly used methods for detecting airtightness and their corresponding sensitivities are shown in the table below: Comparison of sensitivities of common detection methods; Method, Test medium, Sensitivity (Pa·L/s): Hydrostatic test, Water or oil, 4x10-1; Airtightness test, Air, 10-2; Kerosene penetration test, Kerosene, 10-1; Liquid penetration test with dye, 10-4; Halogen leak detection, Freon, 10-4; Ammonia leak detection, Ammonia gas, 10-5; Helium leak test, Helium, 10-12. It is clear that the helium leak test possesses extremely high sensitivity. There are two common methods for helium leak testing: the helium absorption method (overpressure method) and the helium injection method (vacuum method). For pipes subjected to internal pressure, the vacuum method is not suitable for leak detection. In this case, if vacuum testing is used for leak detection, evacuating the pipeline creates external atmospheric pressure on the flanges, which in turn applies additional sealing force to the gaskets. However, when the pipeline is in operation, it is subject to internal pressure, which reduces the sealing force exerted by the bolts. When the internal pressure is high, this factor cannot be ignored, and corrective actions in accordance with relevant standards must be taken before the pipeline can be used. 2.2 The main equipment used for helium leak testing includes helium mass spectrometers for leak detection, helium gas, air compressors, pressure gauges, and sniffing probes. 2.3 The key control parameters for helium leak testing are the test pressure, the method of pressurization, the test temperature, environmental conditions, the distance between the probe and the component being tested, and the scanning speed. 3. Helium leak testing for high-pressure pipelines in coastal LNG projects (pipeline design pressure: 148.9 bar) 3.1 Pipeline helium leak testing is carried out in accordance with Section 345.8 on sensitive leak testing and Section 345.9 on alternative leak testing in ASME B31.3 “Process Piping”, which provides the technical guidelines for conducting pipeline leak tests using helium. ASME standards: The ASME Boiler and Pressure Vessel Code, VOL V – Nondestructive Methods Examination, specifies the criteria for helium testing and acceptance, providing guidance for conducting on-site helium leak tests. In addition, China’s GB50235 \"Code for Construction and Acceptance of Industrial Metal Piping Projects\" and GB/T15823 \"Helium Leak Testing\" contain similar provisions. 3.2 Preparations before the helium leak test for high-pressure pipelines
1) The pipeline system has passed the hydrostatic strength test and been deemed qualified.
2) The pipeline has undergone a pre-pressurization leak test using air as the medium; the test pressure was below 7 kgf/cm³, and it passed the inspection using a foaming agent.

3.3 Parameters for the helium leak test of pipelines
The helium leak test for high-pressure pipelines employs the overpressure method. The test medium consists of 10% helium and 90% nitrogen ； The test pressure shall be set at 7 kgf/cm3 (if air is used as the medium, the test pressure must be at least 148.9 bar, which **increases the risk associated with the test), and the test temperature shall be no less than 50°C. The acceptance criteria follow ASME V, requiring a value of 10*E-6 Pa·m3/s. When testing with 100% helium, the acceptable standard is below 10*E-5; however, since the actual concentration is only 10%, this acceptable standard must be adjusted to 10*-6 Pa·m3/s. 3.4 Pipeline helium leakage test procedure 1) Identify the inspection points in the pipeline system (pipeline flange joints, valve bonnet flange connections) and cover them with film to ensure the sealing of those flange joints. 2) Calibration of instruments and setting of sensitivity (10-6 Pa·m3/s). 3) After the pipeline is properly evacuated, it is first filled with 10% helium followed by 90% nitrogen; once the test pressure of 7 kgf/cm3 is reached, the system is left to stabilize for more than 30 minutes to allow the gases to mix thoroughly. 4) Testing is carried out at each detection point: an appropriate amount of nitrogen is introduced into the membrane cover surrounding the flange joint to displace air, after which a specialized testing probe is inserted into the membrane cover for inspection. 5) Leaks are identified and marked. 6) Pressure is reduced, repairs are made, and testing is repeated until satisfactory results are obtained. 3.5 Test Results: For the Southern Coastal LNG project undertaken by China Global Engineering Corporation, the liquefied natural gas pipelines passed the helium-based testing; no further traditional testing at higher pressures was conducted ； The pipeline system maintains good airtightness after feeding. 4. Conclusion: At present, helium leak testing is widely used in specialized industries such as the nuclear industry, semiconductors, and polyester synthesis. However, in petrochemical projects, it is less utilized in high-pressure pipeline systems due to their complex structure and harsh operational conditions. As society places greater emphasis on safety, leak testing of high-pressure pipeline systems faces the new challenge of \"minimizing the risks associated with such tests while still ensuring the airtightness of the pipeline systems.\" This article presents the successful experience of China Global Engineering Company in conducting helium leak tests for an LNG project along the southern coast, offering a good practical example for the future use of helium leak testing on high-pressure pipelines