Helium mass spectrometry leak detection method for pressure vessels

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With the increase in pressure vessel output and the growing emphasis placed by manufacturers on product quality, the use of helium mass spectrometry for leak detection is also increasing year by year in China’s pressure vessel manufacturing industry. The helium mass spectrometry leak detection method (hereinafter referred to as helium testing) is commonly used on pressure vessels with a high overall leak resistance due to its high sensitivity and accuracy. Helium testing methods can basically be divided into two types: internal pressurization with helium gas and external application of helium after evacuating the interior of the equipment. Since the latter requires the equipment to be completely evacuated, it often necessitates the use of additional testing equipment (such as high and low pressure vacuum pumps, vacuum valves, etc.) as well as special fixtures (such as external pressure reinforcement rings), which increases the cost; therefore, this method is typically used for devices with small volumes but thick walls ； For most pressure vessels, the former method is usually preferred. Principle of operation: A mass spectrometric leak detector is an instrument based on the principles of mass spectrometry, using helium as the probing gas. During the test, once it escapes through the leakage holes, it is drawn into the mass spectrometric leak detector along with other gases. The filament inside the detector emits electrons that ionize the molecules; the positive ions then accelerate under the influence of an acceleration field, forming an ion beam. When this ion beam enters a magnetic field perpendicular to it, it moves in a circular path, the radius of which is determined by the following formula: http://www.chvacuum.com/uploads/userup/0901/131515062W5.gif. In this formula, R represents the radius of the ion’s trajectory (in cm), H represents the magnetic flux density (in T), M/Z represents the ratio of the ion’s mass to its charge, and U represents the acceleration voltage (in V). As can be seen from this formula, when R and H remain constant, changing the acceleration voltage allows ions of different masses to be detected via the detection slit. By placing a collector at a specific position in the instrument analyzer, helium ions can be separated from the remaining ions produced, and converted into a ratio that is displayed on the instrument – a function of the pressure of the helium portion drawn in by the mass spectrometric leak detector. As an exploration gas, helium is present in very small amounts in air and vacuum systems, accounting for only one in two hundred thousand parts in air. This results in low background noise from the instruments. Additionally, helium has a low mass – only greater than that of hydrogen – making it easy to pass through leaks; it also has a high diffusion rate, so even minor leaks can be easily detected. Helium is an inert gas that does not react with the various materials of the equipment being inspected, is not easily adsorbed, and is safe to use. Process: Helium testing should be carried out after the equipment construction and other inspections have been completed. Before the test, the surface and interior of the equipment must be clean and dry; otherwise, it will affect the test results and lead to incorrect conclusions. This article provides a brief description of the process involved in the internal pressurized helium testing method: 2.1 Items and equipment required for the test – Helium mass spectrometer leak detector, suction gun, helium cylinder, hot air device, pressure gauge, plastic film, and tape. 2.2 Operating procedure – Surface treatment and drying of the equipment: Since helium testing relies on helium passing through any leaks, oil residues, slag on the surface of welds, as well as water or dirt inside the equipment can temporarily block the leak points and affect the test results. Therefore, it is necessary to thoroughly clean the interior of the equipment and the surfaces of the welds before the test, and to dry the interior of the equipment completely using a hot air device. Calibration of the mass spectrometry leak detector: After connecting the suction nozzle to the mass spectrometry leak detector using a metal hose, move the suction nozzle to the side of the leak hole where positive pressure is applied, and check the readings of the instrument. The mass spectrometry leak detector should be used after calibration, and calibrated every 1 to 2 hours during testing. The leak detection rate of the mass spectrometry leak detector should be 1 to 2 orders of magnitude higher than the leak rate permitted by the equipment. Internal pressurization: First, place the equipment in a well-lit and well-ventilated area, connect the piping required for testing as well as the pressure gauges. At least two pressure gauges with the same scale and that have been calibrated should be used, and they should be installed at the top of the test container in a location where they can be easily observed. First, increase the pressure in the equipment using nitrogen or another inert gas, and then use pure helium or a helium mixture to raise the internal pressure of the testing equipment to the desired test pressure; the equipment should contain at least 10%–20% helium. The test pressure shall not be higher than 25% of the equipment’s design pressure, but shall not be lower than 0.103 MPa. After the device has been held under pressure for 30 minutes, use a suction gun to draw at a speed of no more than 25 mm/s or slower, within a distance of no more than 3 mm from the weld surface; the suction should be applied from the bottom of the weld upward. Refer to Figure 1. http://www.chvacuum.com/uploads/userup/0901/13151520H41.gif 1- Helium mass spectrometer ； 2- Suction gun ； 3- Item under inspection ； 4- Helium source ； 5- Positive pressure calibration for leak holes: When there are many welds on components such as tube sheets or when the area to be inspected is large, that area can be completely covered with plastic film and sealed with tape, so that any helium leaking out enters the enclosure, as shown in Figure 2. http://www.chvacuum.com/uploads/userup/0901/1315154Cb7.gif Small holes are made in different areas of the plastic cover; initial readings are taken before and after filling it with helium, the holes are then sealed, and new readings are taken at the same locations after 12 hours. If a leak is detected, each weld should be inspected using the method described above until the leak location is found.