Helium detection equipment: How to do helium leak detection for pressure vessels

With the increase in export products of pressure vessels and manufacturers’ emphasis on product quality, the application of helium mass spectrometry leak detection methods in China’s pressure vessel manufacturing industry is also increasing year by year. Helium mass spectrometry leak detection method (hereinafter referred to as helium detection) is usually used for pressure vessels with high overall leak prevention level, with high sensitivity and accuracy.

The helium detection methods can be basically divided into two types: helium internal pressure method and equipment internal vacuum external helium method. As the latter requires the equipment to be completely evacuated, it often requires additional testing equipment (such as high and low pressure vacuum pumps, vacuum valves, etc.) and equipment tools (such as external pressure strengthening rings). Therefore, this method is usually used for small volume and thick wall equipment; For most pressure vessels, the former method is usually preferred.

The mass spectrometer leak detector is an instrument made based on the principles of mass spectrometry, using helium as the exploration gas.

When it leaks out from the leak hole during the experiment, it is sucked into the mass spectrometer leak detector along with other gases. The electrons emitted by the filament in the mass spectrometer leak detector ionize the molecules, and the positive ions undergo accelerated motion under the action of the acceleration field, forming an ion beam. When the ion beam is injected into a magnetic field perpendicular to it, it undergoes circular motion. The orbital radius is determined by the following equation: H is the magnetic induction intensity (T) M/Z is the ratio of ion mass to its charge U is the acceleration voltage (V). It can be seen from the above equation that when R and H are constant values, changing the acceleration voltage can allow ions of different masses to be received and detected through the receiving slit. By setting a collection electrode at a specific location on the instrument analyzer, helium ions can be isolated from the generated ion residue and converted into the ratio displayed on the instrument – the pressure function of the helium portion sucked in from the mass spectrometer leak detector. As an exploration gas, helium has a very low content in air and vacuum systems, accounting for only one in 200000 in air. This reduces the background noise of the instrument, and at the same time, helium has a small mass, only larger than hydrogen, making it easy to pass through leaks and have a high diffusion rate. Even small leaks are easily reflected. Helium is an inert gas that does not react with various materials of the tested equipment, is not easily adsorbed, and is safe to use.

Helium testing should be conducted after equipment construction and other inspections have been completed. The surface and interior of the equipment must be cleaned and dried before the experiment, otherwise it will affect the test results and cause erroneous judgments.

This article briefly describes the process of internal pressurized helium detection method as follows: the required items and equipment for the experiment include helium mass spectrometer leak detector, suction gun, helium cylinder, hot air device, pressure gauge, plastic film and tape. The surface treatment and drying of equipment are affected by the presence of oil stains, welding slag, and accumulated water and dirt inside the equipment, which can temporarily block the leakage holes and affect the testing results. Therefore, before the test, it is necessary to thoroughly clean the interior of the equipment and the surface of the welding seam, and use a hot air device to thoroughly dry the interior of the equipment.

After connecting the suction gun and mass spectrometer leak detector with a metal hose, move the suction gun to the outlet side of the positive pressure calibration leak hole and verify the reading of the instrument. The mass spectrometer leak detector should be used after calibration and calibrated every 1-2 hours during the testing period. The leak detection rate of the mass spectrometer leak detector should be 12 orders of magnitude higher than the allowable leak rate of the equipment.

Firstly, place the equipment firmly in a bright and well ventilated place, connect the test pipeline and pressure gauge, and use at least two calibrated pressure gauges with the same range. They should be installed at the top of the test container for easy observation. First, increase the pressure of the equipment with nitrogen or other inert gas, and then increase the internal pressure of the test equipment to the test pressure with pure helium or a mixture of helium gas, ensuring that the equipment contains at least 1% to 20% helium content inside. The test pressure should not exceed 25% of the equipment design pressure, but not less than 0.103MPa. After the equipment is pressurized for 30 minutes, use a suction gun with a scanning rate of not more than 25mm/s or slower to suck within a range of not more than 3mm from the weld surface, and should run from the bottom of the weld to the top.

1-Helium mass spectrometer; 2- Suction gun; 3- The inspected item; 4-Helium gas source; When there are many leaks in positive pressure calibration holes such as pipe plates or welding seams, or when the detection area is large, the part can be completely covered with plastic film and sealed with tape to allow the leaked helium gas to enter the cover.

Make small holes in different parts of the plastic cover, record the initial readings before and after filling with a certain amount of helium gas, then seal the small holes, and record new readings at the same position after 12 hours. If a leak is found, the weld seam should be inspected one by one according to the above method until the leak point is found.

The observed leakage should be corrected to the percentage of helium used, using the formula: The total pressure of the P mixture in MPa can be considered qualified.

The following points should be noted during the leak detection process:

1. The equipment should undergo this test after all construction work is completed, and no welding or grinding should be carried out after the test is completed.

2. If the equipment is soaked or there is residual liquid, it will affect the leakage of the capillary and the authenticity of the test results.

3. Due to the fact that helium is lighter than air, attention should be paid to the sequence of leak detection, which should be carried out from bottom to top and from near to far.

4. During the leak detection process, if a large amount of helium gas is found to have entered the mass spectrometer leak detector, the suction gun should be immediately removed to prevent delays in the test due to the instrument’s inability to remove helium for a long time.

(The article comes from the Internet. If reprinting is not allowed, please contact our company to delete it.)