Helium mass spectrometer leak detector in pressure vessel leak detection of application
In pressure vessels, the commonly used leak detection methods for helium mass spectrometers can be roughly divided into three types: helium injection method (negative pressure method), suction gun method (positive pressure method), and helium cap method. In terms of sensitivity of several methods, the helium injection method has higher sensitivity. But when the leak detector is installed on a large leak detector, it cannot be precisely said that the sensitivity of the helium injection method is very high on more complex pressure vessel equipment.
Helium injection method is the most commonly used and convenient leakage measurement method. When measuring leaks, use a spray gun connected to a helium cylinder to spray helium onto the suspected leak area. If helium flows into the pressure vessel equipment through the leakage hole, the partial pressure of helium in the equipment will increase, and the helium leakage detector will display the leakage rate. Whether it is qualified depends on the standard leakage rate value, and exceeding the standard leakage rate value can be considered unqualified.
The following issues should be noted:
1. The actual leakage measurement sensitivity is lower than the theoretical calculation value. Firstly, due to the dispersion of the helium flow sprayed by the spray gun, a certain amount of gas mixing reduces the helium concentration. Secondly, the direction of the airflow sprayed by the spray gun cannot be completely aligned with the direction of the leak. In addition, the presence of gas in the recess will reduce the concentration of helium entering the leak hole.
2. Leakage detection sequence: From top to bottom of the tested object, gradually spray helium from the near leak detector to the place far away from the leak detector.
3. The detected large leak should be repaired before finding the small leak.
4. When there are two abnormal leak points that are very close together, one point should be covered first, and then helium should be sprayed with a thinner spray gun.
5. When the spray gun sprays at a certain point, if the display on the leak detector changes, its rising rate is very slow and the display value is unstable, indicating that there are significant vulnerabilities in nearby areas.
6. After detecting the leak, several more re examinations should be conducted.
7. The leakage measurement site should have good ventilation conditions, but it should not be too large and should not affect the direction of the helium airflow in the spray gun.
The suction gun method should first fill the pressure vessel equipment with atmospheric pressure helium or a mixture of helium and other gases, and inspect the outside of the pressure vessel equipment to detect helium leakage from the leak hole to the outside. When searching for helium with a suction gun, once helium is sucked in, the helium leak detector will display the leakage value. In this case, due to the external atmospheric pressure of the pressure vessel equipment, the air pressure inside the pressure vessel equipment needs to be at least one atmospheric pressure. But the pressure should not be too high. If the pressure is too high, the pressure gauge or other plug-in inserted into the equipment will pop out, causing dangerous accidents such as injury. In the helium spray method, the helium spray is likely to completely cover the leak hole, making helium molecules enter the interior through the hole close to the atmospheric pressure difference. On the contrary, in the gun suction method, first use helium to make the internal pressure reach the atmospheric pressure. If the pressure difference cannot reach the atmospheric pressure, helium will not flow into the outer boundary in the same amount, and then helium will immediately diffuse in the air, even though the gun suction port (usually very small) is impossible to inhale all the leaked helium.
Vacuum gun method (positive pressure method) leak detection. When using pure helium detection, helium recovery equipment can be reused to reduce maintenance costs. Alternatively, a helium mixture with a concentration of no less than 5-10% can be used, which can be directly discharged into the air after use. Pay attention to the concentration of helium in the surrounding environment and the flow of air in the surrounding environment, as this can affect the detection accuracy of the suction gun method. The suction gun method (positive pressure method) for leak detection is very similar to halogen leak detection, but it should be noted that the tracer gas filled in the container is different. Even with the same leakage rate value, its meaning is different. It is very important to distinguish this point. In addition to experimental testing comparison method, there is also calculation method Q1 η 1=Q2 η 2 (Q is the gas leakage flow rate, η For gas viscosity coefficient, Q1M11/2=Q2M21/2 (M is the gas content). Attention should be paid to the following issues in leak detection using the suction gun method:
1. Sensitivity is limited by the flow conductivity of the connecting pipe.
2. During the first leak test, the body of the tested item must not be filled with high-pressure helium. Because if there are leaks, helium will leak out of the tested piece, causing significant waste. Helium will spread everywhere and have a significant impact on the leak detector. Therefore, it is generally necessary to first charge low pressure into the inspected part. Low concentration helium should be used, and then high pressure and high concentration helium should be used to measure the small leakage hole. If there are large leaks, you can first pressurize and foam in water to reduce the waste of helium, because helium is too expensive (different concentrations have different prices), to avoid damage caused by helium poisoning in the helium leak detector.
This method uses a leak detection cover (plastic film, etc.) to wrap the tested vessel, first expel the gas inside the cover, and then fill it with helium or helium mixture. The leak detector is connected to the inside of the tested vessel, and when the leak rate is displayed, it indicates a leak. This leakage rate is called the total leakage rate. The total leakage rate is the sum of each leakage rate; The total leakage rate shall not exceed the standard value, and each leakage rate shall not exceed the standard value; If the total leakage rate exceeds the standard value, check whether the leakage rate at each point exceeds the standard value. Then repair the leakage holes that exceed the standard value until they meet the qualified standard.
When detecting leaks, large pressure vessels often use assisted vacuum systems (assisted vacuum systems – vacuum mechanical pumps, booster pumps, vacuum pipes, tees, vacuum gauges, vacuum valves, exhaust valves, etc.) for pre vacuuming. When the vacuum value reaches a certain value, turn off the auxiliary vacuum system or use a vacuum switching valve, and then open the leak detector for leak detection.
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