When storing gas in a pressure vessel, gas leakage is a significant loss to the gas storage of the entire container. For manufacturers of pressure vessels, solving this problem has always been a troublesome and time-consuming task that requires a lot of time, manpower, energy, and financial resources. However, it may not necessarily be able to effectively address the critical issue of leakage. Generally, for leak detection of pressure vessels, the total leakage rate is checked first. When the total leakage rate exceeds the allowable value, the detection of each leakage hole is carried out, and the leakage holes that exceed the allowable value are repaired. Using the pressurized water bubble method to detect pressure vessels can identify larger leak locations, but it is difficult to detect micro leaks and leaks, especially when pressure vessels leak over time (including material leakage). Generally, domestic enterprises often use the following methods for leak detection of pressure vessels:

Pressure placement method: Fill one to three atmospheres of air and leave it for 24 hours or longer to see if the pressure reading on the pressure gauge drops. This method can detect a minimum leakage rate of around 10-1Pa · m3/S.

Coloring method: Fill ammonia gas and color the outside, and a minimum leakage rate of about 10-1Pa · m3/S can be detected.

Ultrasonic method: Fill one to three atmospheres of air and use an ultrasonic detector to detect leaks. The minimum leakage rate can be detected at around 10-2Pa · m3/S.

Pressure water foaming method: After filling with one to three atmospheres of air, place it in water and check for the presence of bubbles (detectable at 10-3 to 10-4 Pa · m3/S). After 24 hours of accumulation, the minimum leakage rate can be detected at around 10-5 Pa · m3/S.

Soap application method: Fill the suspected area with one to three atmospheres of air and apply soap water to check for soap bubbles, which can be detected at 10-2-10-3 Pa · m3/S.

Radioactive gas method: 10-11Pa · m3/S can be detected. There is a risk of radioactive contamination

Halogen leak detection method: can detect 10-5~10-6Pa · m3/S

Helium mass spectrometer leak detector: The suction gun method can detect 10-3-10-7 Pa · m3/S, and the vacuum method can detect 100-13Pa · m3/S.

From the above leak detection methods, it can be concluded that the helium mass spectrometer leak detector has the widest leak detection range, high accuracy, and safety advantages. Helium leak detectors are widely used in foreign and domestic sole proprietorships and joint ventures. Nowadays, domestic enterprises are also gradually increasing the application opportunities of helium leak detectors with the improvement of user requirements, product upgrades, going abroad, and supporting foreign investment.

Helium mass spectrometer leak detector is also one of the various non-destructive testing methods (such as X-ray, eddy current, ultrasound, etc.), and is a form of non-destructive testing for pressure vessels. Widely used in new technologies such as aerospace, aviation, nuclear power, electronic components, vacuum equipment, electric vacuum, power, petrochemicals, semiconductor front-end, air conditioning and refrigeration.

1、 Overview

There are various ways to check for leaks, one of which is to use a helium mass spectrometer leak detector. Using helium as a tracer gas to detect leaks, micro leaks, and leaks (including those of the material itself) in various containers. The use of helium gas for leak detection is considered to have the following advantages: (No need for hydrogen gas, as hydrogen gas has the risk of flammability and explosiveness)

The helium content in air is very low, only 5ppm (about 1/200000 in air);

The residual helium content in the air inside the tested object is also very low;

Due to the small molecular weight and diameter of helium (ranked only after hydrogen in the periodic table), even extremely small leakage holes can easily pass through;

Due to its chemical inactivity, it will not contaminate the exhaust system and the tested object;

Due to its harmless and non flammable properties, it is safe and convenient to use;

The resolution requirement for the analysis tube is not high, and it is easy to achieve high sensitivity;

There is no environmental pollution problem.

2、 The basic principle and main components of helium mass spectrometer leak detector

The performance and characteristics of helium mass spectrometer leak detector depend on the mass spectrometry analysis room. The working principle of the mass spectrometry analysis room is to ionize the ion source gas. The ionized positive ions are accelerated and focused by an electric field through a gap into a uniform magnetic field. Under the action of the magnetic field force, the ion beam rotates 180 or 90 degrees along a certain trajectory to reach the collection electrode. This form is called magnetic deflection analysis. Then, it is processed through small current amplifiers, microprocessors, controllers, and other circuits until the LCD screen displays the size of the reading.

3、 Common methods of helium mass spectrometer leak detector in pressure vessels

In pressure vessels, the commonly used methods for leak detection using helium mass spectrometer leak detectors can be roughly divided into three categories: helium injection method (negative pressure method), suction gun method (positive pressure method), and helium hood method. In terms of sensitivity comparison among several methods, the helium injection method has a higher sensitivity. But when the leak detector is installed on a large and complex pressure vessel device, it cannot be accurately said that the sensitivity of the helium injection method is high.

（1） Helium spray method

The helium spray method is the most commonly used and convenient leak detection method. During leak detection, a spray gun connected to a helium gas cylinder is used to spray helium gas onto the suspected leak area. If helium gas flows into the pressure vessel device through the leak hole, the helium partial pressure inside the device will increase, and the helium leak detector will display a leak rate. Whether it is qualified depends on the standard leakage rate value, and those exceeding the standard leakage rate value are considered unqualified.

The following issues should be noted in helium spray leak detection:

1. The actual leak detection sensitivity is lower than the theoretical calculation value. One reason is that the helium gas flow sprayed by the spray gun is dispersed, so a certain amount of air is added, which reduces the concentration of helium gas. Secondly, the direction of the airflow sprayed by the spray gun cannot be completely aligned with the direction of the leak hole. In addition, the presence of air in the concave area will reduce the concentration of helium entering the leak hole.

2. Leak detection sequence: Perform helium injection inspection point by point from the top to the bottom of the object being inspected, from the location near the leak detector to the location far away from the leak detector.

3. The detected large leaks need to be repaired before looking for small leaks.

4. When there are two suspicious leak points that are very close to each other, one point should be covered first, and then the thinnest spray gun should be used to spray helium on the other point.

5. When the spray gun is sprayed at a certain point, if there is a change in the display on the leak detector, and its rising speed is slow and the display value is unstable, it indicates that there are large leakage holes nearby.

6. After detecting the leak, several retests should be conducted.

7. The leak detection site should have good ventilation conditions, but it should not be too large and should not affect the direction of the helium gas flow sprayed by the spray gun.

（2） Gun suction method

The suction gun method is to first fill the pressure vessel device with one or more atmospheres of helium gas or a mixture of helium gas and other gases, and detect the helium gas leaking from the leak hole to the outside of the pressure vessel device, thereby discovering the leak hole. When using a suction gun to search for helium gas, once helium gas is sucked in, the helium leak detector will display the leakage rate value. In this case, since the external pressure of the pressure vessel device is one atmosphere, the gas pressure filled into the pressure vessel device needs to be at least one atmosphere or higher. But the pressure should not be too high. If the pressure is too high, the pressure gauge or other plug inserted into the device will pop out and cause dangerous accidents such as injury. In the helium spraying method, the sprayed helium gas is likely to completely cover the leak hole, allowing helium molecules to enter the interior through the hole with a pressure difference close to one atmosphere. In contrast, the suction gun method generally uses helium gas to first increase the internal pressure to above one atmosphere. If the pressure difference does not reach above one atmosphere, the helium gas will not flow to the outside in the same amount, and then the leaked helium gas will immediately diffuse into the atmosphere. Even the suction gun (usually very small) cannot suck in all the leaked helium gas.

When using the suction gun method (positive pressure method) for leak detection with pure helium, a helium recovery device can be used for repeated use to reduce operating costs. Alternatively, a helium gas mixture with a concentration of not less than 5-10% can be used, and after use, it can be directly discharged into the atmosphere. Attention should be paid to the concentration of helium gas in the surrounding environment and the flow of air in the surrounding environment, as this will affect the detection accuracy of the suction gun method. The suction gun method (positive pressure method) 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 if there are the same leak rate values, their meanings are different. It is important to distinguish this point. In addition to the experimental comparison method, there are also calculation formulas Q1 η 1=Q2 η 2 (Q is the leakage flow rate of gas, η is the viscosity coefficient of gas), Q1M11/2=Q2M21/2 (M is the molecular weight of gas). The following issues should be noted in leak detection using the suction gun method:

1. Sensitivity is limited by the flow conductivity of the connecting pipeline.

2. During the initial leak detection, do not fill the inspected object with helium gas at an excessively high pressure. Because if there is a leak, helium gas will leak out of the tested object, causing significant waste, and helium gas will spread everywhere, causing significant interference to the leak detector. Therefore, generally, low-pressure and low concentration helium gas should be filled into the inspected object first, and then high-pressure and high concentration helium gas should be filled to detect small leaks. If there is a large leak, the specimen can be first subjected to pressurized water foaming method to detect the large leak. This can not only reduce the waste of helium gas, which is expensive (the price varies depending on the concentration), but also avoid damage to the helium leak detector caused by helium poisoning.

（3） Helium hood method

This method uses a leak detection cover (such as plastic film) to wrap the container being inspected. First, remove the air inside the cover, and then fill it with helium or a mixture of helium gas. The leak detector is connected to the inside of the container being inspected. When the leak detector displays a leak rate, it indicates that there is a leak. This leakage rate is called the total leakage rate. The total leakage rate is the sum of the individual leakage rates; If the total leakage rate does not exceed the standard value, then the individual leakage rates also do not exceed the standard value; If the total leakage rate exceeds the standard value, check whether the leakage rate of each point exceeds the standard value. Then repair the leaks that exceed the standard value until they meet the qualified standards.

During leak detection of large pressure vessels, auxiliary vacuum systems (including vacuum mechanical pumps, booster pumps, vacuum pipelines, tees, vacuum gauges, vacuum valves, vent valves, etc.) are often used for pre vacuuming. When the vacuum degree reaches a certain value, the auxiliary vacuum system is turned off or a vacuum switching valve is used to turn on the leak detector for leak detection.