Its products can be used for plant, biology, microorganism, genetics, virus, medicine, environmental protection, food, petroleum, chemical and other scientific research, education departments for all kinds of biological precision cultivation, genetic engineering research, petrochemical heating and so on. Multifunction oscillator,Shaking table,Digital display multipurpose oscillator Zenith Lab (Jiangsu) Co.,Ltd , https://www.zenithlabo.com
Key words: helium leak detector, leak detector, vacuum system, leak rate, hood method, blowing method
With the rapid development of science and technology, cryogenic liquids are widely used in industry, which makes high-vacuum multi-layer insulation structure transport vehicles widely used, and the quality of vacuum sandwich tank vacuum is related to leak detection technology. The gas rate test is directly related to the life of the tank. Therefore, this paper introduces a physical process for helium leak detection and a method for leak detection of cryogenic liquid transport vehicles. 1 ç‰©ç† physical process of leak detection æ°¦ mass spectrometry leak detection method is recognized as the world's highest sensitivity, non-destructive detection method for leaking holes can be positioned and quantified. When leak detection is performed by a helium mass spectrometer leak detector, all or part of the specimen is in a state of being pressurized or evacuated, and the leak detector is always on the low pressure side of the wall of the specimen. Before the leakage material enters the leak detector through the leak hole on the test piece, part of it is taken away by the auxiliary vacuum system, and the other part enters the leak detector. The leak detector is connected to the tested component and the auxiliary vacuum system, and the system can start working after normal operation. The leak detector background value can be considered zero (or compensated to zero) when helium has not been sprayed onto the leak. After sneezing, helium will enter the leak detection system through the leak hole, and the partial pressure of the enthalpy in the system will start to rise until the steady state (ie, the amount of helium entering is equal to the amount of gas being pumped away). If the sneezing is stopped at this time, the partial pressure of the enthalpy in the system will gradually decrease due to the pumping action of the pump, and finally the initial zero background value will be obtained. 2 Low-temperature liquid transport vehicle tank sandwich vacuum æ°¦ leak detection process The low-temperature liquid transport vehicle tank consists of inner cylinder, outer cylinder, support part, multi-layer insulation layer, adsorbent and suction nozzle, as shown in Figure 1. The outer cylinder and the inner cylinder of the tank of the cryogenic liquid transport vehicle are made of 0Cr18Ni9, and the heat insulating layer refers to a multi-layer heat insulating structure composed of aluminum foil and glass fiber paper on the outer surface of the inner cylinder, the inner cylinder and the outer cylinder. The support material between the bodies is epoxy glass reinforced plastic, the molecular sieve tube is used to fill the adsorbent molecular sieve device, and the outer cylinder body and the inner cylinder body are called interlayers. The cryogenic liquid transport vehicle uses the suction nozzle to evacuate the interlayer to achieve vacuum insulation.æ°¦ Leak detection refers to the process of leak detection of the inner cylinder and the interlayer by the helium mass spectrometer leak detector. The pros and cons of the helium leak detection process is the key link that affects the vacuuming effect. 2.1 Preparation before æ°¦ leak detection If the low temperature liquid transport vehicle has a good vacuum, the leakage rate of the tank interlayer should be strictly controlled to ensure a good sealing performance. Previous tests using ultrasonic, ray, magnetic powder, and penetration have not been able to meet the vacuum requirements (the base metal is ultrasonically tested), and the leaks may still appear anywhere, such as weld beads and base metals. If the helium leak detection technology is adopted, the problem of the leak point can be effectively solved, but it is necessary to prepare for the leak detection before vacuuming. 2.1.1 Cleaning the vacuum system Figure 1 Schematic diagram of the tank of the cryogenic liquid transport vehicle.
Before the leak detection, the vacuum system of the cryogenic liquid transport vehicle should be removed to remove the welding slag, grease and cleaning, and then the tank is dried. The purpose of the cleaning process is to prevent the leaks from being clogged with dirt, oil, organic solutions, etc., and to prevent the leak detection apparatus from being contaminated. Since the equipment and helium gas of the helium mass spectrometer leak detection method are relatively expensive, in order to improve the leak detection effect, helium gas is saved, and the vacuum system should be pre-evacuated before the leak detection.
2.1.2 Pre-evacuation of the vacuum system The tank of the cryogenic liquid transport vehicle is placed in the heating furnace for pre-evacuation. When the vacuum of the interlayer is <10 Pa, the high-purity nitrogen is introduced into the evacuated pipeline to the gauge pressure of 0.1. MPa, then warm to 120 ° C ~ 130 ° C, and keep warm for not less than 12 h. Three thermocouples were placed at different locations in the furnace to observe and control the temperature. The pumping unit was started to pump out for 4-6 hours, and was flushed into the vacuum interlayer with high-purity nitrogen gas to a gauge pressure of 0.1 MPa. After heating for 6 hours, it was evacuated. This process of filling nitrogen, heating and evacuation is generally not less than 8 to 10 times. In this process, in order to keep the temperature constant, a cold trap should be installed between the evacuation unit and the tank, and the liquid nitrogen liquid in the cold trap is ensured. The bit cannot be zero. When the nitrogen is heated, the cold trap should be removed and defrosted and dried, as shown in Figure 2. When the cold trap has no obvious frost and the vacuum reaches 0.1 Pa, the interlayer can be leak tested.
2.2 Leak Detection Method At present, the commonly used leak detection method for cryogenic liquid transport vehicles is the helium mass spectrometer leak detector method. There are two commonly used leak detection methods, namely, the hood method and the blast method. In the case of leak detection, the total leak rate is first measured by the hood method. When the total leak rate exceeds the allowable value, the leaking hole is positioned by the blowing method. The total leakage rate of the inner cylinder of the cryogenic liquid transport vehicle-73 should be less than 2×10 Pa·m/s. The total leakage rate of the interlayer should be less than 6×10 Pa·m/s.
2.2.1 氦 氦 氦 氦 是 是 是 是 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦 氦When there is a change, it indicates that the specimen under the hood has a leak. The process of leak detection of cryogenic liquid transport vehicles is as follows: For the partial hood, it is required to cover the weld with plastic, and the two sides are glued with tape, and the middle is obviously bulged. The overall hood method requires that the entire cylinder be covered with a plastic film, the lap joint of the plastic film is taped, and the bottom edge is pressed to make it 
Form a complete hood. The filling mouth should be opened at the lowest point of the hood. When filling, there must be a special person to monitor the leak detector and report the information to the personnel. After all the hoods have been bulged, the helium is kept in the hood for at least 15 minutes. When the leak indicator output indicates a change, it indicates that the test piece has a leak. If it is found that the leak is large, the leak should be stopped. To the low temperature-73, the liquid transport tank is filled with nitrogen and vacuumed. After ensuring the vacuum inside the container and the bottom of the tank meet the leak detection requirements of the leak detector, spray it. The blow method finds the position of the leak point and marks it, and fills the tank with nitrogen gas for repair welding. After the repair welding is completed, the repair welding position is again subjected to vacuum leak detection. The standard leak in Figure 3 refers to a device that provides a known gas flow to a vacuum system under given conditions. The standard leak is used to calibrate the leak detector [1 Sensitivity and calibration of the size of the leak. .
2.2.2 Injection method The injection method is the most common and convenient method for leak detection. When the total leak rate exceeds the allowable value, the positioning of the leak hole is performed by the blowing method. When leaking, use a nozzle attached to a gas cylinder equipped with helium to blow at a suspected leak. The following points should be noted for the leak detection method: a. The order of leak detection should be from the top to the bottom of the test piece, and sprayed from the leak detector to the point of the leak detector. b. Use a large-diameter nozzle for rough inspection to make the coverage area of ​​the helium flow larger. After finding the area where the leak is located, use a small-diameter nozzle to find the exact position of the leak. c. The large leaks detected are repaired first (blocked with vacuum mud), and then small leaks are found. d. When there are two suspicious leak points that are close together, cover one point first and then spray the other point with the thinnest nozzle. e. When the nozzle blows a point, if the leak indicator indicates a change, but the rate of rise is slow and the indication values ​​are inconsistent, it indicates that there are large leaks elsewhere. f. After the leak is detected, it should be repeated several times. The leak detection environment should be well ventilated, clean and dry. There should be no strong electromagnetic interference and strong vibration. The ambient temperature is 23 °C ± 7 °C. The temperature and atmospheric pressure monitoring devices are required at the site.