In order to ensure that the vacuum equipment or system has a good sealing performance, it is far from enough to find the location of the leak after the installation of the equipment and block the channel of the leak. It is necessary to conduct vacuum leak detection at any time during the design, manufacturing, commissioning and use of vacuum equipment or systems.
Precautions in vacuum equipment design
1. Determine the total maximum allowable leakage rate of the vacuum equipment according to the process requirements of the equipment, and determine the maximum allowable leakage rate of each component according to this total leakage rate.
2. According to the maximum allowable leakage rate of the equipment and other indicators, the leakage detection method to be used will be preliminarily determined in the design stage, and it will be taken as one of the basic principles guiding the commissioning acceptance.
3. According to the maximum allowable leakage rate index of equipment or components, determine the sealing, connection mode and overall processing accuracy of equipment, as well as which dynamic sealing form can meet the requirements. For example, the flange adopts metal seal or rubber seal.
4. When designing the structural strength of the vessel, consider the pressure resistance and structural strength of the tested parts if the pressurization method is used to detect leakage.
5. When selecting the structural materials of parts, consider whether the materials that may be damaged by the working medium and leakage indicating gas are used.
6. During structural design, necessary spare interfaces of leak detection instruments shall be reserved on vessels or systems for leak detection during equipment assembly and commissioning. Especially for large and complex pipeline systems, sectional leakage detection method is usually required, so sectional isolation valves shall be set on the pipeline, and leakage detection instrument interfaces shall be reserved on each isolation section.
7. During the structural design of parts, the design scheme that may interfere with leak detection shall be avoided as far as possible. For example, the screw hole in the vacuum chamber cannot be a blind hole, because the gas in the remaining space inside the screw hole can only escape through the thread gap after installing the screw, forming a virtual leak. This will prolong the pumping time of the system and interfere with the normal leak detection. As shown in the figure, there should be no structure in vacuum leak detection.
8. Similarly, continuous double-sided welds and multi-layer sealing ring structures are not allowed in the structural design, because this will form a “parasitic product” in the middle of the gas will form a virtual leak; However, when the internal and external welds or sealing rings leak at the same time, the “parasitic volume” makes the response time of the leakage indicating gas passing through the double-layer welds too long, which makes it impossible to detect the leakage normally.
9.During the design of welding structure, the welds that cannot be detected after the final assembly shall be minimized.
Leak detection in vacuum equipment manufacturing
In the processing stage of the equipment, it is necessary to follow the processing process (especially the welding process) to detect the leakage of semi-finished parts in a timely manner. For parts that cannot be contacted, detected or repaired after manufacturing, the weld quality shall be strictly detected for leakage. Unqualified parts shall be re welded, repaired and re detected in time. The next process can only be started after meeting the requirements. Especially for the assembly, welding and processing of large vessels, the leak detection in the intermediate process is very critical. If necessary, special leak detection tools (such as leak detection boxes, blind plates, etc.) should be designed and manufactured. For the vacuum chamber body with double-layer chamber wall water-cooled jacket, it is better to weld the inner chamber wall and check the leakage first, and then weld the outer chamber wall after confirming that there is no leakage. In the same way, if there is a thermal insulation layer on the outside of the room wall and other structures that are difficult to disassemble, the room wall must be strictly leak checked before the outer structure can be covered.
If conditions permit, all vacuum flanges and their connecting pipes (including vacuum chamber flange and chamber wall) shall adopt the process of machining flange surface after welding. For flanges not processed after welding, even if the sealing requirements may be met in the installation and commissioning stage, heating, vibration and other factors may also induce the release of welding stress during the use of equipment, resulting in flange deformation and reduced sealing performance.
In the process of processing and manufacturing, strict implementation of vacuum operation hygiene and operation specifications is also helpful to improve the air tightness of vacuum equipment and systems. After the welding groove is polished and formed, it needs to be cleaned and protected in time, which will help to improve the gas tightness of the weld. The dynamic and static sealing surfaces of finished parts shall be provided with protective measures to prevent collision and scratch during storage, handling and assembly. When welding corrugated pipes, metal and ceramic or glass sealing pieces, glass devices and other vulnerable parts are used, careful operation should be carried out, especially to avoid leakage caused by damage after pre inspection.
Leak detection steps during installation and commissioning of vacuum equipment
The installation and commissioning stage is the main part of vacuum equipment or system leak detection. If the airtightness of equipment welds has been ensured through leak detection in the processing stage, the key point of leak detection is to check and ensure the tightness of connection parts during equipment installation and commissioning. It includes the flange connection between pipes and components, dynamic seals and other key suspicious parts. If the leakage detection is conducted on the weld and connection parts at the same time, the workload and difficulty of leakage detection will be increased. For large and complex vacuum equipment, it is better to use sectional leak detection. Each component is installed, its connection and weld joint shall be leak detected once, and the next component shall be installed after meeting the requirements. Because the leakage detection is conducted after all components are fully assembled, it is not only suspected that there are too many parts, but also that multiple leakage holes may leak at the same time, which brings great difficulties to the overall leakage detection. The leakage detection steps during the installation and commissioning of vacuum equipment are as follows:
1. Understand the structure and assembly process of the equipment to be inspected. Master the requirements of the equipment and find out the key suspicious parts that need leak detection.
2. According to the specified maximum allowable leakage rate and whether it is necessary to find the specific location of the leakage hole and other requirements, and based on the principles of economy, speed and reliability, the leakage detection method or instrument shall be correctly selected, and the practical leakage detection procedure shall be formulated after the auxiliary equipment required for leakage detection is prepared.
3. The inspected parts shall be cleaned, and the welding slag and oil dirt shall be taken out and then cleaned according to the vacuum sanitary conditions, and dried. Small devices with high requirements. After cleaning, it can be baked in a vacuum drying oven. After cleaning, it can not only prevent the leak from being blocked by dirt, oil, organic solution, etc., but also protect the leak detector.
4. Calibrate the leak detection sensitivity of the selected leak detection method and equipment, and determine the leak detection time of the leak detection system
5. If vacuum leak detection method is adopted, the tested part shall be pumped to a higher vacuum as far as possible to improve the sensitivity of the instrument.
6. If allowed, more economical leak detection methods and on-site conditions shall be preferred as far as possible
7. When helium mass spectrometer leak detection equipment is used for leak detection, for the parts to be detected that require low leakage detection or have large leakage, helium with low concentration shall be used for leak detection at the initial stage of leak detection, and then leak detection shall be conducted for small leakage holes to save helium.
8. The detected large leak shall be repaired and blocked in time before the small leak shall be detected.
9. Recheck the detected and repaired leak once to ensure that the leak detection results meet the requirements
Precautions during the use of vacuum equipment
During the operation of vacuum equipment, the air tightness of the equipment often decreases and the overall leakage rate increases, which is the main reason for affecting the normal operation of vacuum equipment. The causes of this phenomenon include: mechanical vibration causes the connection parts to loosen; The sealing ring at the frequently disassembled part may be damaged or installed incorrectly; Deformation and fatigue failure due to cold and thermal shock; Some parts or materials are damaged due to corrosion of working medium; Some leaks originally blocked by water, oil or other dirt shall be re released; And cracks caused by stress concentration.
When vacuum equipment is used correctly, leakage detection should be included in the daily maintenance and management specifications of vacuum equipment, such as regular static pressure rise test. The operator shall solve the problem in time if he finds that the air tightness of the equipment decreases, analyze the cause of leakage according to the use of the equipment and the fault phenomenon, and take appropriate leak detection means to detect the location of the leak hole and repair it in time. Do not wait for the equipment to have multiple or multiple leaks, and then go to leak detection and repair when it is unable to work normally.
In addition, preparing sufficient sealing spare parts at ordinary times and replacing wearing parts regularly (not after problems occur) are also the main measures to do a good job of vacuum leak detection and ensure the normal operation of the equipment