Attention to the design of vacuum system equipment
The total maximum allowable leak rate of vacuum equipment and the maximum allowable leak rate of each component should be determined by the technological requirements of the equipment.
According to the maximum allowable leak rate of the equipment and other indicators, the leak detection method to be adopted is preliminarily determined at the design stage, and it is taken as one of the basic principles to guide the commissioning and acceptance.
The maximum allowable leakage rate index of the equipment or components, determines the sealing, connection mode, and overall processing accuracy of the equipment, and which dynamic sealing form can meet the requirements. For example, the flange adopts a metal seal or rubber seal.
The structural strength design of the container needs to consider the pressure resistance and structural strength of the inspected part if the pressure method is used for leak detection.
The selection of structural materials for parts should consider whether materials that may be corroded by the working medium and leaking gas may cause damage.
During structural design, necessary spare interfaces for leak detection instruments should be left on the container or system for use in leak detection during equipment assembly and commissioning. Especially for large and complex pipeline systems, it is usually necessary to adopt a segmented leak detection method. Therefore, segmental isolation valves should be installed on the pipeline, and a leak detection instrument interface should be reserved on each isolation segment.
Part structure design should try to avoid adopting design schemes that may interfere with leak detection work. For example, the screw hole in the vacuum chamber cannot be in the form of a blind hole, because the gas in the remaining space inside the screw hole can only escape through the thread gap after the screw is installed, forming a virtual leak. Thereby prolonging the pumping time of the system and interfering with the normal operation of leak detection. As shown in the figure, the structure should not appear in vacuum leak detection.
Continuous double-sided welds and multi-layer sealing ring structures are not allowed in the structural design, because this will form a "parasitic volume" in the middle and the gas in the gas will form a virtual leak; and when the inner and outer double-sided welds or sealing rings leak at the same time, the "parasitic volume" makes the response time for the leaking gas to pass through the double-layer weld too long, making it impossible to detect leaks normally.
The welded structure design minimizes the welds that cannot be leak detected after final assembly.
Leak Detection in Vacuum System Equipment Manufacturing Process
In the processing of the vacuum equipment, it is necessary to follow the processing technology (especially the welding process) to carry out leak detection on the semi-finished parts in time. For parts that cannot be touched, leak-detected, or repaired after manufacturing, the quality of the welds must be strictly checked for leaks, and unqualified parts should be re-welded, repaired, and leak-detected in time, and the next process can only be carried out after meeting the requirements. Especially for the assembly, welding, and processing of large containers, 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 a vacuum chamber with a double-walled water-cooled chamber, it is best to first assemble and weld the inner chamber wall and check for leaks and then assemble and weld the outer chamber wall after confirming that there are no leaks. In the same way, for the case where there is an insulation layer on the outside of the chamber wall and other structures that are not easily disassembled, strict leak detection must be performed on the chamber wall before the outer structure can be covered.
All vacuum fittings and components (including vacuum chamber flanges and chamber walls) shall adopt the process of processing the flange surface after welding. Flanges without post-welding processing may meet the sealing requirements during the installation and commissioning stage, but during the use of the equipment, factors such as heat and vibration may induce the release of welding stress, resulting in the deformation of the flange and degradation of sealing performance.
In the process of processing and manufacturing, strictly implementing vacuum operation sanitation and operation specifications is also very helpful for improving the airtightness of vacuum equipment and systems. After the welding groove is ground and formed, it needs to be degreased, cleaned, and protected in time, which will help improve the air tightness of the weld. The dynamic and static sealing surfaces of parts that have been processed should have protective measures to prevent bumps and scratches during storage, handling, and assembly. When using vulnerable parts such as welded bellows, metal, ceramic or glass seals, glass devices, etc., more careful work should be done, especially to avoid leaks caused by damage after passing the pre-leakage inspection.
Leak detection steps in the process of vacuum system equipment installation and commissioning
The installation and commissioning stage is the main body of vacuum equipment or system leak detection. If the airtightness of the weld seam of the equipment has been guaranteed through the leak detection in the processing stage, then in the process of equipment installation and commissioning, checking and ensuring the tightness of the connection parts is the focus of the leak detection work. Including key suspicious parts such as various pipelines, flange connections between components, and dynamic seals. If leak detection is performed on the weld seam and the joint at the same time, the workload and difficulty of leak detection will increase. For large and complex vacuum equipment, it is best to use segmental leak detection. Each time a component is installed, leak detection is performed on its connection parts and welds, and the next component is installed after meeting the requirements. Because after all the components are fully assembled, there are not only too many suspected parts, but also multiple leaks may leak at the same time, which brings great difficulties to the overall leak detection. The leak detection steps during the installation and commissioning of vacuum equipment are as follows:
Understand the structural composition and assembly process of the equipment to be inspected. Master the requirements of the equipment, and find out the key suspicious parts that need to be leak tested.
According to the specified maximum allowable leak rate and whether it is necessary to find the specific location of the leak hole and proceed from the principles of economy, speed, and reliability, correctly select the leak detection method or instrument and prepare the auxiliary equipment required for leak detection. Then develop a practical leak detection program.
The inspected parts should be cleaned well, and the welding slag and oil dirt should be removed, and then cleaned according to the vacuum sanitation conditions, and dried. 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, or organic solution but also protect the leak detection instrument.
Calibrate the leak detection sensitivity of the selected leak detection method and leak detection equipment, and determine the leak detection time of the leak detection system.
If the vacuum leak detection method is used, in order to improve the sensitivity of the instrument, the tested part should be pumped to a higher vacuum as much as possible.
Leak detection methods that are more economical and available on-site should be given priority as much as possible.
When using helium mass spectrometry leak detection equipment for leak detection, the parts with large leaks should be detected with low-concentration helium gas at the initial stage of leak detection, and then small leaks should be detected to save helium.
Repair the large leaks that have been detected in time, and then carry out leak detection for small leaks.
Conduct a re-examination of the detected and repaired leaks to ensure that the leak detection results meet the requirements.
Precautions during the use of vacuum equipment
During the operation stage of vacuum equipment, the airtightness of the equipment often decreases and the overall leak rate increases, which is the main reason for affecting the normal operation of vacuum equipment. The reasons for this phenomenon include: mechanical vibration causes the connection part to loosen; the sealing ring of the frequently disassembled part may be damaged or installed incorrectly; deformation and fatigue damage due to cold and thermal shock; Corrosion and damage; some leaks that were originally blocked by water, oil or other dirt are re-released; and stress concentration causes cracks, etc.
To use vacuum equipment correctly, leak detection should be included in the daily maintenance and management of vacuum equipment, such as regular static boost detection experiments. If the operator finds that the airtightness of the equipment has decreased, it should be solved in time, analyze the cause of the leakage according to the use of the equipment and the fault phenomenon, and take appropriate leak detection methods to detect the location of the leak and repair it in time. Don't wait until the equipment has multiple leaks and can no longer work normally before going for leak detection and maintenance.
In addition, preparing sufficient sealing spare parts at ordinary times and replacing vulnerable parts regularly (not after problems occur) are also the main measures to do a good job in vacuum leak detection and ensure the normal operation of equipment.
more information about detecting leaks in a vacuum system