When it comes to vacuum equipment leak picking, each one is important!

In order to ensure that the vacuum equipment or system has good sealing performance, it is not enough to find the location of the leak after the equipment is installed, and the channel that blocks the leak is far from enough. It is necessary to carry out vacuum leak detection at any time in the design, manufacture, commissioning and use of vacuum equipment or systems.

1

Considerations in the design of vacuum equipment

1. According to the process requirements of the equipment, determine the total maximum allowable leakage rate of the vacuum equipment, and determine the maximum allowable leakage rate of each component according to the total leakage rate.

2. According to the maximum allowable leakage rate of the equipment and other indicators, the leak detection method to be adopted is preliminarily determined in the design stage, and it is one of the basic principles to guide the commissioning and acceptance.

3. According to the maximum allowable leakage rate index of the equipment or components, determine the sealing, connection mode and overall processing accuracy of the equipment, as well as what dynamic sealing form can meet the requirements. For example, the flange is sealed with metal or rubber.

4. When designing the structural strength of the container, consider the pressure resistance and structural strength of the tested piece if the pressure method is used to detect leaks.

5. When selecting the structural materials of the parts, consider whether the materials that may be damaged by the corrosion of the working medium and leakage gas are used.

6. When designing the structure, the necessary spare interface of the leak detection instrument should be left on the container or system, so that it can be used for leak detection in the process of equipment assembly and commissioning. Especially for large-scale and complex pipeline systems, it is usually necessary to adopt the segmented leak detection method, so it is necessary to set up sectional isolation valves on the pipeline, and reserve the leak detection instrument interface on each isolation section.

7. When designing the structure of parts, try to avoid using design schemes that may interfere with leak detection. For example, in the vacuum chamber, the screw hole cannot be in the form of a blind hole, because the gas in the remaining space inside the screw hole after installing the screw can only escape through the thread gap, forming a false leak. This prolongs the pumping time of the system and interferes with the normal operation of leak detection. As shown in the figure, the structure that should not appear in vacuum leak detection.

8. Similarly, the structural design does not allow the existence of continuous double-sided welds and multi-layer sealing ring structure, because this will form a "parasitic product" in the middle of the gas will form a 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 of the leakage gas through the double-layer weld too long, and the leakage can not be detected normally.

9. When designing the welded structure, try to minimize the welds that cannot be detected after the final assembly.

2

Leak detection in the manufacturing process of vacuum equipment

In the processing stage of the equipment, it is necessary to follow the processing technology (especially the welding process) to detect the leak of semi-finished parts in a timely manner. For the parts that cannot be contacted, leak detected or repaired after manufacturing, the quality of the weld should be strictly leak detected, and the unqualified ones should be re-welded, repaired and re-tested in a timely manner, and the next process can only be carried out after meeting the requirements. Especially for the assembly welding and processing of large containers, the leak detection of the intermediate process is very critical, and special leak detection tools (such as leak detection boxes, blind plates, etc.) should be designed and manufactured if necessary. For the vacuum chamber body with double-wall water-cooled jacket, it is best to assemble and weld the inner chamber wall and detect the leak first, and then assemble and weld the outer chamber wall after confirming that there is no leakage. In the same way, for the case that there is an insulation layer on the outside of the chamber wall and other structures that are not easy to disassemble, the chamber wall must be strictly tested for leakage before the outer structure can be covered.

If conditions permit, all vacuum flanges and their joints (including vacuum chamber flanges and chamber walls) should be processed after welding. Even if the flange that is not processed after welding may meet the sealing requirements in the installation and commissioning stage, during the use of the equipment, heat, vibration and other factors may also induce the release of welding stress, 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 very helpful to improve the air tightness of vacuum equipment and systems. After the welding groove is ground and formed, it needs to be cleaned by degreasing and protected in time, which will help improve the air tightness of the weld. The dynamic and static sealing surfaces of the parts that have been processed should have protective measures to prevent bumps and scratches during storage, handling and assembly. When using wearing parts such as welded bellows, metal-to-ceramic or glass seals, and glass devices, it is important to work with care, especially to avoid leakage caused by damage after passing the pre-test.

3

Leak detection steps in the process of installation and commissioning of vacuum equipment

The installation and commissioning stage is the main body of leak detection work of vacuum equipment or system. If the air tightness of the weld of the equipment has been guaranteed by the leak detection in the processing stage, then in the process of equipment installation and commissioning, it is the focus of leak detection to check and ensure the tightness of the connection part. Including key suspicious parts such as pipes, flange connections between components and moving seals. If the leak detection of the weld and the connection part is carried out at the same time, the workload and difficulty of leak detection will increase. Large-scale and complex vacuum equipment is best to use segmented leak detection, each time a component is installed, the connection part and weld are tested once, and the next component is installed after meeting the requirements. Because after all the parts are fully assembled and then tested for leaks, not only are there too many parts suspected, but there may also be multiple leaks at the same time, which brings great difficulties to the total leakage inspection. The leak detection steps in the process of installation and commissioning of vacuum equipment are as follows:

1. Understand the structural composition and assembly process of the equipment to be inspected. Grasp the requirements of the equipment and identify the key suspicious parts that need to be tested for leaks.

2. According to the requirements of the maximum allowable leakage rate and whether it is necessary to find the specific location of the leak hole, and 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 to formulate a feasible leak detection procedure.

3. The inspected parts should be cleaned, and the welding slag and grease should be removed and then cleaned according to the vacuum sanitary conditions, and dried. For small devices with high requirements. After the cleaning treatment, it can be baked through the vacuum drying oven, which can not only avoid the leakage hole from being blocked by dirt, oil, organic solution, etc., but also protect the leak detection instrument.

4. 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.

5. If the vacuum leak detection method is used, in order to improve the sensitivity of the instrument, the test piece should be pumped to a higher vacuum as much as possible.

6. Under the premise of permitting, the more economical and on-site qualified leak detection methods should be given priority as far as possible.

7. When using helium mass spectrometry leak detection equipment for leak detection, for the tested pieces that require low leak detection or large leakage, leak detection should be carried out with lower concentration of helium as much as possible in the early stage of leak detection, and then leak detection of small leakage holes should be carried out to save helium.

8. Repair the large leakage holes that have been detected in time and then carry out the leak detection of small leaks.

9. Conduct a review of the leakage holes detected and repaired to ensure that the leak detection results meet the requirements.

4

Precautions during the use of vacuum equipment

In the operation stage of vacuum equipment, the air tightness of the equipment often decreases and the overall leakage rate increases, which is the main reason for the normal operation of vacuum equipment. The causes of this phenomenon include: loosening of the connection due to mechanical vibration, damage or incorrect installation of the sealing ring in the frequently disassembled part, deformation and fatigue damage due to cold and thermal shock, damage of some parts or materials due to corrosion by the working medium, re-release of some leaks that were originally blocked by water, oil or other dirt, and cracks caused by stress concentration.

For the correct use of vacuum equipment, leak detection should be included in the daily maintenance and management specifications of vacuum equipment, such as regular static boost testing experiments. If the operator finds that the air tightness of the equipment has decreased, he should solve it 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 leakage hole and repair it in time. Don't wait until the equipment has multiple leaks and can no longer work properly.

In addition, preparing sufficient sealing spare parts and replacing wearing parts regularly (rather than after problems) are also the main measures to do a good job of vacuum leak detection and ensure the normal operation of the equipment.