The performance of most vacuum systems will change over time, especially for vacuum equipment used in production, which almost inevitably experiences problems such as a decrease in vacuum degree. One common cause of these problems is leakage.
Regular leak detection is important
Large leaks are usually very obvious: the pressure in the vacuum chamber is not decreasing at a normal rate, or the maximum pressure is significantly higher than the normal value. However, sometimes small leaks are difficult to detect due to the ease with which vacuum pumps can handle gas loads caused by leaks. Even if the vacuum gauge reading is still at a normal level, the occurrence of leaks may still bring unexpected gases (such as oxygen) to the vacuum chamber, which can sometimes have a very serious impact on the process (such as certain coating processes). So, regardless of whether there is a significant decrease in pressure in the vacuum system, regular leak testing should be conducted.
Real leakage and virtual leakage
Not all prolonged pumping time and extreme pressure drop are due to leakage. Before using a leak detector to detect leaks, it is necessary to understand how to determine whether the vacuum equipment has really leaked.
The pollutants attached to the inner wall or inner wall of a vacuum chamber continuously release gas under vacuum, and this phenomenon is called deflation. When there is a dead space inside the vacuum chamber, and the dead space is connected to the inside of the chamber through a narrow channel, the gas in the dead space will slowly release under vacuum, forming a phenomenon similar to deflation or leakage, which is usually called virtual leakage.
Real leaks can be found through leak detection, and venting can also be solved by cleaning the inner surface of the vacuum chamber. However, once a virtual leak occurs, it is difficult to detect, and it is necessary to avoid structures or processes that are prone to virtual leaks as much as possible during design and manufacturing, such as threaded connections (hollow bolts can be used if necessary), long slits or capillaries, fully welded chambers on both sides (thicker shells are recommended to be fully welded on the vacuum side and intermittently welded on the atmospheric side), and so on.
The following two methods can be used to determine whether there is a real leak in the vacuum system.
Analysis of pressure drop curve
The historical data of vacuum systems is one of the most valuable tools for understanding the performance of large vacuum systems. Experienced technicians will carefully preserve historical data and quickly determine the cause of problems by comparing the current pressure drop curve with the previous cycle when the system was in good condition. For example, by using the pressure drop curve shown in the following figure, it is possible to determine whether there is a leak.
Under the same process conditions and normal operation of the vacuum pump, if there is a real leak, the gas leaking into the chamber from the outside will cause the pressure in the chamber to drop to a position higher than the normal limit pressure, and then it will no longer decrease or decrease very slowly. The pressure drop curve is similar to the top curve in the above figure. When there is deflation or virtual leakage, the gas slowly releases and the deflation rate decreases. The system can reach the original limit pressure, but the time to reach the limit pressure slows down significantly. The pressure drop curve is similar to the middle curve in the above figure.
Pressure rise test
Pressure rise test, also known as pressure holding test or vacuum holding test. Closing the valve between the vacuum pump and the chamber under vacuum can cause an increase in pressure (rebound) in the chamber due to deflation, virtual leakage, or leakage. By dividing the pressure increase by the elapsed time, the pressure rise rate of the vacuum system can be calculated and plotted as a curve as shown in the following figure. The speed of pressure rise and fall is usually expressed in Pa/hr. For general industrial vacuum equipment, a pressure rise rate exceeding 1Pa/hr during pressure holding testing needs to be investigated and resolved. Some high vacuum equipment requires a pressure rise rate of 0.5Pa/hr or even lower.
The pressure drop and pressure rise tests will not locate leaks, but only indicate the cumulative effect of all gas sources (actual leaks and deflation or virtual leaks). If there is suspicion of a real leak, the next step is usually to use a helium leak detector for leak detection.
External leakage and internal leakage
The leakage we usually refer to refers to external leakage, that is, leakage from the outside to the inside of the vacuum chamber or pipeline; Internal leakage refers to the leakage that occurs between two vacuum chambers that should have been isolated, between two vacuum pipelines isolated by valves, or between a vacuum chamber and a pipeline.
External leaks can be easily detected through leak detectors, while internal leaks can only be pre judged through segmented pressure maintenance and other methods. Then, the pipeline on one side of the suspicious valve can be removed and leak detection can be carried out (or a leak detector can be connected to one side of the suspicious valve and helium gas can be filled into the pipeline on the other side for leak detection).
Dynamic leakage
In vacuum systems where there is motion transmission, dynamic seals are used. These dynamic seal structures are likely to seal well in static conditions but leak during motion; For example, valves that use rubber shaft seals for dynamic sealing (between the valve stem and valve body) are more prone to this phenomenon. The probability of this phenomenon occurring is not high, but due to the fact that the valve is usually in an open or closed state during leak detection, this dynamic leakage is difficult to detect.
For critical processes, choosing valves sealed with corrugated pipes can greatly reduce the probability of dynamic leakage; If the valve is still sealed with a shaft seal, during leak detection, spraying helium at the valve stem position while operating the valve can quickly determine whether there is dynamic leakage in the valve.
