Leakage analysis and maintenance of pump mechanical seals

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There are many types of mechanical seals for pumps with different models, but there are five main leakage points.: 1) Seal between sleeve and shaft ； 2) Seal between moving ring and shaft sleeve ； 3) Seal between dynamic and static rings ； 4) Seal between static ring and static ring seat ； 5) Seal between the sealing end cover and the pump body. Generally speaking, leaks between the shafts where the shaft sleeve extends, and between the sealing end cover and the pump body are easier to find and solve, but they require careful observation. The rest of the leaks are difficult to identify and judge intuitively. Only by observing, analyzing, and judging leak symptoms based on long-term management and maintenance practices can we draw correct conclusions. 1. Leakage cause analysis and judgment ⑴ Leakage during installation and static test After the mechanical seal is installed and debugged, a static test is generally performed to observe the leakage amount. If the leakage is small, there is usually a problem with the dynamic or static ring seal. ； When the leakage is large, it indicates that there is a problem between the dynamic and static friction pairs. On the basis of preliminary observation of the leakage amount and judgment of the leakage location, and then manually turning the machine to observe, if the leakage amount does not change significantly, there is a problem with the static and dynamic ring seals. ； If the leakage changes significantly during cranking, it can be concluded that there is a problem with the dynamic and static ring friction pairs. ； If the leaking medium is sprayed in the axial direction, the problem is mostly caused by the dynamic ring sealing ring. If the leaking medium is sprayed around, it is mostly caused by the failure of the static ring sealing ring. In addition, leakage channels may also exist at the same time, but generally there is a distinction between primary and secondary. As long as you observe carefully and are familiar with the structure, you will be able to judge correctly. ⑵Leakage that occurs during trial operation. After the mechanical seal of the pump has been statically tested, the centrifugal force generated by high-speed rotation during operation will inhibit the leakage of the medium. Therefore, mechanical seal leakage during trial operation is basically caused by damage to the dynamic and stationary ring friction pairs after excluding the failure of the shaft and end cover seals. The main factors causing friction pair seal failure are:: 1) During operation, due to abnormal phenomena such as evacuation, cavitation, and pressure suppression, large axial forces are caused, causing the contact surfaces of the dynamic and static rings to separate. ； 2) When installing the mechanical seal, the compression amount is too large, resulting in serious wear and scratches on the end face of the friction pair. ； 3) The sealing ring of the moving ring is too tight and the spring cannot adjust the axial float of the moving ring. ； 4) The static ring sealing ring is too loose. When the moving ring floats axially, the static ring separates from the static ring seat. ； 5) There are granular substances in the working medium, which enter the friction pair during operation and scratch the sealing surfaces of the dynamic and static rings. ； 6) The design selection is wrong, the specific pressure of the sealing end face is low or the sealing material has high cold shrinkage, etc. The above phenomenon often occurs during trial operation. Sometimes it can be eliminated by appropriately adjusting the static ring seat, etc., but most of them require re-disassembly and replacement of seals. ⑶Sudden leakage during normal operation: A few centrifugal pumps suddenly leak during operation. A few are due to normal wear and tear or have reached the end of their service life, while most are caused by large changes in working conditions or improper operation and maintenance. 1) Evacuation, cavitation or pressure suppression for a long time, leading to seal damage ； 2) The actual output of the pump is too small, a large amount of medium circulates in the pump, and heat accumulates, causing the medium to vaporize, leading to seal failure. ； 3) The return flow is too large, causing sediment to appear at the bottom of the container (tower, kettle, tank, pool) on the side of the suction pipe, damaging the seal. ； 4) If the machine is shut down for a long time and there is no manual turning when restarting, the friction pair will tear the sealing surface due to adhesion. ； 5) Increased corrosive, polymeric, and gelling substances in the medium ； 6) Rapid changes in ambient temperature ； 7) Frequent changes or adjustments to working conditions ； 8) Sudden power outage or malfunction, etc. If a centrifugal pump suddenly leaks during normal operation, if it cannot be detected in time, it will often lead to major accidents or losses, so attention must be paid and measures must be taken. 2. Several misunderstandings in the maintenance of pump mechanical seals (1) The greater the spring compression, the better the sealing effect. Excessive spring compression can lead to rapid wear of the friction pair and instant burnout. ； Excessive compression causes the spring to lose its ability to adjust the end face of the moving ring, resulting in seal failure. ⑵The tighter the dynamic ring sealing ring, the better. Too tight a dynamic ring sealing ring is harmful and useless. One is to intensify the wear between the sealing ring and the shaft sleeve and cause premature leakage. ； Second, the resistance to axial adjustment and movement of the moving ring is increased, making it impossible to adjust in a timely manner when working conditions change frequently. ； Third, the spring is easily damaged due to excessive fatigue. ； The fourth is to deform the dynamic ring sealing ring, affecting the sealing effect. ⑶The tighter the static ring sealing ring, the better. The static ring sealing ring is basically in a static state. If it is relatively tight, the sealing effect will be better, but too tight is also harmful. First, it causes the static ring seal to be excessively deformed, affecting the sealing effect. ； Second, the static ring is mostly made of graphite, which is generally brittle and can easily cause fragmentation due to excessive stress. ； Third, it is difficult to install and disassemble, and the static ring is easily damaged. ⑷The tighter the impeller lock nut, the better. Among mechanical seal leaks, leakage between the sleeve and the shaft (inter-shaft leakage) is relatively common. It is generally believed that inter-shaft leakage means that the impeller lock nut is not locked. In fact, there are many factors that lead to inter-shaft leakage, such as failure of the inter-shaft pad, offset, impurities in the shaft, large shape error in the fit between the shaft and the sleeve, damage to the contact surface, gaps between the components on the shaft, excessive thread length of the shaft head, etc., which will lead to inter-shaft leakage. Excessive locking of the lock nut will only lead to premature failure of the inter-shaft pad. On the contrary, lock the lock nut appropriately so that the inter-shaft pad always maintains a certain compression elasticity. During operation, the lock nut will automatically lock in time to keep the shafts in a good sealing state. ⑸The new one is better than the old one. Relatively speaking, the effect of using a new mechanical seal is better than the old one. However, when the quality or material of the new mechanical seal is not selected properly, the large error in the fit size will affect the sealing effect. ； In polymeric and permeable media, if the static ring is not excessively worn, it is better not to replace it. Because the static ring is in a static state for a long time in the static ring seat, the polymer and impurities are deposited into one body, which plays a better sealing role. ⑹It is better to dismantle and repair than not to dismantle and repair. Once the mechanical seal leaks, you will rush to dismantle and repair it. In fact, sometimes the seal is not damaged, and the leakage can be eliminated by adjusting the working conditions or adjusting the seal appropriately. In this way, you can avoid waste and verify your fault judgment ability, accumulate maintenance experience and improve the quality of maintenance.