Repair of mechanical seals for pumps

Thread Content

Repair of mechanical seals for pumps: There are a wide variety of mechanical seals used in pumps, with different models, but there are mainly five locations where leaks can occur: (l) The seal between the shaft sleeve and the shaft； 　　(2) Sealing between the rotating ring and the shaft sleeve ； 　　(3) Sealing between the rotating and stationary rings ； 　　(4) Sealing between the stationary ring and the stationary ring seat ； 　　(5) Sealing between the sealed end cover and the pump body. 　　Generally, leaks between the shafts where the shaft sleeves protrude, and between the sealing end caps and the pump body are relatively easy to detect and resolve, but careful observation is required, especially when the working medium is a liquefied gas or a high-pressure, toxic, or harmful gas, in which case it becomes more difficult. The remaining leaks are difficult to identify and assess intuitively; it is necessary to observe, analyze, and evaluate the symptoms of these leaks based on long-term management and maintenance practices in order to draw correct conclusions. 　　I. Analysis and determination of the cause of leakage 　　1. Leakage during static installation testing. After the mechanical seal is installed and adjusted, a static test is generally carried out to observe the leakage rate. If the leakage amount is small, it is usually due to a problem with the seal ring of the rotating ring or the stationary ring ； A large leakage amount indicates a problem in the friction pair between the rotating and stationary rings. Based on the initial assessment of the leakage amount and identification of the leakage location, manual rotation of the shaft is carried out for further inspection; if there is no significant change in the leakage amount, then there is a problem with the static or dynamic ring seals ； If there is a significant change in the leakage amount during cranking, it can be concluded that there is a problem with the friction pair between the rotating and stationary rings ； If the leaking medium sprays axially, there are usually problems with the rotating ring seal; if the leaking medium sprays in all directions or leaks out through the water cooling holes, it is typically due to a failure of the stationary ring seal. Furthermore, leakage channels can also exist simultaneously, but there is usually a primary and secondary distinction; with careful observation and familiarity with the structure, it is definitely possible to make the correct judgment. 　　2. Leaks that occur during trial operation. After undergoing a static test, the mechanical seal used in pumps prevents leakage of the medium due to the centrifugal force generated by high-speed rotation during operation. Therefore, during commissioning, mechanical seal leakage, after excluding failures in the shaft-separation and end-cap seals, is basically caused by damage to the friction pair of the rotating and stationary rings. The main factors that cause failure in the sealing of friction pairs are: (l) During operation, abnormal conditions such as evacuation, cavitation, and pressure buildup generate significant axial forces, resulting in the separation of the contact surfaces between the moving and stationary rings ； 　　(2) Excessive compression during the installation of the mechanical seal leads to severe wear and scarring of the end faces of the friction pairs ； 　　(3) The rotating ring seal is too tight, preventing the spring from adjusting the axial float of the rotating ring ； 　　(4) The static ring seal is too loose; when the dynamic ring moves axially, the static ring detaches from its seat ； 　　(5) There are particulate substances in the working medium, which enter the friction pair during operation, affecting the sealing surfaces of the moving and stationary rings used for flaw detection ； 　　(6) Incorrect design selection, low specific pressure at the sealing surface, or high cold shrinkage of the sealing material, etc. The above phenomenon occurs frequently during trial operation; sometimes it can be eliminated by making appropriate adjustments to the static ring seat, etc., but in most cases it is necessary to disassemble and reassemble the components and replace the seals. 　　3. Sudden leakage during normal operation. Sudden leakage from a centrifugal pump during operation is caused in a few cases by normal wear or reaching the end of its service life, while in most cases it is due to significant changes in operating conditions or improper operation and maintenance. 　　(1) Vacuum, cavitation, or prolonged pressure retention leading to seal failure ； 　　(2) The actual output volume of the pump is too low; a large amount of fluid circulates within the pump, heat accumulates, which causes the fluid to vaporize and leads to seal failure ； 　　(3) Excessive backflow causes the sediment at the bottom of the container on the suction pipe side (tower, reactor, tank, pool) to be displaced, thereby damaging the seal ； 　　(4) During a prolonged period of shutdown, when restarting without performing manual turning of the shaft, the friction pairs cause the sealing surfaces to be damaged due to sticking ； 　　(5) Increase in corrosive, polymeric, and gelling substances in the medium ； 　　(6) Sudden changes in ambient temperature ； 　　(7) Frequent changes or adjustments in operating conditions ； 　　(8) Sudden power outages or machine failures, etc. If a centrifugal pump starts leaking suddenly during normal operation, and this is not detected in time, it can often lead to serious accidents or losses; therefore, it is necessary to pay close attention to this issue and take effective measures. 　　II. Several misconceptions in the maintenance of mechanical seals for pumps 1. The greater the spring compression, the better the sealing effect. In fact, this is not the case; excessive compression of the spring can lead to rapid wear of the friction pair, resulting in instant burnout ； Excessive compression causes the spring to lose its ability to adjust the end face of the moving ring, resulting in seal failure. 　　2. The tighter the dynamic ring seal, the better. In fact, an overly tight dynamic ring seal is more harmful than beneficial. First, it exacerbates the wear between the sealing ring and the shaft sleeve, leading to premature leakage ； Secondly, it increases the resistance to axial adjustment and movement of the moving ring, making it impossible to make timely adjustments when operating conditions change frequently ； Third, excessive fatigue of the spring can lead to damage ； Fourth, it deforms the rotating ring seal, affecting the sealing performance. 　　3. The tighter the static ring seal, the better. The static ring seal is essentially at rest, which results in a better sealing effect; however, being too tight is also harmful. First, it causes excessive deformation of the static ring seal, thereby affecting its sealing performance ； Secondly, the material of the static ring is mostly graphite, which is generally brittle; excessive stress can easily cause it to crack ； Third, it is difficult to install and remove, and the static ring is very prone to damage. 　　4. The tighter the impeller lock nut, the better. Among mechanical seal leaks, leakage between the shaft sleeve and the shaft (inter-shaft leakage) is relatively common. It is generally believed that inter-shaft leakage is caused by the failure to properly secure the impeller lock nut; however, there are many factors that can lead to inter-shaft leakage. Issues such as failed inter-shaft gaskets, misalignment, contaminants inside the shaft gap, significant geometric errors at the junction between the shafts and their sleeves, damaged contact surfaces, gaps between various components on the shaft, and excessively long shaft threads can all cause inter-shaft leakage. Excessive tightening of the lock nut will only lead to premature failure of the gasket between the shafts; on the other hand, moderate tightening of the lock nut ensures that the gasket maintains a certain degree of compressive elasticity. During operation, the lock nut will tighten automatically at the right time, keeping the area between the shafts in a good sealed state. 　　5. The new is better than the old. Relatively speaking, new mechanical seals perform better than old ones; however, if the quality or material selection of the new mechanical seals is improper, significant dimensional errors can affect their sealing performance ； In polymeric and permeable media, it is better not to replace the static ring unless there is excessive wear. Since the stationary ring remains stationary in its seat for an extended period of time, polymers and impurities accumulate together, thereby providing an effective sealing effect. 　　6. It’s better to repair it than not to do anything at all. Once a leakage occurs in the mechanical seal, there is a rush to repair it. In fact, sometimes the seal is not damaged at all; adjusting the operating conditions or making appropriate adjustments to the seal can eliminate the leakage. This not only prevents waste but also allows one to test their ability to diagnose faults, thereby gaining repair experience and improving the quality of maintenance work.