Mechanical seal

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Introduction to mechanical seals: There are a wide variety of mechanical seals with different models, but there are mainly five leakage points: (1) 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 seal end cover and the pump body. 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 with the friction pairs of 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 pairs of the rotating and stationary rings ； If the leaking medium is ejected axially, there are usually problems with the rotating ring seal; if the leaking medium sprays in all directions or leaks out from the water cooling holes, it is mostly 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 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-to-shaft and end-cap seals, is basically caused by damage to the friction pair of the rotating and stationary rings. What are the causes of seal failure and leakage in friction pairs? (l) During operation, abnormal conditions such as vacuum formation, cavitation, or pressure buildup can generate significant axial forces, causing the contact surfaces of the moving and stationary rings to separate ； (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 comes loose 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, too low specific pressure at the sealing surface, or high cold shrinkage of the sealing material, etc. The above phenomenon occurs frequently during commissioning; sometimes it can be eliminated by making appropriate adjustments to the stationary ring seat, but in most cases it is necessary to disassemble and reassemble the components and replace the seals. What is the cause of leakage due to the loss of the lubrication film at the two sealing surfaces? 1. Due to the end-face sealing load, dry friction occurs when the pump is started with no liquid in the sealing chamber ； 2. The medium’s pressure being below the saturated vapor pressure causes the liquid film on the end faces to flash, resulting in a loss of lubrication ； 3. If the medium is a volatile substance, when scaling or blockage occurs in the mechanical seal cooling system, the friction at the end faces and the heat generated by the rotating elements stirring the liquid cause the saturated vapor pressure of the medium to rise, which also results in the medium pressure being lower than its saturated vapor pressure. Causes of mechanical seal leakage due to corrosion? 1. Pitting on the sealing surface, even penetration. 2. Due to the welding of the tungsten carbide ring to the stainless steel seat, intergranular corrosion is likely to occur in the stainless steel seat during use ； 3. Welded metal bellows, springs, etc. are prone to rupture under the combined effect of stress and medium corrosion. Reasons for mechanical seal leakage caused by high-temperature effects: 1. Thermal cracking is the most common failure phenomenon in high-temperature oil pumps, such as slurry pumps, re-refining oil pumps, and bottom pump in atmospheric and vacuum distillation towers. Radial cracks in the ring surface can occur at the sealing surface due to dry friction, sudden interruption of cooling water, intrusion of contaminants into the sealing surface, evacuation, and other such conditions. 2. Graphite carbonization is one of the main reasons for seal failure when carbon-graphite rings are used. In use, if the graphite ring exceeds its allowable temperature (usually between -105 and 250°C), resin will precipitate on its surface; the resin near the friction surface will carbonize. In the presence of a binder, this will cause the material to foam and soften, thereby increasing leakage at the sealing surface and leading to seal failure ； 3. Auxiliary seals (such as fluororubber, EPDM, and all-rubber types) will rapidly age, crack, and harden, losing their elasticity when exposed to temperatures above their operating range. The flexible graphite currently in use has good high-temperature resistance and corrosion resistance, but its resilience is poor. It is also prone to cracking and can be easily damaged during installation. Seal failure caused by wear of the sealed end face? 1. Factors such as materials used in the friction pair having poor wear resistance, a high friction coefficient, and an excessive end-face specific pressure (including spring-specific pressure) can all reduce the service life of mechanical seals. For commonly used materials, in order of wear resistance: silicon carbide—carbon graphite, cemented carbide—carbon graphite, ceramics—carbon graphite, sprayed ceramics—carbon graphite, silicon nitride ceramics—carbon graphite, high-speed steel—carbon graphite, surfacing with cemented carbide—carbon graphite. 2. For media containing solid particles, the entry of solid particles into the sealing surface is the main cause of seal failure. Solid particles that enter the end faces of the friction pair act as abrasives, causing severe wear on the seal and leading to its failure. A proper gap in the sealing surface, the degree of balance of the mechanical seal, as well as the flashing of the liquid film on the sealing surface, are all major causes of the sealing surface opening and allowing solid particles to enter. 3. The balance degree β of the mechanical seal also affects the wear of the seal. Under normal circumstances, a balance level of around β=75% is most suitable. β