Several issues related to the failure of mechanical seals
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Mechanical seal leakage is a common issue; the main reasons for this are as follows: Please feel free to provide your guidance. Several problems related to the failure of mechanical seals: There are many different types and models of mechanical seals used in pumps, but there are mainly five locations where leakage can occur: (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 sealed end cover and the pump body. I. Analysis and determination of leakage causes 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 moving 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 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 ruling out 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 causing seal failure in friction pairs are: (1) During operation, abnormal conditions such as evacuation, cavitation, and pressure buildup generate significant axial forces, leading to the separation of the contact surfaces between the moving and stationary rings ； (Excessive compression during the installation of the mechanical seal led to severe wear and scarring of the end surfaces 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 seal ring of the stationary ring is too loose; when the moving ring moves axially, the stationary 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, 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 static ring seat, etc., but in most cases it is necessary to disassemble and reassemble the components and replace the seals. Failure resulting from the loss of the lubrication film at the two sealing surfaces: a) Dry friction occurs when the pump is started with no liquid in the sealing chamber, due to the sealing load on the surfaces ； b) 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 ； c) 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. Mechanical seal failure caused by corrosion: a) Pitting on the seal surface, even penetration. b) 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 ； c) Welded metal bellows, springs, etc. are prone to rupture under the combined effect of stress and medium corrosion. Mechanical seal failure caused by high-temperature effects: a) Thermal cracking is the most common failure phenomenon in high-temperature oil pumps, such as sludge 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, contamination entering the sealing surface, evacuation, and other such conditions ； b) 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 ； c) Auxiliary seals (such as fluororubber, EPDM, all-rubber types) will rapidly age, crack, and harden and lose their elasticity when exposed to temperatures above their allowable limits. 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. Sealing failure due to wear of the sealed end face: a) Factors such as poor wear resistance of the materials used in the friction pair, high friction coefficients, and excessive end face pressure (including spring 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. b) For media containing solid particles, the penetration of solid particles into the sealing surface is the main cause of seal failure. Solid particles that enter the end surfaces 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. c) 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
It’s well-explained; very detailed and appropriate
Very good material, I’ve learned from it. Thank you
1. For the materials used in friction pair assemblies, a \"hard against hard\" configuration is recommended; commonly, tungsten carbide against tungsten carbide (with YG6-YG6 being the choice) and tungsten carbide against silicon carbide are used. When adopting a \"hard-to-hard\" design, the following points must be taken into consideration: 1) The cooling system must be ensured to function properly; any interruption in the cooling water flow must be avoided, as this can cause the end face to rise, leading to the evaporation of the lubricating film and thereby reducing the lubrication at the sealed end face, which in turn increases wear ； 2) During the installation of the mechanical seal, some lubricant (either engine oil or butter) should be applied to the sealing surface. To prevent this during pump startup. Dry friction at the sealed end face due to a lack of lubrication ； 3) Using external flushing with clean fluid is one of the relatively effective methods for dealing with the accumulation of solvent particles. However, this method is quite wasteful. Moreover, the fluids, temperatures, and pressures required for various pumps differ (it is generally necessary that the pressure of the flushing fluid be 0.07–0.12 MPa higher than the pressure on the medium side), which makes the structure of the external flushing system more complex. In addition, the costs associated with installing such flushing facilities and their maintenance can sometimes result in more disadvantages than advantages, especially for small and medium-sized enterprises. As a result, the oil sealing systems in many enterprises are either not used at all or not installed at all. In such cases, it is recommended to use a multi-seal structure with an isolating medium, such as in slurry pumps and re-refining pumps, by employing double-end face mechanical seals with an isolating medium (such as clean engine oil) filled between the two seal surfaces, as shown in Figure 3. This structure can effectively extend the service life of mechanical seals, which can generally reach over 6,000 to 8,000 hours. Additionally, this approach takes the following two points into consideration: ① The sealing surface materials near the impeller are selected to have a \"hard-on-hard\" configuration (such as YG6-YG6) ； And the set of sealing end faces located near the mechanical seal gland can use copper- or antimony-doped carbon-graphite against tungsten carbide or silicon carbide ； ②The isolation medium selected for high-temperature oil pumps should have a high thermal decomposition temperature, autoignition point, and flash point (generally above 260°C), good thermal oxidation stability, and low evaporation loss at high temperatures. Selection of mechanical seals for liquid hydrocarbon pumps: Liquid hydrocarbon media are low-temperature liquefied gases with characteristics such as low boiling points, low viscosity, and high vapor pressures. Mechanical seals used under such operating conditions can cause the sealing material to become cold-sensitive; water vapor in the atmosphere will freeze on the atmospheric side of the sealing device, and the liquid film on the surfaces of the frictioning parts tends to vaporize. Especially when there is a slight leakage in the medium, the leaked liquid hydrocarbons vaporize immediately on the atmospheric side, carrying away a large amount of heat. As a result, the temperature of the environment surrounding the mechanical seal drops sharply; ordinary sealing materials such as rubber or polytetrafluoroethylene become brittle, leading to seal failure and an increasing leakage that becomes uncontrollable. Some companies use double-end mechanical seals, with an isolation chamber installed at both the medium side and the atmospheric side; seal oil is passed through this chamber to mitigate the effects of low temperatures. But this structure is complex and requires a sealing fluid system. Based on experience, it is better to use bellows mechanical seals; this is mainly achieved by replacing the auxiliary sealing rings with metal bellows and flexible graphite, thereby solving the problems of cold brittleness and loss of elasticity in the sealing ring materials, as well as the issue of cushioning effect. Examples of such seals include the DBM type and the YH-604/606/609 types. a) For metal bellows materials, those with low-temperature resistance, as well as good plasticity and toughness, such as Har-C, AM350, Carpenter20, etc., should be selected ； b) The materials for friction pairs are selected under two special circumstances: 1) For equipment that operates continuously, when the medium contains a large amount of solid particles, it is better to use a \"hard-on-hard\" configuration (in practice, YG6–YG6 is chosen); generally, such equipment can operate continuously for over 8,000 hours ； 2) For intermittently operating equipment, tungsten carbide or silicon carbide paired with special graphite is used for the friction pairs. c) Since vaporization at the surfaces of the friction pair under low-temperature conditions has a significant impact on the performance of mechanical seals, in addition to selecting appropriate materials, it is necessary to choose an appropriate surface pressure ratio (mainly the compression amount of the bellows; generally, a compression amount 15%–30% higher than that used under normal conditions is advisable). Cooling water at around 25°C is introduced on the side of the mechanical seal elements that is exposed to the atmosphere, in order to improve the lubrication conditions of the friction pair.
Great material, thanks to the original poster; I’ve learned something from it. A rare experience.