Fault analysis of the hottest mechanical seal

  • Detail

Mechanical seal failure analysis

causes and analysis of seal failure (I) seal failure mainly has the following three reasons: 1. When repairing mechanical seals, 85% of seal failure is not caused by wear, but has leaked before wear. As soon as the sealing surface is opened, the solid particles in the medium enter the sealing surface under the action of liquid pressure. After the sealing surface is closed, these solid particles are embedded in the surface of the soft ring (usually the right ink ring), which actually becomes a "grinding wheel", which will damage the surface of the hard ring. Because the moving ring or rubber ring is fastened on the shaft (shaft sleeve), when the shaft moves in series, the moving ring cannot fit in time, so that the sealing surface opens and the sealing surface lags closed, so that solid particles enter the sealing surface. At the same time, there are also solid particles between the shaft (shaft sleeve) and the sliding parts, which affect the sliding of the rubber ring or moving ring (relative to the dynamic sealing point, common faults). In addition, the medium will also produce crystals at the friction part between the rubber ring and the shaft (shaft sleeve), and there will also be solid substances at the spring, which will open the sealing surface. 2. Due to overheating, heat will be generated on the sealing surface, so the service temperature of the rubber ring should be lower than the design specification. Fluororubber and polytetrafluoroethylene electronic universal testing machine is a new generation of double space microcomputer controlled electronic universal testing machine specially designed for colleges and universities and scientific research institutes. The service temperature of ethylene is 216 ℃, and the service temperature of nitrile rubber is 162 ℃. Although both of them can withstand high temperature, the heat generated by the sealing surface is high, so the rubber ring is in danger of continuous vulcanization, and eventually loses elasticity and leaks. (cold brittleness is considered in the cold zone) heat will also cause the crystallization of the medium between the sealing surfaces, such as carbon deposition, causing the sliding parts to stick and the sealing surface to condense. Moreover, some polymers are coking due to overheating, and some fluids lose lubrication due to overheating, and even flash fire. Overheating can not only change the condition of the medium, but also aggravate its corrosion rate. For the deformation of metal parts, the cracking of alloy surface, and the cracking of some coatings, the balanced mechanical seal should be selected in the design to reduce the specific pressure and prevent overheating. 3 out of tolerance and correct assembly tolerance are necessary for the installation of mechanical seals. The shaft (shaft sleeve) must have appropriate surface roughness and correct size, but the manufacturer rarely provides tolerance data, which is very critical for installation. (rely on experience and common sense) the dimensional accuracy and geometric tolerance of mechanical seal must meet the requirements of the drawing, and out of tolerance will lead to seal failure in advance. (2) Analysis of the causes of seal failure. The sealing surface itself will also provide signs of seal failure. For example, during vibration, there will be wear marks on the transmission parts. If the marks are not obvious, it is generally caused by improper assembly. For the graphite ring (moving ring) with poor quality, there are many internal pores, which is caused by the expansion of the gas gathered in the graphite during the manufacturing process, blowing out the carbon particles. Therefore, the carbon particles of this low-quality graphite ring are easy to fall off during the sealing and use, so that the sealing surface sticks when the sealing is stopped. The scars on the inner cylindrical surface of the sealing surface are likely to be caused by foreign matters entering the sealing surface or improper installation. Most of the annular grooves on the sealing surface are caused by the deposition of solid particles on the sealing surface. The cracks of graphite ring (moving ring) are caused by the vibration of transmission parts, the expansion of rubber ring and the internal stress of graphite ring itself, while the coking is caused by high temperature, which is common in high-temperature hot oil medium in oil refineries. Fuming sulfuric acid, nitric acid, hydrofluoric acid, sodium hypochlorite, aqua regia, peroxides and other strong oxidants that have corrosive effect on graphite, their corrosive effect intensifies with the increase of temperature. Usually, overheating of the hard ring (stationary ring) surface will cause serious wear of the sealing ring, such as a vertical pump without cooling. Under high temperature and high pressure, excessive spring compression and shaft series movement will cause excessive wear of the sealing surface. Four signs should be noted when checking the surface of the hard ring: (1) the ceramic ring is broken; (2) Hot cracking; (3) Nicks; (4) Peeling of coating. Over tight assembly of ceramic rings is the main reason for cracking, and improper assembly is also a common reason. Due to the different linear expansion rate between the coating material and the base material, cracks will appear on the ring surface when the temperature rises, especially in stellite alloy. Among the higher coating materials, cobalt based tungsten carbide is not as good as nickel based coating. Cooling the sealing surface can effectively prevent thermal cracking. The solid particles remaining on the sealing surface often damage the surface. For example, the sand particles on the grinding wheel will damage the surface of the hard ring during grinding, resulting in the opening of the sealing surface or the formation of crystals between the sealing surfaces. After Regrinding the graphite ring, the abrasive will be embedded in the surface of the graphite ring. The failure of the rubber ring is related to the way of use. Usually, high pressure is one of the reasons for the failure of the pressed O-ring. When it is found that the O-ring becomes rectangular or the ring becomes hard, it is necessary to adjust the compression amount, otherwise it will heat up. Therefore, it is necessary to know the service temperature of a synthetic rubber. Most of the swelling of synthetic rubber rings is caused by chemical erosion. They all have some respective characteristics, such as fluororubber is resistant to high temperature, while ethylene and propylene O-rings will swell when used in petroleum lubricants, and ozone has an corrosive effect on nitrile rubber. Therefore, nitrile rubber products should not be installed in motors, so high temperature and chemical corrosion are usually the main reasons for the hardening and cracking of rubber products. During installation, the rubber parts are cut and the surface is scored, which is also a common cause of seal failure. Pay attention to the old fixing screws, keyways, spline shafts, sharp shaft shoulders and other signs on the maintenance and protection of the shaft, which will damage the rubber parts. Here, the following points need to be added to the wear marks of the sealing surface. Checking the wear marks can help analyze the fault. 1. Wear widens: it indicates that the pump is seriously misaligned. The reasons are: (1) bearing damage; (2) Shaft vibration or shaft deformation; (3) Shaft bending; (4) Pump cavitation produces vibration; (5) The coupling is not aligned; (6) The pipe is seriously deformed; (7) The seal stationary ring is inclined. 2. Grinding mark narrowing: the grinding mark is narrower than the minimum width of the two sealing surfaces, which indicates that the sealing surface is deformed by overpressure, pressure or temperature. 3. No wear marks: it means that the sealing surface is not bonded. Check whether the compensation mechanism such as spring slips or is blocked. 4. The sealing surface has no wear marks but bright spots. If the sealing surface is warped, there will be bright spots without wear marks. The pressure is too high, the gland bolt is not screwed or clamped, or the pump surface is rough, which can form bright spots. When the gland with two bolts is used, its stiffness is not enough, and deformation is also a reason for the bright spot. The appearance of this symptom indicates that the seal may leak as soon as driving. 5. The sealing surface has trimming: This is because the sealing surface is too open, and it breaks when closing. Flash (gasification) is a common reason for the separation of sealing surfaces, especially when there is condensate in the hot water system or fluid, water expands from liquid to vapor, which can separate the sealing surfaces. (cold medium gasification will also cause) sealed metal parts, such as springs, set screws, transmission parts and metal sleeves, may become the root cause of seal failure. Corrosion of springs under alternating stress is its primary problem, because metals will corrode rapidly under stress, stainless steel springs are vulnerable to chloride stress corrosion, and there are many chlorides in the world, so foreign departments recommend not to use stainless steel springs, but Hastelloy springs with high corrosion resistance. In addition, spring fatigue caused by improper assembly is another reason for failure. The fixing screws used for mechanical seals should not be made of hardened materials, because heat treatment will reduce the corrosion resistance of metal. 4 Automatic storage: automatic storage of test data: it can store the experimental data, judgment results and test curve of the gas spring according to the production date. The soft fixing screw can be fastened on the shaft. Vibration, deflection and misalignment will wear the transmission parts. For example, when the sealing surface sticks when starting, the transmission parts will be bent or even damaged. The trans isoprene rubber, dynamic vulcanized rubber, long carbon chain nylon and biological polyamine produced by friction, which have won the national science and technology award, represent the achievements of our country in this field, and the heat often intensifies corrosion. The wear marks on the outer surface of the metal sleeve may be caused by solid particles entering the sleeve from the sealing side, which interfere with the follow-up ability of the seal. It may also be caused by deflection and non concentricity. The color of metal should be changed in the process of temperature rise. When stainless steel is used, attention should be paid to the color at the following temperatures. Light yellow - - the temperature is 700 ~ 800 ℉ (about 370 ~ 432 ℃) - Brown - - the temperature is 900 ~ 1000 ℉ (about 486 ~ 540 ℃) - Blue - - the temperature is 1100 ℉ (about 590 ℃) - Black - - the temperature is 1200 ℉ (about 648 ℃). When the seal failure does not meet any of the above, the maintenance is more difficult, but the leakage of the following conditions can be used for reference: 1. The pump shaft sleeve leaks, and many shaft sleeves do not extend out of the sealing box, Therefore, it is difficult to determine the source of the leak. The leakage of the shaft sleeve is usually stable, while the leakage of the sealing surface is often increased or decreased. After the leakage of the sealing surface, the surface is uneven, but sometimes it will run in to the original state. (sometimes don't rush to overhaul, but observe it for a period of time.) 2. If the surrounding of the seal is wet, and there is no leakage. The centrifugal force generated by the pump operation at the start-up makes the leaked liquid return to the sealing surface, acting as a barrier. The liquid leaked from the flange or joint on the pump drips into the stuffing box. 3. Thermal expansion can loosen the graphite ring embedded in the metal parts, or the O-ring may lose its elasticity due to low temperature, resulting in leakage. 4. The fluctuation of flushing pressure will cause seal failure. The flushing pressure must be higher than the pressure of the sealing chamber. Enabling the solenoid valve and time-delay switch installed in front of the pump can ensure that the residues in flushing are flushed before the pump is started or after shutdown. IF quenching method is used to control the temperature, the pressure of the sealing chamber must be maintained. 5. If a layer of scale is deposited on the cooling spacer, we can install a graphite bushing at the bottom of the sealing cavity and use its heat barrier function to solve these problems. 6. For the leakage of heat exchanger, it is often the crud on the cooling surface that hinders the heat transfer, the fluid flow rate in the cooler is accelerated, or the direction of the heat exchanger is installed in the opposite direction

Copyright © 2011 JIN SHI