Mechanical seal also known as the end seal, which has a pair of perpendicular to the axis of rotation of the end surface, the end of the fluid pressure and compensate mechanical external elastic force, depending on the auxiliary seal with the other end to maintain fit and slide relative to prevent fluid leakage. A leakage due to pressure (1) high pressure and pressure wave caused by the mechanical seal leakage due to the spring than the pressure and the total pressure design is too large and the pressure in the sealed chamber exceeds 3MPa, will make the sealing end pressure is too large, the liquid Difficult to form the film, seal face serious wear and tear, increased heat, resulting in sealing surface thermal deformation. Countermeasures: In the assembly machine seal, the amount of spring compression must be carried out according to regulations, not allowed to have too large or too small, under high pressure mechanical seal should take measures. In order to make the end face force reasonable and minimize the deformation, we can adopt materials with high compressive strength such as cemented carbide and ceramics, and strengthen the cooling lubrication measures. (2) Mechanical seal leakage caused by vacuum operation During pump start-up and shut-down, due to blockage of pump inlet and gas in pumping fluid, negative pressure may appear in sealed chamber. If negative pressure is applied in sealed chamber, Causing seal surface dry friction, built-in mechanical seal will have leaks (water) phenomenon, the difference between vacuum seal and positive pressure seal is the direction of the object, and mechanical seal also has its own direction of adaptability. Countermeasures: The use of double-sided mechanical seal, which will help improve the lubrication conditions, improve the sealing performance. Second, due to leakage caused by the medium (1) Most of the submersible sewage pump mechanical seal dismantling, the stationary ring and moving ring auxiliary seal inelastic, and some have been decay, resulting in a large number of machine seal leakage and even grinding Shaft phenomenon. Due to the high temperature, the weak acid and weak base in the sewage corrode the static ring and moving ring auxiliary rubber seal, resulting in excessive mechanical leakage. The material of static and dynamic ring rubber sealing ring is nitrile-40, Acid and alkali, when the water is acidic alkaline easily corroded. Countermeasures: corrosive media, rubber parts should be used high temperature, weak acid, weak base of fluorine rubber. (2) Mechanical Seal Leakage Caused by Solid Particle Impurities If solid particles enter the seal face, it will scratch or speed up the wear on the seal face. The accumulation of scale and grease on the shaft (sleeve) surface exceeds the wear rate of the friction pair, causing The moving ring can not compensate for the wear displacement. The working life of the hard-to-hard friction pair is longer than that of the hard-to-graphite friction pair because the solid particles are embedded in the sealing surface of the graphite sealing ring. Countermeasures: easy access to solid particles in place should be used tungsten carbide pairs of mechanical friction of the mechanical seal. Third, the common phenomenon of leakage Leakage of mechanical seals accounted for more than 50% of all maintenance pumps, mechanical seals directly affect the operation of the normal operation of the pump, are summarized as follows. 1. Periodic leakage (1) pump rotor axial play large, auxiliary seal and shaft interference large, flexible ring can not move on the axis of flexibility. In the pump flip, moving, static ring wear, not to compensate for displacement. Countermeasures: When assembling the mechanical seal, the axial displacement of the shaft should be less than 0.1mm, the interference of the auxiliary seal and the shaft should be moderate, while ensuring the radial seal, the flexible ring can be flexibly moved on the shaft after being assembled (The ring pressing the spring can be free to bounce back). (2) insufficient lubricant on the sealing surface causes dry friction or nap sealing of the end surface. Countermeasures: oil chamber cavity lubricating oil height should be added to the moving, stationary ring sealing surface. (3) Rotor periodic vibration. The reason is the misalignment between the stator and the upper and lower end caps or imbalance between the impeller and the main shaft, cavitation or bearing damage (wear), which can shorten the seal life and create leakage. Countermeasures: According to the maintenance standards to correct the above problems. 2. Small submersible sewage pump seal leakage caused by grinding shaft phenomenon (1) 715kW the following pump seal failure often produce grinding shaft, grinding shaft position mainly in the following: moving ring auxiliary seal at the ring position, a few spring Grinding axis phenomenon. (2) the main reason for grinding shaft: ①BIA double mechanical seal, the back pressure state is a bad working condition, the media of particles, impurities can easily enter the sealing surface, the seal failure. The main parts of the grinding shaft are rubber bellows, and because the sealing surface of the upper end is in an unheated lubrication state, the friction moment between the static and the stationary rings is larger than the transmission torque between the rubber bellows and the shaft, and relative rotation occurs. ③ moving, static seal auxiliary seal due to the weak acid in the sewage, weak alkali corrosion, rubber parts have been inelastic. Some have been decayed, lost their due function, resulting in the phenomenon of grinding shaft. (3) In order to solve the above problems, the following measures are taken: (1) To ensure the cleanliness of the lower end cap and oil chamber, and to prohibit the assembly of unclean lubricating oil. ② machine seal oil chamber cavity line should be higher than the static and dynamic ring sealing surface. ③ According to different media use different structure of the machine seal. The high-lift pump should be redesigned machine seal structure, corrosive medium rubber should be resistant to weak acid, weak base of fluorine rubber. Machine seal ring should be added to prevent pin. Fourth, due to other problems caused by the mechanical seal leakage Mechanical seal also exists in the design, selection, installation, etc. not reasonable place. (1) The amount of compression spring must be carried out according to regulations, not allowed to have too large or too small, the error ± 2mm, excessive compression to increase the end pressure, friction heat too much, resulting in sealing surface thermal deformation and accelerated wear on the end , The amount of compression is too small static and dynamic pressure than the end face of the ring, you can not seal. (2) The end surface of the shaft (or sleeve) on which the moving ring seal is installed and the seal gland (or casing) on ​​which the stationary ring seal is installed should be chamfered and smoothed to avoid bruising the static and dynamic ring seal during assembly. The above summarizes the more common causes of mechanical seal leakage. Mechanical seal itself is a high precision parts, the design, machining, assembly quality has high requirements. In the use of mechanical seals, the mechanical seal should be used to analyze the various factors that make the mechanical seal suitable for a variety of pump technical requirements and the use of media requirements and adequate lubrication conditions, so as to ensure the long-term reliable seal operation.
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