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.
First, the common leakage phenomenon
The proportion of mechanical seal leakage accounted for more than 50% of all maintenance pump, mechanical seal the operation of a direct impact on the normal operation of the pump, are summarized as follows.
Periodic leakage
(1) The axial displacement of the pump rotor is large, the interference between the auxiliary seal and the shaft is large, and the moving ring can not move flexibly on the shaft. In the pump flip, moving, static ring wear, not to compensate for displacement.
Countermeasures: When assembling the mechanical seal, the shaft axial displacement should be less than 0.1mm, secondary seal and shaft interference should be moderate, while ensuring the radial seal at the same time, moving ring pump equipped with flexible on the shaft to ensure flexibility Move (move the ring against the spring to bounce back freely).
(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 small pump seal failure often produce grinding shaft, grinding shaft position mainly in the following: moving ring auxiliary seal, static ring position, a few spring grinding shaft phenomenon.
(2) the main reason for grinding shaft: â‘ BIA 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. â‘¢ 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. â‘¡ machine seal oil chamber cavity line should be higher than the static and dynamic ring sealing surface. â‘¢ 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.
Second, leakage due to pressure
(1) High pressure and pressure wave caused by the mechanical seal leakage As the spring pressure and the total pressure than the design is too large and the pressure inside the sealed chamber exceeds 3MPa, the pressure will seal the end surface is too large, the liquid film is difficult to form, the sealing surface wear serious , 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 pumping of gas in the medium, negative pressure may appear in the seal chamber. If negative pressure is applied in the seal 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.
Third, due to leakage caused by the media
(1) Most of the submersible sewage pump mechanical seal dismantling, 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.
Fourth, due to other problems caused by mechanical seal leakage
There are also mechanical seals in the design, selection, installation, etc. not reasonable place.
(1) the amount of compression spring must be required to pump, not allowed to have too large or too small, the error ± 2mm, the amount of compression is too large to increase the pressure on the end face, friction heat too much, resulting in sealing surface thermal deformation and acceleration of the face Wear, compression is too small static and dynamic ring End pressure is not enough, you can not seal.
(2) Install the end surface of the shaft (or sleeve) of the moving ring seal and the seal of the stationary ring seal The end surface of the seal gland (or casing) of the pump should be chamfered and light-edged so as to avoid touching the static and dynamic ring seal .
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