Common leakage phenomenon The proportion of leakage of mechanical seals accounts for more than 50% of all maintenance pumps. The operation of mechanical seals directly affects the normal operation of pumps. The summary analysis is as follows.
1. 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 cannot move flexibly on the shaft. After the pump is turned over and the moving and stationary rings are worn, the compensation displacement is not obtained.
Countermeasure: When assembling the mechanical seal, the axial yaw of the shaft should be less than 0.1mm, and the interference between the auxiliary seal and the shaft should be moderate. While ensuring the radial seal, the moving ring can be flexibly moved on the shaft after assembly ( Pressing the moving ring against the spring can bounce back freely).
(2) Insufficient amount of lubricating oil on the sealing surface causes dry friction or brushed end faces.
Countermeasure: The height of the lubricating oil surface in the oil chamber cavity should be added to the upper and lower sealing surfaces.
(3) The rotor periodically vibrates. The reason is that the stator is not centered with the upper and lower end caps or the impeller and the main shaft are unbalanced, cavitation or bearing damage (wear), which shortens the seal life and creates leakage.
Countermeasures: The above problems can be corrected according to the maintenance standards.
2. The grinding shaft phenomenon caused by leakage of small submersible sewage pump seals (1) The failure of small pump seals below 715 kW often produces grinding shafts. The grinding shaft positions mainly include the following: moving ring auxiliary sealing ring, static ring position, A few springs have a grinding shaft phenomenon.
(2) The main reason for grinding the shaft:
1BIA type double-face mechanical seal, the back pressure state is a bad working state, particles and impurities in the medium can easily enter the sealing surface, making the seal invalid.
2 The main part of the grinding shaft is a rubber bellows, and because the upper end sealing surface is in a poor lubrication state, the friction torque between the dynamic and static rings is greater than the transmission torque between the rubber bellows and the shaft, and relative rotation occurs.
3 The moving and static ring auxiliary seals are inelastic due to the corrosion of weak acid and weak alkali in the sewage. Some have rotted, lost their proper function, and produced a phenomenon of grinding the shaft.
(3) In order to solve the above problems, the following measures are taken:
1 Ensure the cleanliness of the lower end cover and oil compartment, and prohibit assembly of unclean lubricant.
The oil level line in the chamber of the 2 machine seal chamber should be higher than the seal surface of the static and dynamic ring.
3 According to different media, choose different structure of the machine seal. For the high lift pump, the machine seal structure should be redesigned. For the corrosive medium rubber, the weak acid and weak base fluororubber should be selected. The machine seal static ring should be added to prevent resale.
Leakage due to pressure (1) Mechanical seal leakage caused by high pressure and pressure wave. Because the spring specific pressure and total specific pressure design are too large and the pressure in the sealed chamber exceeds 3 MPa, the sealing end face is overpressured and the liquid film is difficult. Forming, the sealing end face is seriously worn, and the heat generation is increased, causing thermal deformation of the sealing surface.
Countermeasure: When assembling the machine seal, the spring compression must be carried out according to the regulations. It is not allowed to be too large or too small. The mechanical seal under high pressure should take measures. In order to make the end face force reasonable and minimize the deformation, it is possible to use materials with high compressive strength such as cemented carbide and ceramics, and to strengthen the cooling lubrication measures, and select the transmission mode such as keys and pins.
(2) Mechanical seal leakage caused by vacuum operation During the start and stop process, due to the blockage of the pump inlet, the pumping medium contains gas, etc., it may cause negative pressure in the sealed chamber, if the negative pressure in the sealed chamber, The dry end face of the seal is caused to dry, and the built-in mechanical seal will cause air leakage (water). The difference between the vacuum seal and the positive pressure seal lies in the directional difference of the seal object, and the mechanical seal also has adaptability in one direction.
Countermeasure: Double mechanical seals are used to help improve lubrication and improve sealing performance.
Leakage caused by the medium (1) After the mechanical seal of most submersible sewage pumps is disassembled, the auxiliary seals of the static ring and the moving ring are inelastic, and some have decayed, resulting in a large leakage of the machine seal or even a grinding shaft. phenomenon. Due to the high temperature, the weak acid and weak alkali in the sewage, the corrosion of the static ring and the moving ring auxiliary rubber seals, the mechanical leakage is too large, and the dynamic and static ring rubber sealing ring material is nitrile-40, which is not resistant to high temperature. Not resistant to acid and alkali, it is easy to corrode when the sewage is acidic and alkaline.
Countermeasures: For corrosive media, rubber parts should be made of fluororubber with high temperature resistance, weak acid resistance and weak alkali.
(2) Mechanical seal leakage caused by solid particle impurities If the solid particles enter the sealing end face, it will scratch or accelerate the wear of the sealing end face, and the accumulation speed of scale and oil on the shaft (sleeve) surface exceeds the wear speed of the friction pair, resulting in The moving ring cannot compensate for the wear displacement, and the hard-to-hard friction pair has a longer operating life than the hard-to-graph graphite friction pair because the solid particles are embedded in the sealing surface of the graphite sealing ring.
Countermeasure: The mechanical seal of tungsten carbide to tungsten carbide friction pair should be selected at the position where solid particles are easy to enter.
Mechanical seal leakage caused by other problems There are still some unreasonable places in the mechanical seal of design, selection and installation.
(1) The amount of spring compression must be carried out according to the regulations. It is not allowed to be too large or too small. The error is ±2mm. The compression amount is too large to increase the end face specific pressure. The friction heat is too much, which causes the sealing surface to be thermally deformed and accelerate the end face wear. If the compression amount is too small, the end face of the static and dynamic ring is insufficient, so it cannot be sealed.
(2) The end face of the shaft (or bushing) on ​​which the moving ring seal ring is mounted and the end face of the seal gland (or casing) on ​​which the static ring seal ring is mounted shall be chamfered and polished to avoid damage to the static and dynamic ring seal ring during assembly.
The above summarizes the more common causes of leakage in mechanical seals. The mechanical seal itself is a highly demanding precision component, which has high requirements for design, machining and assembly quality. When using mechanical seals, the various factors of using mechanical seals should be analyzed to make the mechanical seals suitable for the technical requirements of various pumps and the use of media and sufficient lubrication conditions to ensure long-term reliable operation of the seals.
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