Common leakage phenomena and countermeasures of pump mechanical seals

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This article summarizes the common causes of leakage in mechanical seals. The mechanical seal itself is a high-precision component with high requirements on design, machining, and assembly quality. When using mechanical seals, various factors for the use of mechanical seals should be analyzed to ensure that the mechanical seals are suitable for the technical requirements of various pumps and medium requirements and have sufficient lubrication conditions, so as to ensure long-term reliable operation of the seals. Mechanical seals are also called end seals. They have a pair of end faces perpendicular to the axis of rotation. Under the action of fluid pressure and mechanical external elastic force, the end faces rely on the cooperation of the auxiliary seal to keep fit with the other end and slide relative to each other to prevent fluid leakage. 1. Common leakage phenomena: The proportion of mechanical seal leakage accounts for more than 50% of all repaired pumps. The operation of the mechanical seal directly affects the normal operation of the water pump. The summary and analysis are as follows. 1. Periodic leakage (1) The axial movement of the 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 dynamic and static rings are worn, the displacement cannot be compensated. Countermeasures: When assembling the mechanical seal, the axial movement of the shaft should be less than 0.1mm, and the interference between the auxiliary seal and the shaft should be moderate. While ensuring radial sealing, the moving ring must be able to move flexibly on the shaft after assembly (press the moving ring against the spring to bounce back freely). (2) Insufficient lubricating oil on the sealing surface causes dry friction or roughening of the sealing end surface. Countermeasures: The height of the lubricating oil level in the oil chamber should be higher than the dynamic and static ring sealing surfaces. (3) The rotor vibrates periodically. The reason is that the stator and the upper and lower end covers are not aligned or the impeller and main shaft are unbalanced, cavitation or bearing damage (wear), which will shorten the sealing life and cause leakage. Countermeasures: The above problems can be corrected according to maintenance standards. 2. Shaft grinding phenomenon caused by mechanical seal leakage of small submersible pumps (1) Small pumps below 715kW often produce shaft grinding due to mechanical seal failure. The main locations of the grinding shaft are as follows:: There is shaft grinding at the auxiliary sealing ring of the moving ring, the position of the static ring, and a few springs. (2) Main reasons for shaft grinding: ①BIA type double-end mechanical seal, the back pressure state is a poor working state, and particles and impurities in the medium can easily enter the sealing surface, causing the seal to fail. ②The main part of the grinding shaft is the 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, causing relative rotation. ③The dynamic and static ring auxiliary seals are corroded by weak acids and weak alkali in sewage, and the rubber parts have become inelastic. Some have rotted and lost their proper functions, resulting in shaft grinding. (3) In order to solve the above problems, the following measures are taken:: ①Ensure the cleanliness of the lower end cover and oil chamber. It is prohibited to assemble unclean lubricating oil. ②The oil level line in the machine seal oil chamber should be higher than the sealing surface of the dynamic and static rings. ③Mechanical seals with different structures are selected according to different usage media. For high-lift pumps, the mechanical seal structure should be redesigned, and for corrosive media, fluorine rubber that is resistant to weak acids and weak alkali should be used. The mechanical seal static ring should be equipped with an anti-rotation pin. 2. Leakage due to pressure (1) Mechanical seal leakage caused by high pressure and pressure waves. When the spring specific pressure and total specific pressure design are too large and the pressure in the sealing chamber exceeds 3MPa, the specific pressure of the sealing end face will be too large, the liquid film will be difficult to form, the sealing end face will be seriously worn, and the heat will increase, causing thermal deformation of the sealing surface. Countermeasures: When assembling the mechanical seal, the spring compression must be carried out in accordance with regulations. It is not allowed to be too large or too small. Measures should be taken for mechanical seals under high pressure conditions. In order to make the end face stress reasonable and minimize deformation, materials with high compressive strength such as cemented carbide and ceramics can be used, and cooling and lubrication measures can be strengthened, and reliable transmission methods can be selected, such as keys, pins, etc. (2) Mechanical seal leakage caused by vacuum operation. During the startup and shutdown process of the pump, due to the blockage of the pump inlet and the presence of gas in the pumped medium, there may be negative pressure in the seal cavity. If there is negative pressure in the seal cavity, it will cause dry friction on the seal end face, and the built-in mechanical seal will cause air (water) leakage. The difference between vacuum seals and positive pressure seals lies in the directional difference of the sealing object, and the mechanical seal also has its adaptability in a certain direction. Countermeasures: Use double-end mechanical seal, which helps to improve lubrication conditions and improve sealing performance. 3. Leakage due to medium (1) After the mechanical seals of most submersible sewage pumps are disassembled, the auxiliary seals of the static ring and the moving ring are inelastic, and some have rotted, causing a large amount of leakage of the mechanical seal and even shaft grinding. Due to the corrosive effect of high temperature, weak acid and weak alkali in the sewage on the auxiliary rubber seals of the static ring and the moving ring, excessive mechanical leakage occurs. The material of the moving and static ring rubber seals is nitrile-40, which is not resistant to high temperatures, acids and alkalis, and is prone to corrosion when the sewage is acidic and alkaline. Countermeasures: For corrosive media, rubber parts should be made of fluorine rubber that is resistant to high temperatures, weak acids, and weak alkali. (2) Mechanical seal leakage caused by solid particle impurities. If solid particles enter the seal end face, it will scratch or accelerate the wear of the seal end face. The accumulation speed of scale and oil on the shaft (sleeve) surface exceeds the wear speed of the friction pair, causing the moving ring to be unable to compensate for the wear displacement. The operating life of the hard-to-hard friction pair is longer than that of the hard-to-graphite friction pair, because the solid particles will be embedded in the sealing surface of the graphite seal ring. Countermeasures: A mechanical seal of tungsten carbide to tungsten carbide friction pair should be used where solid particles are easy to enter. 4. Mechanical seal leakage caused by other problems There are still unreasonable design, selection and installation issues in the mechanical seal. (1) The compression amount of the spring must be carried out according to the regulations. It is not allowed to be too large or too small. The error is ±2mm. If the compression amount is too large, the end face specific pressure will be increased, and the friction heat will be too much, causing thermal deformation of the sealing surface and accelerated end face wear. If the compression amount is too small, the end face specific pressure of the dynamic and static ring will be insufficient, and the seal cannot be sealed. (2) The end face of the shaft (or sleeve) where the dynamic ring sealing ring is installed and the end face of the sealing gland (or housing) where the static ring sealing ring is installed should be chamfered and smoothed to avoid damaging the dynamic and static ring sealing rings during assembly. 5. Conclusion The above summarizes the common causes of leakage of mechanical seals. The mechanical seal itself is a high-precision component with high requirements on design, machining, and assembly quality. When using mechanical seals, various factors for the use of mechanical seals should be analyzed to ensure that the mechanical seals are suitable for the technical requirements of various pumps and medium requirements and have sufficient lubrication conditions, so as to ensure long-term reliable operation of the seals.