Mechanical seal is also called end face seal, usually called "mechanical seal". It is a key component of submersible pump waterproofing. The phenomenon of mechanical seal leakage in the use of submersible pumps is more common in submersible pump failures. At the same time, mechanical seals The operation of the seal directly affects the normal operation of the pump. In order to ensure the normal operation of the submersible pump and avoid accidents caused by the leakage of the mechanical seal during operation, the leakage of the mechanical seal is summarized and analyzed as follows.
A. Periodic leakage
1. The axial movement of the submersible pump rotor is too large, and 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 static ring is worn out, the pump cannot be compensated for displacement.
Corresponding solution: When we assemble the submersible pump mechanical seal, the axial movement of the submersible pump 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 should be assembled After that, it is guaranteed to be able to move flexibly on the shaft (press the moving ring to the spring to be able to bounce back freely).
2. The insufficient amount of lubricating oil on the mechanical sealing surface of the submersible pump will also cause dry friction or roughening of the sealing end surface. The corresponding solution: the height of the lubricating oil surface in the oil chamber of the submersible pump should be kept higher than the mechanical sealing surface of the dynamic and static rings.
3. Periodic vibration of the rotor of the submersible pump. It is because the stator and the upper and lower end caps are misaligned or the impeller and the main shaft are unbalanced, cavitation or bearing damage (wear), this situation will shorten the seal life and cause leakage. Corresponding solutions: the above problems can be corrected according to the maintenance standards.
B. The phenomenon of shaft grinding of the submersible pump caused by the leakage of the submersible pump seal
The mechanical seal failure of small pumps below 715kW often produces grinding shafts. The grinding shaft positions of submersible pumps are mainly as follows: the auxiliary sealing ring of the moving ring, the position of the static ring, and a few springs have the phenomenon of shaft grinding.
The main causes of shaft grinding:
1. BIA type double end face mechanical seal, the back pressure state is a bad working state, particles and impurities in the medium can easily enter the sealing surface and cause the seal to fail.
2. The main part of the submersible pump grinding shaft is a rubber bellows, and because the upper sealing surface is in a poor lubrication state, the friction torque between the moving and static rings is greater than the transmission torque between the rubber bellows and the shaft, and relative rotation occurs.
3. The dynamic and static ring auxiliary seal is corroded by the weak acid and weak alkali in the sewage, and the rubber parts have no elasticity. Some have rotted and lost their proper function, resulting in the phenomenon of shaft grinding.
Solution:
1. Ensure the cleanliness of the lower end cover and oil chamber, and it is forbidden to assemble dirty lubricating oil.
2. The oil level line in the oil chamber of the engine seal should be higher than the sealing surface of the static and dynamic rings.
3. Choose mechanical seals of different structures according to different media used. For high-lift pumps, the mechanical seal structure should be redesigned. For corrosive medium rubber, fluorine rubber that is resistant to weak acids and weak bases should be selected. The machine-sealed static ring should be equipped with anti-rotation pins.
C.The mechanical seal leakage caused by the high pressure of the submersible pump
1. Mechanical seal leakage caused by high pressure and pressure waves. When the spring specific pressure and total specific pressure are designed to be too large and the pressure in the seal cavity exceeds 3MPa, the specific pressure of the seal end face will be too large, the liquid film will be difficult to form, and the seal end face will be severely worn. Increased heat generation causes thermal deformation of the sealing surface.
Corresponding solution: When assembling the mechanical seal, the spring compression must be carried out in accordance with the regulations, and it is not allowed to be too large or too small. Measures should be taken for the mechanical seal under high pressure. In order to make the end face force reasonable and minimize deformation, materials with high compressive strength such as cemented carbide and ceramic can be used, and cooling lubrication measures should be strengthened, and reliable transmission methods such as keys and pins can be selected.
2. Mechanical seal leakage caused by vacuum operation. During the start and stop of the pump, due to the blockage of the pump inlet and the gas contained in the pumping medium, it may cause negative pressure in the sealed cavity. If there is negative pressure in the sealed cavity, it will cause The dry friction of the seal end surface will cause air leakage (water) in the built-in mechanical seal. The difference between vacuum seal and positive pressure seal is the difference in the directionality of the sealed object, and the mechanical seal also has its adaptability in a certain direction.
Corresponding solution: adopt double end face mechanical seal, which helps to improve lubrication conditions and improve sealing performance
D. Leakage caused by the medium
1. After most of the submersible pump mechanical seals 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 the phenomenon of shaft grinding. Due to the corrosive effect of high temperature, weak acid and weak alkali in the sewage on the static ring and the auxiliary rubber seal of the moving ring, excessive mechanical leakage has been caused. It is not acid and alkali resistant, and it is easy to corrode when the sewage is acidic and alkaline. Corresponding solution: For corrosive media, rubber parts should be fluorine rubber resistant to high temperature, weak acid and weak alkali.
2. Mechanical seal leakage caused by solid particles and 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 movement The ring cannot compensate for the wear displacement, and the operating life of the hard-to-hard friction pair is longer than that of the hard-to-graphite friction pair, because solid particles will be embedded in the sealing surface of the graphite sealing ring. Corresponding solution: the mechanical seal of tungsten carbide to tungsten carbide friction pair should be selected at the position where solid particles are easy to enter.
E.Mechanical seal leakage caused by other problems. There are still unreasonable places in the design, selection, and installation of the mechanical seal.
1. The compression of the spring must be carried out in accordance with the regulations. It is not allowed to be too large or too small. The error is ±2mm. Too much compression increases the specific pressure of the end surface, and excessive frictional heat causes thermal deformation of the sealing surface and accelerated end surface wear. If the compression is too small, the specific pressure of the dynamic and static ring end faces is insufficient, and the seal cannot be sealed.
2. The end face of the shaft (or sleeve) where the moving ring seal ring is installed and the end face of the sealing gland (or housing) where the static ring seal ring is installed should be chamfered and trimmed to prevent damage to the moving and static ring seal rings during assembly.
