The proportion of mechanical seal leakage accounts for more than 50% of all maintenance pumps, and the operation of mechanical seal directly affects the normal operation of pumps. The summary and analysis are as follows.

1. Periodic leakage

(1) The axial movement of the pump rotor is large, and the interference between the auxiliary seal and the shaft is large, so the moving ring cannot move flexibly on the shaft. After the pump turns 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. At the same time, the radial seal should be ensured, and the movable ring can move flexibly on the shaft after assembly (the movable ring can rebound freely when pressed against the spring).

(2) Insufficient lubricating oil on the sealing surface causes dry friction or roughness on the sealing end surface.

Countermeasures: the height of lubricating oil surface in oil chamber cavity should be higher than that of dynamic and static ring sealing surface.

(3) The rotor vibrates periodically. The reason is that the stator is misaligned with the upper and lower end covers, or the impeller is unbalanced with the main shaft, and cavitation or bearing damage (wear) will shorten the sealing life and cause leakage.

Countermeasures: The above problems can be corrected according to the maintenance standards.

2. Shaft grinding caused by mechanical seal leakage of small submersible sewage pump.

The mechanical seal failure of small pumps below (1) 7 15kW often leads to shaft grinding. The main positions of the shaft grinding are: the auxiliary sealing ring of the moving ring and the static ring, and a few springs have shaft grinding.

(2) The main reasons for grinding the shaft: ①BIA double-ended mechanical seal, the back pressure state is a bad working state, and particles and impurities in the medium can easily enter the sealing surface, making the seal invalid. ② The main component of the grinding shaft is rubber bellows. Because the sealing surface at the upper end is in a bad 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 auxiliary seal of dynamic and static rings is corroded by weak acid and alkali in sewage, so the rubber parts are inelastic. Some have rotted, lost their proper functions, and appeared the phenomenon of grinding shafts.

(3) To solve the above problems, take the following measures: ① Ensure the cleanliness of the lower end cover and oil chamber, and prohibit the assembly of unclean lubricating oil. (2) The oil level in the mechanical seal oil chamber cavity should be higher than the sealing surface of the dynamic and static rings. ③ Choose mechanical seals with different structures according to different media. The mechanical seal structure of high-lift pump should be redesigned, and the corrosive medium rubber should be fluorine rubber resistant to weak acid and alkali. Mechanical seal static ring should be added with anti-rotation pin.

Second, leakage due to pressure.

(1) Mechanical seal leakage caused by high pressure and pressure wave. When the specific pressure of the spring and the total specific pressure are too large, and the pressure in the sealing cavity 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 calorific value will increase, resulting in the thermal deformation of the sealing end face.

Countermeasures: When assembling the mechanical seal, the compression of the spring must be carried out according to the regulations, and it is not allowed to be too large or too small. Measures should be taken for mechanical seal under high pressure. In order to make the stress on the end face reasonable and minimize the deformation, high compressive strength materials such as cemented carbide and ceramics can be used, and cooling lubrication measures can be strengthened, and transmission modes such as keys and pins can be selected.

(2) Leakage of mechanical seal caused by vacuum operation In the process of starting and stopping the pump, negative pressure may appear in the sealed cavity due to the blockage of the pump inlet and the gas contained in the pumping medium. If there is negative pressure in the sealing cavity, it will cause dry friction of the sealing end face and air leakage (water) of the built-in mechanical seal. The difference between vacuum seal and positive pressure seal lies in the different directionality of the sealing object, and mechanical seal also has its adaptability in a certain direction.

Countermeasures: Adopting double-end mechanical seal is helpful to improve lubrication conditions and sealing performance.

Third, the leakage caused by medium.

(1) After the mechanical seals of most submersible sewage pumps are dismantled, the auxiliary seals of the static ring and the dynamic ring are inelastic, and some of them have rotted, resulting in a large number of mechanical seals leaking, and even shaft grinding. Due to the high temperature in sewage and the corrosion of weak acid and weak alkali to the auxiliary rubber seals of static ring and moving ring, the mechanical leakage is too large. The rubber seal of the dynamic and static rings adopts nitrile -40, which is not resistant to high temperature and acid and alkali, and the sewage is easy to corrode when it is acid and alkali.

Countermeasures: For corrosive media, fluororubber with high temperature resistance, weak acid resistance and weak alkali resistance should be selected for rubber parts.

(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 stain on the surface of shaft (sleeve) exceeds the wear speed of friction pair, which makes the moving ring unable to compensate the wear displacement. The service life of hard-to-hard friction pair is longer than that of hard-to-graphite friction pair because solid particles will be embedded in the sealing surface of graphite sealing ring.

Countermeasures: the mechanical seal of tungsten carbide to tungsten carbide friction pair should be selected in the position where solid particles can easily enter.

Four, other problems caused by mechanical seal leakage.

There are still some unreasonable places in the design, selection and installation of mechanical seals.

(1) The compression of the spring must be carried out according to the regulations, and it is not allowed to be too large or too small, with an error of 2mm. Excessive compression, increased end-face specific pressure and excessive friction heat will cause thermal deformation of sealing surface and accelerate end-face wear. If the compression is too small, the end face of the dynamic and static ring will not be sealed.

(2) The end face of the shaft (or shaft sleeve) where the dynamic ring sealing ring is installed and the end face of the sealing gland (or shell) where the static ring sealing ring is installed shall be chamfered and polished to avoid damaging the dynamic and static ring sealing rings during assembly.