Reasons and analysis of leakage of mechanical seal of constant pressure water supply pump

The main forms of mechanical seal leakage of constant-pressure water supply pumps are: leakage due to pressure, leakage due to media, periodic leakage of the mechanical seal, dry friction or roughening of the seal end face caused by insufficient lubricating oil on the sealing surface, periodic vibration of the rotor, mechanical seal leakage caused by other problems. There are also unreasonable design, selection, installation and other aspects of the mechanical seal.

Definition of mechanical seal of water pump: Also known as end face seal, it has a pair of end faces perpendicular to the axis of rotation. Under the action of fluid pressure and compensating mechanical external elastic force, the end faces rely on the cooperation of the auxiliary seal to maintain fit with the other end and slide relative to each other, thereby preventing fluid leakage.

1. Leakage caused by media

(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 and moving rings, excessive mechanical leakage occurs. The material of the movable 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 use fluororubber 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 high-temperature gear pump 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.

2. Periodic vibration of the rotor. 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.

3. Leakage due to pressure

(1) Mechanical seal leakage caused by high pressure and pressure wave. When the spring specific pressure and total specific pressure design are too large and the pressure in the sealing chamber exceeds 3MPa, the sealing end face specific pressure 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 according to regulations. It is not allowed to be too large or too small. Measures should be taken for the mechanical seal of the gear pump under high pressure conditions. In order to ensure reasonable stress on the end face and minimize deformation, hard materials can be used.

(2) Mechanical seal leakage caused by vacuum operation. During the startup and shutdown process of the thermal oil 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 seals, which will help improve lubrication conditions and improve sealing performance.

4. 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.

5. Periodic leakage of mechanical seals

The explosion-proof self-priming pump rotor has large axial movement, 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 the radial seal, 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).

6. 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. The lubricating oil pump will cause thermal deformation of the sealing surface and accelerate the 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 polished to avoid damaging the dynamic and static ring sealing rings during assembly.

(Source: New Horizons in Water Treatment)