Common Leakage Phenomena of Mechanical Seals

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 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 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 should be able to move flexibly on the shaft after assembly. (Press the moving ring toward the spring so it can 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 is not aligned with the upper and lower end covers 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. 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 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 seriously worn. , the heat generation increases, 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 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.

(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 a negative pressure in the sealing chamber. If there is negative pressure in the sealing chamber, it will Causes dry friction on the sealing end face, and the built-in mechanical seal will produce air (water) leakage. The difference between vacuum sealing and positive pressure sealing 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.

3. 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 corrosion effect of high temperature, weak acid and weak alkali in sewage on the auxiliary rubber seals of the static ring and the moving ring, excessive mechanical leakage is caused. The material of the moving and static ring rubber sealing rings is nitrile-40, which is not resistant to high temperatures. It is not resistant to acid and alkali 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, resulting in The moving 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.

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 some unreasonable design, selection and installation issues in mechanical seals.

(1) The spring compression amount must be carried out according to regulations. It is not allowed to be too large or too small. The error is ±2mm. Excessive compression amount increases the end face specific pressure and excessive friction heat, causing thermal deformation of the sealing surface and accelerated end face wear. , if the compression amount is too small, the end face pressure of the dynamic and static rings is insufficient, and the sealing cannot be achieved.

(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.