COMMON PROBLEMS AND COUNTERMEASURES OF MECHANICAL SEALS OF WATER PUMPS

The mechanical seal itself is a high precision component, which has high requirements for design, machining, and assembly. Commonly used mechanical seals are spring seals, elastomer bellow seals, etc. When using mechanical seals, various factors should be analyzed, making the mechanical seals suitable for the technical requirements of various pumps and the use of media and having sufficient lubrication conditions, so that it can ensure the long-term reliable operation of the seals.

The definition of the mechanical seal of the water pump: also known as end seal, it has a pair of end faces perpendicular to the axis of rotation. Under the action of fluid pressure and compensation of the external elastic force of the machine, the end face depends on the cooperation of the auxiliary seal to keep attached to the other end and slide relatively to prevent fluid leakage. Next we will analysis several types of leakages of pump seals.

1. Leakage due to pressure

1) Leakage of mechanical seals caused by high pressure and pressure wave.

When the design of spring specific pressure and total specific pressure is too large and the pressure in the sealing cavity exceeds 3MPa, the specific pressure of the seal end face will be too large, and the liquid film will be difficult to form. Increased heat causes thermal deformation of the sealing surface.

Countermeasure: When assembling the mechanical seal, the spring compression must be carried out according to the regulations, and excessive or too small spring compression is not allowed. The mechanical seal of the gear pump under high pressure should be taken care of. In order to make the pressure of the end face reasonable and minimize the deformation, materials with high compressive strength such as cemented carbide and ceramics can be used, and lubrication measures for cooling are strengthened. Transmission methods such as keys and pins can be used.

2) Leakage of mechanical seals caused by the vacuum operation. 

During the start-up and shutdown of the thermal oil pump, due to the blockage of the pump inlet and the gas contained in the pumped medium, there may be negative pressure in the sealed cavity. If the pressure in the sealed cavity is negative, it will cause dry friction at the seal end, the built-in mechanical seal will cause air leakage or water leakage. The difference between vacuum sealing and positive pressure sealing is the difference in the direction of the sealing object, and the mechanical seal also has its adaptability in a certain direction.

Countermeasure: Use double end face mechanical seals, which will help to improve the lubrication conditions and improve the sealing performance.

2. Leakage due to media

1) Leakage of mechanical seals caused by weak acid and weak alkali. 

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 the phenomenon of grinding the shaft. Due to the corrosive effect of weak acid and weak alkali in sewage on auxiliary rubber seals of static ring and moving ring, the leakage is too large. The material of rubber seals of moving and static ring is nitrile-40, which is not resistant to high temperature, acid and alkali, it is easy to corrode when the sewage is acidic or alkaline.

Countermeasure: For corrosive media, the rubber parts should be made of fluorine rubber with high temperature resistance, weak acid resistance and weak alkali resistance.

2) Leakage of mechanical seals caused by solid particles 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 water scale and oil dirt of high-temperature gear pumps on the surface of the shaft (sleeve) exceeds the wear speed of the friction pair , so that the moving ring can not compensate the wear and displacement, 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 seal ring.

Countermeasure: Where the solid particles are easy to enter, the mechanical seal of the tungsten carbide to tungsten carbide friction pair should be selected

3. Periodic leakage of mechanical seals

The axial movement of the explosion-proof self-priming 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 wear, no compensation displacement can be obtained.

Countermeasure: 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. Press the moving ring against the spring, and it can spring back freely.

4. Insufficient amount of lubricating oil on the sealing surface causes dry friction.

Countermeasure: The lubricating oil surface height in the oil chamber should be higher than the dynamic and static ring sealing surfaces.

5. The rotor periodically vibrates.

The reason is that the stator is not aligned with the upper and lower end caps or the impeller and main shaft are unbalanced, this situation will shorten the seal life and cause leakage.

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