Keywords: mechanical seal, leakage phenomena 

A mechanical seal is also called an end face seal. It has a pair of end faces perpendicular to the axis of rotation. Under the action of fluid pressure and compensating the external elastic force of the machine, the end face relies on the cooperation of the auxiliary seal to maintain a fit with the other end, and relatively slide, thereby preventing fluid leakage.

1. Common leakage phenomena

The proportion of mechanical seal leakage accounts for more than 50% of all repaired pumps. The performance of mechanical seals directly affects the normal operation of the pump. The summary analysis is 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, and the moving ring cannot move flexibly on the shaft. After the pump is turned over and the moving and static rings are worn out, 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 movable ring can be flexibly moved on the shaft after assembly (Press the moving ring against the spring and spring back freely).

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

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

(3) The rotor vibrates periodically. The reason is the misalignment of the stator and the upper and lower end caps or the imbalance of the impeller and the main shaft, cavitation or bearing damage (wear). This situation will shorten the seal 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

(1) The mechanical seal failure of small pumps below 715kW often produces grinding shafts. The grinding shaft positions 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.

(2) The main reasons for shaft grinding: ①BIA type double-end 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. ②The main part of the 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. ③Due to the corrosion of the weak acid and alkali in the sewage, the moving and static ring auxiliary seals have no elasticity. Some have rotted and lost their proper function, resulting in the phenomenon of shaft grinding.

(3) In order to solve the above problems, the following measures are now taken: ①Ensure the cleanliness of the lower end cover and oil chamber, and it is forbidden to assemble dirty lubricating oil. ②The oil level line in the oil chamber of the mechanical seal should be higher than the sealing surface of the static and moving rings. ③Choose mechanical seals of different structures according to different media. For high-lift pumps, the mechanical seal structure should be redesigned, and for corrosive medium rubber, fluorine rubber that is resistant to weak acids and weak alkalis should be selected. The static ring of mechanical seal should be equipped with anti-rotation pins.

2. Leakage due to pressure 

(1) Mechanical seal leakage caused by high pressure and pressure waves. When the spring specific pressure and total specific pressure 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 severely 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 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 and 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 pumped medium, it may cause negative pressure in the sealed cavity. If there is negative pressure in the sealed cavity, it will cause dry friction on 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 directional difference of the sealing object, and the mechanical seal also has its adaptability in one direction.

Countermeasure: Adopt double end face mechanical seal, which helps to improve lubrication conditions and improve sealing performance.

3. Leakage caused by medium 

(1) After most of the mechanical seals of submersible sewage pumps are disassembled, the auxiliary seals of the stationary 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 is caused. The material of the moving and static ring rubber seal is nitrile-40, which is not resistant to high temperature. It is not resistant to acid and alkali, and it is easy to corrode when the sewage is acidic and alkaline.

Countermeasures: 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 end face of the seal, it will scratch or accelerate the wear of the end face. The accumulation of scale and oil on the surface of the shaft (sleeve) exceeds the wear rate of the friction pair, resulting in the moving ring cannot 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 solid particles will be embedded in the seal surface of the graphite seal ring.

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

4. Mechanical seal leakage caused by other problems  

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

(1) The compression of the spring must be carried out in accordance with the regulations, and it is not allowed to be too large or too small. The error is ±2mm. The compression is too large to increase the specific pressure of the end surface, and the frictional heat is too much, which causes the sealing surface to be thermally deformed and accelerates the end surface wear. If the compression is too small, the specific pressure of the moving and static ring ends is insufficient, and it 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 avoid damage to the moving and static ring seal rings during assembly.

5. Concluding remarks  

The above summarizes the common causes of leakage of mechanical seals. The mechanical seal itself is a high precision component, which has high requirements on design, machining, and assembly quality. When using a mechanical seal, the various factors of using the mechanical seal should be analyzed to make the mechanical seal suitable for the technical requirements and medium requirements of various pumps and have sufficient lubrication conditions, so as to ensure the long-term and reliable operation of the seal.