Mechanical seals are one of the basic mechanical components with high precision and complex structure. They are the key components of various pumps, synthetic kettles of reaction mechanical seals, turbine compressors, submersible motors and other equipment. Its sealing performance and service life depend on many factors, such as type selection, machine accuracy, correct installation and use, etc.

The mechanical seal is a sealing device consisting of two sealing elements (static ring and dynamic ring) which are perpendicular to the axis, and the smooth and straight surface of the mechanical seal joins each other and rotates relatively.

It is based on the pressure of the sealing medium to produce appropriate pressure on the contact surface (end face) of the rotating dynamic ring and static ring, so that the two ends are closely combined, and a very thin liquid film is maintained between the ends to achieve the purpose of sealing. This liquid film has hydrodynamic pressure and static pressure, and plays the role of lubrication balance force.

1. Cleaning and Inspection of Mechanical Seals

1) Check whether there are scratches, cracks and other defects on the surface of dynamic and static rings, which will cause serious leakage of mechanical seals. Conditionally, special tools can be used to check whether the sealing surface is flat or not, and the sealing surface is not even. Pressure water will enter the dynamic and static ring sealing surface of the mechanical seal after assembly. The dynamic and static rings will be separated and the mechanical seal will fail. If necessary, it is possible to make hydraulic test of tooling before assembly.

2) The working principle of mechanical seal requires that there is no impurity inside the mechanical seal. Before assembling mechanical seals, it is necessary to thoroughly clean moving rings, stationary rings, axle sleeves and other components.

3) Check whether there is any defect affecting the sealing of the dynamic and static rings. For example, whether there are defects on the mating surface between the dynamic and static ring seat and the dynamic and static ring seal ring.

4) Check whether the mechanical seal compensation spring is damaged and deformed, and whether the stubborn coefficient changes.

5) Check whether there are burrs, grooves and other defects in the sealing sleeve.

6) Mechanical seals with pumping mechanism also need to check whether there are cracks or broken lines in the helix of the screw pump.

7) Clean and inspect all sealant rings for cracks, air holes and other defects, and measure whether the diameter of the rubber rings is within tolerance range.

2. Dimension Check of Mechanical Seal Assembly Technology

1) Measure the size of the sealing surface of the dynamic and static ring. This data is used to verify the radial width of the dynamic and static ring. When different friction materials are selected, the radial width of the friction surface of hard material should be 1-3 mm larger than that of soft material, otherwise, the edges and corners of the hard material end face are easily embedded in the end surface of soft material.

2) Check the clearance between the dynamic and static ring and the shaft or bush. The inner diameter of the static ring is generally 1-2 mm larger than that of the shaft. For the dynamic ring, the inner diameter is 0.5-1 mm larger than the diameter of the shaft to compensate for the vibration and deflection of the shaft, but the clearance should not be too large, otherwise, the seal ring of the dynamic ring will be stuck in and the mechanical seal function will be damaged.

3) Check the tightness of mechanical seals. We usually talk about the mechanical seal tightening force is the end specific pressure, the end specific pressure should be appropriate, too large, will make the mechanical seal friction surface heating, accelerate the end wear, increase the friction power; too small, easy to leak. The specific pressure of the end face is determined in the design of mechanical seals. We can only determine it by measuring the tightness of mechanical seals when assembling. Normally, the measurement method makes it possible to measure the vertical distance between the fixed static ring end face and the cover end face, and then the vertical distance between the moving ring end face and the cover end face. The difference between the two is the tightening force of the mechanical seal.

4) Measure whether the length of the compensation spring has changed. The change of spring performance will directly affect the specific pressure of mechanical seal face. Generally, the length of the spring will be shortened after a long time operation, and the mechanical seal of the compensating spring on the moving ring will also be deformed due to centrifugal force.

5) Measure the length and pinhole depth of the static ring anti-dumping pin to prevent the pin from being too long and the static ring from not being assembled in place. This situation will damage the mechanical seal.

3. Grinding of the end faces of the moving ring and the static ring

1) After removal of the movable ring, it is grinded, roughly grinded first, and then finely grinded, so that polishing can be carried out if conditions permit.

2) When rough grinding, 80-160 Then more than 160 # Abrasives can be used for fine grinding, so that the finish meets the design requirements. Cemented carbide or ceramic movable rings need to be polished by a polishing machine after finishing grinding. Boron carbide M28-M5 can be selected as the strength of the polishing machine. After polishing, it reaches the mirror. Ceramic rings can be polished with chromium oxide after being polished with agate powder of M5.

3) Graphite filled PTFE stationary rings can be polished with kerosene, gasoline or clean water because of their soft materials, without adding abrasives. In the running-in process can also be self-study, so the requirements of finish is not too high.

4) Grinding method, there is a grinder can be grinded on the grinder, no grinder can be used on flat glass 8-shaped manual grinding method.

4. Inspection of axle sleeve

After the axle sleeve is repaired and disassembled, the corrosion and wear can be checked. If the corrosion or wear is slight, fine sand paper can be used to polish and reuse. If the corrosion or wear is serious, the method of electroplating after processing or replacing the axle sleeve can be adopted.

5. Spring

1) For mechanical seals with assembly boxes, clean the boxes and check whether the grooves are worn or deformed for correction and repair, re-grooving or replacement.

2) After repairing the mechanical seals, the mechanical seals are reassembled and the pressure test is also carried out after assembly, and then put into normal operation.

3) If the spring rust is not serious, it can maintain the original elasticity, but can not be replaced. If the corrosion is serious or the elasticity decreases a lot, a new spring needs to be replaced.

6. Sealing ring

After a period of service time, the sealing ring loses elasticity or aging in most cases. In general, new rings need to be replaced.