Quality Inspection Process of Mechanical Seal

Author:Site Editor     Publish Time: 2020-12-08      Origin:Site

The mechanical seal is a sealing device composed of two sealing elements (stationary ring and moving ring) perpendicular to the shaft, smooth and flat surfaces attached to each other, and relative rotation. It relies on the pressure of the sealing medium to generate the appropriate pressing force on the contact surface (end surface) of the rotating moving ring and the static ring so that the two end surfaces are tightly combined, and a very thin liquid film is maintained between the end surfaces to achieve the sealing purpose. This liquid film has hydrodynamic pressure and static pressure, which plays the role of lubrication and balance.

 

Cleaning and inspection of mechanical seal


 1) The working principle of a mechanical seal requires no impurities inside the mechanical seal. Before assembling the mechanical seal, clean the moving ring, static ring, shaft sleeve, and other parts thoroughly.

 2) Check whether there are defects such as scratches and cracks on the surface of the moving and static rings. The existence of these defects will cause serious leakage of the mechanical seal. If possible, you can use special tools to check whether the sealing surface is flat or not. Pressure water will enter the dynamic and static ring sealing surfaces of the mechanical seal after assembly, separating the dynamic and static rings, and the mechanical seal will fail. If necessary, the tooling can be made for hydraulic test before assembly.

 3) Check whether the dynamic and static ring seat has defects that affect the seal. For example, whether there are defects in the mating surfaces of the dynamic and static ring seat and the dynamic and static ring seals.

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

 5) Check whether the seal sleeve has defects such as burrs and groove marks.

 6) Clean and check all sealing rubber rings for cracks, pores, and other defects, and measure whether the diameter of the rubber ring is within the tolerance range.

 7) For mechanical seals with pumping mechanisms, check the spiral line of the screw pump for cracks, disconnections, and other defects.

 

Technical dimension check of the mechanical seal assembly

 

1) Measure the size of the sealing surface of the static and dynamic rings. This data is used to verify the radial width of the moving and static rings. When using different friction materials, the radial width of the friction surface of the hard material should be 1~3mm larger than that of the soft one, otherwise, the edges and corners of the end face of the hard material will be embedded in the end face of the soft material. 

 

mechanical seal


2) Check the gap between the dynamic and static rings and the shaft or sleeve. The inner diameter of the static ring is generally 1~2mm larger than the shaft diameter. For the dynamic ring, to ensure the floatability, the inner diameter is 0.5~1mm larger than the shaft diameter to compensate for shaft vibration. But the gap should not be too large, otherwise, the moving ring seal ring will be stuck in and the mechanical seal function will be damaged.

 

3) Check the tightness of the mechanical seal. The mechanical seal tightening force we usually talk about is the end-face specific pressure. If the end-face specific pressure is appropriate, if it is too large, it will cause the friction surface of the mechanical seal to heat up, accelerate the wear of the end face, and increase the friction power; if it is too small, it is easy to leak. The specific pressure of the end face is determined during the design of the mechanical seal, and we can only determine it by measuring the tightness of the mechanical seal during assembly. The usual measurement method is to measure the vertical distance from the end face of the installed static ring to the end face of the gland, and then measure the vertical distance from the end face of the moving ring to the end face of the gland. The difference between the two is the tight force of the mechanical seal.

 

4) Measure whether the length of the compensation spring has changed. Changes in spring performance will directly affect the specific pressure of the mechanical seal. Under normal circumstances, the length of the spring will be shortened after a long time of operation, and the mechanical seal of the compensation spring on the moving ring will also be deformed due to centrifugal force.

 

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

 

Grinding of the end faces of the moving and static rings


1) After the moving ring is dismantled, after grinding, rough grinding is carried out first, then fine grinding and polishing can be carried out if possible.

 

2) When rough grinding, choose 80~160# grit abrasives, first grind away the processing traces. Then use 160# or more abrasives for fine grinding to make the finish meet the design requirements. Carbide or ceramic moving ring needs to be polished with a polishing machine after fine grinding. The strength of the polishing machine can be M28~M5 boron carbide. A mirror surface is achieved after polishing. The ceramic ring can be finely ground with M5 agate powder and then polished with chromium oxide.

 

3) The static ring of graphite-filled PTFE, because the material is soft, can be finely ground with kerosene, gasoline, or clean water, without the need for abrasives. Self-study is also available during the running-in process, so the finish requirements are not too high.

 

4) Grinding method, if there is a grinder, you can grind on the grinder, and if you don’t have a grinder, you can use the 8-shaped manual grinding method on the flat glass.

 

Bushing inspection

 

Check the rust and wear after removing the shaft sleeve. If the rust or wear is slight, you can polish it with fine sandpaper and use it again. If the rust or wear is serious, you can use electroplating after processing or replace the shaft sleeve. 

 

Sealing o-ring

 

After a period of use, the sealing ring loses its elasticity or ages in most cases and generally needs to be replaced with a new ring.

 

Spring


1) If the spring is not severely corroded and can maintain the original elasticity, it is not necessary to replace it. If the rust is severe or the elasticity is reduced a lot, a new spring needs to be replaced.

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

3) After the mechanical seal element is repaired, re-assemble it, and perform the same pressure test after assembly, and then put it into normal operation.


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