Overhaul of mechanical seal

To master the repair and maintenance of mechanical seals, you must first know the working principle of mechanical seals. The mechanical seal is a sealing device composed of two sealing elements (static ring and moving ring) that are perpendicular to the shaft, smooth and flat surfaces are attached to each other and rotate relative to each other. It relies on the pressure of the sealing medium to generate an appropriate pressing force on the contact surfaces (end faces) of the rotating moving ring and the stationary ring, so that the two end faces are tightly combined, and a very thin liquid film is maintained between the end faces to achieve sealing. purpose. This layer of liquid film has hydrodynamic pressure and static pressure, which play the role of lubrication and balancing force. The maintenance process of the mechanical seal of the water pump mainly includes the following aspects:

01. Mechanical seal cleaning and inspection

The working principle of the mechanical seal requires that there are no impurities inside the mechanical seal. Before assembling the mechanical seal, thoroughly clean the moving ring, static ring, shaft sleeve and other components. Key check:

1. Whether there are scratches, cracks and other defects on the surface of the dynamic and static rings (5, 6), the existence of these defects will cause serious leakage of the mechanical seal. If conditions permit, special tools can be used to check whether the sealing surface is flat. If the sealing surface is not flat, the pressure water will enter the sealing surface of the dynamic and static rings of the mechanical seal after assembly, separate the dynamic and static rings, and the mechanical seal will fail. If necessary, tooling can be made for hydrostatic test before assembly.

2. Check whether the dynamic and static ring seat (2) has defects that affect the sealing. For example, whether there are defects such as scars on the mating surface of the dynamic and static ring seat and the dynamic and static ring sealing rubber ring.

3. Check whether the mechanical seal compensation spring (7) is damaged and deformed, and whether the stubborn coefficient changes.

4. Check the sealing bushing (8) for defects such as burrs and groove marks.

5. Clean and check whether all sealing rubber rings have cracks, pores and other defects, and measure whether the diameter of the rubber ring is within the working tolerance range.

6. The mechanical seal with the pumping mechanism should also check whether the helix of the screw pump has defects such as cracks and broken wires.

02. Mechanical seal assembly technical dimension check

The mechanical seal maintenance process is relatively complicated. To ensure that the assembled mechanical seal has no leakage, the verification of the technical size of the mechanical seal is essential.

1. Measure the size of the sealing surface of the moving ring and the static ring (5, 6). This data is used to verify the radial width of the dynamic and static rings. When different friction materials are selected, the radial width of the friction surface of the hard material should be 1-3mm larger than that of the soft material, otherwise it will easily cause the edges and corners of the end face of the hard material to be embedded in the soft material. on the end.

2. Check the clearance between the moving ring, the static ring (5, 6) and the shaft or the shaft sleeve (8). The inner diameter of the static ring is generally 1-2 mm larger than the shaft diameter. For the moving ring, in order to ensure floating, the inner diameter is larger than the shaft diameter. Larger than 0.5-1mm, it is used to compensate for the vibration and deflection of the shaft, but the gap should not be too large, otherwise the moving ring seal will be stuck 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 specific pressure of the end face, and the specific pressure of the end face should be appropriate. If it is too large, the friction surface of the mechanical seal will be heated, which will 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 (2), and when measuring 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 tightness of the mechanical seal.

4. Measure whether the length of the compensation spring (7) changes.

The change of spring performance will directly affect the specific pressure of the mechanical seal end face. In general, the length of the spring will be shortened after long-term 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 pin hole to prevent the static ring from being assembled in place if the pin is too long. This condition can damage the mechanical seal.

03. Assembly of the mechanical seal

After cleaning and inspection, the mechanical seal can be re-installed after technical dimension verification. Pay attention to the following points during this process:

1. All sealing rings should be coated with soapy water and other lubricants during assembly, so as to avoid damage to the rubber rings during assembly. Grease is applied between the sealing surfaces of the dynamic and static rings to prevent the sealing surfaces of the dynamic and static rings from being worn before the pump starts.

2. When assembling the floating ring, be careful not to knock down the spring of the floating ring, so as not to affect the floating performance of the floating ring after the spring is knocked down. After the floating ring is assembled, you can press the floating ring lightly to determine whether it has good floating performance.

3. When installing the seal, handle it with care to prevent damage to the seal, and clean the seal and cavity during installation.

4. When tightening the mechanical seal gland, the tightening bolts should be evenly stressed to prevent uneven stress from damaging the mechanical seal. For the quick-installed mechanical seal, don't forget to move the positioning piece radially to a position away from the shaft to fix it after the whole assembly is completed.