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Common Leakage Phenomenon
- Oct 09, 2018 -

The proportion of leakage of mechanical seals accounts for more than 50% of all maintenance pumps. The operation of mechanical seals directly affects the normal operation of pumps. The summary analysis is as follows:

1, periodic leakage

(1) The axial displacement of the 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 moving and stationary rings are worn, the compensation displacement is not obtained.

Countermeasure: When assembling the mechanical seal, the axial yaw 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 moving ring can be flexibly moved on the shaft after assembly. (Press the moving ring against the spring to bounce back freely).

(2) Insufficient amount of lubricating oil on the sealing surface causes dry friction or brushed end faces.

Countermeasure: The height of the lubricating oil surface in the oil chamber cavity should be added to the upper and lower sealing surfaces.

(3) The rotor periodically vibrates. The reason is that the stator is not centered with the upper and lower end caps or the impeller and the main shaft are unbalanced, cavitation or bearing damage (wear), which shortens the seal life and creates leakage.

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

2. Leakage due to pressure

(1) Mechanical seal leakage caused by high pressure and pressure wave. Because the spring specific pressure and total specific pressure design are too large and the pressure in the sealed chamber exceeds 3 MPa, the sealing end face is overpressured, the liquid film is difficult to form, and the sealing end face is seriously worn. The heat is increased, causing thermal deformation of the sealing surface.

Countermeasure: When assembling the machine seal, the spring compression must be carried out according to the regulations. It is not allowed to be too large or too small. The mechanical seal under high pressure should take measures. In order to make the end face force reasonable and minimize the deformation, it is possible to use materials with high compressive strength such as cemented carbide and ceramics, and to strengthen the cooling lubrication measures.

(2) Mechanical seal leakage caused by vacuum operation During the start and stop process, due to the blockage of the pump inlet, the pumping medium contains gas, etc., it may cause negative pressure in the sealed chamber, if the negative pressure in the sealed chamber, The dry end face of the seal is caused to dry, and the built-in mechanical seal will cause air leakage (water). The difference between the vacuum seal and the positive pressure seal lies in the directional difference of the seal object, and the mechanical seal also has adaptability in one direction.

Countermeasure: Double mechanical seals are used to help improve lubrication and improve sealing performance.

3, due to leakage caused by the medium

(1) After the mechanical seal of most submersible sewage pumps is disassembled, the auxiliary seals of the static ring and the moving ring are inelastic, and some have rotted, resulting in a large amount of leakage of the machine seal and even the phenomenon of grinding the shaft. Due to the high temperature, the weak acid and weak alkali in the sewage, the corrosion of the static ring and the moving ring auxiliary rubber seals caused the mechanical leakage to be too large. The material of the dynamic and static ring rubber sealing ring is nitrile-40, which is not resistant to high temperature. Not resistant to acid and alkali, it is easy to corrode when the sewage is acidic and alkaline.

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

(2) Mechanical seal leakage caused by solid particle impurities If the solid particles enter the sealing end face, it will scratch or accelerate the wear of the sealing end face, and the accumulation speed of scale and oil on the shaft (sleeve) surface exceeds the wear speed of the friction pair, resulting in The moving ring cannot compensate for the wear displacement, and the hard-to-hard friction pair has a longer operating life than the hard-to-graph graphite friction pair because the solid particles are embedded in the sealing surface of the graphite sealing ring.

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

4. Mechanical seal leakage due to other problems

There are also unreasonable places in the mechanical seal for design, selection, and installation.

(1) The amount of spring compression must be carried out according to the regulations. It is not allowed to be too large or too small. The error is ±2mm. The compression amount is too large to increase the end face specific pressure. The friction heat is too much, which causes the sealing surface to be thermally deformed and accelerate the end face wear. If the compression amount is too small, the end face of the static and dynamic ring is insufficient, so it cannot be sealed.

(2) The end face of the shaft (or bushing) on which the moving ring seal ring is mounted and the end face of the seal gland (or casing) on which the static ring seal ring is mounted shall be chamfered and polished to avoid damage to the static and dynamic ring seal ring during assembly.