Centrifugal pump mechanical seal leakage

Posted by angroup on April 27, 2022

A mechanical seal, also known as an end seal, has a pair of end faces perpendicular to the axis of rotation, the end face under the action of fluid pressure and compensating mechanical external elasticity, relying on the auxiliary seal with the other end to maintain the fit and relative sliding, thus preventing fluid leakage
The mechanical seal itself is a demanding precision part, with high requirements for design, machining, assembly, and quality. In the use of mechanical seals, should analyze the use of mechanical seals of various factors, so that mechanical seals are suitable for a variety of pump technology and the use of media requirements and adequate lubrication conditions, so as to ensure long-term reliable operation of the seal.



The more common causes of leakage of centrifugal pump mechanical seal 

The proportion of mechanical seal leakage accounts for more than 50 % of all maintenance pumps, the operation of the mechanical seal directly affects the normal operation of the pump, which is summarized and analyzed as follows:

Periodic Leakage

The pump rotor axial runout is large, the auxiliary seal and shaft interference is large, and the dynamic ring can not move flexibly on the shaft. After the pump is turned over and the movable and static rings are worn, no compensation displacement is available
Solution: When assembling the mechanical seal, the axial runout of the shaft should be less than 0.1mm, and the interference between the auxiliary seal and the shaft should be moderate to ensure radial sealing while the dynamic ring is assembled to ensure that it can move flexibly on the shaft (the dynamic ring can spring back freely when pressed against the spring)

Insufficient amount of lubricant on the sealing surface causes dry friction or pulls hair on the sealing end face
Solution: the height of the lubricant surface in the oil chamber should be increased to a level higher than the sealing surface of the moving and static rings.

The rotor vibrates periodically. The reason is that the stator is not aligned with the upper and lower end caps or the impeller and the main shaft are not balanced, cavitation or bearing damage (wear), and this condition will shorten the seal life and produce leakage.  
Solution: the above problems can be corrected according to the maintenance standards.  

Leakage due to pressure

High pressure and pressure wave caused by mechanical seal leakage due to the spring than the pressure and the total pressure ratio design is too large and the pressure in the seal cavity more than 3MPa, will make the seal end face pressure is too large, liquid film is difficult to form, the seal end face wear serious, heat generation increased, resulting in thermal deformation of the seal surface.  
Solution: When assembling the machine seal, the spring compression must be carried out in accordance with the provisions, not allowing too large or too small phenomenon, the mechanical seal under high pressure conditions should take measures. In order to make the end surface force reasonable, minimize deformation, can use carbide, ceramic and other materials with high pressure strength, and strengthen the cooling lubrication measures.

Mechanical seal leakage caused by vacuum state operation pump in the start-up, shutdown process, due to pump inlet blockage, pumping medium containing gas and other reasons, it is possible to make the seal cavity negative pressure, the seal cavity if the negative pressure, will cause the seal end face dry friction, internal mechanical seal will produce gas (water) leakage phenomenon, vacuum seal and positive pressure seal is the difference between the direction of the sealing object, and mechanical seal also There is a certain direction of adaptation.  
Solution: the use of double-end mechanical seal, which helps to improve lubrication conditions, improve sealing performance

Leakage due to media

Most of the submersible pump mechanical seals after disassembly, static ring and dynamic ring auxiliary seals are not flexible, some have rotted, resulting in a large number of leakage of the mechanical seal and even the phenomenon of grinding shaft. Due to high temperature, weak acid and alkali in the sewage on the static ring and moving ring auxiliary rubber seal corrosion, resulting in excessive mechanical leakage, moving, static ring rubber seal material is nitrile-40, not high temperature, not acid and alkali, when the sewage is acidic alkali easy corrosion.  
Solution: for corrosive media, rubber parts should be selected for high-temperature resistance, resistance to weak acids, and weak alkali fluorine rubber.

Solid particles of impurities are caused by mechanical seal leakage. If solid particles into the seal face, will scratch or accelerate the wear of the seal face, the scale and oil in the shaft (sleeve) surface accumulation rate exceeds the wear rate of the friction side, resulting in the dynamic ring can not compensate for wear displacement, hard on hard friction side of the operating life than hard on graphite friction side of the long, because the solid particles will be embedded in the graphite seal ring sealing surface.  
Solution: In the location where solid particles can easily enter the tungsten carbide to tungsten carbide friction sub mechanical seal should be used.

Mechanical seal leakage caused by other problems 

There are also mechanical seals in the design, selection, and installation of other places that are not reasonable enough.  
Spring compression must be carried out in accordance with the provisions, do not allow too large or too small phenomenon, error ± 2mm, too much compression to increase the end surface pressure, too much friction heat, resulting in thermal deformation of the sealing surface and accelerate end wear, too little compression static ring end surface pressure is insufficient, it can not be sealed.  
Install the moving ring seal shaft (or sleeve) end face and install the static ring seal sealing gland (or shell) end face should be chamfered and repair light, so as not to touch the static ring seal during assembly

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