【Excerpts】Steps for replacing the machine seal and overhaul misconceptions

Mechanical seal overhaul

        Mechanical seal should be assembled, clean and check not to forget. First look at the two rings and then look at the shaft, do not miss the scarring cracks. Finally, look at the rubber ring, the diameter of the air hole should be clear. Various sizes need to be measured, the gap between the two rings should be adjusted. The tightening force must be checked, the small spring is the key. The bolt should be evenly stressed, and it is a good habit to hold it lightly.

Precautions for mechanical seal disassembly

        When dismantling mechanical seals, the use of hand hammers and flat shovels is strictly forbidden to avoid damage to the sealing components. If there is a mechanical seal at both ends of the pump, care must be taken during dismantling to prevent losing sight of the other. For working mechanical seals, if the gland is loosened when the sealing surface moves, the static and dynamic ring parts should be replaced and should not be re-tightened for further use. Because after loosening, the friction vice original running trajectory will change, the contact surface of the seal will be easily damaged.

        If the sealing element is bonded by dirt or condensation, the condensation should be removed before the mechanical seal is dismantled. 

1、Mechanical seal disassembly

The disassembly order of mechanical seal is opposite to the installation order. In the process of dismantling the mechanical seal must be careful, not to use hand hammer, flat shovel, not to knock the machine seal, not to damage the sealing surface of the dynamic and static ring of the machine seal, if too much dirt because of oil and other dirt, can not be dismantled, shall be cleaned with paraffin and other cleaning before demolition, so as to keep the parts of the machine seal intact.

2、Mechanical seal maintenance

After removing the seal, first check the pump body where the seal is installed, and then overhaul the mechanical seal.

    (1) Check the components of the mechanical seal. The components must be complete, no damage, no scratches, bruises and other defects on the sealing surface of the dynamic and static rings, if damage is found, the seal must be repaired or replaced.

    (2) Check whether the spring force is small, such as small to replace the spring.

    (3) Check whether there is any damage to the inner wall of the seal and the inner surface of the seal end cover, if any problem is found, repair it in time.

3、Preparation and precautions for the installation of mechanical seals

After the above overhaul work is completed, the machine seal needs to be reinstalled. Before installation, preparatory work must first be done as follows.

    (1) If replacing the new machine seal, the type and specification of the mechanical seal shall be checked to see if it is correct and if the quality is up to standard.

    (2) static ring tail anti-rotation groove end and the top of the anti-rotation pin should maintain 1mm-2mm axial clearance to avoid buffer failure.

    (3) moving, static ring end face with alcohol clean, the rest of the metal parts with gasoline cleaning and clean compressed air blow dry, carefully check to ensure that the moving, static ring sealing surface without any damage. Before assembly, two pieces of "0" shaped rubber seal should be coated with a layer of lubricating oil paste, moving, static ring end face is not allowed to apply oil.

4、Installation of mechanical seals

The sequence of mechanical seal installation and precautions are as follows.

    (1) After the relative position of the rotor and the pump body is fixed, determine the installation position of the mechanical seal and, according to the installation size of the seal and the position of the static ring in the gland, calculate the positioning size of the seal on the shaft or sleeve.

    (2) Install the dynamic ring of the mechanical seal, after the installation of the dynamic ring shall ensure its flexible movement on the shaft.

    (3) Assemble the static ring part and the dynamic ring part.

    (4) Fit the seal end cap into the seal body and tighten the screws.

5、mechanical seal overhaul misconceptions

（1）The higher the spring compression the better the sealing effect

In fact, the spring compression is too large, which can lead to sharp wear and tear of the friction sub, instant burnout; excessive compression makes the spring lose the ability to adjust the dynamic ring end face, resulting in seal failure.

（2）The tighter the dynamic ring sealing diagram, the better

In fact, the moving ring seal ring is too tight and harmful. First, intensify the wear between the seal ring and the sleeve, premature leakage; Second, increase the dynamic ring axial adjustment, moving resistance, in frequent changes in working conditions can not be adjusted in a timely manner; Third, excessive spring fatigue easy to damage; Fourth, the dynamic ring seal deformation, affecting the sealing effect. 

（3）The tighter the static ring seal, the better

Static ring seal is basically in a static state, relatively tight seal effect will be better, but too tight is also harmful. One is caused by the static ring seal due to excessive deformation, affecting the sealing effect; Second, the static ring material to graphite mostly, generally more brittle, excessive force is very easy to cause shattering; Third, installation, disassembly difficulties, very easy to damage the static ring.

（4）Multi-stage centrifugal pump impeller locking master the tighter the better

Mechanical seal leakage, the leakage between the sleeve and the shaft (inter-shaft leakage) is more common. In fact, there are many factors that lead to inter-shaft leakage, such as inter-shaft pad failure, offset, impurities in the shaft, shaft and sleeve with a large form error, contact surface damage, the gap between the various parts of the shaft, the shaft head thread is too long, etc. can lead to inter-shaft leakage. On the contrary, a moderate locking of the locking mother will always maintain a certain compression elasticity, and the locking mother will automatically lock at the right time during operation, so that the shaft is always in a good sealing condition.

（5）New is better than old

Relatively speaking, the pump using the new mechanical seal is better than the old, but the quality of the new mechanical seal or material selection is not appropriate, with a larger size error will affect the sealing effect; in polymeric and permeable media, static ring such as no excessive wear, or not to replace the good. Because the static ring in the static ring seat for a long time in a static state, so that the polymer and impurities deposited as one, played a better sealing role.

（6）Better to demolish than not to demolish at all

Once there is a mechanical seal leakage will be eager to dismantle and repair, in fact, sometimes the seal is not damaged, just adjust the working conditions or appropriate adjustment of the seal can eliminate leakage. This will avoid waste and can verify their ability to determine the fault, and accumulate maintenance experience to improve the quality of maintenance.

——Source: Zhulong Geotechnical

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