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Current Position:Question and Answer - What are the reasons for the failure and leakage of the mechanical seal of the centrifugal pump

What are the reasons for the failure and leakage of the mechanical seal of the centrifugal pump

Centrifugal pumps are basically sealed in the form of mechanical seals, and magnetic pumps are sealed in the form of static seal isolation sleeves. There are many types and models of mechanical seals in centrifugal pumps, but there are mainly five leakage points: the seal between the shaft sleeve and the shaft; the seal between the moving ring and the shaft sleeve; the seal between the dynamic and static rings; the seal between the static ring and the static ring seat The seal between the seal; the seal between the seal end cover and the pump body. It is very troublesome for pump seal failure to leak. Whether it is a centrifugal pump or a magnetic pump, liquid leakage is an important factor causing accidents in production. The following is an analysis and solution to the leakage problem caused by seal failure.

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1. Leakage during trial operation

  The centrifugal force generated by the high-speed rotation during the operation of the mechanical seal for the pump will inhibit the leakage of the medium after the static test. Therefore, the leakage of the mechanical seal during the test run is basically caused by the damage of the friction pair of the dynamic and static rings after the failure of the seal between the shafts and the end cover is excluded.

The main factors that cause the failure of the friction pair seal are:


  1. During operation, due to abnormal phenomena such as evacuation and cavitation, a large axial force is caused, and the contact surfaces of the dynamic and static rings are separated;


  2. When the mechanical seal is installed, the amount of compression is too large, resulting in serious wear and scratches on the end face of the friction pair;


  3. The sealing ring of the moving ring is too tight, and the spring cannot adjust the axial floating amount of the moving ring;


  4. The sealing ring of the static ring is too loose. When the moving ring floats axially, the static ring is separated from the static ring seat;


  5. There are granular substances in the working medium, which enter the friction pair during operation, and the end faces of the dynamic and static ring seals are tested for flaws;


  6. The design and selection are wrong, the specific pressure of the sealing end face is low or the cold shrinkage of the sealing material is large. The above phenomenon often occurs in the trial operation, and sometimes can be eliminated by properly adjusting the static ring seat, etc., but most of them need to be disassembled and replaced.

2. Leakage during installation static test


  After the mechanical seal is installed and debugged, a static test is generally performed to observe the leakage. If the leakage is small, it is mostly due to the problem of the dynamic ring or the static ring sealing ring; when the leakage is large, it indicates that there is a problem between the friction pair of the dynamic and static rings. On the basis of preliminary observation of the leakage amount and judgment of the leakage location, manual cranking is performed to observe, if there is no obvious change in the leakage amount, there is a problem with the static and dynamic ring seals; There is a problem with the friction pair of the static ring; if the leakage medium is sprayed along the axial direction, there are many problems with the dynamic ring seal. In addition, leakage channels can also exist at the same time, but generally there are primary and secondary differences. As long as you observe carefully and are familiar with the structure, you will be able to judge correctly.


  3. Failure caused by the loss of lubricating film on the two seal end faces


  1. Due to the existence of the seal load on the end face, dry friction occurs when the pump is started when the seal cavity lacks liquid;


  2. When the pressure of the medium is lower than the saturated steam pressure, the liquid film on the end face flashes and the lubrication is lost;

3. If the medium is a volatile product, when the mechanical seal cooling system is fouled or blocked, the saturated vapor pressure of the medium will rise due to the end face friction and the heat generated by the rotating element stirring the liquid, which also causes the medium pressure to be lower than its saturated vapor. pressure condition.


  4. Mechanical seal failure due to corrosion


  1. Pitting corrosion on the sealing surface, or even penetration.


  2. Due to the welding of the tungsten carbide ring and the stainless steel seat, the stainless steel seat is prone to intergranular corrosion during use;


  3. Welded metal bellows, springs, etc. are prone to rupture under the combined action of stress and medium corrosion.


  5. Seal failure due to wear of seal end face


  1. The balance degree β of the mechanical seal also affects the wear of the seal. Under normal circumstances, the balance degree β=75% is suitable. β<75%, although the amount of wear decreases, the leakage increases, and the possibility of opening the sealing surface increases. For mechanical seals with high load (high PV value), due to the large frictional heat of the end face, β is generally 65% to 70%. For the influence of frictional heat, β should be taken as 80% to 85%.


  2. The materials used in the friction pair have poor wear resistance, large friction coefficient, and excessive end face specific pressure (including spring specific pressure), which will shorten the service life of the mechanical seal. For commonly used materials, the order of wear resistance is: silicon carbide-carbon graphite, cemented carbide-carbon graphite, ceramics-carbon graphite, spray ceramics-carbon graphite, silicon nitride ceramics-carbon graphite, high-speed steel --Carbon graphite, surfacing carbide--carbon graphite.


  3. For the medium containing solid particles, the entry of solid particles into the sealing surface is the main reason for the failure of the seal. The solid particles enter the end face of the friction pair to act as an abrasive, which causes the seal to wear violently and fail. The reasonable clearance of the sealing surface, the balance of the mechanical seal, and the flashing of the liquid film on the sealing end face are the main reasons for the opening of the end face and the entry of solid particles.


  6. Mechanical seal leakage caused by errors in installation, operation or the equipment itself


  1. The mechanical seal leaks due to poor installation. Mainly in the following aspects:


  1) The contact surfaces of the dynamic and static rings are uneven, and they are bruised or damaged during installation;


  2) The size of the dynamic and static ring seals is wrong, damaged or not pressed;


  3) There are foreign objects on the surface of the dynamic and static rings;


  4) The direction of the V-shaped sealing ring of the dynamic and static rings is reversed, or the reverse side is installed;


  5) There is leakage at the shaft sleeve, the sealing ring is not installed or the pressing force is not enough;


  6) The spring force is uneven, the single spring is not vertical, and the multiple springs have different lengths;


  7) The verticality between the end face of the sealing chamber and the shaft is not enough;


  8) There are corrosion spots at the movable part of the sealing ring on the shaft sleeve.


  2. During the operation of the equipment, the main reasons for the leakage of the mechanical seal are as follows:


  1) The axial movement of the pump impeller exceeds the standard, the periodic vibration of the rotating shaft and the unstable process operation, and the frequent changes in the pressure in the sealing chamber will lead to periodic leakage of the seal;


  2) The friction pair is damaged or deformed and cannot run-in to cause leakage;


  3) Improper selection of sealing ring material, swelling and loss of elasticity;


  4) The steering of the big spring is wrong;


  5) The vibration of the equipment is too large when it is running;


  6) The scale formed between the dynamic and static rings and the shaft sleeve causes the spring to lose its elasticity and cannot compensate for the wear of the sealing surface;


  7) The seal ring is cracked, etc.


  3. The pump leaks when it is restarted after stopping for a period of time. This is mainly due to the solidification and crystallization of the medium near the friction pair, scale on the friction pair, corrosion of the spring, and blockage, resulting in the loss of the bomb.


  7. Mechanical seal failure due to high temperature effect


  1. Thermal cracking is a common failure phenomenon of high temperature oil pumps, such as oil residue pumps, oil return pumps, atmospheric and vacuum tower bottom pumps, etc. In the case of dry friction at the sealing surface, sudden interruption of cooling water, impurities entering the sealing surface, evacuation, etc., radial cracks will appear on the annular surface;


  2. Graphite carbonization is one of the main reasons for seal failure when carbon-graphite rings are used. Because in use, if the graphite ring exceeds the allowable temperature (generally -105 ~ 250 ℃), resin will precipitate on its surface, and the resin will be carbonized near the friction surface. When there is a binder, it will foam and soften. Increase the leakage of the sealing surface and the sealing failure;


  3. Auxiliary seals (such as fluororubber, ethylene-propylene rubber, and all-rubber) will rapidly age, crack, harden and lose elasticity after exceeding the allowable temperature. The flexible graphite currently used has good high temperature resistance and corrosion resistance, but its resilience is poor. And it is brittle and easy to be damaged during installation.



Mr. Chu

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