Analysis of Causes of Seal Failure of Mechanical Seals

Pumps have a wide range of mechanical seals and different models, but there are five main leak points:
(1) a seal between the sleeve and the shaft;
(2) sealing between the moving ring and the sleeve;
(3) Seal between moving and static rings;
(4) sealing between the stationary ring and the stationary ring seat;
(5) Sealing between the sealed end cap and the pump body.

1. Leakage during installation static test After the mechanical seal is installed and debugged, it is generally necessary to perform a static test to observe the leakage. If the leakage is small, there are problems with the moving ring or static ring seal; when the leakage is large, it indicates that there is a problem between the friction pairs of the moving and stationary rings. On the basis of preliminary observation of the leakage amount and judging the leakage location, the vehicle can be manually observed. If there is no significant change in the leakage amount, the static and dynamic ring seals have problems; if there is a significant change in the leakage amount during the cranking, it can be determined that it is moving. There is a problem with the static ring friction pair; if the leakage medium is sprayed in the axial direction, there are many problems with the moving ring seal ring, and the leakage medium is sprayed to the periphery or leaks from the water cooling hole, and the static ring seal ring is mostly invalid. In addition, the leakage channel can also exist at the same time, but there are generally primary and secondary differences. As long as the observation is meticulous and familiar with the structure, it can be judged correctly.
2. Leakage that occurs during commissioning. After the static test of the mechanical seal of the pump, the centrifugal force generated by the high-speed rotation during operation will suppress the leakage of the medium. Therefore, the leakage of the mechanical seal during the test run is basically caused by the failure of the friction pairs of the moving and stationary rings after the failure of the seal between the shafts and the end cap seal. The main factors causing the failure of the friction pair seal are:
(1) During operation, due to abnormal phenomena such as evacuation, cavitation, and rolling, a large axial force is caused to separate the moving and static ring contact surfaces;
(2) The amount of compression is too large when installing the mechanical seal, resulting in severe wear and abrasion of the end face of the friction pair;
(3) The moving ring seal ring is too tight, and the spring cannot adjust the axial floating amount of the moving ring;
(4) The static ring seal ring is too loose, when the moving ring is axially floating, the static ring is separated from the static ring seat;
(5) There are granular materials in the working medium, and the friction pair is entered during operation to detect the moving and static ring sealing end faces;
(6) The design selection is wrong, the sealing end face is lower than the pressure deviation or the sealing material is more cold-shrinkable. The above phenomenon often occurs during trial operation, and can sometimes be eliminated by appropriately adjusting the static ring seat, etc., but most of them need to be reassembled and replaced.

Failure due to loss of lubricating film on both sealed end faces:
A) due to the presence of the end face seal load, the pump is activated when the seal chamber lacks liquid and dry friction occurs;
B) The medium is lower than the saturated vapor pressure, causing the liquid film on the end surface to flash and lose lubrication;
C) If the medium is a volatile product, when the mechanical seal cooling system is fouled or blocked, the saturated vapor pressure of the medium rises due to the friction of the end face and the heat generated by the rotating component stirring the liquid, and the medium pressure is lower than the saturated steam. The situation of pressure.

Mechanical seal failure due to corrosion:
A) pitting and even penetration of the sealing surface;
B) Due to the welding of the tungsten carbide ring and the stainless steel seat, the stainless steel seat in use is prone to intergranular corrosion;
C) Welded metal bellows, springs, etc. are prone to cracking under the combined action of stress and medium corrosion.

Mechanical seal failure due to high temperature effects:
A) Thermal cracking is the most common failure phenomenon of high temperature oil pumps, such as oil slag pumps, refining oil pumps, and atmospheric and vacuum tower pumps. At the sealing surface, due to dry friction, sudden interruption of cooling water, impurities entering the sealing surface, evacuation, etc., will cause radial cracks in the annulus;
B) Graphite carbonization is one of the main reasons for seal failure when using carbon-graphite rings. Since, in use, if the graphite ring exceeds the allowable temperature (generally at -105 to 250 ° C), the resin will precipitate on the surface, and the resin will be charred near the friction surface. When there is a binder, it will foam and soften. Increase leakage of sealing surface and seal failure;
C) Auxiliary seals (such as fluoroelastomer, ethylene propylene rubber, and all rubber) will rapidly age, crack, and harden out of the bomb after exceeding the allowable temperature. The flexible graphite used now has high temperature resistance and corrosion resistance, but its resilience is poor. It is also brittle and easy to damage during installation.

Seal failure due to wear of the seal face:
A) The material used for the friction pair has poor wear resistance, large friction coefficient, and excessive end face 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, hard alloy - carbon graphite, ceramic - carbon graphite, sprayed ceramic - carbon graphite, silicon nitride ceramic - carbon graphite, high speed steel -- Carbon graphite, hardfacing cemented carbide - carbon graphite.
B) For media containing solid particles, the entry of the sealing surface into the solid particles is the main cause of failure of the seal. The solid particles enter the end face of the friction pair and act as an abrasive, causing the seal to wear out and fail. Reasonable clearance of the sealing surface, as well as the balance of the mechanical seal, as well as 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.
C) The degree of balance β of the mechanical seal also affects the wear of the seal. In general, the degree of balance β = 75% is most suitable. β "75%, although the amount of wear is reduced, but the leakage increases, the possibility of opening the sealing surface increases. For high-load (high PV) mechanical seals, β is generally taken as 65% to 70% due to the large frictional heat of the end face. For low-boiling hydrocarbon media, etc., the temperature is sensitive to the gasification of the medium, so as to reduce The influence of frictional heat is preferably 80% to 85% of β.

Mechanical seal leakage due to errors in installation, operation or the equipment itself:
A) The mechanical seal leaks due to poor installation. Mainly in the following aspects:
1) The surface of the moving and static rings is not flat, and it is bumped or damaged during installation;
2) The size of the moving and static ring seal is incorrect, damaged or not pressed;
3) There are foreign objects on the surface of the moving and stationary rings;
4) The direction of the V-ring seal of the moving and static ring is reversed, or the reverse side is installed;
5) Leakage at the bushing, the seal ring is not installed or the pressing force is insufficient;
6) The spring force is not uniform, the single spring is not vertical, and the length of the multiple springs is different;
7) The end face of the sealing cavity is not perpendicular to the axis;
8) There is a corrosion point at the active part of the seal on the sleeve.
B) The main reasons for leakage of mechanical seals during operation are:
1) The axial turbulence of the pump impeller exceeds the standard, the periodic vibration of the rotating shaft and the unstable operation of the process, and the frequent changes of the pressure in the sealed chamber will lead to periodic leakage of the seal;
2) The friction pair is damaged or deformed and cannot run to cause leakage;
3) The sealing ring material is improperly selected, and the swelling is lost;
4) The large spring turns wrong;
5) The vibration of the equipment is too large during operation;
6) The formation of scale between the moving and static rings and the sleeve causes the spring to lose its elasticity and cannot compensate for the wear of the sealing surface;
7) Cracking of the seal ring.
C) The pump leaks when it starts again after stopping for a period of time. This is mainly due to the solidification and crystallization of the medium near the friction pair, the scale on the friction pair, the corrosion of the spring, and the blockage and the loss of the bomb.
D) The pump shaft is too noisy.