Analysis of seal failure of the hottest mechanical

2022-08-08
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Analysis of seal failure causes of mechanical seals

mechanical seals for pumps have a wide variety of types and models, but there are five main leakage points:

(L) seal between shaft sleeve and shaft

(2) seal between moving ring and shaft sleeve

(3) sealing between dynamic and static rings

(4) seal between the stationary ring and the stationary ring seat

(5) seal the seal between the end cover and the pump body

1. Leakage during installation static test

after the installation and commissioning of mechanical seal, static test is generally carried out to observe the leakage. If the leakage is small, there are problems with the moving ring or static ring sealing ring; When the leakage is large, it indicates that there is a problem between the dynamic and static ring friction pairs. On the basis of preliminary observation of the leakage and judgment of the leakage position, manually turn the gear for observation. If there is no obvious change in the leakage, there is a problem with the static and dynamic ring seals; If there is a significant change in the leakage during turning, it can be concluded that there is a problem with the friction pairs of the dynamic and static rings; If the leakage medium is sprayed along the axial direction, the problems of the moving ring seal are most common. If the leakage medium is sprayed around or leaked from the water cooling hole, the static ring seal is mostly invalid. In addition, leakage channels can also exist at the same time, but there are generally primary and secondary differences. As long as you observe carefully, (5) regularly check the transmission of the sprocket, be familiar with the structure, and you will be able to make a correct judgment

2. Leakage during test run. After the static test of the mechanical seal for the pump, the centrifugal force generated by the high-speed rotation during operation will inhibit the leakage of the medium. Therefore, during the test run, the mechanical seal leakage is basically caused by the destruction of the dynamic and static ring friction pairs after excluding the failure of the seal between the shafts and the end cover. The main factors causing the seal failure of the friction pair are:

(L) during operation, due to abnormal phenomena such as evacuation, cavitation, pressure holding, etc., large axial force is caused, which separates the contact surfaces of the dynamic and static rings

(2) when installing the mechanical seal, the compression is too large, resulting in serious wear and abrasion of 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 stationary ring is too loose. When the moving ring floats axially, the stationary ring disengages from the stationary ring seat

(5) there are granular substances in the working medium, which enter the friction pair during operation, and the sealing end faces of the dynamic and static rings are detected

(6) wrong design and selection, low specific pressure of sealing face or large cold shrinkage of sealing material, etc. The above phenomena often occur in the test run, and sometimes can be eliminated by properly adjusting the static ring seat, but most of them need to be disassembled and reinstalled to replace the seal

failure caused by the loss of lubricating film on the two seal faces:

a) due to the existence of end face seal load, dry friction occurs when the pump is started when the seal cavity is lack of liquid

b) the medium is lower than the saturated steam pressure, causing the liquid film on the end face to flash and lose lubrication

c) if the medium is a volatile product, when scaling or blocking occurs in the mechanical seal cooling system, the saturated steam pressure of the medium rises due to the heat generated by the end face friction and the liquid stirred by the rotating element, which also causes the medium pressure to be lower than its saturated steam pressure

mechanical seal failure caused by corrosion:

a) pitting corrosion or even penetration of sealing surface

b) due to the welding of tungsten carbide ring and stainless steel seat, the stainless steel seat is prone to intergranular corrosion in use

c) welded metal bellows and springs are prone to fracture under the combined action of stress and medium corrosion

mechanical seal failure due to high temperature effect:

a) hot cracking is the most common failure phenomenon of high temperature oil pumps, such as oil residue pumps, refining oil pumps, atmospheric and vacuum tower bottom pumps. Radial cracks will appear on the annular surface due to dry friction at the sealing surface, sudden interruption of cooling water, impurities entering the sealing surface, evacuation, etc

b) graphite carbonization is one of the main reasons for seal failure when using carbon graphite rings. In use, if the graphite ring exceeds the allowable temperature (generally - 105 ~ 250 ℃), resin will precipitate on its surface, and the resin near the friction surface will carbonize. When there is binder, it will foam and soften, increasing the leakage of the sealing surface and sealing failure

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

seal failure caused by the wear of the seal end face:

a) poor wear resistance of the materials used in the friction pair, large friction coefficient, excessive end face specific pressure (including spring specific pressure), etc., 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, sprayed ceramics carbon graphite, silicon nitride ceramics carbon graphite, high-speed steel carbon graphite, surfacing cemented carbide carbon graphite

b) for the medium containing solid particles, the preformed fiber fabric particles that enter the solid and infiltrate the sealing surface are the main reasons for the seal failure. Solid particles enter the end face of the friction pair to act as an abrasive, causing severe wear and failure of the seal. The reasonable clearance of the sealing surface, the balance degree 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

c) balance degree of mechanical seal β It also affects the wear of the seal. In general, the degree of balance β= About 75% is the most suitable. β 75%. Although the wear is reduced, the leakage is increased, and the possibility of opening the sealing surface is increased. For mechanical seals with high load (high PV value), due to the large friction heat on the end face, β Generally, 65% - 70% is appropriate. For hydrocarbon media with low boiling point, because the temperature is sensitive to medium gasification, in order to reduce the impact of friction heat, β It's better to take 80% - 85%, so we don't need to develop new products from scratch

mechanical seal leakage caused by errors caused by installation, operation or the equipment itself:

a) mechanical seal leakage caused by poor installation. It is mainly manifested in the following aspects:

1) the contact surface of the dynamic and static rings is uneven, and they are bumped and damaged during installation

2) the size of the dynamic and static ring seals is wrong, damaged or not compressed by various signal elements

3) there are foreign matters on the surfaces of the moving and stationary rings

4) the direction of the V-shaped sealing ring of the dynamic and static rings is reversed, or the side is reversed during installation

5) the shaft sleeve leaks, 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 length of multiple springs is different

7) the perpendicularity between the end face of the sealing cavity and the shaft is not enough

8) there are corrosion spots at the moving part of the sealing ring on the shaft sleeve

b) during the operation of the equipment, the main reasons for the leakage of the mechanical seal are:

1) the axial displacement of the pump impeller exceeds the standard, the periodic vibration of the rotating shaft and the unstable process operation, and the frequent change of the pressure in the seal cavity will lead to the periodic leakage of the seal

2) the friction pair is damaged or deformed and cannot run in, causing leakage

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

4) the large spring turns incorrectly

5) the equipment vibrates too much during operation

6) the scale formed between the dynamic and static rings and the shaft sleeve makes the spring lose elasticity and cannot compensate for the wear of the sealing surface

7) the sealing ring cracks, etc

c) the pump leaks when it is restarted after stopping for a period of time, which is mainly due to the solidification and crystallization of the medium near the friction pair, the scaling, spring corrosion and blockage on the friction pair and the loss of elasticity

d) the interference of pump shaft is too large

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