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Leakage Causes of Mechanical Seals for Pumps

Leakage Causes of Mechanical Seals for Pumps


Generally speaking, it is easier to find and solve the leakage between the shafts where the shaft sleeves protrude, and the leakage between the sealing end cover and the pump body, but careful observation is required, especially when the working medium is liquefied gas or high-pressure, toxic and harmful gas, it is relatively difficult some. The rest of the leaks are difficult to distinguish and judge intuitively. Only by observing, analyzing and judging the symptoms of leakage on the basis of long-term management and maintenance practice can a correct conclusion be drawn.

1. Leakage during installation static test

After the mechanical seal is installed and debugged, it is generally necessary to conduct a static test to observe the leakage.

If the leakage is small, it is mostly due to the problem of the moving ring or the static ring sealing ring. Manual cranking observation, if there is no obvious change in the leakage, there is a problem with the static and dynamic ring seals.

When the leakage is large, it indicates that there is a problem between the dynamic and static ring friction pairs.

In addition, leakage channels can also exist at the same time, but generally there is a difference between primary and secondary. As long as you observe carefully and are familiar with the structure, you can definitely make a correct judgment.

2. Leakage during test run

After the mechanical seal of the pump has passed the static test, the centrifugal force generated by the high-speed rotation during operation will inhibit the leakage of the medium. 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 seals between the shafts and the end cover is excluded.

The main factors causing friction pair seal failure are:

(1) During operation, due to abnormal phenomena such as evacuation, cavitation, and pressure holding;

(2) The compression amount is too large when installing the mechanical seal;

(3) The moving ring sealing ring is too tight;

(4) The static ring sealing ring is too loose;

(5) There are granular substances in the working medium;

(6) Failure caused by the loss of lubricating film on the two sealing end faces:

a) Due to the existence of the load on the end face seal, dry friction occurs when the pump is started when the seal chamber lacks liquid;

b) The pressure of the medium is lower than the saturated vapor pressure, so that the liquid film on the end surface flashes and loses lubrication;

c) If the medium is a volatile product, when scaling or blockage occurs in the mechanical seal cooling system, the saturated vapor pressure of the medium will rise due to the friction of the end face and the heat generated by the stirring liquid of the rotating element, which will also cause the pressure of the medium to be lower than its saturated vapor pressure situation.

(7) For a longer period of outage, there is no manual cranking when restarting;

(8) The design selection is wrong.

The above phenomenon often occurs during trial operation, and sometimes it can be eliminated by properly adjusting the static ring seat, etc., but most of them need to be disassembled and replaced to replace the seal.

3. During the operation of the equipment, the main reasons for the leakage of the mechanical seal are:

Centrifugal pumps suddenly leak during operation, a few are due to normal wear or have reached the end of their service life, and most are caused by large changes in working conditions or improper operation and maintenance.

1) The axial displacement of the pump impeller exceeds the standard;

2) Periodic vibration of the rotating shaft, unstable process operation, and frequent changes in the pressure in the sealing chamber will cause periodic leakage of the seal;

3) Evacuation, cavitation or pressure holding for a long time, resulting in damage to the seal;

4) The actual output of the pump is relatively small, and a large amount of medium circulates in the pump, and the heat accumulates, causing the medium to vaporize and resulting in seal failure;

5) Corrosive, polymeric and gelling substances in the medium increase;

6) Frequent changes or adjustments in working conditions;

7) Scale is formed between the dynamic and static rings and the shaft sleeve, which makes the spring lose its elasticity and cannot compensate for the wear of the sealing surface;

8) Sudden power failure or shutdown due to failure, etc.

9) The equipment vibrates too much during operation;

10) The friction pair is damaged or deformed and cannot run in, causing leakage;

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

12) The large spring turns incorrectly;

13) Cracks in the sealing ring, etc.

Centrifugal pump leaks suddenly during normal operation. If it cannot be found in time, it will often lead to major accidents or losses. It must be paid attention to and effective measures should be taken.

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

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

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

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

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

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

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

7) The spring force is uneven, the single spring is not vertical, and the multiple springs are of different lengths;


5. Mechanical seal failure caused by corrosion, high temperature, wear of sealing end face, etc.

1) Mechanical seal failure due to corrosion:

a) The sealing surface is corroded or even penetrated.

b) Since the tungsten carbide ring is welded to the stainless steel seat, etc., the stainless steel seat is prone to intergranular corrosion during use;

c) Welded metal bellows, springs, etc. are prone to rupture under the joint action of stress and medium corrosion.

2) Mechanical seal failure due to high temperature effect:

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

b) Graphite carbonization is one of the main causes of seal failure when using carbon-graphite rings

c) Auxiliary seals (such as fluorine rubber, ethylene propylene rubber, and full rubber) will rapidly age, crack, harden and lose their elasticity when the temperature exceeds the allowable temperature. The flexible graphite currently used has good high temperature resistance and corrosion resistance, but its resilience is poor, it is easy to be brittle at high temperature, and it is easy to be damaged during installation

3) Seal failure due to wear of the sealing end face

a) The materials used in the friction pair have poor wear resistance, high friction coefficient, and excessive end surface specific pressure (including spring specific pressure), etc., which will shorten the service life of the mechanical seal.

b) For media containing solid particles, the entry of solid particles into the sealing surface is the main cause of seal failure. Solid particles enter the end face of the friction pair and act as abrasives, causing severe wear and tear of the seal to fail. The reasonable clearance of the sealing surface, the balance degree of the mechanical seal, and the flash evaporation of the liquid film on the sealing end surface are the main reasons for the opening of the end surface and the entry of solid particles.

4) Leakage occurs when the pump starts 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, and spring corrosion and blockage to lose the bullet.

5) The pump shaft is too disturbed.

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