Causes of Seal Failure

Author:Site Editor     Publish Time: 2020-08-26      Origin:Site

The sealing surface itself will also provide signs of seal failure. For example, when vibration, there will be traces of wear on the transmission parts. If the traces are not obvious, it is generally caused by improper assembly. For the graphite ring (moving ring) of poor quality, there are more pores inside. This is because, during the manufacturing process, the gas accumulated in the graphite expands and blows out the carbon particles. Therefore, this low-quality graphite The carbon particles of the ring are easy to fall off when the seal is activated, and the sealing surface sticks when the seal is deactivated.

The scars on the inner cylindrical surface of the sealing surface are probably caused by foreign matter entering the sealing surface or improper installation. The annular groove on the sealing surface is mostly caused by solid particles deposited on the sealing surface. The cracks of the graphite ring (moving ring) are caused by the vibration of the transmission parts, the expansion of the rubber ring and the internal stress of the graphite ring itself, while the coking is caused by high temperature. This is common in high-temperature thermal oil media in refineries.

Several strong oxidants, such as fuming sulfuric acid, nitric acid, hydrofluoric acid, sodium hypochlorite, aqua regia, and hydrogen peroxide, have corrosive effects on graphite, and their corrosive effects increase with increasing temperature. Usually, the overheating of the surface of the hard ring (stationary ring) will cause severe wear of the sealing ring, such as a vertical pump without cooling. Under the conditions of high temperature, high pressure, excessive spring compression, and excessive shaft string movement, excessive wear of the sealing surface will be caused.

 

standard mechanical seals


There are four signs to pay attention to when inspecting the surface of the hard ring: a. The ceramic ring is broken; b. The hot crack; c. Notches; d. The coating is peeling off.

 

The over-tight assembly of the ceramic ring is the main cause of cracking, and improper assembly is also a common cause.

Due to the different linear expansion rates between the coating material and the base material, cracks will appear on the ring surface when the temperature rises, and Stellite alloy is particularly serious. Among the higher-level coating materials, cobalt-based tungsten carbide is inferior to nickel-based coatings. Cooling the sealing surface can effectively prevent thermal cracking. The solid particles remaining on the sealing surface often damage the surface. For example, sand particles on the grinding wheel will damage the surface of the hard ring during grinding, causing the sealing surface to open or on the sealing surface. Crystals are formed from time to time, and after grinding the graphite ring again, the abrasive will be embedded in the surface of the graphite ring.

The failure of the rubber ring is related to the way of use. Usually, high pressure is a reason for the failure of the pressed O-ring. When the O-ring becomes rectangular or the ring becomes hard, it is necessary to adjust the compression amount, otherwise, it will generate heat. Therefore, it is necessary to understand the operating temperature of the synthetic rubber. Most of the swelling of synthetic rubber rings are caused by chemical attack. They all have their own characteristics. For example, fluorine rubber is resistant to higher temperatures, while ethylene and propylene ○-shaped rings will swell when used in petroleum lubricants. Rubber has a corrosive effect, so do not install nitrile rubber products in the motor, so high temperatures and chemical corrosion are usually the main reasons for the hardening and cracking of rubber products.

The rubber parts are cut and scratched during installation, which is also common reasons for seal failure. The old fixing screws, keyways, spline shafts, sharp shoulders, and other signs on the shaft will damage the rubber parts.

 

Checking the Signs of Wear


Here, the following points need to be added to the signs of wear on the sealing surface. Checking the signs of wear can help analyze the failure.

(1) Widened wear and tear: It indicates that the pump has a serious misalignment. The reason is:

a. the bearing is damaged;

b. Shaft vibration or shaft deformation;

c. The shaft is bent;

d. Pump cavitation produces vibration;

e. The coupling is misaligned;

f. The pipe is severely deformed;

g. The static sealing ring is inclined.

(2) The wears scar is narrowed: the wear scar is narrower than the minimum width of the two sealing surfaces, which means that the sealing surface is deformed by overpressure, pressure, or temperature.

(3) No wear marks: It means that the sealing surface is not bonded. Check whether the compensation mechanism such as the spring is slipping or obstructed.

(4) The sealing surface has no wear marks but bright spots.

There will be bright spots on the sealing surface warping without wear marks. If the pressure is too high, the gland bolts are not tightened or clamped, or the pump surface is rough, bright spots can be formed. When a gland with two bolts is used, its rigidity is insufficient, and deformation is also a cause of bright spots. The appearance of this symptom indicates that the seal may leak as soon as the vehicle is driven.

(5) There are cut edges on the sealing surface:

This is due to the sealing surfaces being separated too far and breaking when closed. Flashing (vaporization) is a common cause of separation of the sealing surfaces, especially when there is condensate in the hot water system or fluid, the water expands from the liquid to vapor, which can separate the sealing surfaces. (The vaporization of cold medium will also cause it)

mechanical shaft seal

Sealed metal parts, such as springs, fixing screws, transmission parts, and metal sleeves, may become the root cause of seal failure. Corrosion of springs subjected to alternating stress is its primary problem because metals will corrode rapidly under stress, stainless steel springs are susceptible to stress corrosion by chlorides, and there are many chlorides in the world, so there are foreign departments It is recommended not to use stainless steel springs, but to use springs made of Hastelloy steel with higher corrosion resistance. In addition, spring fatigue caused by the improper assembly is another cause of failure.

Do not use hardened materials for fixing screws used in mechanical seals, because heat treatment will reduce the corrosion resistance of the metal, and softer fixing screws without heat treatment can be fastened to the shaft. Vibration, deflection, and misalignment will wear the transmission parts. For example, when the sealing surface is stuck when starting, the transmission parts will bend or even be damaged, and the heat generated by friction often aggravates corrosion. The wear marks on the outer surface of the metal sleeve may be caused by solid particles entering the sleeve from the sealing side, which interferes with the follow-up ability of the seal. It may also be caused by skewness and disagreement.

 

Metal needs to change its color during the temperature rise. When using stainless steel, pay attention to the color at the following temperatures.

Light yellow----Temperature is 700~800℉ (about 370~432℃)

Brown-the temperature is 900~1000℉ (approximately 486~540℃)

Blue-the temperature is 1100°F (about 590°C)

Black-the temperature is 1200°F (about 648°C)

 

Other Leak Cause

 

When the seal failure does not meet any of the above, it is more difficult to overhaul. The following leaks are for reference:

(1) pump shaft sleeve leakage

Many shaft sleeves do not extend out of the sealed box, so it is difficult to determine the source of leakage. The leakage of the shaft sleeve is usually stable, while the leakage of the sealing surface tends to increase or decrease. After the sealing surface leaks, it makes the surface uneven, but it sometimes runs into its original shape. (Sometimes don't rush to overhaul, you can observe for a period of time)

(2) If the area around the seal is damp, and there is no leakage. The centrifugal force generated by the operation of the pump at startup causes the leaked liquid to return to the sealing surface, acting as a barrier. The liquid leaking from the flange or joint on the pump drips into the stuffing box.

(3) Thermal expansion can loosen the graphite ring inlaid in the metal part, or it may be due to the low temperature that the O-ring loses its elasticity, causing leakage.

(4) Flushing pressure fluctuations will cause seal failure. The flushing pressure must be higher than the pressure in the sealed chamber. The solenoid valve and delay switch installed in front of the pump can ensure that the residue in the flushing is flushed before the pump is started or after it is stopped. Clean, such as using quenching to control the temperature, the pressure in the sealed cavity must be maintained.

(5) If a layer of scale deposits on the cooling spacer, we can install a graphite bushing at the bottom of the sealed cavity and use its thermal barrier function to solve these problems.

(6) Leakage of the heat exchanger is often caused by the fouling on the cooling surface that hinders the heat transfer, and the fluid flow rate in the cooler increases or the direction of the heat exchanger is reversed.

 

 

WenDong, as a mechanical seal manufacturer in China, provides professional mechanical seals knowledge and high-quality mechanical shaft seals. If you have any needs, please contact us now!


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