Mechanical seal failures are generally known by people after abnormal leakage, abnormal wear, abnormal torque and other phenomena. The reasons for the failure are as follows: 1. The design and selection of mechanical seal are incorrect; 2. Poor mechanical seal quality; 3. The accuracy of the machine using or installing mechanical seals cannot meet the requirements; 4. Machine operation error.

Specific analysis of the causes of seal failure and analysis (I) There are three main reasons for seal failure: 1. When the sealing surface is opened and the mechanical seal is repaired, 85% of seal failure is not caused by wear, but has leaked before wear.

As soon as the sealing surface is opened, solid particles in the medium enter the sealing surface under the action of liquid pressure. After the sealing surface is closed, these solid particles are embedded on the surface of the soft ring (usually the right ink ring), which is actually a "grinding wheel" that will damage the surface of the hard ring.

Since the moving ring or rubber ring is fastened on the shaft (shaft sleeve), when the shaft is in series, the moving ring cannot fit in time, which causes the sealing surface to open and the sealing surface to close behind time, so that solid particles enter the sealing surface.

At the same time, there are also solid particles between the shaft (sleeve) and the sliding parts, which affect the sliding of the rubber ring or dynamic ring (relative dynamic sealing point, common fault). In addition, the medium will also produce crystals at the friction part between the rubber ring and the shaft (shaft sleeve), and there will also be solid substances at the spring, which will open the sealing surface.

2. Overheating will generate heat on the sealing surface, so the use temperature of the rubber ring should be lower than the design specification. The service temperature of fluororubber and polytetrafluoroethylene is 216 ℃, and the service temperature of nitrile rubber is 162 ℃. Although they can withstand higher temperatures, the rubber ring is in danger of continuous vulcanization due to the high heat generated by the sealing surface, and the rubber ring will lose its elasticity and leak. (Cold brittleness is considered in cold zone)

Heat will also cause crystallization of the medium between the sealing surfaces, such as carbon deposition, which will cause the sliding parts to be stuck and the sealing surface to be condensed. Moreover, some polymers become coking due to overheating, and some fluids lose lubrication due to overheating, and even flash.

Overheating can not only change the condition of the medium, but also aggravate its corrosion rate. For deformation of metal parts, cracking of alloy surface, and some coating cracks, balanced mechanical seal shall be selected for design to reduce specific pressure and prevent overheating.

3. Out of tolerance correct assembly tolerance is necessary for the installation of mechanical seal. The shaft (shaft sleeve) has proper surface roughness and correct size, but the manufacturer rarely provides tolerance data, which is critical for installation. (Rely on experience and common sense)

The dimensional accuracy and geometric tolerance of the mechanical seal meet the requirements of the drawing, and out of tolerance will lead to premature failure of the seal.

（2） Analysis of seal failure causes The sealing surface itself will also provide signs of seal failure. For example, when vibrating, there will be wear marks on the transmission parts. If the marks are not obvious, it is generally caused by improper assembly.

For the poor quality graphite ring (moving ring), there are many internal pores, which is caused by the expansion of the gas gathered in the graphite during the manufacturing process, blowing out the carbon particles. Therefore, the carbon particles of this low-quality graphite ring are easy to fall off when the seal is started, which makes the sealing surface stick when the seal is stopped.

The scars on the cylindrical surface inside the sealing surface may be caused by foreign matters entering the sealing surface or improper installation. Most of the annular grooves on the sealing surface are 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, which is common in high-temperature hot oil media in refineries.

Several strong oxidants, such as fuming sulfuric acid, xiao acid, hydrofluoric acid, sodium hypochlorite, aqua regia, and peroxides, which have corrosive effects on graphite, will intensify with the increase of temperature.

Generally, overheating of the hard ring (stationary ring) surface will cause serious wear of the sealing ring, such as vertical pump without cooling. Under high temperature, high pressure, excessive spring compression, and excessive shaft movement, the sealing surface will be excessively worn. When checking the surface of the hard ring, there are four signs to note: (1) the ceramic ring is broken; (2) Hot cracking; (3) Nick; (4) The coating falls off.

Tight assembly of ceramic ring is the main reason for fracture, and improper assembly is also a common reason.

Because the linear expansion rate of the coating material is different from that of the base material, cracks will appear on the ring surface when the temperature rises, especially for Stellite alloy. In coating materials, cobalt based tungsten carbide is inferior to nickel based coating. Cooling the sealing surface can effectively prevent hot cracking. The solid particles left on the sealing surface often damage the surface. For example, the sand particles on the grinding wheel will damage the hard ring surface during grinding, causing the sealing surface to open or generate crystals between the sealing surfaces. After regrinding the graphite ring, the abrasive will be embedded in the surface of the graphite ring. The failure of rubber ring is related to the way of use. Usually, high pressure is one of the reasons for the failure of the pressed O-ring. When it is found that the O-ring becomes rectangular or the ring becomes hard, the compression amount needs to be adjusted, otherwise it will generate heat. Therefore, it is necessary to know the service temperature of a synthetic rubber. The swelling of synthetic rubber rings is mostly caused by chemical erosion, and they all have their own characteristics. For example, fluororubber is resistant to high temperature, while ethylene and propylene O-ring will swell when used in petroleum lubricating oil, and ozone will erode NBR. Therefore, NBR products should not be installed in the motor, so high temperature and chemical corrosion usually cause rubber products to harden Main causes of cracks.

The rubber parts are cut and nicked on the surface during installation, which is also a common reason for seal failure. The old set screw, keyway, spline shaft, sharp shoulder and other signs on the shaft will damage the rubber parts.

Here, the following points need to be added for the wear trace of the sealing surface. Checking the wear trace can help analyze the fault.

1. Widened wear: it indicates that the pump is seriously misaligned.

The reasons are: (1) the bearing is damaged; (2) Shaft vibration or shaft deformation; (3) Shaft bending; (4) Vibration caused by pump cavitation; (5) Coupling is not aligned; (6) The pipe is severely deformed; (7) The seal stationary ring is inclined.

2. Narrow wear scar: the wear scar is narrower than the width of the two sealing surfaces, which indicates that the sealing surface is deformed due to overpressure, pressure or temperature.

3. No abrasion mark: it indicates that the sealing surface is not bonded. Check whether the compensation mechanism such as spring is slipping or blocked.

4. The sealing surface is free of abrasion marks but has bright spots.

The warped sealing surface will have bright spots without abrasion marks. If the pressure is too high, the gland bolt is not screwed or clamped properly, or the pump surface is rough, bright spots can be formed. When the gland with two bolts is used, its rigidity is not enough, and deformation is also a reason for forming bright spots.

This symptom indicates that the seal may leak as soon as the vehicle is started.

5. There is trimming on the sealing surface: this is because the sealing surface is too open, and it breaks when closing. Flash evaporation (gasification) is a common reason for sealing surface separation, especially when there is condensate in the hot water system or fluid, water expands from liquid to vapor, which can separate the sealing surface. (Cold medium gasification will also cause)

Sealed metal parts, such as springs, set screws, transmission parts and metal sleeves, may become the source of seal failure. Corrosion of springs subjected to alternating stress is its primary problem, because metal will rapidly corrode under stress, stainless steel springs are vulnerable to stress corrosion of chlorides, and there are many chlorides in the world, so foreign departments recommend not to use stainless steel springs, but to use Hastert alloy steel springs with high corrosion resistance. In addition, spring fatigue caused by improper assembly is another reason for failure.

The set screw used for mechanical seal shall not be made of hardened material, because heat treatment will reduce the corrosion resistance of metal, and the soft set screw without heat treatment can be fastened on the shaft.

Vibration, deflection and misalignment will wear the transmission parts. If the sealing surface sticks when starting, the transmission parts will be bent or even damaged, and the heat generated by friction often aggravates corrosion.

The abrasion marks on the outer surface of the metal sleeve may be caused by solid particles entering the sleeve from the sealing side, which interfere with the follow-up ability of the seal. It may also be caused by deflection and misalignment.

The color of metal shall be changed in the process of temperature rise, and the color of stainless steel at the following temperatures shall be noted during use.

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

Brown -- temperature 900~1000 ℉ (about 486~540 ℃)

Blue -- 1100 ℉ (about 590 ℃)

Black -- 1200 ℉ (about 648 ℃)

When the seal failure does not conform to any of the above, it is difficult to repair, but the leakage in the following cases can be referred to: 1. Many shaft sleeves do not protrude out of the seal box, so it is difficult to determine the source of the leakage. The leakage of the shaft sleeve is usually stable, while the leakage of the sealing surface is often increased or decreased. After the sealing surface leaks, the surface is uneven, but sometimes it will run in to the original state. (Sometimes don't rush to overhaul, but observe for a while.)

2. If the surrounding of the seal is wet, and no leakage can be seen. This centrifugal force generated by the pump operation during startup makes the leaked liquid return to the sealing surface and acts as a barrier. The liquid leaked from the flange or joint on the pump drops into the stuffing box.

3. The thermal expansion can make the graphite ring embedded in the metal part loose, or it may be that the low temperature makes the O-ring lose its elasticity, resulting in leakage.

4. The fluctuation of flushing pressure will cause seal failure. The flushing pressure is higher than the pressure of the seal chamber. Enabling the solenoid valve and time delay switch installed in front of the pump can ensure that the residues in flushing are washed clean before the pump starts or after the pump stops. If quenching is used to control the temperature, the pressure of the seal chamber must be maintained.

5. If a layer of scale is deposited on the cooling spacer, we can install a graphite bushing at the bottom of the seal chamber to solve these problems by using its heat barrier function.

6. The leakage of the heat exchanger is often caused by the fouling on the cooling surface, which hinders the heat transfer, and the flow rate of the fluid in the cooler is accelerated, or the direction of the heat exchanger is reversed.