Causes of mechanical seal failure

1. There are three main reasons for seal failure:

(1) Open the sealing surface

When repairing mechanical seals, 85% of seal failures are not caused by wear, but leakage before wear.

When the sealing surface is opened, the 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 in the surface of the soft ring (usually graphite ring), which actually becomes a "grinding wheel" ” will damage the hard ring surface.

Since the moving ring or rubber ring is fastened to the shaft (shaft sleeve), when the shaft moves in series, the moving ring cannot fit in time, causing the sealing surface to open, and the sealing surface lags to close, allowing solid particles to 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 to the dynamic sealing point, a common fault). In addition, the medium will also produce crystals at the friction part between the rubber ring and the shaft (sleeve), and there will also be solid matter at the spring, which will cause the sealing surface to open.

(2), overheating

Because heat will be generated on the sealing surface, the operating temperature of the rubber ring should be lower than the design specification. The service temperature of fluorine rubber and polytetrafluoroethylene is 216°C, and the use temperature of nitrile rubber is 162°C. Although they can withstand higher temperatures, the heat generated by the sealing surface is higher, so the rubber ring continues to vulcanize. The danger is that it will eventually lose its elasticity and leak. (Consider cold crispiness in the cold area)

The heat between the sealing surfaces will also cause the crystallization of the medium, such as carbonization, causing the sliding parts to be stuck and the sealing surfaces to be condensed. Moreover, some polymers coke due to overheating, and some fluids lose lubrication and even flash fire due to overheating.

In addition to changing the condition of the medium, overheating will also increase its corrosion rate. Causes deformation of metal parts, cracks on the alloy surface, and cracks in some coatings. The design should use a balanced mechanical seal to reduce the specific pressure and prevent overheating.

(3), out of tolerance

Correct assembly tolerances are necessary for installing mechanical seals. The shaft (sleeve) must have appropriate surface roughness and correct size, but manufacturers rarely provide tolerance data, which are critical to installation. of. (Rely on experience and common sense)

The dimensional accuracy and geometric tolerance of the mechanical seal must meet the requirements of the drawing. Excessive tolerance will cause the seal to fail prematurely.

2. Analysis of seal failure reasons

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

For poor quality graphite rings (moving rings), there are more pores inside. This is because during the manufacturing process, the gas accumulated inside the graphite expands and blows out the carbon particles. Therefore, this low-quality graphite When the ring is in use for sealing, its carbon particles can easily fall off, causing the sealing surface to stick when the seal is out of service.

The scratches on the inner cylindrical surface of the sealing surface are most likely caused by foreign matter entering the sealing surface or improper installation. The annular grooves on the sealing surface are mostly caused by the deposition of solid particles on the sealing surface.

Cracks in 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 coking is caused by high temperature, which is the case in the high-temperature hot oil medium of a refinery. Common.

Several strong oxidants such as oleum, nitric acid, hydrofluoric acid, sodium hypochlorite, aqua regia, and perhydroxide are corrosive to graphite, and their corrosive effects intensify as the temperature increases.

Usually overheating of the hard ring (stationary ring) surface will cause severe wear of the sealing ring, such as in vertical pumps without cooling. Under high temperature, high pressure, excessive spring compression, and excessive shaft movement, excessive wear of the sealing surface will occur.

There are four signs to look for when inspecting hard ring surfaces:

a. Ceramic ring rupture; b. Hot cracking; c. Scores; d. Falling off of coating.

Over-tightening of the ceramic ring is the main cause of rupture, and improper assembly is also a common cause.

Since the linear expansion rates of the coating material and the base material are different, cracks will appear on the ring surface when the temperature rises, especially for Stelite alloys. Among higher-grade coating materials, cobalt-based tungsten carbide is inferior to nickel-based coatings. Cooling the sealing surface can effectively prevent hot cracking. Solid particles remaining on the sealing surface often damage the surface. For example, the sand on the grinding wheel during grinding will damage the surface of the hard ring, causing the sealing surface to open or between the sealing surfaces. Crystals are formed in between, and after the graphite ring is reground, the grinding material will be embedded in the surface of the graphite ring.

The failure of the rubber ring is related to the way it is used. Usually high pressure is one of the reasons for the failure of the pressed ○-shaped ring. When it is found that the ○-shaped shape becomes rectangular or the ring becomes hard, the compression amount needs to be adjusted, otherwise it will generate heat. So it is necessary to know the service temperature of a synthetic rubber. The swelling of synthetic rubber rings is mostly caused by chemical erosion. They all have their own characteristics. For example, fluorine rubber is resistant to higher temperatures, while ethylene and propylene rings will swell when used in petroleum lubricants. Ozone is resistant to Dingqing. Rubber has corrosive effects, so NBR products should not be installed in motors. Therefore, high temperature and chemical corrosion are usually the main reasons for hardening and cracking of rubber products.

Cuts and scratches on the rubber parts during installation are also common causes of seal failure. Old fixing screws, keyways, spline shafts, sharp shoulders and other signs on the shaft will damage rubber parts.

Here, the following points need to be added regarding the wear marks on the sealing surface. Checking the wear marks can help analyze the fault.

(1) Widening wear: This indicates serious misalignment of the pump. The reasons are:

a. Bearing damage;

b. Shaft vibration or shaft deformation;

c. Shaft bending;

d. Pump cavitation produces vibration;

e. The coupling is not aligned;

f. The pipe is severely deformed;

g. The static sealing ring is tilted.

(2) Narrowing of the wear marks: The wear marks are narrower than the minimum width of the two sealing surfaces, which indicates that the seal is overpressured and the pressure or temperature deforms the sealing surface.

(3) No wear marks:

Indicates that the sealing surface is not bonded. Check whether the spring and other compensation mechanisms are slipping or blocked.

(4) There are no wear marks on the sealing surface but there are bright spots.

If the sealing surface is warped, there will be bright spots but no wear marks. The pressure is too high, the gland bolts are not tightened or clamped properly, or the pump surface is rough, which can cause bright spots. When a gland with two bolts is used, its stiffness is not enough, and deformation is also a reason for the formation of bright spots.

The appearance of this symptom indicates that the seal may leak as soon as the car is started.

(5) The sealing surface has trimmed edges:

This is due to the sealing surfaces being spread too far apart and breaking when they come together. Flash evaporation (vaporization) is a common cause of sealing surface separation, especially in hot water systems or when there is condensation in the fluid. Water expands from liquid to vapor, which can cause the sealing surface to separate. (The vaporization of the cold medium will also cause it)

Sealed metal parts, such as springs, fixing 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 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 Hastelloy steel springs with higher corrosion resistance. In addition, spring fatigue caused by improper assembly is another cause of failure.

The fixing screws used in mechanical seals should not be made of hardened materials, because heat treatment will reduce the corrosion resistance of the metal, and softer fixing screws without heat treatment can be tightened on the shaft.

Vibration, deflection, and non-concentricity will cause the transmission parts to wear. For example, if the sealing surface sticks during startup, 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 deviation or inconsistency.

Metal changes color during temperature rise. When using stainless steel, you should pay attention to the color at the following temperatures.

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

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

Blue color——temperature is 1100℉ (about 590℃)

Black————The temperature is 1200℉ (about 648℃)

When the seal failure does not meet any of the above conditions, maintenance will be more difficult, but the following leakage situations are available for reference:

(1) Leakage of pump shaft sleeve

Many bushings do not extend beyond the seal box, so determining the source of a leak can be difficult. 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, the surface will be uneven, but sometimes it will be worn back to its original state. (Sometimes don’t rush to repair, you can wait for a while before making any decisions)

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

(3) Thermal expansion can cause the graphite ring embedded in the metal component to loosen, or the low temperature may cause the O-ring to lose its elasticity, causing leakage.

(4) Fluctuations in flushing pressure will cause seal failure. The flushing pressure must be higher than the seal chamber pressure. Using the solenoid valve and delay switch installed in front of the pump can ensure that the residue in flushing is flushed before starting or after stopping the pump. Clean. If quenching is used to control temperature, the pressure in the seal chamber must be maintained.

(5) If a layer of scale deposits on the cooling spacer, a graphite bushing can be installed at the bottom of the sealing chamber and its thermal barrier function can be used to solve these problems.

(6) The leakage of the heat exchanger is often caused by the scale on the cooling surface that hinders the heat transfer, the fluid flow rate in the cooler is accelerated, or the direction of the heat exchanger is reversed.