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Analysis Of The Cause Of Seal Failure
- Oct 29, 2018 -

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

For the poor quality graphite ring (dynamic ring), there are many internal pores, because during the manufacturing process, the gas accumulated inside the graphite expands to blow out the carbon particles, so the low-quality graphite When the ring is activated, its carbon particles are easily detached, and the sealing surface is stuck when the seal is deactivated.

The flaw on the cylindrical surface of the sealing surface is likely to be caused by foreign debris entering the sealing surface or improperly installed. The annular grooves on the sealing surface are mostly caused by solid particles deposited on the sealing surface.

The crack of the graphite ring (dynamic ring) is caused by the vibration of the transmission member, the increase of the rubber ring and the internal stress of the graphite ring itself, and the coking is caused by the high temperature, which is in the high temperature hot oil medium of the refinery. Common.

Several strong oxidants, such as fuming sulfuric acid, nitric acid, hydrofluoric acid, sodium hypochlorite, aqua regia, and persalt, which attack the graphite, have a corrosive effect with increasing temperature.

Usually the overheating of the hard ring (static ring) surface can cause severe wear of the seal ring, such as a vertical pump without cooling. Under high temperature, high pressure, when the spring compression is too large, and the shaft movement is too large, it will cause the transition wear of the sealing surface.

There are four signs to be aware of when examining the hard ring surface:

(1) ceramic ring rupture; (2) hot cracking; (3) nicking; (4) delamination of the coating.

Tight assembly of ceramic rings is the main cause of cracking, and improper assembly is also a common cause.

Since the linear expansion rate of the plating material and the base material are different, cracks may occur on the surface of the ring when the temperature is raised, and the Stellite alloy is particularly serious. In higher grade coating materials, cobalt-based tungsten carbide is not as good as a nickel-based coating. Cooling the sealing surface can effectively prevent thermal cracking. The solid particles remaining on the sealing surface often damage the surface. For example, the sand on the grinding wheel will damage the hard ring surface during grinding, causing the sealing surface to open or the sealing surface. Crystallization is formed between them, and after the graphite ring is reground, the abrasive is embedded in the surface of the graphite ring.

The failure of the rubber ring is related to the way of use. Usually, the high pressure is one of the reasons for the failure of the press-formed O-ring. When it is found that the O-shape becomes a rectangle or the ring becomes hard, it is necessary to adjust the compression amount, otherwise it will generate heat. Therefore, it is necessary to know the temperature of use of a synthetic rubber. Most of the swelling of synthetic rubber rings is caused by chemical erosion. They all have their own characteristics. For example, fluororubber is resistant to higher temperatures, while ethylene and propylene ○ rings will swell in petroleum lubricating oils. Rubber has an erosive effect, so the nitrile rubber products should not be installed in the motor. Therefore, high temperature and chemical corrosion are the main causes of hardening and cracking of rubber products.

The rubber parts are cut and the surface is nicked during installation, which is also a common cause of seal failure. The old fixing screws, keyways, spline shafts, sharp shoulders and other signs on the shaft can damage the rubber parts.

Here, the following points must be added to the wear marks of the sealing surface to check the wear marks, which can help analyze the fault. 1, wear and widening: indicates that the machine pump has a serious misalignment.

The reason is:

(1) bearing damage;

(2) Shaft vibration or shaft deformation;

(3) shaft bending;

(4) The pump cavitation generates vibration;

(5) The coupling is not centered;

(6) The pipe is severely deformed;

(7) Sealing the static ring tilt.

2, the wear scar is narrow: the wear scar is narrower than the minimum width of the two sealing faces, which means that the seal overpressure, pressure or temperature deforms the sealing surface.

3, no wear scars:

Note that the sealing surface is not bonded. Check if the compensation mechanism such as the spring is slipping or obstructed.

4, the sealing surface has no wear marks but there are bright spots.

There will be bright spots and no wear marks on the warpage of the sealing surface. The pressure is too high, the gland bolts are not screwed or not clamped, or the pump surface is rough to form a bright spot. When two guilloté glands are used, their 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 car is driven. 5, the sealing surface has trimming:

This is because the sealing faces are too open and break when closed. Flashing (gasification) is the reason why the more common sealing surfaces are separated, especially when there is condensate in the hot water system or fluid, and the water expands from the liquid into a vapor, which can separate the sealing faces. (The cold medium gasification will also cause)

Sealed metal parts such as springs, set screws, transmissions and metal sleeves can be the source of seal failure. The corrosion of a spring subjected to alternating stress is its primary problem, because the metal will corrode rapidly under stress, the stainless steel spring is susceptible to stress corrosion by chloride, 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 with high corrosion resistance Hastelloy steel. In addition, improper assembly causes spring fatigue to be another cause of failure.

The fixing screws used for mechanical seals should not be made of hardened materials. The heat treatment reduces the corrosion resistance of the metal, while the softer fixing screws without heat treatment can be fastened to the shaft.

Vibration, deflection, and distraction will cause the transmission member to wear. If the sealing surface is stuck when it is started, the transmission member will be bent or even damaged, and the heat generated by the friction will often exacerbate the corrosion.

The wear scar 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 sealing ability of the seal. It may also be skewed, caused by different reasons.

The metal should change color during the temperature rise process. When using stainless steel, pay attention to the color at the following temperatures.

Light yellow————The temperature is 700~800°F (about 370~432°C) Brown————The temperature is 900~1000°F (about 486~540°C) Blue————The temperature is 1100°F (about 590 ° C)

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

When the seal failure does not meet any of the above, the overhaul is more difficult, but the following situations can be used for reference:

1. Pump bushing leakage

Many bushings do not extend out of the sealed box, so it is difficult to determine the source of the leak. Leakage of the bushing is usually stable, while leakage of the sealing face tends to increase or decrease. After the sealing surface leaks, the surface is not flat, but sometimes it will wear into the original shape. (Sometimes don't rush to overhaul, you can observe it for a while)

2. If the seal is moist around, there is no leakage. This centrifugal force generated by the operation of the pump at the start of the operation causes the leaked liquid to return to the sealing surface and acts as a barrier. The liquid leaking from the flange or joint on the pump drops into the stuffing box.

3. Thermal expansion can loosen the graphite ring embedded in the metal part, or it may cause the O-ring to lose its elasticity due to low temperature, resulting in leakage.

4. The fluctuation of the flushing pressure will cause the seal to fail. The flushing pressure must be higher than the pressure of the sealed chamber. The solenoid valve and the delay switch installed in front of the pump can ensure that the residue in the flushing is flushed before the pump starts or after the stop. If quenching is used to control the temperature, the pressure in the sealed chamber must be maintained.

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

6. The leakage of the heat exchanger is often caused by the accumulation of scale on the cooling surface, the flow of the fluid in the cooler is accelerated, or the direction of the heat exchanger is reversed.