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Analysis Of Seal Failure
Apr 24, 2018

(II) Analysis of Seal Failure

Sealing surface itself will also provide signs of seal failure, such as vibration, there will be signs of wear on the transmission parts, such as traces are not obvious, it is generally caused by improper assembly.

For poor quality graphite rings (rings), there are many internal pores. This is because during the manufacturing process, the gas accumulated inside the graphite expands due to carbon particles blowing out, so this kind of low-grade graphite When the ring is in the seal enable, its carbon particles can easily fall off, causing the sealing surface to stick when the seal is disabled.

The flaw on the cylindrical surface of the sealing surface is likely to be caused by outside impurities entering the sealing surface or being improperly installed. The annular groove on the sealing surface is mostly caused by the deposition of solid particles on the sealing surface.

The cracks in the graphite ring (ring) are caused by the vibration of the transmission element, the increase of the rubber ring, and the internal stress of the graphite ring itself. The coking is caused by the high temperature. This is in the hot oil medium of the refinery. Common.

Several strong oxidants such as fuming sulfuric acid, nitric acid, hydrofluoric acid, sodium hypochlorite, aqua regia, and peroxohydroxide, which have erosion effects on graphite, are exacerbated as the temperature increases.

In general, overheating of the surface of the hard ring (static ring) causes severe wear of the seal ring, such as a vertical pump without cooling. In the case of high temperature, high pressure, excessive compression of the spring, and excessive shaft movement, excessive wear of the sealing surface can occur.

There are four signs to pay attention to when checking hard surface:

(1) Cracked ceramic rings; (2) Hot cracks; (3) Nicks; (4) Shedding of coatings.

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The over-tightening of ceramic rings is the main cause of cracking. Improper assembly is also a common cause.

Since the linear expansion rates of the plating material and the base material are different, the cracks appear on the surface of the ring when the temperature rises, and the Stellite alloy is particularly serious. In higher grade 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 when the grinding is performed, resulting in the sealing surface being opened or on the sealing surface. Crystals are formed in-between, and after re-grinding the graphite ring, the abrasive is embedded in the surface of the graphite ring.

The failure of the rubber ring is related to the mode of use. Usually 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, the amount of compression needs to be adjusted, otherwise it will generate heat. Therefore, it is necessary to understand the use temperature of a synthetic rubber. Synthetic rubber ring swelling is mostly caused by chemical attack, they all have some of their own characteristics, such as high temperature resistance of fluoro rubber, and ethylene, propylene ○ ring in the use of petroleum lubricating oil will swell, ozone on nitrile Rubber has an erosive effect, so NBR rubber products should not be installed in the motor, so high temperature and chemical corrosion are usually the main causes of hardening and cracking of rubber products.

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