Causes and Analysis of Mechanical Seal Failure

Causes and Analysis of Mechanical Seal Failure

1. There are three main reasons for seal failure:

(1), the sealing surface is opened

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

When 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 in the surface of the soft ring (usually the right ink ring), which actually becomes a“Grinding wheel”Will damage the hard ring surface.

Since the moving ring or rubber ring is fastened on the shaft (shaft sleeve), when the shaft moves in series, the moving ring cannot be fitted in time, so that the sealing surface is opened, and the sealing surface is closed with a delay, so that solid particles enter the sealing surface .

At the same time, there are solid particles between the shaft (shaft sleeve) and the sliding parts, which affect the sliding of the rubber ring or the moving ring.

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

Because heat will be generated on the sealing surface, the service temperature of the rubber ring should be lower than the design specification.

The operating temperature of fluororubber and polytetrafluoroethylene is 216°C, and that of nitrile rubber is 162°C. Although they can withstand higher temperatures, the rubber ring will continue to vulcanize due to the higher heat generated by the sealing surface. The danger is that it will eventually lose its elasticity and leak.

The heat will also cause the crystallization of the medium between the sealing surfaces, such as carbon formation, 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 due to overheating, and even flash fire. In addition to changing the condition of the medium, overheating will also aggravate its corrosion rate. Cause deformation of metal parts, cracking of alloy surface, and some coating cracks, the design should choose a balanced mechanical seal to reduce the specific pressure to prevent overheating.

(3), out of tolerance

Correct assembly tolerances are necessary for the installation of mechanical seals. The shaft (shaft sleeve) must have appropriate surface roughness and correct dimensions, but manufacturers rarely provide tolerance data, which are critical to installation of.

The dimensional accuracy and shape and position tolerance of the mechanical seal must meet the requirements of the drawing, otherwise the seal will fail prematurely.

2. Analysis of the cause of seal failure

The sealing surface itself will also provide signs of seal 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 internal pores. This is because during the manufacturing process, the gas accumulated inside the graphite expands and the carbon particles are blown out. 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 likely to be caused by foreign matter entering the sealing surface or improper installation. Most of the annular grooves on the sealing surface are caused by the deposition of solid particles 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, and the coking is caused by high temperature, which is the most important in the high-temperature hot oil medium of the refinery. Common.

Fuming sulfuric acid, nitric acid, hydrofluoric acid, sodium hypochlorite, aqua regia, perhydroxide and other strong oxidizing agents have corrosive effects on graphite, and their corrosive effects will intensify as the temperature increases.

Usually the overheating of the hard ring (stationary ring) surface will cause severe wear of the sealing ring, such as vertical pumps without cooling.

Under high temperature, high pressure, excessive spring compression, and excessive shaft movement, excessive wear of the sealing surface will be caused.