CAUSES OF BELLOWS SEAL FAILURE

Bellows seal has the double functions of good sealing effect and compensation buffer. It has high safety factor, compact structure and wide applicability. Due to the high cost of bellows seal, the material and production process adopted by some manufacturers are not ideal, so bellows seal often fails in use. Therefore, the bellows seal can not be accepted by enterprises and users. Most enterprises still use non contacting mechanical seal and gland packing, which leads to the slow process of popularizing bellows seal technology.

Structure and Sealing Characteristics of Bellows

The bellows seal can be divided into metal bellows seal, elastomer bellows seal, etc. Mechanical seal adopts the sealing form of "four points and one side". "Four points" refers to the sealing points between the dynamic ring and the shaft sleeve, the shaft sleeve and the shaft, the static ring and the gland, and the gland and the pump body. "One side" refers to the contact surface between the dynamic ring and the static ring. In the structure of bellows seal, the sealing points of the dynamic ring and the shaft sleeve are transformed from the dynamic sealing points to the static sealing points, and the sealing points of the static ring and the gland are transformed from the dynamic sealing points to the static sealing points. The performance of bellows seal is better than spring seal. Using bellows instead of spring can enhance the anti fatigue ability of compensation buffer, reduce the leakage rate and the failure rate, and extend the service life of seal. However, it is found that there are still some failures in the bellows seal, which affect the stable operation of the equipment.

Causes of Bellows Seal Failure

Generally, when the friction pair of the bellows is subjected to great friction, the torque of the bellows reaches the limit value, which will cause the bellows to twist and crack. However, the friction force between the two ends of the seal will not change under the same working condition of the medium. Therefore, the failure of bellows may be related to its material and rigidity. In terms of material selection of bellows, some manufacturers use materials with good welding performance and corrosion resistance, high strength and fatigue resistance, while some manufacturers use 1Cr18Ni9Ti and other similar materials in order to save cost, and these materials often lose elasticity or the welding accuracy fails to meet the requirements under high temperature.

When the pressure in the seal chamber reaches the limit value of the design pressure, there is a large medium pressure in the bellows chamber, which may cause explosion damage or fatigue torsion crack along the outer arc of the bellows in the seal chamber, resulting in seal failure. However, under the current working condition, explosion damage will not occur generally. Because the pump seal is mostly used in the form of pressure reducing ring and seal oil flushing cooling. The sealing oil pressure is controlled at 0.05 to 0.1MPa higher than the pressure in the sealing chamber. It can not only take away part of the friction heat, but also wash and lubricate the two ends of the friction pair, which ensures that the pressure difference inside and outside the seal chamber is a constant value, so it is impossible for the inner chamber of the bellows to burst and make the bellows seal fail.