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Several Types Of Mechanical Seals
May 14, 2018

Ordinary Bellows Mechanical Seal Failure Modes Currently used bellows mechanical seals and its auxiliary systems cause seal failures mainly in the following types.

1. Seal failure due to missing or broken bellows. During the use of the bellows, the stiffness of the mechanical seal or elasticity of the fire pump will gradually decrease, and this reduction in elasticity is often referred to as insufficiency. Experiments and practical applications show that when the amount of lost bellows exceeds 18% to 20% of the initial design compression, the entire bellows mechanical seal will leak. When the metal bellows machine is sealed at a temperature lower than 200°C, the insension phenomenon is not obvious, but when it is used at a high temperature (over 300°C), the pump soon leaks after driving, and the mechanical seal to be used is removed to measure the mechanical seal. The amount of metal bellows assembly was found to be 2 to 3 mm lower than the average before installation. Corrugated tube loss is mainly due to long-term deformation or mechanical relaxation of the multi-stage pump stress relaxation, there is a situation where the working medium crystal precipitation or solidification in the gap in the bellows, so that the bellows deformation ability to reduce or lose deformation ability. Therefore, the high temperature and load are the main reasons for the loss of bellows. In addition, in the case of high temperature and bellows compression or contact pressure is large, the bellows will break; the higher the temperature, the greater the load, the higher the probability of the bellows breaking.

2. Seal failure caused by excessive wear of the graphite ring in the paired friction pair. In general, the inspection of the failed seal removed revealed that the graphite seal ring was heavily worn. Through analysis and experiments, it was found that the specific pressure of the end face of the metal bellows machine seal is affected by the effective diameter of the bellows, and the effective diameter changes with the pressure. Due to the excessive pressure, the friction of the friction pair is severe and the graphite ring wears excessively to cause leakage.

3. Sealing failure caused by hot cracking of cemented carbide rings in paired friction pairs. The inspection found that the cemented carbide ring sealing the friction pair appeared many radial cracks which were diverging outwards from the center of the hard surface. This was the sealing failure caused by thermal cracking. The main cause of thermal cracking is excessively high local thermal stress, in which the linear expansion coefficients of hard alloy rings and ring blocks are different, and the type or flushing of the rinsing fluid in the sealed cooling rinsing system adopts a surfacing structure or an integral structure. Whether the size of the method and flow is appropriate may cause hot cracking of the seal face.