What are the main forms of seal failure caused by the bellows mechanical seal and its auxiliary system currently used? -
The bellows mechanical seal and its auxiliary system currently used mainly have the following forms of seal failure:
(1) Seal failure caused by the loss of elasticity or rupture of the bellows Seal failure caused by the loss of elasticity or rupture of the bellows:
The stiffness or elasticity of the bellows will gradually decrease during use. This decrease in elasticity is usually called loss of elasticity. Experiments and practical applications show that when the loss of elasticity of the bellows exceeds 18% to 20% of the design initial compression, the entire bellows mechanical seal will leak. The mechanical seal of Jinli bellows when the temperature is lower than 200℃ Figure 9-1 The mechanical seal of bellows for hot oil pump Fig. 9-1 The phenomenon of dust loss when the mechanical seal of bellows for hot oil pump is not obvious, but at high temperature (over 300℃) The pump leaked soon after driving. The mechanical seal used was removed and the measurement found that the height of the metal bellows assembly was reduced by an average of 2-3mm compared to before installation. The loss of elasticity of the bellows is mainly due to permanent deformation or stress relaxation. Another situation is that the working medium crystallizes or solidifies in the gaps of the bellows, which reduces or loses the deformability of the bellows. Therefore, the high mild load is the main reason for the loss of the bellows.
In addition, in the case of high temperature and larger compression or contact pressure of the bellows, the bellows will break (Figure 9-4); the higher the temperature and the greater the load, the more likely the bellows will break. high.
(2) Seal failure caused by excessive wear of graphite ring in the mating friction pair:
Generally, the failure of the mechanical seal is removed and inspected, and it is found that the graphite sealing ring is severely worn. Through analysis and experiments, it is 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. When the bellows is under external pressure, its effective diameter gradually decreases with the increase of pressure; when the bellows is under internal pressure, its effective diameter gradually increases with the increase of pressure. Due to the excessive pressure, the friction of the friction pair is serious, and the graphite ring is excessively worn to cause leakage. Figure 9-5 shows the abrasion and abrasion of the graphite sealing ring end face of the vacuum tower bottom pump of a certain factory.
Figure 9-4 Broken bellows
Figure 9-5 Dry friction of the sealing surface causes abrasion and leakage of the sealing surface
(3) The sealing failure caused by the surface thermal cracking of the cemented carbide ring in the matched friction pair:
It is found that the cemented carbide ring of the sealing friction pair has many radial cracks of different thicknesses that diverge from the center of the hard face outwards. This is a seal failure caused by thermal cracking. The main cause of thermal cracking is excessive local thermal stress. Among them, the coefficient of linear expansion of the two materials of the cemented carbide ring and the ring seat is different, whether the surfacing structure or the overall structure is used, the type of flushing fluid in the sealed cooling flushing system, and the flushing Whether the method and the size of the flow are appropriate may cause thermal cracks on the sealing end face. Figure 9-6 shows that the end face of the mechanical seal of a decompression tower bottom pump of a certain factory suddenly entered high-temperature heavy oil, causing the sealing surface to crack and cause leakage.
Figure 9-6 Hot cracks on the sealing surface
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