At present, the main failure modes of bellows mechanical seal and its auxiliary system are as follows:
(1) Seal failure caused by the failure or breakage of bellows seal failure caused by the failure or breakage of bellows:
The stiffness or elasticity of bellows will decrease slowly during the use. This kind of elastic reduction is usually called the loss of elasticity. The experimental and practical application show that when the loss of elasticity of bellows exceeds 18% - 20% of the design initial compression, the mechanical seal of the bellows will leak. The mechanical seal of Jinli bellows is used at a temperature lower than 200 ℃, but it is used at a high temperature (more than 300 ℃). After the pump is started, the pump will leak quickly. It is found that the height of metal bellows assembly is reduced by 2-3mm on average compared with that before installation. The bellows are mainly lost in elasticity due to permanent deformation or stress relaxation. Another is that the working medium crystallizes and settles in the cracks of the bellows, which reduces or loses the deformation capacity of the bellows. Therefore, high and moderate load is the main reason for the bellows to lose its elasticity.
(2) Seal failure caused by excessive wear of graphite ring in matched friction pair:
Generally, the failure of the machine seal removal inspection found that the graphite seal ring is seriously worn. Through analysis and test, it is found that the specific pressure of the mechanical seal of metal bellows is affected by the effective diameter of the bellows, and the effective diameter is changed with the pressure. When the bellows is under external pressure, the effective diameter decreases with the increase of pressure; When the bellows is under internal pressure, the effective diameter increases with the increase of pressure. Because of the pressure too much, the friction of friction pair is serious, and the graphite ring is excessively worn and leakage is caused.
(3) Sealing failure caused by thermal cracking of cemented carbide ring in pair friction pair:
It is found that there are many radial cracks in the hard alloy ring of the seal friction pair, which are caused by thermal cracking. The main reason for thermal cracking is high local thermal stress. The difference of linear expansion coefficient between cemented carbide ring and ring seat, the overlay structure or the overall structure are adopted. Whether the type, flushing method and flow rate of flushing fluid in the seal cooling flushing system are suitable may cause thermal cracking on the sealing end face.