The failure forms of common bellows mechanical seal are mainly as follows, which are caused by bellows mechanical seal and its auxiliary system.
1. Seal failure caused by bellows loss or fracture. During the use of corrugated pipe, its stiffness or elasticity of the mechanical seal of the fire pump will slowly decrease, which is usually referred to as the phenomenon of loss of elasticity. The experiment and practical application show that when the elastic loss of the bellows exceeds 18%~20% of the designed initial compression, the whole bellows mechanical seal will leak. When the temperature of metal bellows mechanical seal is lower than 200 ℃, the phenomenon of elasticity loss is not obvious, but when it is used under high temperature (more than 300 ℃), the pump will leak soon after startup. When the used mechanical seal is removed and measured, it is found that the height of metal bellows components is 2~3mm lower than that before installation. The elasticity loss of the bellows is mainly due to permanent deformation or relaxation of the mechanical seal of the stress multistage pump. In another case, the working medium crystallizes and precipitates or solidifies in the gap of the bellows, reducing or losing the deformation capacity of the bellows. Therefore, high temperature and load are the main reasons for the loss of elasticity of bellows . In addition, the bellows will break at high temperature and when the compression or contact specific pressure of the bellows is large; The higher the temperature and the greater the load, the higher the probability of bellows fracture.
2. Seal failure caused by excessive wear of graphite ring in the paired friction pair. In general, it is found that the graphite seal ring is seriously worn after the failed mechanical seal is removed for inspection. Through analysis and test, it is found that the face specific pressure of metal bellows mechanical seal is affected by the effective diameter of the bellows, and the effective diameter changes with the change of pressure. Due to the excessive pressure, the friction pair is severely rubbed, and the graphite ring is excessively worn, causing leakage.
3. Sealing failure caused by surface hot cracking of cemented carbide ring in paired friction pair. Through inspection, it was found that the cemented carbide ring of the sealing friction pair had many radial cracks of different thickness from the center of the hard surface to the outside, which was the seal failure caused by hot cracking. The main cause of hot cracking is high local thermal stress. The difference of linear expansion coefficient between cemented carbide ring and ring seat, whether surfacing structure or integral structure is adopted, and whether the type, method and flow of flushing fluid in the seal cooling flushing system are appropriate may cause hot cracking on the sealing end face.