In the fluid machinery, the parts with the most severe working conditions are the mechanical seal parts for pumps, which bear the axial force like a thrust bearing and conduct the generated heat like a heat exchanger. It is already difficult to achieve static sealing in high temperature, high pressure, corrosion-prone parts, and it is even more difficult to achieve dynamic sealing on the sealing end face with a width of several millimeters in high-speed rotation.
In order to solve this thorny problem, after long-term practice, people have found two ways: one is to start with the sealed structure and material to make it suitable for harsh working conditions. The cost of this approach is high, however, it still does not solve the problem fundamentally. Once the seal fails, the consequences are serious. Another way is to improve the working environment through auxiliary facilities.
At present, mechanical seals without any auxiliary facilities in actual work are rare. Corresponding auxiliary facilities must be used, such as serial mechanical seals used in high pressure and mechanical seal parts of high temperature pumps. Through auxiliary facilities, some harsh working environments are changed to those acceptable to the seal, so that the effect of the mechanical seal is greatly improved and the scope of use is wider.
The frictional heat generated in the work of the mechanical seal parts will increase the temperature of the sealing end face. If the corresponding facilities are not adopted, many adverse consequences will occur.
1) The rise in temperature causes the liquid film between the seal end faces to vaporize, aggravate the wear, and make the mechanical seal parts fail;
2) The increase in temperature causes thermal deformation of the dynamic and static rings, increasing leakage and aggravating wear;
3) The increase in temperature also aggravates the corrosion of the medium to mechanical seal parts;
4) The temperature rise makes the auxiliary sealing ring age, deteriorate and fail;
5) When the temperature rises, the graphite impregnated with synthetic resin will decrease in performance due to carbonization of the resin, and the graphite ring impregnated with metal will leak due to the melting of the metal.
It can be seen that the unacceptable temperature rise is the enemy of mechanical seal parts
The purpose of the auxiliary facilities is to reduce this undesired temperature rise, maintain a good lubrication state between the sealing end faces, and make all parts of the mechanical seal work well and normally; It does not vaporize; it can play the role of heat preservation and heat supply in the sealing of cryogenic pumps; for the sealing of crystallization and strong corrosive media with solid particles, it can also keep the sealing from being damaged. Therefore, some countries refer to the auxiliary system as the protection system.
Practice has proved that the structure is reasonable and the materials are good, but auxiliary facilities are neglected, resulting in difficult working conditions and early failure of the seal. On the contrary, in some occasions, the structure and materials are not enough to meet the requirements of working conditions, and effective auxiliary facilities are adopted, as a result, the seal meets the requirements of working conditions and runs smoothly.
Some mechanical seals do not have high working temperature, pressure and speed, and can work without auxiliary facilities. However, from the viewpoints of seal stability and service life, it is advantageous to employ auxiliary facilities. Therefore, auxiliary facilities should be regarded as an integral part of the mechanical seal, and different auxiliary facilities should be used according to different working conditions. The international standard of the American Petroleum Institute (Centrifugal Pump APl610) has made specific regulations on this. The correct and reasonable selection of auxiliary facilities is of great significance not only to the stability of parts of mechanical seal but also to prolonging their service life; it has a certain effect on safe production, reducing leakage, reducing maintenance workload and reducing production costs, and must be given enough attention. Auxiliary facilities are called sealed circulation protection systems, or simply called sealed systems or protection systems.