Choosing Materials for Mechanical Seal

Description
Mechanical seal is composed by many parts of different materials which can be classified into four types, namely, frication materials, secondary sealing materials, elastic components and other structural materials.

Main Classification
1. Frication Materials
As we all know, frication materials are influenced by its mechanical properties, chemical properties and frication properties. To guarantee long service life and good sealing performance, we should pay attention to the principles listed below when choosing such materials.
1) They should be of good wear resistance and corrosion resistance, leading long service life.
2) Frication materials should feature strong mechanical strength, to avoid damaging and deformation of surface.
3) They should characterize excellent heat resistance and superior heat conduction, to reduce some disadvantages that frication causes.
4) Frication materials should be low in coefficient of friction and have a degree if lubrication itself. The coefficient of friction directly influences the frication heat.
5) Air impermeability should be good. Tight structure and no holes lead no leakage.
6) They should be easy to mold and process, reducing cost and guaranteeing machining precision.
Till now, no materials can satisfy all the principles listed above. So we should choose such materials based on their application and properties.
Carbon-graphite is one of suitable materials due to their properties, featuring good lubrication, low coefficient of frication, good corrosion and heat resistance, excellent heat conduction, and easy to process.
2. Secondary Sealing Materials
Secondary sealing materials had better feature good elasticity, suitable hardness, small deformation, good high/low temperature resistance, good aging quality and excellent compatibility of medium.
Actually, none of materials can possess all the properties. Selection of the materials used in mechanical seals is based on experience. Under normal circumstances, material selection is the responsibility of the seal manufacturer and it is thus essential that he is provided with full details of the fluid coming into contact with the seal. These should include minor chemical constituents and off design conditions that can occur in service. For instance, pumps operating in the food or pharmaceutical industries may be subject periodically to special sterilization procedures which can involve flushing with different chemical fluids from those normally handled.
Since their inception, the reliability and mean time between failures of mechanical seals fitted to pumps have been inconsistent. Operational experience shows that seal performance varies relative to application and pump type. A significant amount of research in seal face materials, hydraulic and thermal face distortion, face lubrication, seal cooling, etc., has been carried out by a number of mechanical seal makers. And large strides in technology and seal performance have been achieved. The ultimate breakthrough that will give consistent high reliability on mechanical seals has still to be made. As a contribution towards achieving this step changes in operation, the pump maker should recognize that those designs and manufacturing aspects of pump will affect the seal performance. So if it is necessary, alter pump design philosophy to ensure the pump to seal interface. This is idealized not only from the standpoint of the pump, but also from that of the mechanical seals. Here followed some requirements for the pump.
A. Radial runout of shaft < 0.03mm
B. Coaxiality of shaft and sealing chamber > 0.02mm
C. Verticality of shaft and sealing face < 0.05mm
D. Axial float <0.01 mm
E. Amplitude of vibration of the shaft during running <10um

Failure Analysis
There are a lot of factors that cause failure of mechanical seals, such as the problems in design and manufacture, inappropriate type and installation, or problem in equipment itself. Therefore, we should analyze the reasons of failure and take measures.
1. First, we should take a general analysis on the leakage path of mechanical seal.
2. Check the materials if they are of good wear and corrosion resistance.
3. Inspect the wear trace, because it can reflect abrasive conditions and some useful clues for analyzing failure.
4. Check the secondary sealing materials whether they are in good condition. High hardness, low elasticity and permanent deformation are the appearance of the failure of mechanical seals.
5. Inspect the spring. Pay attention to the elasticity and fracture.
There are other failures of mechanical seals, like inappropriate operating way. Therefore, we should analyze comprehensively according to different applications.

Shanghai Trisun Parts Manufacture Co., Ltd. is a professional manufacturer and supplier of various fluid seals and seal materials in China. Our main mechanical seals include dry gas seals, agitator seals, cartridge seals, metal bellows seals, etc. And our secondary sealing materials consist of silicon carbide seals, tungsten carbide seals, rubber seals and graphite seals. Adhering to the principle of “gaining mutual benefits with our customers”, Trisun does its utmost to serve every customer.