There are many opportunities for mechanical seals to fail, and the proportion is relatively large. The common damage forms can be divided into three types: corrosion damage, thermal damage and mechanical damage. Due to the special structural form of mechanical seals and the vastly different working environments, there are also various forms of corrosion. sexual characteristics.
(1) Uniform corrosion on the surface
If the surface of the metal mechanical seal ring contacts the corrosive medium, and the metal itself is not resistant to corrosion, surface corrosion will occur, and the phenomena are leakage, early wear, damage, and sound. The uniform corrosion of the metal surface has two forms: film formation and no film. The corrosion of metal without film is very dangerous, and the corrosion process proceeds at a certain speed, which is mainly caused by the wrong material selection. In film-forming corrosion, the passivation film usually has the characteristics of protection, but the passive film on the surface of the seal ring material used for the metal sealing ring, such as stainless steel, cobalt, chromium alloy, etc., is destroyed in the friction of the end face, and the new film is formed under the condition of lack of oxygen. It is difficult to generate, which makes galvanic corrosion aggravate.
(2) Stress corrosion cracking
Under the simultaneous action of corrosion and tensile stress, the metal first produces cracks in the weak area, and then develops to the depth, resulting in cracks, which is called stress corrosion cracking. The use of surfacing hard alloy and cast iron, tungsten carbide, titanium carbide and other mechanical seal rings is prone to stress corrosion cracking. Seal ring cracks are generally radially divergent and can be one or more. These cracks communicated the entire mechanical seal end face, accelerated the wear of the end face, and increased the leakage.
(1) Graphite ring corrosion
There are three reasons for the corrosion of resin-impregnated impermeable graphite seal rings: one is that when the end face is overheated and the temperature is greater than 180°C, the impregnated resin will separate from the graphite ring, which will reduce the wear resistance of the ring; Improper selection will cause chemical changes in the medium, which will also reduce the wear resistance; the third is that the resin impregnation depth is not enough, and the wear resistance will decrease when the impregnated layer is removed. Therefore, in the establishment of a sealed cooling system, it is necessary to select a corrosion-resistant impregnating resin, use high-pressure impregnation, and increase the impregnation depth.
(2) Oxidation of graphite rings
In an oxidizing medium, when the end face is dry rubbed or poorly cooled, the temperature of 350-400 °C can cause the graphite ring to react with oxygen to generate CO2 gas, which can roughen the end face and even break it. Non-metallic rings can also crack under the simultaneous action of chemical media and stress.
(3) Corrosion of PTFE (tetrafluoroethylene gasket) sealing ring
F4 is filled with glass fiber, graphite powder, metal powder, etc. to improve its temperature resistance and wear resistance. The corrosion of the filled F4 PTFE seal ring mainly refers to the selective corrosion, dissolution or deterioration of the filler. For example, in hydrofluoric acid, the glass fiber molecules are thermally corroded, so what is filled in the mechanical seal should be determined on a case-by-case basis.
(1) Corrosion of auxiliary sealing ring
Different types of rubber have different corrosion resistance. Due to the corrosion and aging of the rubber, the surface of the failed rubber becomes rough and loses elasticity after being corroded, and it is easy to break. The oil resistance of rubber varies depending on the variety. The rubber that is not oil resistant is easy to swell, the friction force increases, and the floating property is not good, which makes the seal invalid. Rubber and F4 have poor temperature resistance, and silicone rubber has the best temperature resistance and can be used at 200 °C.
(2) Corrosion of the contact part with the auxiliary sealing ring
Mechanical seal moving ring, shaft sleeve, static ring, static ring seat, there is no large relative movement in contact with rubber or F4 auxiliary seal ring, where the liquid is relatively static and easy to form a dead angle. , static ring seat and sealing body, etc. cause special corrosion, mainly crevice corrosion, friction corrosion, contact corrosion, three kinds of corrosion exist at the same time, alternately, so the corrosion surface is wider and deeper. Observe that when the surface depth is 1-1.5 times the diameter of the sealing ring, and the corrosion degree is not less than 0.01mm, the sealing leakage is serious.