Mechanical seals are commonly used two methods of selection is introduced

1. Selection according to the working parameters of p, v, t p here refers to the medium pressure, sealed cavity place according to the size of the p values can preliminarily determine whether to choose balance type structure and balance degree. For high medium viscosity and good lubricity, p 0 or less. 8 mpa, or low viscosity, lubricity, low dielectric, p 0 or less. 5 mpa, we usually choose the balanced structure. P value exceeds the scope, should consider to choose balanced structure. When p> 15 mpa, general single face structure balance it is hard to meet the sealing requirements, this time can choose tandem end face seal. Nu is refers to the average diameter of circumferential velocity on sealing surface, according to the size of the nu value to determine whether elastic element with the axis of rotation, namely USES the rotary spring or spring stationary type structure, general nu & lt; 20 ~ 30 m/s spring rotary can be used, under the condition of higher speed, due to the imbalance of the rotating parts quality easy cause strong vibration, had better use spring stationary type structure. If the value of p and nu are all high, can consider to choose the fluid dynamic pressure type structure. T is refers to the medium temperature of the sealed cavity, according to the size of the t determine the auxiliary seal ring material, sealing surface cooling method and its auxiliary system. Temperature t within 0 ~ 80 ℃, the auxiliary seal ring usually choose NBR O ring; - - - - - - 50℃≤t< 150 ℃, according to the corrosive medium strength, can choose fluorine rubber, silicone rubber or ptfe forming filler sealing ring: temperature & lt; - - - - - - 50 or t 150 ℃ or higher, rubber and ptfe can produce low temperature embrittlement or high temperature aging, metal bellows structure can be used at this time. Medium turbidity higher than 80 ℃, usually in the field of seal is to consider as high temperature, the corresponding cooling measures must be taken at this time. 2. Types according to characteristics of weak type corrosive medium, usually choose built-in mechanical seal, the transverse stress state and the direction of the medium leaking than external type is reasonable. For strong corrosive medium, because of the spring material selection is difficult, can choose the external type or ptfe bellows type mechanical seal, but usually only for p 0 or less. 2 to 0. The scope of 3 mpa. Easy crystallization and easy to solidification and high viscosity medium, big spring rotary structure should be adopted. Because of the small spring easy to solid plug, high viscosity medium can make the small spring axial compensation movement is blocked. Flammable, explosive, toxic media, in order to ensure that nothing more than medium leakage, should adopt a sealing liquid ( Spacer) The double end face structure. According to the working parameters and dielectric characteristics of the selected structure is often just a preliminary scheme, finalized must also consider the characteristics of the host and some special requirements for sealing. For example, the rocket engine seal life just a few minutes, but require a short period of time is not leak. The host on the ships sometimes in order to obtain a more effective space, to seal the size and location of the often very strict request, and as the drainage pump on the submarine, in the process of submarine ups and downs, large variations in pressure, etc. In these cases, there will be no structure by conventional selection standard, and must make a special design of specific conditions, and take necessary auxiliary measures.