Mechanical seal structure type selection is an important step in the design aspects must be investigated: the operating parameters - media pressure, temperature, shaft diameter and speed. Media properties - density, viscosity, corrosive, with or without solid particles and fibers impurities are easily vaporized or crystallization. Host work characteristics and environmental conditions - continuous or batch operation; host installation in indoor or outdoor; atmosphere surrounding the nature and temperature changes.
Host allowable leakage sealing, leak direction (internal leakage or leakage) requirements; life and reliability requirements. Host restrictions on seal structure size. Stability operations and production processes.

1 According to the operating parameters p, v, t Selection
Here p refers to the sealed chamber at medium pressure, can initially determine whether to select a balanced structure and degree of balance based on the size of the p values. For medium viscosity, lubricity good, p≤0.8MPa, or low viscosity, lubricity, poor media, p≤0.5MPa, the usual selection of non-equilibrium structure. When the p value exceeds the above range, you should consider using balanced structure. When p> 15MPa, the general structure of single-end balance is difficult to achieve sealing requirements, then you can use multiple tandem seal.
υ is the peripheral speed of the mean diameter of the sealing surface, according to the size of the υ values ​​to determine whether the elastic member rotating with the shaft, i.e., using a spring or spring-type rotary static Structure, υ <20 ~ 30m / s can be a spring of a rotary type conditions at higher speeds, due to unbalanced mass of rotating parts can lead to strong vibration, preferably spring stationary structure. If p and υ values ​​are high, you can consider using hydrodynamic structure.
t is the temperature of the medium inside the sealed cavity, the size of t is determined according to the material of the secondary seal, the cooling method and the sealing surface of the auxiliary systems. Temperature t in the range 0 ~ 80 ℃, secondary seal is usually used nitrile rubber O-ring; -50 ℃ ≤t <150 ℃, according to media corrosion strength, the choice of fluorine rubber, silicone rubber or PTFE ethylene forming filler ring: temperature <-50 or t≥150 ℃, the rubber and PTFE will produce low or high temperature aging embrittlement, this time can be metal bellows structure. When the media turbidity higher than 80 ℃, usually in sealed areas should be considered by the high temperature, then must take appropriate measures to cool.

The dielectric properties of Selection
Weak corrosive medium, usually used built-in mechanical seals, end face the stress state and the direction of media leaks than the external reasonable. For corrosive media, due to the more difficult spring selection, the choice of external or PTFE bellows mechanical seal, but generally only applicable within the scope of p≤0.2 ~ 0.3MPa.
Easy to crystallize, easy solidification and high viscosity media, should be using a large spring rotating structure. Because of the small spring easily blocked solids, high viscosity media will compensate small spring axial movement blocked.
Flammable, explosive, toxic media, in order to ensure that the media does not leak, should have sealed using liquid (liquid separation) of the double-end structure.
Above operating parameters and dielectric properties selected structures are often only a preliminary plan, and ultimately determine the host must also consider the characteristics and sealed some special requirements. For example, the rocket engine seal life just a few minutes, but requires a short time absolutely no leakage. Sometimes on the host ship in order to obtain a more efficient space, the size of the seal and mounting position are often made ​​very demanding, and if the drainage pump submarines, submarine ups and downs in the process, a lot of pressure change range and so on. In these cases, it is not the usual selection criteria structure, and must be specifically designed for special conditions, while taking the necessary complementary measures.