The selection of mechanical seal structure type is an important step in the design process. It must be investigated first: 1) working parameters - medium pressure, temperature, shaft diameter and speed. (2) Medium properties - concentration, viscosity, corrosiveness, presence or absence of solid particles and fibre impurities, whether it is easy to vaporize or crystallize, etc. (3) Main engine working characteristics and environmental conditions - continuous or intermittent operation; main engine installed in indoor or open air; ambient atmosphere properties and temperature changes. (4) Requirements for permissible leakage, leakage direction (internal or external leakage) and service life and reliability of the main engine. The restriction of the main engine on the size of the sealing structure. _Stability of operation and production process.

1. Selection according to working parameters p, V and t

Here P refers to the medium pressure in the sealing chamber. According to the value of p, the balance structure and the balance degree can be initially determined. For medium with high viscosity and good lubricity, P < 0.8MPa, or medium with low viscosity and poor lubricity, P < 0.5MPa, the unbalanced structure is usually used. When the value of P exceeds the above range, the balanced structure should be considered. When P > 15MPa, it is difficult to meet the sealing requirements of the general single-end balanced structure. At this time, the series multi-end seal can be chosen.

_refers to the circumferential velocity of the average diameter of the sealing surface. According to the magnitude of_value, it is determined whether the elastic element rotates with the axis, i.e. spring rotating or spring stationary structure is adopted. Generally, spring rotating type can be adopted when_is less than 20-30m/s. Under the condition of higher speed, strong vibration is easily caused by the unbalanced mass of the rotating part, and it is best to adopt spring rotating type. Spring static structure is adopted. If the values of P and_are both high, hydrodynamic pressure structure can be considered.

T refers to the medium temperature in the sealing chamber. The material of the auxiliary sealing ring, the cooling method of the sealing surface and the auxiliary system are determined according to the size of T. The O-ring of NBR is usually used as auxiliary sealing ring in the temperature range of 0-80 (?) C; - 50 (?) T < 150 (?) C. According to the corrosiveness of the medium, fluororubber, silicone rubber or polytetrafluoroethylene (PTFE) can be used as filler sealing ring. When the temperature is below-50 or t (?) 150 (?), the rubber and PTFE will produce brittle cracks at low temperature or aging at high temperature. At this time, metal bellows can be used. When the medium turbidity is higher than 80 C, the high temperature is usually considered in the field of sealing, and corresponding cooling measures must be taken at this time.

2. Selection according to dielectric properties

For the medium with weak corrosiveness, the built-in mechanical seal is usually used. The stress state of the end face and the direction of leakage of the medium are more reasonable than those of the external seal. For highly corrosive media, because of the difficulty in selecting materials for springs, external or PTFE bellows mechanical seals can be used, but generally only in the range of P < 0.2-0.3 MPa.

The medium with easy crystallization, solidification and high viscosity should adopt large spring rotating structure. Because small springs are easily blocked by solids, high viscosity media will hinder the axial compensation movement of small springs.

Flammable, explosive and toxic media, in order to ensure that the media do not leak, should adopt a double-end structure with sealing fluid (isolating fluid).

The structure selected according to the above working parameters and medium characteristics is often only a preliminary scheme, and the final determination must also take into account the characteristics of the main engine and some special requirements for sealing. For example, the sealing life of rocket engine only takes a few minutes, but it requires absolute leakage in a short time. Sometimes, in order to obtain more effective space, the main engine on the ship often puts forward very stringent requirements for the size and installation position of the seal, such as the drainage pump on the submarine, the pressure changes greatly during the submarine sinking and floating. In these cases, the standard structure can not be selected according to the conventional method, but special design must be made for specific working conditions, and necessary auxiliary measures must be taken at the same time.