Seal selection parameters: medium pressure, temperature, shaft diameter and speed.
The selection of mechanical seal structure type is an important step in the design process, and must be investigated first: (1) working parameters - medium pressure, temperature, shaft diameter and speed. (2) medium characteristics a concentration, viscosity, corrosion, whether there are solid particles and fiber impurities, whether easy to vaporize or crystallize. ③ The host working characteristics and environmental conditions of a continuous or intermittent operation: the host installed in the indoor or outdoor: the nature of the surrounding atmosphere and temperature change. (4) the host to seal the allowable leakage, leakage direction (leakage or leakage) requirements: life and reliability requirements. (5) the limit of the sealing structure size of the host. ⑥ Stability of operation and production process.
1, according to the working parameters P, V, T selection
Here P refers to the medium pressure in the mechanical seal chamber. According to the size of P value, we can preliminarily determine whether to choose the balanced structure and the balance degree. For medium viscosity is high, good lubricity, P ≤0.8mpa, or low viscosity, poor lubricity medium, P ≤0.5mpa, usually choose the unbalanced structure. When the P value exceeds the above range, the balanced structure should be considered. When P ≥15mpa, the general single-end balanced structure is difficult to meet the sealing requirements, so the series multi-end seal can be used.
V refers to the mechanical seal sealing surface average diameter of circumferential velocity, according to the size of the v value to determine whether elastic element with the axis of rotation, namely USES the bullet rotary or spring stationary type structure, general v < 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 static storage structure. If the values of p and V are high, fluid dynamic pressure structure can be considered.
T refers to the temperature of the medium in the mechanical seal cavity. According to the size of T, the material of the auxiliary sealing ring, the cooling method of the sealing surface and its auxiliary system are determined. Temperature t in the range of 0~80 deG C, the auxiliary sealing ring is usually selected nitrile rubber 0 sealing ring: -50 deG C ≤ T <150 DEG C, according to the corrosive strength of the medium, fluorine rubber, silicone rubber or ptfe molding packing sealing ring can be selected: When the temperature is less than -50℃ or T ≥150℃, rubber and ptfe will produce low temperature brittle cracking or high temperature aging, and metal bellows can be used at this time. When the temperature of the medium is higher than 0℃, it is usually considered as high temperature in the sealing field, and corresponding cooling measures must be taken at this time.
2, according to the characteristics of the medium selection
Corrosion is weak quality, usually choose the built-in mechanical seal, its end face force state and medium leakage direction are reasonable than the external type. For strong corrosive media, due to the difficult selection of spring material, external or ptfe bellows mechanical seal can be selected, but generally only applicable to the range of P ≤0.2-0.3mpa.
Easy to crystallize, easy to solidify and high viscosity medium, should adopt large spring rotary structure mechanical seal. Because the small spring is easy to be blocked by solids, high viscosity medium will make the small spring axial compensation movement blocked.
Flammable, explosive, toxic medium, in order to ensure that the medium does not leak, should use sealing liquid (isolation liquid) double end structure mechanical seal.
According to the above working parameters and medium characteristics of the selected structure is often only a preliminary scheme, the final determination must also consider the characteristics of mechanical sealing host and some special requirements for sealing. For example, the sealing life of a rocket engine is only a few minutes, but it is absolutely leakproof for a short period of time. In order to obtain more effective space, the main engine on the ship sometimes puts forward very strict requirements on the size and installation position of the mechanical seal, such as the drainage pump on the submarine, the pressure varies greatly in the process of diving and floating. In these cases, the standard structure cannot be selected as a general rule, but a special design must be made for specific conditions, and necessary auxiliary measures must be taken.