Mechanical seal structure type selection is an important step in the design process, must be the first investigation of mechanical seals: work parameters - media pressure, temperature, shaft diameter and speed. The characteristics of the medium - concentration, viscosity, corrosion, there is no solid particles and fiber impurities, whether it is easy to vaporization or crystallization, etc.. Host operating characteristics and environmental conditions, continuous or intermittent operation; the host installed in the indoor or open air; the nature of the surrounding atmosphere and temperature changes, etc.. The host of the seal allows leakage, leakage direction (internal leakage or leakage) required life and reliability requirements. The restrictions on the size of the sealing structure of the host. The stability of the operation and production process.
1 according to the working parameters p, V, T type selection
Here P refers to the seal chamber of the media pressure, according to the size of the p value can be determined initially to determine whether the pump mechanical seal to choose the balance of the structure and the degree of balance. For the medium high viscosity, good lubrication, P = 0.8MPa, or low viscosity, poor lubrication medium, P = 0.5MPa, usually choose non balanced structure. When the p value exceeds the above range, the balanced structure should be considered. When p > 15MPa, the general single end balance structure is difficult to achieve the sealing requirements, at this time can use a series of multi end seal.
V refers to the circumferential speed of the average diameter of the sealing surface, according to the principles determined whether the elastic element rotates along with the shaft, the rotary spring or spring static structure, general V < 20 ~ 30m/s with rotary spring, higher speed conditions, because of the rotating parts unbalance caused by strong vibration, the best use of the static structure of spring. If P and V values are high, consider using a hydrodynamic structure.
T is the seal cavity of the medium temperature, according to the size of the T to determine the auxiliary seal ring material, the sealing surface of the cooling method and its auxiliary system. The temperature T at 0 to 80 DEG C, auxiliary seals usually choose nitrile rubber O ring; -50 < T < 150 DEG C, according to the corrosive medium strength, can choose fluorine rubber, silicone rubber or PTFE packing seal: the temperature of less than -50 or T less than 150 DEG C, rubber and poly tetrafluoroethylene will produce low temperature cracking or high temperature aging, the use of metal bellows structure. When the turbidity of the medium is higher than 80, it is usually considered to be at high temperature in the field of sealing, and the corresponding cooling measures must be taken at this time.
2 according to the characteristics of the medium selection
Weak corrosive medium, usually use a built-in mechanical seal, the end force of the state and the media leakage direction than the external type. For the strong corrosive medium, because the spring material is difficult, can use external or PTFE bellows type mechanical seal, but generally only applicable range of P ~ 0.3MPa in less than 0.2.
Yi Jiejing, easy to coagulation and high viscosity of the media, should adopt a large spring rotating structure. Because the small spring is likely to be blocked by solid material, high viscosity medium will make the small spring axial compensation movement blocked.
Flammable, explosive, toxic media, in order to ensure that the media is nothing more than leakage, should be used to seal the liquid (isolation liquid) of the double end structure.
According to the above mentioned work parameters and the structure of the selected medium is usually only a preliminary program, the final determination must also consider the characteristics of the host and some special requirements of the seal. For example, the rocket engine seal life is only a few minutes, but requires a short period of time is absolutely not leak. Sometimes the host vessels in order to obtain a more effective space, the size of the seal and installation of the proposed location is often very demanding, and as the drainage pump on the submarine, the submarine in the process of ups and downs, the magnitude of the pressure change. Under these circumstances, it is not necessary to choose standard structure according to the conventional choice, but must make special design for the specific working conditions, and take necessary auxiliary measures.