First, understand the classification of mechanical seals
According to the contact state of the seal end face, the mechanical seal can be divided into two types: contact type and non-contact type, among which the contact type seal is widely used. Contact-type mechanical seals are widely used in single-face seals and double-face seals. Compared with the single-end seal, the double-face seal adds a pair of friction pairs, and is generally used for equipment with higher sealing levels.
According to the balance coefficient, mechanical seals are divided into unbalanced type (applicable pressure ≤ 0.8 MPa), incomplete balance type (commonly known as balanced type), and fully balanced type (generally applicable pressure > 0.8 MPa). The incompletely balanced type is widely used, which can balance the effect of a part of the liquid pressure on the end face, can improve the wear of the end face, and has a large bearing capacity; the fully balanced type is rarely used.
Second, the selection needs to be clear
When selecting mechanical seals, it is necessary to investigate the type of the main engine and the parts that need to be sealed, the characteristics of the medium (such as viscosity, pH value, whether it is corrosive, whether there are suspended particles and fiber impurities, whether it is easy to vaporize or crystallize, etc.), and the pressure of the sealing cavity. , fluid temperature, linear speed, shaft vibration and deflection, shaft diameter, allowable effective installation space, allowable leakage, leakage direction requirements, support system conditions, etc. The level of mechanical seal technology is closely related to the sealing material. The selection of mechanical seal should pay special attention to the selection of friction pair material, compensation original material, auxiliary sealing ring material and other component materials. (Related reading: Principles and parameters of mechanical seal selection)
Third, the properties of the material
When selecting the material of the mechanical seal friction pair group, pay attention to the material of the group pair should have wear resistance, corrosion resistance, good heat resistance and thermal conductivity, mechanical strength, small friction coefficient, air tightness, and compatibility with the group material. sex, etc. A certain emergency pump mainly used for emergency drainage and long-distance large-flow water supply, the material of the machine seal group is nickel-chromium-based cemented carbide.
The material of the auxiliary sealing ring should be able to adapt to the characteristics of the medium, with good aging resistance, high temperature resistance, low temperature resistance, appropriate permanent deformation, anti-knock performance under high pressure, and economical efficiency should be considered under the conditions of use. . The elastic element material should have high strength, medium corrosion resistance, fatigue resistance, and long-term working elasticity will not be reduced.
Fourth, the design of mechanical seal
The complete mechanical seal design includes the mechanical seal body and the support system, both of which are equally important. The mechanical seal support system mainly includes flushing, cooling, filtering and other components. The flushing and cooling medium of the machine seal can be the circulating cooling water introduced from the outlet side of the emergency pump or the lubricating oil in the sealing cavity. When using circulating cooling water as a flushing and cooling medium, the content of impurities or suspended particles in the circulating cooling water should be controlled from the pump body flow channel and the structure of the pump. When the cooling medium is used, the oil cavity should be sealed with the help of other sealing types, and the oil cavity should be cleaned before adding oil.
In the design calculation of the mechanical seal, the specific pressure Pc of the end face and the specific pressure Ps of the spring should be considered for the calculation of the structure size, the calculation of the geometric size, and the calculation of the friction power of the mechanical seal, and the Rcv value of the selected friction pair group to the material should be checked.
The mechanical seal seal design has the matching tolerance of the inner diameter of the moving ring and the pump shaft, the outer diameter of the static ring and the matching hole of the pump cover, the radial circular runout tolerance of the pump shaft in the installation section of the mechanical seal, the dimensional tolerance of the outer diameter of the pump shaft and the roughness Ra, Mechanical seal installation dimensions have high requirements, and these key dimensions must be processed in strict accordance with tolerance requirements.