How is the end-face specific pressure of a mechanical seal calculated?

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Selection of the face pressure for mechanical seals: The force per unit area acting on the sealing faces is known as the face pressure. To ensure that mechanical seals have a long service life and good sealing performance, it is necessary to select an appropriate value for this face pressure. The principles are as follows: 1. To keep the sealing faces in close contact at all times, the face pressure must be positive, that is, greater than zero. 2. The end-face specific pressure must be greater than the vapor pressure of the material at the sealed end face. 3. The end-face specific pressure is an important factor determining the presence of a liquid film between the end faces; it should not be too high to prevent the evaporation of the liquid film and increased wear. However, from the perspective of leakage, it should not be too small either, to prevent a decline in sealing performance. In pump mechanical seals, for internal-mounted mechanical seals, the face pressure is generally set at 0.3–0.5 MPa ； The face pressure of external mechanical seals is generally set at 0.5–0.4 MPa ； For media with high viscosity, the end-face specific pressure can be increased appropriately; generally, a value of 0.5–0.7 MPa is chosen. For materials with poor lubricity and high volatility, a lower specific pressure is used, typically 0.25–0.45 MPa. The end-face specific pressure for external mechanical seals used in reaction vessels is slightly higher than that used in pumps. Selection of spring specific pressure: The force exerted by the spring per unit area on the sealing surface is known as the spring specific pressure, and it constitutes an important part of the surface specific pressure. The function of the spring specific pressure is to maintain a certain end-face specific pressure even when the medium pressure is low or fluctuates, thereby preventing leakage of the medium. 1. The selection of spring specific pressure is based on the average linear velocity at the end face. For high-speed mechanical seals, that is, when the average linear velocity at the end faces is greater than 30 m/s, the spring compression ratio is set at (0.5 – 2.0) × 9.8 × 10^4 Pa. For medium-speed mechanical seals, that is, when the average linear velocity at the end faces is between 10 and 30 m/s, the spring compression ratio is set at (1.5 – 3) × 9.8 × 10^4 Pa. As for low-speed mechanical seals, that is, when the average linear velocity at the end faces is less than 10 m/s, the spring compression ratio is set at (1.5 – 6) × 9.8 × 10^4 Pa. Based on current usage, the spring compression should not be too high; excessive compression will lead to excessively high temperatures at the sealing surfaces, thereby accelerating wear. According to relevant information, the spring specific pressure for unbalanced mechanical seals used in pumps is 0.08 MPa, 0.15 MPa for balanced types, while 0.22 MPa is more suitable for those used in reactors. The spring preload of mechanical seals used in reactors is set at a higher value due to the operating characteristics of such reactors: low rotation speeds, a gaseous medium, significant shaft oscillation, as well as unstable operating pressures, temperatures, and phase conditions.