Study On Dynamic Characteristics Of Mechanical Seal For Static Pressure Gas Lubrication

The traditional gas film mechanical seal is primarily applied in high-speed rotating machinery such as centrifugal compressors. In the condition of high speed, the seals have a strong dynamic pressure effect; large opening force and high film stiffness can be obtained to achieve stable running. But at low speed, the dynamic pressure effect will be so weak that the seals are difficult to achieve a good seal. Aerostatic gas lubricated mechanical seal is a new mechanical seal technology which is suitable for low speed conditions because of its advantages with low friction loss, low heat generation, large opening force and high gas film stiffness at low speed operation situation. In this paper, the steady state characteristics, dynamic characteristics and kinematic characteristics of aerostatic gas lubricated mechanical seal have been investigated to provide useful reference to the designing and application of the seal.On the steady state characteristics, a series of derivation about the steady state parameters of aerostatic gas lubricated mechanical seal which were based on N-S equations had been made by the analytical method. An analytical expression of the gas film stiffness was obtained and verified with the reference. The finite difference method (FDM) was adopted to discrete the steady Reynolds Equation and compared with the calculated results of the analytical method. The influences of orifice diameter, process gas pressure and supplying gas pressure on the gas stiffness and the leakage of aerostatic gas lubricated mechanical seal were analyzed. The results showed that choosing smaller orifice diameter could get larger gas film stiffness and smaller leakage as to the small working gas thickness situation; the big thickness situation was opposite. Improving supplying gas pressure and reducing process gas pressure are effective to increase the gas stiffness, while the leakage to the process fluid was reducing.On the dynamic characteristics, the dynamic Reynolds Equations of aerostatic gas lubricated mechanical seal which under the condition of three degrees of freedom perturbation were derived by small perturbation method in detail. Relevant differential iterative scheme were given and some examples were checked the scheme. The effects of squeeze number, frequency number, orifice diameter, groove depth and supplying gas pressure on dynamic characteristic parameters of aerostatic gas lubricated mechanical seal were analyzed. The results showed that squeeze number, frequency number, orifice diameter, groove depth and supplying gas pressure influence the axial stiffness and damping of aerostatic gas lubricated mechanical seal significantly. Angle crossed stiffness and damping corresponding to opposite number relationship.On the kinematic characteristics, a kinematic analysis model of aerostatic gas lubricated mechanical seal which considered three degrees of freedom perturbation on stationary ring and rotor’s axial movement was established. To obtain the expressions of the seal rings axial, angular and end face gas film thickness displacement response, the kinematic equations had been solved by the complex modal theory. The influence of frequency number, orifice diameter, supplying gas pressure and the mass of the seal rings on aerostatic gas lubricated mechanical seal’s kinematic characteristics were studied. The results showed that increasing frequency number, orifice diameter, supplying gas pressure are all reducing the amplitude of the seal rings axial, angular and end face gas film thickness displacement response. Frequency number would affect the vibration frequency of dual-mass system significantly.