Face Seal Balance Ratio Selection For Two Phase Single And Multicomponent Mixtures
It is well known that the fluid pressure distribution between the faces of a mechanical seal greatly affects seal performance. In cases where a sealed fluid changes phase from liquid to vapor as it leaks across the seal faces, the load support derived from fluid pressure can be considerably greater than for the all liquid case. Furthermore, unlike for all liquid seals, this load support changes with seal temperature. Thus, using seals with balance ratios selected based on liquid applications can result in excessive leakage. On the other hand, the selection of balance ratio at too high a level can cause excessive wear of the seal faces by forcing all vapor operation. Thus, balance ratio selection is critical to the successful operation of a two phase seal. The question of how to select balance ratio for two-phase applications has not been clearly resolved. A simplified method of balance ratio selection for two phase applications is presented herein. By assuming isothermal faces, it is shown how the equilibrium face temperature can be calculated by iterative methods. Equations are presented which are used to calculate critical balance ratio. There is a minimum critical balance ratio and maximum critical balance ratio corresponding to the two undesirable operating conditions above. The method is applied first for the case of water where it is shown how a balance ratio selection graph can be constructed which relates critical balance ratio to pressure and temperature. One graph covers applications over all temperatures and several pressure values. The technique is then extended to a more typical refinery product stream, a two component solution of propane and methane. The behavior of this solution in a two phase seal application is somewhat different than the single component. Studies are made which show how critical balance ratio is affected by changes in design, specifically face width, temperature coefficient, secondary seal type, and radial taper. The results illustrate the sensitivity of critical balance ratio to both design parameters and to operating conditions. In conclusion, it is shown that knowledge of the product stream, coupled with calculations of critical balance ratio as outlined herein, can lead to a rational choice for seal balance ratio.
Lebeck, Alan O. (1988). Face Seal Balance Ratio Selection For Two Phase Single And Multicomponent Mixtures. Turbomachinery Laboratories, Department of Mechanical Engineering, Texas A&M University. Available electronically from