A Simplified Model for Two Phase Face Seal Design

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Abstract
Subcooling is a critical factor in determining the stability of face seals. A simplified quasi-isothermal low leakage laminar model is presented to analyze the stiffness and stability characteristics of two phase face seals with real fluids. Sample calculation results for low leakage operations using the laminar model are presented along with those for high leakage operations using an adiabatic turbulent model. The seal characteristics predicted using the two extreme models tend to overlap with each other, giving confidence that the model may be a useful tool for seal design. It is shown that as the sealed liquid approaches saturation there is a critical temperature where the stiffness coefficient becomes negative and may lead to instability. However, depending on the seal balance, a seal may still pop open and fail at a temperature lower than this critical value. The effects of coning are also studied and it is shown that even a small amount of coning can have a dramatic effect on seal behavior. For the same balance, a coned seal has a higher leakage rate than a parallel face seed. The simplified method of face seal analysis presented here is intended to serve as a tool for the seal designer to enable a better choice of the seal balance ratio to suit the anticipated operating conditions.