A mixed friction hydrostatic mechanical face seal model is presented. Load support and friction due to mechanical contact and the effect of a phase change are considered. The results show that a phase change within the seal interface leads to a greater fraction of the load being supported by fluid film pressure than for an all liquid or all gas phase seal. As seal operating temperature approaches the boiling point of the sealed fluid, very high leakage is predicted. An explanation for puffing is offered. The effect of various design parameters on two phase seal operation is examined. On a theoretical basis, operation at a higher temperature reduces seal wear rate and friction. The model can be used by seal designers to predict instability and performance for a two phase seal.

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