A saturated liquid-vapor mixture discharging from a pressurized thermal-insulated vessel through a nozzle or a valve is considered as an unsteady-flow problem. It is shown that in the case of relatively small energy losses a valve may be modeled as a converging nozzle. After losses exceed certain value a condition of constant enthalpy typical for throttling devices is applicable to a valve. On the basis of such consideration, the step by step calculation method for small time intervals or pressure drops is developed. The proposed approach allows obtaining all characteristics of the discharging process as functions of time. The expression defining the speed of sound in the case of discharging of wet vapor is derived. A special technique is proposed to calculate and correct the usually recommended value of critical pressure ratio. A numerical example of calculation is provided. To estimate the accuracy and limits of the applicability of the results presented in the paper a comparison with corresponding experimental data is needed.

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