A review of status is presented, discussing hydraulic power recovery of fluid streams expanding with or without gas evolution. Point of departure for the treatise given, is embedded in basic thermodynamics and fluid mechanics. Formulae for estimating isentropic power recovery are derived, which hold asymptotically for low gas to liquid mass fraction. These formulae provide simple means for calculating power recovery with gas evolution, given a certain vapor void fraction. The key element in the entire approach is to attribute the extra power recovery due to gas evolution to the change in density of the fluid stream. The formulae and discussions presented in the paper may assist the (application) engineer in evaluating the effect of vapor evolution with regard to hydraulic performance of hydraulic power recovery turbines. The paper further also discusses the condition of (runaway) overspeed, as this should not be omitted from a review on basic considerations for hydraulic power recovery turbines. Formulae and calculation methods are given for calculating runaway speed, choke flow and time to runaway speed.

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