This paper is concerned with the computation of the theoretical critical flow of dry saturated steam through passages over a range of 1 psia (7 kPa) to the critical pressure of 3208.2 psia (22.12 MPa). Two computational methods are used: a theoretical method using ideal gas relations, and a flow maximization method using actual saturated steam properties. An equation is developed and based on the theoretical equation that yields flow rates that have an average deviation of 0.1 percent and a maximum deviation of 0.3 percent from the flow rate found by flow maximization. It is also demonstrated that Napier’s equation currently recommended by PTC 25.3-1976 “Safety and Relief Valves” is unsatisfactory for the calculation of theoretical critical flow rates.

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