Safety relief valve vents in power plants are typically sized by treating steam flow as a perfect gas flow. ANSI B31.1 [1] and an ASME paper by G. S. Liao [2] provided the power industry with the guidance for using this approximation. The perfect gas approximation of the steam flow furnished the engineers with the simple formulations currently available in References 1 and 2. Modern computational techniques allow the accuracy of vent sizing calculations to be improved by eliminating the perfect gas assumption. The following methodology uses ASME steam property tables [3] assuming adiabatic flow through one or more constant area pipe sections connected by reducers. Compressible flow Fanno-Line differential equations for continuity, momentum, energy, and state [4] are generalized to handle the steam thermodynamic properties. These equations are simultaneously solved numerically to determine the flow thermodynamic state at any location along the vent. The differential equations are integrated utilizing steam thermodynamic functions [3], in conjunction with the Math-Cad’s Runga-Kutta method [5]. Both choking and non-choking conditions are considered at the vent exit. Closed discharge backpressures and open discharge blowback are addressed. Amendment to the ASME ANSI B31.1 Power Piping Code [1], to include the differential equations formulation for the constant area adiabatic steam flow is also proposed.

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