The calculation of the mass flow rate through throttling devices is difficult when handling two-phase flow, especially when boiling liquids flow into these fittings. Control valves and orifices are typically oversized in industry and the control range of those valves often does not fit the control requirements. In this paper the HNE-DS method is proposed for the sizing of control valves, orifices, and nozzles in two phase flow. It extends the omega-method, originally developed by Leung, by adding a boiling delay coefficient to include the degree of thermodynamic nonequilibrium at the start of the nucleation of small, vapor mass fractions upstream of the fitting. The additional introduction of a slip correction factor, to take account of hydrodynamic nonequilibrium (slip), also makes it possible to calculate reliably the flow rate through control valves and orifices in both flashing and nonflashing,flow. In Part 2 the HNE-DS method for short nozzles, orifices, and control valves is considered. Part 1 describes the sizing of safety valves using the same method. Additionally, the derivation of the HNE-DS model is given there in detail. The predictive accuracy of the HNE-DS model has been checked with reference to more than 1300 sets of experimental data. (C) 2005 American Institute of Chemical Engineers.