TABLE OF CONTENTS As components of a pressure system, pressure relief valves have been treated in Chapter 12. In this section some further consideration is given in respect of the flow through such devices. Dispersion of the discharge is considered in Section 15.20 and, for dense gases in Section 15.43.
Relations for single-phase flow of gases and liquids have been presented in Section 15.1. Formulae derived from these are given in the codes for pressure relief. The API and BS formulae are stated in Chapter 12.
Two-phase vapour liquid flow has been discussed in Sections 15.2 15.4, from which it will be apparent that this is a much more complex topic. The various models for two-phase flow do not lend themselves to formulae for calculating the area of the pressure relief valve orifice that are comparable in simplicity with those for single-phase flow. Moreover, with two-phase flow the determination of the flow through the valve orifice by no means exhausts the problem. The flows in the inlet and outlet lines are also complex.
The treatment here deals primarily with flow through the valve orifice and is confined to an account of the widely used American Petroleum Institute (API) method and of a method given by Leung which has gained some acceptance, together with a overview of the wider aspects of the design.
API RP 520 Part 1: 1990 gives the following method for sizing a two-phase gas liquid relief. It describes...
