Old Method
API 520 Part I (9th edition) selected three (3) methods to size two-phase PRV. One of them is the HDI (Homogeneous Direct Integration) method described in section C.2.1. HDI method involves generating multiple data points over an isentropic range of pressure from the inlet to the discharge. These data are used to evaluate the mass flux integral by direct numerical integration.
New Easier Method
An easier HD (Homogeneous Direct) method without integration, yet producing identical mass flux, is presented. The HD (Homogeneous Direct) method compares the velocity of fluid and the velocity of sound along the isentropic expansion path. The choke point is determined when the velocity of fluid equals the velocity of sound. The mass flux is then simply calculated by multiplying mass density and the velocity of fluid at the choke point. A detailed example is given in the presentation. The HD method is compared against the established data in “Benchmarking of two-phase flow through safety relief valves and pipes” by Shawn Adair and Harold Fisher. The mass flow rate through a 4P6 relief valve by HD method is well within the reasonable range, among other established methods.
