Easily size relief devices and piping for two-phase flow

This article will provide practicing engineers with a unified methodology for sizing relief devices--both rupture disks (RDs) and pressure relief valves (PRVs)--and associated relief piping for two-phase flow. Sizing methods for two-phase flow, in particular flashing flow, have been lacking and are often misunderstood. The American Petroleum Institute (API) method for flashing flow is probably the most widely used in the chemical process industries (CPI), but it lacks both a sound theoretical basis and validated experimental data. Despite its origin dating back to the 1950s and its popularity over the years, this method is regarded as obsolete and can lead to undersizing of relief devices. The present approach is based on the classical homogeneous equilibrium model (HEM) that assumes equal velocities (or no slip) and thermal equilibrium between both phases. HEM is favored by AIChE`s Design Institute for Emergency Relief Systems (DIERS) for emergency relief sizing design. Furthermore, the flow capacity curve for saturated water discharge in the current ASME pressure vessel code is in close agreement with the HEM prediction. This article will: present a unified treatment for sizing PRVs and RDs in two-phase service, covering flashing as well as nonflashing two-phase flow; emphasize the pencil-and-paper approach for amore » better understanding of the theory and method, although the underlying equations and formulae can be easily programmed into handheld calculators or laptop computers; provide applications and worked examples to illustrate the sizing methodology; and reduce to the API gas-sizing and liquid-sizing methods in the limits.« less