by Robert A. Sadowski, P.E. / May 01, 2019

The increased emphasis on safety in the petrochemical industries has produced greater sophistication in the analysis of overpressure concerns. One area where this is particularly evident is in the study of two-phase flow in pressure relief devices (PRDs). Two-phase flow can develop in pressure vessel systems requiring overpressure protection. Of particular concern are situations involving the flashing of liquids through PRDs because the resulting volumetric flowrate will be much more than if the fluid remained in a liquid phase. Not applying proper sizing techniques can result in undersized relief valves, thus resulting in overpressuring equipment. Two-phase PRD sizing calculations are more complex than the ideal-nozzle concepts employed for single-phase gasses and liquids. Over the years, many techniques have been proposed to estimate two-phase flow, each of which may be based on a specific set of assumptions that are valid for certain specific conditions but may not be accurate for others. Proper sizing of a PRD requires knowledge of the conditions upstream and downstream of the device, the physical and thermal properties of the fluids, and a model that accurately predicts the mass flux through the device. API Standard 520, Part I, Annex C presents three distinct methods for sizing the required area of a relief device according to the fluid conditions entering the relief device.

  • The Homogeneous Direct Integration Method
  • The Two-Point Omega Method
  • The Omega Method for Subcooled Liquids

These methods should be applicable for the majority of applications encountered in the refining and petrochemical industries. This article will provide an overview of the API STD 520 methodologies and will assist the user in choosing and utilizing the proper method for determining the required relief device area for the fluid conditions encountered.