The Effect of Dissolved and Entrained Gases on Flow Through a Process Control Valve

Abstract

Process fluids flowing through control valves undergo thermodynamic throttling that induces a decrease in the mean fluid pressure. Substantial pressure reduction can result in the formation of a stable, secondary compressible phase within the liquid continuum. Without distinction this phenomenon is generally referred to as flashing, and may have a significant impact on the magnitude of flow through the control valve. The formation of the compressible phase may be the result of two different underlying sources: vaporization, wherein a small portion of the liquid undergoes a phase change from liquid to vapor; or out-gassing, wherein dissolved or entrained gases come out of solution. The industry standard equations used to determine an appropriate process control valve size include an adjustment for the effect of vaporization. Implicit in this method is the assumption that the fluid is vaporizing according to a corresponding states model. This paper distinguishes between the two compressible phase forming mechanisms and derives a comparable methodology that is appropriate to the out-gassing process. A revised prediction of the choked or limiting flow pressure differential is presented and density corrections are incorporated into the industry standard methodology.