Transient analysis of two-phase flow during blow down of a pipeline carrying flashing liquids

Abstract

In this study, the controlled or uncontrolled release of a vapor-liquid mixture during the blowdown of a pipeline carrying a flashing liquid has been numerically simulated. The liquid may be pure such as propane or a mixture of two or more components, such as liquefied petroleum gas (LPG). With the help of a mathematical model, transient pressure and temperature profiles, quality of a 2-phase vapor-liquid mixture and the amount of the liquid remaining in the pipe during a typical blowdown are predicted. The theoretical analysis based on the assumption of a homogeneous two-phase flow (no slip condition between phases) allows for external heat transfer from the ambient to the liquid stored in the pipe. The simulation results show that the sudden depressurization of the liquid stored under supersaturated conditions may result in rapid cooling of the liquid possibly to the temperature below the ductile brittle transition temperature (DBTT) of the material of construction of the pipe. This may be hazardous as the pipe may rupture at weak locations along the length. The simulation results also show significant effects of ambient temperature, use of a low vapor pressure liquid as an additive, and pipewall roughness on total blowdown time of a long pipe. The present study is important from the point-of-view of maintenance and operation of pipelines carrying flashing liquids, especially in chemical industries and offshore oil and gas exploration operations. The model simulation results have been validated, wherever possible, with those published in literature.