Simulation of Liquor Blow, Thrust Force, and Risk of Condensation Hydraulic Shock in a Pulp-Digester Piping System

Abstract

This study is an industrial case study for the application of a validated flashing and hydraulic shock modeling approach to the safety and design of a reactor blow line. The maximum flowrate is important for sizing of downstream components. The high pressure of the blow and flashing of the liquid can result in significant forces on pipe bends and other geometrical features. Analysis and prediction of such forces are of importance for the structural design and anchoring of the piping. Another concern for a liquid blow under high pressure is the potential for condensation-induced hydraulic shock. The collapse of the flashed vapor to the liquid phase creating shock waves of large amplitudes is a serious safety concern. The computational fluid dynamics model used the homogeneous mixture model with a flashing model for phase change of the fluid. The properties of the fluid were defined by a custom function which interpolated between tabulated values of the thermodynamic and transport properties. The simulations predicted the occurrence of a condensation hydraulic shock when the blow down is initiated with empty pipes and demonstrated that a hydraulic shock could be prevented with liquid-filled condition. The pipework geometry was also optimized to reduce the forces acting at the junctions. The vapor quality at the outlet as a result of flashing was estimated, which is necessary for the design of downstream systems.