Investigation and Modeling of the Single- and Two-Phase Flow in High-Pressure Relief Systems

Relief devices for high-pressure systems are mostly calculated using empirical methods as outlined in the API 520 [1], which often neglect the process conditions of the mixture. The sizing of these relief devices uses a discharge coefficient that acts as a correction factor to account for flow frictions, valve geometry factors, and other things that result in a lower relief flow than theoretically expected. These values are determined by manufacturers using non-reactive substances under standard conditions. The present work is intended to provide an in-depth understanding of the relief process of polymerization mixtures under process conditions. Therefore, the transient relief model of ZINK [2] based on an isentropic nozzle model and a real gas equation of state is used to describe the single-phase flow. For the description of a two-phase flow, a transient model based on the homogeneous non-equilibrium model from DIENER and SCHMIDT (HNE-DS) [3] is developed. Further, the development of a model to calculate the temperature and pressure loss at the downstream valve body is presented. Both models are compared with pressure and temperature data from high-pressure relief experiments. The combination of both models allows to investigate the influence different parameters like gas type, starting pressure or temperature have on the relief process.