Modelling of flow and ventilation within petroleum process plants

Abstract

Adequate natural ventilation in petroleum process plants is of crucial importance when classification of hazardous areas is considered. The term “adequate” is most often referring to either the number of air changes in a given area or to the frequency of occurence of wind speeds less than a given value, typically 0.5 m/s or 2 m/s. In an attempt to specify these parameters accurately, and to aid in the assessment of area classification, CFD can be used to simulate the wind flow field and gas dispersion around and within process plants. A case study from a hydrocarbon production platform in the North Sea is presented. For this installation, a computer model comprising 50.000 grid elements were used. Wind simulations were carried out for three different wind speeds and eight wind directions, using a ‘finite volume’ solution of the governing equations with the standard k- κ model of turbulence applied. Combined with information from the wind rose for the actual site, the simulations yielded the frequency of occurrence of number of air changes on all areas of the installation considered, as well as air velocity distributions. The results obtained this way were used in the area classification considerations, to establish whether the requirements for adequate natural ventilation were met. Given the wind flow field, gas dispersion simulations were also carried out to consider the likelihood of having ignitable gas-air concentrations into non-hazardous areas of the plant in the case of a gas release. An example is given on a calculated gas concentration profile around the platform.