Inerted vessels: Avoiding hazards caused by gas buoyancy

Performing hot work on a process vessel that previously contained a flammable hydrocarbon liquid poses a significant explosion and fire hazard. To reduce the combustion hazard potential, the facility operator may choose to purge and blanket the vessel with an inert gas such as nitrogen or carbon dioxide. Numerous accidents have occurred during hot work due to inadequate inerting operations. Oftentimes the source of the problem was inadequate gas composition control caused by gas buoyancy.A useful paradigm for analyzing the inerting process is the well-stirred control volume with a spatially uniform chemical composition (i.e., perfect mixing). Certain features of the vessel construction, in concert with the physical properties of the inert gas, can interfere with the complete mixing of the inert gas with the vessel atmosphere. This paper discusses how to evaluate the potential for buoyant flows to disrupt and interfere with the design goal of perfect mixing. Three case studies of accident investigations are used to illustrate the potentially detrimental effects of buoyancy on inerting operations. Finally, recommendations are presented on how to use buoyancy to improve the effectiveness of inerting operations.