Venting Flammable Gas to a "Safe Area": An Objective Review of Best Practices and Guidelines

Venting, purging, or blowing down flammable gas-containing piping systems and vessels may pose unintended fire and explosion hazards to personnel.  In fact, several high-profile incidents have occurred in recent years highlighting this hazard.  Industrial safety standards and guidelines exist as a basis for safely designing venting systems, but can a designer rely solely upon these standards for every situation?  Certainly, site-specific constraints can be incorporated into a hazard analysis to understand potential adverse outcomes of a venting process.  In this paper we give a brief overview of the existing safety standards applicable to blowdown and gas purging operations of flammable gas-containing systems.  In a blowdown, the piping system is relieved of contained fuel gas pressure, typically in order to make the system safe.  In gas purging, a fuel gas system is freed of air and replaced with fuel gas in order to bring the system into service.  We employ consequence modeling software tools to illustrate the influence of relevant system parameters on vapor cloud dispersion and compare the results with the safety guidelines.  Guidelines will be presented for the designer to consider when evaluating their specific venting systems.