The Mitigation Conundrum: When Reducing One Hazard May Increase Another

Siting regulations for onshore LNG facilities in the United States require the consequences of credible accidental scenarios to be confined within the facility’s boundaries – that is, no harm should occur to the public or public property from the worst credible events.  Compliance with regulations based on worst credible events is often challenging and may require mitigation methods.  One example is that of vapor barriers being used to control the dispersion of a flammable cloud from a release within the facility, in order to keep the cloud from extending beyond the property boundaries before dissipating below ½-LFL.

However, using barriers to keep the flammable cloud from dispersing too far downwind is likely to result in a larger volume of gas remaining near the release location and possibly within congested areas inside the facility.  This may therefore increase the hazard footprint from the worst credible overpressure scenarios.  Similarly, the flow obstruction posed by the barriers may decrease the air flow through the area increasing the overpressure hazard from smaller releases.  Since small releases tend to occur more frequently, increasing their potential severity is likely to increase the overall risk to which facility personnel are exposed.  Fortunately, not all barrier installations will have an adverse impact on overpressure hazard and/or personnel risk. 

This paper will examine the impact of vapor barriers on overpressure hazards for different barrier configurations.  The case study will apply CFD modeling to realistic scenarios, inside a detailed 3D model of a typical LNG liquefaction facility.