Applicability of Vapor Barrier Devices

Van Zele and Diener (1990) state that vapor fences can reduce the near-field concentrations of hazardous materials, but that concentrations increase with increasing downwind distance. Wind tunnel data show that vapor fences will reduce near-field concentrations by factors of 2 to 9, but the concentrations downwind at approximately 1000 m from the release point will eventually equal concentrations when no fence is in place. For a heavier-than-air cloud, there is no appreciable delay in the cloud s arrival time. 

Three factors strongly influence the effect any vapor barrier can have on cloud dilution. First, the release scenario, including the rate and duration of release, and the wind direction, will influence the barrier s effectiveness. Second, site layout, such as the location of process units and the equipment and buildings in them (e.g., tanks, towers, and pipe racks located upwind of the release point), can influence local air currents and the direction the release will take. Third, the height of the barrier and its relationship to other equipment and the source must be considered. 

For cryogenics, vapor boxes may be more effective. Wind tunnel data indicate that vapor boxes have the potential to reduce near-field concentrations by a factor of 4 to 15 (Arthur D. Little, Inc., 1974). Additionally, far-field concentrations are lowered by the reduction in the rate the gas is discharged from the vapor box. This downwind reduction, when compared to a no-vapor-box case, can vary by a factor of 1 to 4. The degree of reduction is dependent on the release rate, the duration of the release, and the volume of the vapor box. 

---