Hot Work Safety Zones - What if there is a Leak?

One of the main objectives of hot work safety is prevent the interaction of an ignition source and a flammable mixture. This seemingly simple objective can be complicated by the geometry, environment, and conditions surrounding the hot work. The hazard may not always be obvious or recognized. When flammable materials ignite, the resulting fire or explosion can cause asset damage, injury, or fatality. None of these are desirable outcomes from hot work activities. There are many rules, regulations, and safe work practices that are intended to prevent ignition of a flammable material during hot work, but history has shown that incidents still occur. We present case studies from experience and from the literature where a flammable mixture was released, ignited, and caused an incident. The root causes for such incidents often involve changing conditions, undetected leaks, breakdown of hazard communication, or an inadequate job safety analysis. In this paper, we offer comparisons from screening models and CFD models of the release dynamics for leaks from vessels for buoyant and dense gases and vapors. We also evaluate complex geometry that is commonly involved in situations that makes detection of flammable atmosphere difficult. Finally, we compare the calculated safety zones to rules of thumb and common guidance including NEC Class 1 Division 1 and Division 2 boundaries.