Is Your Hot Work Safety Zone Actually Safe?

Hot work safety zone boundaries are often based on fixed criteria, which fail to take into account changing conditions at the worksite and undetected gas leaks.

This article is based on a presentation given at the AIChE 2018 Spring Meeting and 14th Global Congress on Process Safety (Orlando, FL; Apr. 22–25, 2018).

The term “hot work” can be applied to any activity that involves cutting, welding, soldering, grinding, or other activity that is capable of generating hot surfaces, sparks, or flames of sufficient intensity to ignite a flammable gas mixture and cause a fire or explosion.

One of the main objectives of hot work safety is to 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. Hazards 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 which are desirable outcomes of hot work activities. Many rules, regulations, and safe work practices exist that are intended to prevent ignition of a flammable material during hot work, but history has shown that incidents still occur despite adherence to regulations. The root causes of such incidents often involve changing conditions, undetected leaks, breakdown of hazard communication, or an inadequate job safety analysis.

This article presents case studies in which a flammable mixture was released, ignited, and caused an incident. It compares modeled release profiles of vapor leaks from vessels, and evaluates complex geometries that make detection of flammable ambient atmospheres difficult. Finally, it compares the calculated safety zones to rules of thumb and common guidance including National Electric Code (NEC) Class 1 Div. 1 and Div. 2 specifications.