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.

Author Bios: 

Delmar Morrison

Dr. Morrison’s practice areas encompass both product and process safety pertaining to the control of hazardous energy sources such as fires, explosions, and chemical releases. He specializes in both field and laboratory investigations of origin, cause, and engineering issues related to hazardous chemicals accidents, fires, explosions, and chemical technology. Dr. Morrison’s expertise includes chemical engineering, fire dynamics, process hazard analyses, and the system safety of products and processes.Read more

Ryan J. Hart, P.E.

Ryan J. Hart, PhD, P.E., is a managing engineer in the Thermal Sciences practice at Exponent, Inc. (Email:, where he assists in the investigation and analyses of fires, explosions, and chemical process safety incidents. His investigations have focused on mid-scale to large-scale chemical production facilities, industrial material handling and processing facilities, power plants, refineries, and biofuel production facilities. The investigations have focused on fire and explosion cause analysis, origin determination, and evaluation...Read more

Eric Peterson

Eric Peterson, PhD, is a senior process safety and risk consultant for Physical Safety, LLC (Email: He has more than 20 years of experience focused on strategic planning, compliance consulting, and advanced process safety and risk analyses regarding infrastructure projects worldwide for industrial clients, particularly those involved in oil and gas, hydrocarbon transport, and the chemical industry. He provides guidance on the performance and direction of numerous air dispersion studies to support permitting, compliance, and...Read more

Morgan Reed

Morgan Reed, D.Eng., is a senior process safety and risk consultant for Physical Safety, LLC (Email: He has nine years of offshore and onshore experience in the upstream, midstream, and downstream sectors of the oil, gas, and petrochemical industries. He applies thermodynamics, fluid dynamics, and heat-transfer principles to study combustion processes (and consequences/responses), including jet/pool fires, explosions, structural response, and a variety of heat-transfer processes for technical process safety studies. Reed’s primary...Read more

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