How hot work ignites fires and explosions

To control hot work fires, it helps to understand exactly how the heat gets from the torch or arc to the fuel. Investigators who study hot work losses keep finding the same few pathways: sparks and spatter that travel farther than anyone expected, slag that drops through openings to floors below, heat that is conducted along metal to the far side of a wall, and vapors or dusts that ignite instantly when they meet a spark. This lesson explains each pathway and the conditions that make it more likely.

Sparks, spatter and slag

Sparks are small particles of metal or abrasive heated until they glow. Spatter is molten metal thrown from a welding arc. Slag is the molten or solid residue left by cutting and some welding processes, often in large globules. All three carry heat away from the work. A single spark may contain very little energy, but it is hot enough to ignite fine, dry materials such as sawdust, lint, paper dust, cotton waste, dry leaves or the fluffy surface of some insulation. Larger globules of slag hold far more heat and can ignite wood, cardboard and plastics directly.

How far sparks travel depends on the process, the height of the work, the direction of the cut and any air movement. Welding throws spatter a few meters. Grinding sends a concentrated stream in one direction, often several meters. Oxy-fuel and plasma cutting blow molten metal out of the kerf with high-velocity gas and can scatter slag over 10 m on a level floor. When the work is above floor level, sparks fall and then bounce and roll, so the distance grows with height. This is why codes and standards define a protective zone around the work: 11 m (35 ft) in CSA W117.2, NFPA 51B and OSHA's rules, and 15 m in the National Fire Code of Canada.

Sparks do not stop at walls and floors if there is a way through. They roll under doors, drop through floor openings, cable penetrations and gaps around pipes, slip between floorboards and fall down shafts and chutes. They can land on stored goods on the floor below or in a ceiling space full of dust. Every opening within the protective zone needs to be identified and covered or sealed before hot work starts.

Conduction and radiant heat

Not all hot work fires start with sparks. Metal conducts heat well, so welding or cutting a steel beam, pipe, duct or wall heats the metal well beyond the visible work. On the other side of a steel wall or deck there may be insulation, wood blocking, roofing materials or stored goods. Heat conducted through the metal can ignite them without any spark crossing the wall. Codes specifically require precautions when combustibles are on the opposite side of metal walls, partitions, ceilings or roofs, including checking the far side and posting a second fire watch there if needed.

Pipes are especially deceptive. A copper pipe heated with a soldering torch can carry enough heat along its length to scorch wood framing or ignite insulation inside a wall cavity. Steel pipe that passes through a wall can conduct heat to combustible pipe insulation on the other side. Long sections of ductwork can carry heat into ceiling spaces.

Radiant heat from a large flame, a big weld or a hot workpiece can ignite nearby materials or damage plastic pipes, cables and sprinkler heads. Hot workpieces placed down on a bench or floor can ignite dust, rags or packaging long after the arc is off. Recently welded or cut metal should be marked as hot, and some regulations require it.

Ignition of dusts, vapors and gases

Some fuels need far less energy to ignite than solid materials. Flammable gases and vapors from liquids, and many combustible dusts suspended in air, can be ignited by a tiny spark if the mixture is within its flammable range. The result may be a flash fire or an explosion rather than a slowly growing fire.

• A flammable gas or vapor mixed with air burns only between its lower explosive limit (LEL) and upper explosive limit (UEL). Below the LEL the mixture is too lean; above the UEL it is too rich, but it can become flammable if air gets in.
• Combustible dusts, such as flour, sugar, wood, grain, plastics, metals and many chemicals, can explode when dispersed in air in a confined space. A layer of dust on beams or equipment can be thrown into the air by a first small explosion, causing a much larger secondary explosion.
• Vapors can travel. Many vapors are heavier than air and flow along floors, into pits, sumps, drains and trenches, and can be ignited far from the source.

Lesson 7 explains how to measure flammable atmospheres with a combustible gas detector, which reads in percent of the LEL, and why hot work is normally stopped well before any flammable mixture can form.

Hidden and concealed spaces

Many of the worst hot work fires start in spaces that nobody could see. Wall and ceiling cavities, the voids inside roof assemblies, raised floors, ductwork, pipe chases, insulated panels with foam cores and the spaces under stages and platforms can all hold combustible materials and dust. Fire can smolder and spread inside them, out of sight, until it breaks out somewhere else, often after the workers have left. Insulated metal panels with expanded polystyrene or polyurethane cores have been involved in serious fires when welding or cutting on or near them ignited the core.

Before hot work near a concealed space, the permit authorizing individual should find out what is inside it. If it cannot be inspected or made safe, the work should be moved, a different method chosen, or the space opened up and protected. A fire watch should be able to see or check the concealed space, which may need inspection openings or thermal imaging after the work.

Conditions that increase risk

Some conditions make every pathway more dangerous. The permit authorizing individual looks for them when deciding whether hot work can proceed and what precautions are needed.

• Impaired fire protection: sprinklers shut off, fire alarms disabled or detection bypassed. Codes generally forbid hot work in sprinklered areas while sprinklers are impaired, unless specific additional precautions are taken.
• Dry, dusty or untidy conditions, especially in woodworking, food, paper, textile and recycling facilities.
• Strong drafts or wind that carry sparks farther or fan smoldering fires.
• Work at height above combustibles or openings.
• Recently painted surfaces, coatings and adhesives that give off flammable vapor or burn readily.
• Construction and renovation, when fire protection is incomplete and combustible packaging, formwork and debris are everywhere.
• Time pressure, shift changes and end-of-day work, when the fire watch is most likely to be cut short.

Putting it together

Most hot work fires combine two or three of these pathways with a gap in the controls. Sparks find a gap under a door because no one sealed it. Heat conducts through a wall because no one checked the other side. Vapor from an empty tank ignites because no one tested inside it. Understanding the pathways turns the precautions in the following lessons from a checklist into common sense: you protect against sparks by distance, removal and covers; against conduction by checking the far side; against vapors and dusts by testing and cleaning; and against hidden fires by watching longer and checking again later.

How quickly a small fire grows

Fires started by hot work often begin as tiny glowing spots in dust or fabric. Once open flame appears, growth can be startlingly fast. A fire in a stack of cardboard or plastic crates can grow from the size of a hand to involving the whole stack in a few minutes, producing thick, toxic smoke that fills a room and hides exits. That is why the first minute matters so much: a fire watch with an extinguisher can put out a small fire in seconds, but the same fire left alone for five minutes may be beyond any portable extinguisher. Every pathway described in this lesson has the same answer at the end: someone trained, watching and ready, with the right equipment at hand.

Think of each hot work job as a search for pathways. Before lighting up, ask where sparks will fly, where they will land, where heat will conduct, what vapors or dusts could be present and what spaces cannot be seen. If you can name the pathway, you can usually close it.