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The Fire Triangle, And How It Relates To Oxy-Fuel Cutting

What is the “Fire Triangle”

A reader recently asked us to mention the “Fire Triangle”, and how it works in relation to oxy-fuel cutting. The fire triangle is a great way to illustrate the three elements needed to create fire, namely 1) fuel, 2) oxygen, and 3) heat. Sometimes they are represented differently, such as:

  • Fuel – Oxidizer – Energy
  • Fuel – Oxidizer – Ignition Source
  • Fuel – Air – Heat

Many Possibilities

When looking at oxy-fuel cutting, the fire triangle can be seen in several different places, some of them desirable, some not so much. I’ll discuss the two primary examples of the preheat flame and the burning process, as well as two other examples of which the cutting machine operator should be made aware.


The Fire Triangle of the Preheat Flame

The fire triangle and oxy-fuel cutting

Fuel: Fuel Gas

Oxygen: Preheat Oxygen Gas

Heat: Ignition Source, such as an automatic ignitor or a manual striker


The oxy-fuel cutting process requires a preheat flame. This is a perfect example of the fire triangle, where fuel gas is mixed with pure oxygen and then ignited by an outside spark or flame. One of the characteristics of the fire triangle is that if you remove any one leg of the triangle, the fire is extinguished. So the preheat flame actually presents two separate examples of the triangle. The first example is the ignition of the flame. But after the initial spark is withdrawn, you have a chain reaction that continues to heat and burn itself, because the burning flame becomes the heat source that ignites the fuel oxygen/fuel gas mixture that continues to flow from the nozzle.

You can also see this principle in action if you turn off one of the two gas valves, either removing the fuel gas or the oxygen gas. In either case, the flame will usually go out (if the preheat oxygen valve is closed, and the fuel gas pressure is low enough, it may continue to burn because of oxygen in ambient air).


The Fire Triangle of the Steel Burning Process

The fire triangle and oxy-fuel cutting

Fuel: Steel

Oxygen: Cutting Oxygen

Heat: Preheat flame + exothermic reaction


During the actual steel burning process we see a slightly different example of the fire triangle. Once the cutting process has begun, a jet of pure oxygen is being fed into the molten steel, feeding a rapid oxidation process, or “burning” of the steel. During this process, the chemical reaction of oxidizing steel is “exothermic”, meaning that it gives off more heat that it takes to start the reaction. After the plate is preheated and the burning starts, the preheat flame is turned down to a lower setting. But the preheat flame continues to provide the heat & ignition source, while the cutting oxygen stream provides the oxidizer, and the steel itself is the fuel.


The Fire Triangles You Don’t Want

The fire triangle and oxy-fuel cutting

Fuel: Clothing, rags, paper, boxes, wood, trash

Oxygen: Ambient air, or compressed oxygen

Heat: Hot/molten sparks from the cutting process


Here are some examples of the fire triangle that you don’t want to encounter. Any cutting machine operator should be aware of these hazards, and take steps to prevent the three elements from coming together in a dangerous way. This is why any cutting machine documentation will contain safety hazard warnings that inform the operator to keep the working area clean, and clear of any combustible materials. A thermal cutting machine can be a prolific source of hot sparks, molten spatter, and even stray radiation. Any of those potential ignition sources can cause a nearby fuel to burst into flames.

Another example that any machine operator should be warned about is the hazard of using the oxygen from a hand-held torch as if it were a compressed air hose. Most people have heard horror stories of “some guy” who dusted off his clothing with the oxygen stream from a hand torch, while he had a lit cigarette in his mouth, then promptly had his clothing burst into flames. The high concentration of pure oxygen makes the clothing highly flammable, so that even the smallest spark is enough to start the fire.


The Extremes of High Pressure Oxygen

The fire triangle and oxy-fuel cutting

Fuel: The smallest particles of hydrocarbon contaminants

Oxygen: High Pressure Pure Oxygen Supply

Heat: Ambient warmth


Another example of the fire triangle is the explosion hazard associated with high pressure oxygen. Any time pure oxygen is pressurized, it becomes even more combustible. If a high pressure oxygen hose, pipe, valve, or fitting has not been properly cleaned for oxygen service, or has become contaminated with hydrocarbons or other flammable elements (fuel), then an explosion can result even without an ignition source. This is because pure oxygen under high pressure becomes very reactive, lowering the ignition temperature to below the typical ambient temperatures. That is why it is so important to keep oxygen hoses and fittings clean and covered during installation, or during any maintenance procedures that break open the oxygen lines.

Why study the fire triangle?

More than just an interesting discussion, studying the fire triangle helps understand how the oxy-fuel process works, which can aid in troubleshooting as well as proper operation. Studying examples of the fire triangle can also help machine operators and other factory workers recognize and avoid potential fire dangers and explosion hazards wherever they may exist.  


Posted in Cutting Systems , Tagged with Oxy-Fuel, Safety