How Blowtorches Work

Gas blowtorches, even professional ones, are fairly simple constructions. Their basic design has not really changed in the past century. A professional blowtorch usually consists of a cylinder of fuel gas, a cylinder of oxygen, two gas regulators, two hoses, the torch and the tip.

Regulators on the cylinder heads control the pressure and flow of the gas. Adjusting them to the appropriate pressures is essential to preventing explosions. A regulator has two gauges: the high-pressure gauge reads the gas pressure in the tank, and the low-pressure gauge reads the gas pressure for delivery to the hose and torch. Learn more about pressure and how to measure it in How Tire Pressure Gauges Work.

The hoses that connect the steel cylinders to the torch are made of strong, flexible material, usually rubber. To avoid confusion, the hose containing oxygen typically is green, while the fuel-gas hose is red.

The torch head has two needle valves, which are circular knobs that control the flow of either the oxygen or the fuel gas. The tip attaches to the torch head, and people change it according to the task being performed and the gas used. When a blowtorch is used for cutting metal, an oxygen pipe sends an additional stream of oxygen directly to the center of the flame, increasing its intensity. Finally, an igniter, or pilot light, much like the one on a gas stove, actually lights the torch. It's a bad idea to ignite these torches with a cigarette lighter or a match because of the dangerously high temperature of the flames. 

What prevents this thing from blowing up in your face? Well, the following attachable safety devices usually come standard on blowtorches sold today, but can also be sold separately.

Next, we'll take a close look at the different gases blowtorches consume to attain these high temperatures.

Blowtorches can use different gases depending on the kind of work done. Professional gas torches use acetylene or MAPP gas to mix with oxygen, while smaller, home torches typically use either butane or propane.

Let's start with acetylene. Although highly effective, acetylene (C₂H₂) is a volatile fuel gas. The volatility stems from acetylene's triple carbon bond. This kind of bond can hold a considerable amount of energy that releases when ignited. However, the bond's unstable nature means it can suddenly explode unless it is kept at the proper pressure (between 15 psi and 29.4 psi, where psi stands for pounds per square inch). Even sudden bumps and small shocks can cause an explosion. Dissolving acetylene in acetone helps keep it more stable, but it is still dangerous and must be stored properly; that is, upright, otherwise the acetone can came come in contact with and harm the other equipment, such as the regulators, hoses or torch head.

Compared to acetylene, MAPP gas, which is made up of methylacetylene and propadiene, is much less dangerous. Unlike acetylene, MAPP gas is not prone to explosion if the cylinder is hit or bumped. It also can withstand higher pressures, which means it's good for underwater work, like repairing ships. Although MAPP gas flames cannot burn as hot as those of acetylene, some claim that it meets or exceeds the performance of acetylene in welding [source: Fundamentals of Professional Welding].

Because oxygen is needed to sustain any flame, it also plays an essential role in the use of all blowtorches. But why, if the gas is naturally present in the air, is a cylinder of compressed oxygen necessary? Because without it, neither acetylene nor MAPP gas would burn nearly as hot as they do. Oxygen acts as an accelerant, which means that it helps the fuel to burn at a higher temperature.

Welding torches typically use oxygen and acetylene (hence the name "oxyacetylene torch") because together they produce flames ranging from 5000 degrees Fahrenheit to 6000 degrees Fahrenheit (2760 degrees Celsius to 3316 degrees Celsius). In fact, the oxyacetylene combination produces hotter flames than any other gas combinations. Adding pure oxygen to the flame increases the performance of acetylene by more than 1000 degrees Fahrenheit (538 degrees Celsius), and that of MAPP gas by more than 1500 degrees Fahrenheit [source: Bernzomatic].

Considering the blistering flames, it's important to know what you're doing before you decide to light up a blowtorch. In the next section, we'll take a look at the safety measures involved in starting one.

Before turning on a professional-grade blowtorch, welders should put on a few protective items, like gloves, an apron and, most important, goggles or full face masks with tinted lenses (because of the intense brightness of the flame). In addition, ensuring good ventilation, having a fire extinguisher handy, frequently checking for leaks and wearing clothes without grease or oil stains are critical steps in preventing injuries. Even better: wear flame-retardant clothes and hard-toed shoes.

You'll be grateful for this layer of protection if something goes wrong while you're using a blowtorch, like a backfire or flashback. Backfire occurs when the flame extinguishes with a loud pop. Insufficient gas pressure or touching the torch tip against the work causes these backfires, which, although startling, won't hurt you. Flashback happens when the flame retreats into the torch because of improper pressures or a clog. If you think flashback is occurring, immediately shut off both gas cylinders, as it is extremely dangerous.

A shower of sparks flying from a blowtorch workstation also can be trouble. Despite their beauty, an abundant amount of sparks can indicate problems. When the sparks sputter out excessively and far, this can mean that the gas pressures are too high or you may be in danger of causing a fires [source: Finch].

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After gearing up, a welder follows certain steps to turn a blowtorch on and off. As different torches have different safety procedures, refer to the specific manufacturer instructions before operating a blowtorch. A step-by-step procedure for turning on an oxyacetylene torch goes like this:

But to switch off the torch, you follow these steps:

[Source: Oregon State University]

Intimidated by all those steps? You may want to try arc welding, which is eliminating the need for welding with gas blowtorches. Arc welding heats metal by means of electric discharge and is generally faster and more efficient than the blowtorch process since it focuses heat more precisely [source: Fundamentals of Professional Welding]. However, gas blowtorches still are used in certain fabrication processes, as well as in other capacities, such as plumbing, which we'll discuss next.

Plumbers use blowtorches to solder and fix copper pipes. The word "solder" (pronounced "sodder," with a silent L) refers both to the process of joining the pipes and to the substance used in the process. To solder, a plumber heats the pipes and then applies solder material, which softens in the heat. As the solder melts, it flows into the gap between the pipes and creates a tightly fitted joint. To learn about a different method of soldering, read about cold heat soldering irons.

In addition to fixing leaky pipes, many people use smaller, handheld blowtorches for cooking, stripping paint and thawing frozen pipes. These simplified torches are relatively inexpensive. To produce a flame, they use pressurized fuel gas, like propane or butane. Rather than having a separate cylinder of oxygen, they rely on the oxygen in the surrounding air to sustain their flame. These kinds of torches can reach temperatures of 3200 to 3800 degrees Fahrenheit (1760 degrees Celsius to 2093 degrees Celsius), lower than those attained by oxyacetylene torches, but still plenty hot.

Handheld butane or propane blowtorches can come in handy in the kitchen. Most commonly, cooks fire them up to caramelize the top layer of sugar on crème brûlée -- a custardy dessert -- add a glossy exterior to chocolate ganache or brown the top of lemon meringue pie. Their focused heat also works well for roasting vegetables and melting cheese. But remember, if you do keep a blowtorch for home use, it's always smart to have a working fire extinguisher and smoke detectors nearby.

In the past, blowtorches commonly were used for stripping paint. After being heated with a blowtorch, paint is soft enough for easy removal with a scraper or putty knife. However, several potential dangers can come from this practice. For instance, the flame can ignite the wood or dust on the paint, injuring the user. In addition, if you apply the blowtorch to a lead-based paint, it can release toxic fumes, which are dangerous to inhale. Because of these reasons, new and safer techniques have been developed for paint removal, such as electric heat guns, and the use of blowtorches for this purpose has generally been abandoned.

Similarly, some people grab their blowtorches to thaw out frozen water pipes. However, experts strongly discourage this practice as it is a safety hazard. If the water inside the pipe starts to boil, the pipe can explode. People have started fires trying to thaw out pipes this way [source: CBC News]. For more ideas on thawing frozen pipes, check out the related section of this article.

Although blowtorches can be a dangerous instrument in the hands of untrained amateurs, people continue to use them. If you're one of them, read more about related subjects, including how fire works, in the Lots More Information page.