Oxy-fuel cutting has been around for more than 100 years, and the most commonly used gas for the process, acetylene, has been around even longer. Its longevity certainly proves that it’s an effective gas for cutting. But is it still the best? Advancements over the past century have led to more quality cutting options, and gas is an important part of that equation. Here’s a look at the pros and cons of not only acetylene, but also alternate gases that can be used in oxy-fuel cutting.
What factors can affect cut quality?
Don Bobyk, vice president of marketing, sales, training and technical support at Gas Innovations, said cut quality is determined by a number of factors:
- Type of fuel gas used with oxygen
- Thickness of the steel to be cut
- Cut type (straight vs. bevel, for example)
- Cutting tip
- Cutting speed
- Cutting method (by hand, by machine or by robot.)
“In fabricating steel today, 85% (of the cost) is labor, 10% is oxygen and 5% is the fuel gas,” Bobyk said. “The objective is to reduce labor costs.” Choosing the correct fuel gas and cutting tip will have the greatest impact on reducing the cost of labor.
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Acetylene: Pros and Cons
There’s a reason acetylene has been used in oxy-fuel cutting for more than a century — it’s readily available everywhere and is well known to fabricators. Acetylene is the hottest fuel gas, with a flame temperature approaching up to 5,700ºF and a triple-bond C2H2. It works with multiple applications, including heating, cutting and brazing, is slightly lighter than air, and its high flame velocity with oxygen (22.7 ft./sec.) means the torch is easy to light.
Despite acetylene’s availability, however, its limited number of producers makes it expensive. The cylinders, which typically use a filler of gypsum and acetone for stability, are expensive too. In addition, acetylene is limited to only 15 psi and isn’t available in bulk. “You must manifold multiple cylinders together” in an upright position to prevent leaks, Bobyk explained.
Cutting speeds using acetylene are limited in material that’s more than ½” thick — its large primary flame BTU output and limited secondary flame BTU output cause slag to stick to the cut steel at faster speeds.
Finally, acetylene is the most unstable of all the fuel gases and has a higher tendency to flashback. In fact, acetylene above 15 PSI or if the cylinder is dropped it could detonate.
Propylene vs. Acetylene
An alternative to acetylene is propylene, a byproduct of petroleum refinement or natural gas processing that has a flame temperature of 5,312F (slightly less than acetylene) and a double-bond C3H6. Like acetylene, it’s readily available and can be used for a number of applications, including cutting, heating, gouging, beveling, brazing, flame hardening and metalizing.
But that’s where the similarities end. Bobyk said that propylene is typically 50% cheaper than acetylene, is 20 times more stable with a very low flashback tendency, and one, 100-pound cylinder can do the work of five large acetylene cylinders.
Because acetylene’s secondary flame BTU output is less than propylene it has a longer preheat time. But once the metal is heated, propylene cuts 25-30% faster than not just acetylene, but any other fuel gas, with slag-free cuts and no grinding required. It achieves smooth, finished cuts and can cut cold steel up to 25 inches.
“Propylene increases production, makes quality, slag-free cuts, eliminates grinding and has faster preheat times,” Bobyk said. And when the objective is to reduce labor costs, stability, cost and speed all meet the mark.
Other Fuel Gases
In addition to acetylene and propylene, three other commonly used fuel gases are propane, Chemtane and natural gas.
Propane’s high number of BTUs makes it a good choice for heating large parts and slow cutting speeds on steel thicker than one inch, Bobyk said. For smaller parts, like shafts, propylene is better because of its focused primary and large secondary flames. Propylene is also preferred for flame spray or metalizing, cutting thick steel, bevel cutting and scrap cutting.
Natural gas in an inexpensive gas, but has long preheat times and is very slow to cut. In addition, it’s typically delivered via pipeline vs. cylinders — not very practical for a small or mid-sized fabrication shop.
Gases and Cutting Tips
Choosing the right fuel gas isn’t the only important part of a quality cut — the cutting tip itself is essential as well. “With multiple cutting tips on the market today, there are only a few tips that have outstanding performance and reliability to give you the best results,” Bobyk said. The difference? A stainless steel insert. It may cost a little more, but lasts three to five times longer, he said.
Don Bobyk is the VP of Marketing and Sales at Gas Innovations, a producer, purifier and packager of high purity hydrocarbon specialty gases.