The Nature of Acetylene Explained

by: Chris Ray

Valerie was in the back of the shop, straightening out some of the gear and rolling cylinders of oxygen and acetylene around the floor to the wall, where she then secured the tanks with a clankey rusty chain. "Ooff" she exclaimed as she stooped over to pick up a wrench and her hand grabbed her back, as if to squeeze it into place. "Ohhh, that smarts, she thought, I've gotta hire a schlepper to do this stuff, this is too much sometimes." She then wheeled around, startled by the scraping and clanging noises coming from the entry door. "Who's there" she called out. "It's just me Val" came the distant reply through the rubble of noise. "I've got a problem with this piece I'm working on and wonder if you could help me." Oh damn, she thought, it's him again, what a nudge. "Okay, I'm in the back Hertzel, just bring it back here and we'll take a look at it."

"Just set that thing down there and let's sit down, I'm bushed." said Valerie, in her weary voice. "Now what seems to be your problem this time?" " Well Val, I'm tryin' to get all this metal stuck together and look at it will ya, it's all crappy lookin'." "Yeah, I can see you got a problem alright, what are you using in your torch this time? I don't know why I ever bother explaining anything to you, you never listen." "Well Val, I thought this bottle of propane I got at the hardware store would be okay to use, but I ain't havin' any luck with it." "Okay Hertzel, I'll go over this one more time, but after this you're on your own, no more pulling hot irons out of the fire for you." "I'll listen real careful, Val, I promise."

VALERIE: Sighing out her frustration: "For the very thin gauge material, many people use an air-acetylene torch. The torch is designed to take air into several intake holes near the torch tip/torch body connection. Again, good for light gauge work. Oxygen is added to burning fuel to increase combustion. The fuels we use in conjunction with oxygen are called hydrocarbons. They each have a different combination of carbon to hydrogen. MaPP gas (Methyl Acetylene Propadiene), Propane, Liquid Propylene, Natural gas and Methane are some of the hydrocarbons. Hydrogen can also be used as a fuel gas."

HERTZEL: All ears and paying full attention: "Yeah, I remember you tellin' me that before, but I just thought the propane would be okay so I wanted to use it. Besides I only needed a little bit and this little bottle seemed to have just the right amount. I guess I was wrong, huh? Well if I switch over to acetylene, what makes it better?"

VALERIE: Groaning in dismay at this hopeless student: "The unique thing about acetylene is that its combination of 2 carbons to 2 hydrogens is an unstable combination. It only takes 15 psi (pounds per square inch) pressure to make the carbon and the hydrogen explosively disassociate (boom). That is why the red area on the regulator."

HERTZEL: Lighting up, smiling: "Oh I get it now, you mean I gotta be extra careful with this gas. Well, okay, I suppose I can do that, but if I want to weld up that real heavy section, ain't I supposed to use a real big torch and turn the gage handle all the way up? Oh man, I could melt down the whole building if I wanna huh? Wow, what a sense of power that must give ya. So how high can I crank up that sucker?"

VALERIE: Shaking her head, wiping away what appears to be a tear of frustration: "Then the question: How come we can't use acetylene above 15 psi when there is 250 psi in the cylinder? This is important. When you knock on the acetylene cylinder and the oxygen cylinder, you'll notice that they sound quite different from each other. The oxygen one is hollow and rings. The acetylene cylinder is filled with what they call "monolithic filler". Used to be sawdust, corncobs, etc. Now is a kind of foamed concrete type material. Into this is added acetone. In the most common size cylinder there is 14 lbs of it added (by weight not by gallon). The acetone absorbs the acetylene into the spaces between its molecules (into its interstices). In this way, the acetylene is stabilized. It is similar to the way pop is made in a restaurant: The CO2 gas is pumped into the pop syrup and that liquid absorbs it into its interstices. If you let it sit there, the CO2 will everveses (yikes, can't spell it - bubbles) back out into the atmosphere. When we open up the cylinder of acetylene, the acetylene comes out of suspension and is a gas in your hose. You cannot pull more than one-seventh of the volume of an acetylene cylinder in an hour or you will start to take the acetone out too. This is dangerous. You compromise your safety. This can happen if you try to use the cylinder on its side. You can sometimes smell it (acetone is nail-polish remover) and you can see the flame get purplish. If liquid comes out of the torch -stop- and turn it off. When using too large of a torch (a rose-bud), it is easy to pull too much acetylene too fast from the cylinder, especially if you have one of the smaller ones."

HERTZEL: Fidgeting with the strap of his overalls, buckling and unbuckling it: "Uh, so I guess your tellin' me that I don't need so much of the gas. But hey man, if I got a big cylinder drug into the shop, you mean I gotta lift that heavy bongo bottle up off the floor? I can't just leave it lyin' there and just use it? Bummer. Maybe I better use sumthin' else that ain't so much work."

VALERIE: What have I ever done to deserve this torture, she winced. "Why use alternative gases? There isn't the same safety issue with them as with acetylene. BUT, Acetylene has the hottest temperature neutral flame (~5700 degrees, depends on your reference material - some say 5900). It also uses the least amount of fuel in relation to oxygen. Acetylene uses one part oxygen to one part acetylene in the neutral cone of the flame (an additional one and one-half parts of oxygen is taken from the atmosphere to give the complete combustion formula). MAPP uses one part oxygen to 2-1/2 parts Mapp gas. Propane uses 3.5 parts fuel to one part oxy. Propylene uses 4.5 parts fuel to one part oxy. Natural gas uses two parts fuel gas to one part oxy. Sometimes, when stuff seems cheaper, it might not be. You use more volume of the alternative gases. Propylene and Propane are great for use when flame cutting, but not as good for welding and heating up more than light gauge stuff."

HERTZEL: Now standing near his work, hugging it and glassy eyed. "I guess I'll take your advice Val, I think I understand everything now. I can just roll in a big tank of acetylene and hook it into the oxygen thingie and use the big blow torch to get everything stuck together. Yeah, I got it now and thanks Val. See yah."

"Oh damn," thought Valerie, "summer is just around the corner, thank goodness. I can't wait to get away from this madhouse. It's a zoo around here. Why do I ever bother?"

Contributor: - Valerie Weihman - vweihman@madison.tec.wi.us