Is Acetylene Flammable?

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Welding is the process of joining two pieces of metal together using extreme heat. The most popular fuel for welding torches is acetylene. When acetylene is burned it produces a very high heat, which can be directed easily to the join and quickly fuse metal to metal. But does this mean that acetylene is flammable and if so, is there anything we should know about the risks of acetylene?

Acetylene is considered to be a very flammable gas that can ignite at -0.7 degrees Fahrenheit (-18.15 degrees Celsius). It is imperative that precautions are taken when working with it or storing it.

When using a gas that is so flammable like acetylene, it is important to understand what it is and how it behaves, in order to stay safe. Lets take a closer look.

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Also read: What Makes Something Flammable?

What Is Acetylene?

acetylene welding

Acetylene (or ethyne) is a hydrocarbon with the formula C2H2

This makes it the very simplest alkyne and it is a colorless gas at room temperature.

The simplicity of its chemical formula makes acetylene highly reactive, and it is prized for use as a fuel.

This is, in particular, due to the triple bond between the two carbon atoms in the molecule with only single bonds attaching each hydrogen atom.

Interestingly, the acetylene molecule forms a perfect straight line with the angles between each atom in the molecule at exactly 180 degrees from each other. 

What is acetylene?

How Dangerous Is Acetylene?

Acetylene is considered a very hazardous chemical to work with.

It is chemically unstable. 

It burns very easily, and it must be stored and transported with great care.

In environments where acetylene is used in work, typically welding but also in other workplaces, individuals working with acetylene need proper training, and health and safety assessments must be carried out and all precautions adhered to.

In short, acetylene is very dangerous under the wrong circumstances and thus, it’s vital to make sure that those circumstances never arise. 

Also read: Is Benzene Flammable?

Is Acetylene Flammable?

The flashpoint of acetylene is -0.7 degrees Fahrenheit or -18.15 degrees Celsius, which is a long way below the 199.4 degrees Fahrenheit or 93 degrees Celsius top limit for flammability. 

This means that acetylene is very flammable, indeed.

What Temperature Does Acetylene Burn With Oxygen At?

Once acetylene starts burning in oxygen things get hot very quickly, indeed. 

In an ordinary air mixture where Oxygen is not the only gas present, acetylene burns at a temperature of around 4000 degrees Fahrenheit or 2200 degrees Celsius. 

This is a very useful temperature for welding aluminum or repairing a radiator with, however, it’s not hot enough for somebody to weld steel together with. 

For that, you need a pure oxygen supply to burn with the acetylene and the temperature of the flame produced shoots up to 5730 degrees Fahrenheit (3166 degrees Celsius)!

This is one of the reasons that acetylene is considered so dangerous, it’s not just that it burns super easily but also that once it burns, it burns extremely hot. 

GCEacademy (1) - Flammability of oxygen, acetylene and a mixture of these

What About Without Oxygen?

In an even more frightening development, acetylene that is stored for long enough, can start to break down into its constituent parts. 

That is carbon and hydrogen.

And in doing so, it starts to release heat.

In fact, it can release enough heat that it causes the remaining acetylene to ignite even though there’s no oxygen present!

This makes acetylene nearly unique in that it doesn’t require oxygen to burn. 

At What Temperature Does It Become Unstable?

Acetylene does not need a flame to burn, and it will auto-ignite (spontaneously combust) at temperatures of around 763-824 degrees Fahrenheit. 

This is quite high and it’s not the sort of temperature that you would encounter without fire, but, if you compress acetylene as you do to store it, by the time the pressure reaches 30 psi, the acetylene will spontaneously combust at room temperature!

Thus, free acetylene must never be stored in high pressure cylinders. 

Instead, it’s pumped into cylinders with acetone in them which can then absorb the acetylene. 

It’s worth noting that you must keep them upright or the acetone and acetylene will separate and potentially become hazardous.

You cannot freeze these cylinders either or there’s a chance that the acetone will exit the cylinder first, clogging any regulators used and leaving a cylinder full of acetylene behind. 

Is Acetylene Toxic?

Acetylene is non-toxic thought if used in high enough quantities it can have an anesthetic effect which would make someone drowsy or weak and possibly dizzy too. 

What Happens If You Inhale It? 

While acetylene is non-toxic, it doesn’t mean that inhaling it is good news.

Ignoring the anesthetic effect, if you are inhaling it in an oxygen rich environment, you won’t suffocate, but only because it will catch fire first and you would almost certainly burn to death.

If you are inhaling it in an environment which is oxygen deficient, you would, eventually suffocate to death. 

It is also worth noting that while acetylene is not toxic on skin contact, the acetone that is dissolved in can irritate your skin. 

Does It Cause Cancer?

Acetylene is not known to be carcinogenic but the compound acetylene tetrabromide, a common byproduct of acetylene, is known to cause cancer in animals and is suspected to cause stomach cancer in human beings too. 

Is Acetylene Explosive?

Acetylene is not, technically, explosive, but because of the violence of the reaction combined with a very low ignition point, acetylene reactions can become explosive if allowed to get out of control. 

Can Acetylene Tanks Explode?

If an acetylene tank gets above 39 psi (200 kPa) then it will explode. 

Most acetylene tanks are low pressure cannisters but, of course, if the acetylene catches fire and starts to heat an acetylene tank, then the pressure will rapidly increase (thanks to the extreme heat of the acetylene flame) and it’s very easy for them to exceed 39 psi.

My Firefighter Nation has some shocking pictures of the damage that even a small acetylene cylinder can do if it gets to the point of exploding. 

Take a look:

Footage: Gas cylinders explode at facility in east China

What Do You Turn Off First Oxygen Or Acetylene?

If you’re looking to use an oxy-acetylene welding torch then you should be aware that you must turn off the oxygen supply prior to switching off the acetylene supply

This helps to ensure that there’s no carbon build up inside the torch that damage the torch and make it unusable, it also means that you should be able to detect any leaks from within the two tanks more easily. 

Can You Smell Acetylene?

Acetylene is an odorless gas but that doesn’t mean that you can’t smell it when it is leaking from an industrial cylinder. 

That’s because there are certain impurities in the acetylene including phosphine and arsine, these two chemicals have a sort of garlic-like aroma to them, and this can act as an early warning sign that you have an acetylene leak.

Is Acetylene Bad For The Environment?

With all the precautionary data for acetylene, you might expect it to be bad for the environment but, in fact, the reverse is true. 

The gas cannot build up in any real concentrations without combusting and in very low concentrations it’s both non-toxic and harmless to the environment as an air pollutant. 

Can Acetylene Set Off A Carbon Monoxide Detector?

A carbon monoxide detector is an essential piece of fire safety equipment in most homes and offices. 

Carbon monoxide is colorless, odorless and can cause you to suffocate by bonding to your red blood cells, permanently, in the sites where oxygen normally bonds.

Each year in the United States alone, 430 people die from Carbon Monoxide poisoning and 50,000 are hospitalized with it. 

A carbon monoxide detector is an electrical sensor system which detects carbon monoxide and then sets off an alarm if a certain threshold level of carbon monoxide is breached.

Unfortunately, there are other gases which can set off a carbon monoxide detector and acetylene is one of those gases. 

That means a carbon monoxide detector may not operate efficiently in a space where acetylene is used on a regular basis. 

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