Acetylene is a colorless gas. It is odourless in pure form but industrial acetylene has some odour due to impurities. 20 per cent of acetylene is used for gas welding and cutting, due to its high flame temperature of 3300 °C when mixed with oxygen. It has the third hottest natural flame. Oxy-acetylene welding is used quite a lot as it is very versatile. It is preferred as the torch is best used for some kinds of steel or iron welding and it can also be used for bending, brazing, tempering and other applications. It is usually handled in solution (in metal containers) as it is unstable in pure form. This makes it safe for transport and use, given proper handling. Where acetylene is used in welding and cutting, its pressure (safe use is 15 psig) must be controlled by a regulator or it may cause an explosion. Acetylene is an extremely flammable gas and safety regulations must be adhered to, to ensure proper use.
Under certain conditions acetylene can decompose explosively into its constituent elements i.e carbon and hydrogen. This can cause an explosion. There are three main conditions where this can happen i.e the gas is not at the correct pressure, the gas hoses are not purged (flushed out) and the cylinders are not protected with flashback arresters. A flash back can occur when there is a flammable mixture of fuel gas and oxygen in the hoses when the torch is lit. If not stopped, the flame can ignite the mixture, travel along the hoses to the cylinders and cause an explosion. However, there would be time for emergency action and evacuation most times as it can take a while for the decomposition of the gas. This is due to the porous material used to encase acetylene cylinders. For safe welding, the acetylene pressure should not exceed 0.62 bar (9psi). Purging the hoses involves opening up the gas supply to each hose for a few seconds to flush out any flammable mixes before use. Flash arresters should be fitted to the regulators and the cylinders.
The HSE is currently consulting on proposals to consolidate and modernise the current legislative arrangements for acetylene. The new regulations will consolidate the rules currently in place. The current rules are fragmented at the moment due to then coming from different sets of regulations and amendments. The proposed consolidation will not alter the existing set of regulations already in place for the use of acetylene, but simply make them easier to understand. The risks posed by acetylene are currently governed under the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). However, the HSE points out the DSEAR does not fully address, that, under certain conditions, even in the absence of oxygen, acetylene could undergo decomposition that can lead to an explosion. The explosives regulations does not fully address this issue at the moment. The proposed regulations are named the Control of Acetylene Regulations 2014 (CAR 2014). Public consultation commenced July 2013 and all comments on the consultation must be reviewed by 24 September 2013. CAR 2014 will modernise acetylene regulation. CAR 2014 has been developed with a working group including representatives from trade unions, fire and rescue services, government departments, security services and the Department for Transport. CAR 2014 will consolidate the current regulations and ensure that legislation includes the safe production, storage and use of acetylene.