Acetylene is a very dangerous substance. But it doesn’t fit well with other types of hazard and has been shoe-horned into explosives regulations rather than the dangerous substances legislation.
Most of us who deal with risk in the workplace probably haven’t given the legislative background much thought. There are, after all, established guidelines for its handling and use, and we tend to just follow those without needing to dig further.
In fact, the legal requirements begin with the Explosives Act 1875 and then move forward in time through a total of 13 further Orders, Regulations, and Certificates of Exemption. The outcome is an overarching prohibition on liquid acetylene and compressed acetylene gas, with its use then allowed by exempting certain activities from the prohibition. Site-specific approvals are even used for the riskiest activities — high-pressure manufacture, compression, and cylinder filling.
Acetylene is an extremely flammable gas, which can burn in oxygen at up to 3,500 degrees Celsius. It’s so hazardous because it’s unstable. Under certain conditions, it can decompose explosively into its constituent elements, carbon and hydrogen. Those triggering conditions include raised temperature and/or pressure. Due to its instability it must be handled with care.
An acetylene cylinder has a different design from most other pressurized gas cylinders. It contains a porous mass that completely fills the cylinder. The acetylene gas is dissolved in acetone, which is then absorbed by the porous mass. This method of storing it is designed to slow down or stifle any decomposition of the gas.
If you want to see what happens when acetylene destablizes, this video clip gives a handy illustration:
It can take hours to cool down a cylinder when a reaction has started. For that reason fire and rescue services (FRSs) used to have a standard procedure which puts in place a 200 metres exclusion zone for 24 hours if the cylinders are involved in a fire (studies of cylinders which have failed catastrophically have shown that pieces can penetrate masonry and may be fired a distance of 200 metres). Clearly in our highly populated cities, that was expensively disruptive.
Despite the fact that several FRS websites still suggest this fire fighting policy is in place, in fact revised Hazmat guidance allows for a much more flexible approach involving at least one hour of cooling and a further hour of monitoring, followed by further cooling as needed.The zone around the fire is redesignated as a ‘danger zone’ other than in exceptional circumstances when total exclusion may be required.
In 2007 the Communities and Local Government Department produced a paper Safety of acetylene containing cylinders during and after involvement in a fire. It concluded that "Water deluge appears to work very well and a deluged cylinder has never been known to fail catastrophically".
Reading the consultative document which discusses the planned new Acetylene Safety Regulations 2014, there was a small part of me that hoped to see plans to introduce a method used in Sweden to control fire risk in these cylinders. In some cases it is permitted to perforate the cylinder with at least two holes by rifle shot, a method that has been shown to prevent uncontrolled rupture of a hot acetylene cylinder. But it’s clearly not yet time to arm our FRSs! There doesn’t seem to be an appetite for it in the UK.
As it turns out, the proposals are pretty free from any kind of excitement. The aim is to simply take the current fragmented legislative regime and put the same requirements into a single set of regulations. This would mean that licenses would still be required for acetylene gas compression processes and the filling of cylinders.
The HSE’s guidance would also be rationalized and made more user friendly. At present this includes INDG327, “Take care with acetylene“; and HSG139, “The safe use of compressed gases in welding, flame cutting and allied processes.”
This guidance would be aimed at the average user, while those with more advanced needs would be expected to refer to technical standards and industry guides, as they do now.
There’s also a proposal to keep the UK colour coding for acetylene: maroon.
Overall this review and rationalization shouldn’t do any harm. It will be an opportunity for FRSs to address complacency and remind those who use and store this substance of its dangers.
If it’s something you have an interest in, you may wish to respond to the HSE’s consultation here. The deadline for comments is September 24, 2013, and the new regulations and guidance are expected in October 2014.
This article was updated on 29th August at 5pm to reflect updated advice to Fire Service’s on fire fighting strategy when a fire involves an acetylene cylinder..
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