Although a highly versatile and functional gas, acetylene also has hazardous potential. Understanding such hazards as cylinders in fires is essential.
As we recently provided a gas profile of acetylene, it’s perhaps important to explore the safety aspects of handling, using and storing this potentially hazardous and disruptive gas.
When handled and stored correctly, acetylene is both safe and an invaluable commodity for industry. As a highly flexible gas, acetylene finds application in a wide range of industries and processes, though it is perhaps most renowned for its almost irreplaceable use in metal cutting and welding technologies.
While other such technologies may exist, acetylene is thought to be the preferred option due to quicker operation and the high flame temperature which oxy-acetylene provides. Such versatility and effective use counts for little however, if not handled and used safely.
All gas cylinders present a risk if exposed to fire, but where acetylene is concerned this can be a heightened sense of caution.
Because of the special potential for a delayed action rupture with acetylene cylinders, observing a safe distance during and after is advised, which can cause serious local disruption due to emergency services establishing a 200m cordon zone around the scene of an incident.
The British Compressed Gases Association (BCGA) notes in its May 2008 document, titled ‘Model Risk Assessment for the Storage and Use of Oxyacetylene Cylinders’, that most reported incidents would not have occurred if:
Users fully understood the properties of the gas and the associated hazards
Users were properly trained on how to use oxyacetylene equipment
Users followed guidance on the safe handling and storage of gas cylinders.
So why do acetylene cylinders require quite such diligence and caution?
Dissolved Acetylene (DA) has a distinct property which requires special precautions.
Acetylene cylinders are capable of reacting differently to other gas cylinders after exposure to fire – due to the chemical reaction of decomposition.
This exothermic reaction emits a great deal of heat and can cause a DA cylinder to reheat even after the fire is extinguished, meaning that in rare circumstances acetylene cylinders are capable of still being hazardous some hours after the fire is out.
Acetylene cylinders are actually designed to minimise the risk of decomposition, containing a porous mass and a solvent in which the acetylene is dissolved. Although this porous mass acts as a stabiliser if decomposition were to begin, if unchecked the exothermic reaction can cause the process to accelerate and eventually rupture the cylinder – with potentially disastrous consequences.
Decomposition can be initiated by exposure to intense heat, shock to a warm, fire-damaged cylinder, or if the cylinder has endured multiple flashbacks due to operational error.
But myth has grown around the hazards of acetylene. In fact, as part of the type approval for DA cylinders used across Europe they are tested for the ability to withstand dropping, elevated temperatures and a very significant single flashback.
It is also clear that whilst not to be recommended, cold, undamaged DA cylinders will withstand major mechanical shock without decomposition being initiated.
The BCGA has recently offered guidance to Fire Service, Police and Highways Agency staff for example, that where cylinders have been involved in road crashes and have not been directly exposed to fire and are not leaking, then there is no risk of decomposition
Dealing with the danger
It goes without saying that all users of acetylene cylinders should fully understand the gas’ properties and hazards. Users should also have been properly trained and follow the appropriate guidance on handling, use, and storage.
Safety data sheets for gases are readily available from suppliers and while these points should be adhered to, incidents are testament to the fact that too many users clearly aren’t aware and need greater encouragement.
All gas companies should now be applying a vibrant new reflective label to acetylene cylinders too, which should at least make identification easier and alert users to the gases and circumstances they are dealing with.
If an incident is already underway however, the most effective way to control an exothermic reaction is to cool the cylinder by applying copious amounts of water. In the eyes of the Fire and Rescue Service (FRS), this is quite an extensive and time-consuming task.
In the event of an incident, the alarm should have been sounded, the area evacuated, and the FRS contacted an in attendance.
The FRS current protocol for dealing with a DA decomposition issue is at odds with the rest of the world and involves cooling the cylinders for a precautionary period of 24 hours.
Initially a 200m hazard zone is placed around the fire-exposed cylinder, but whilst this is supposed to be reduced at the earliest opportunity on a dynamic risk assessment basis, the hazard zone is often left in place, which can lead to unnecessary road and other infrastructure disruption.
The BCGA is currently sponsoring research which will hopefully justify a reduction in the UK FRS procedure, to at least be in line with what operates safely in other countries.
In order to reduce the risk of flashbacks, some simple steps can be taken that are applicable to a wide range of gas cylinders and could just avoid a hazardous situation.
Ensure appropriate risk assessments have been conducted in line with current regulations.
Do not use oxy/fuel gas equipment unless the user has been appropriately trained.
Always ignite the fuel gas before introducing the oxygen stream, with the nozzle pointing upwards for acetylene and downwards for propane.
Inspect equipment regularly and replace anything damaged or out of date.
Ensure that valves, regulators and flashback arrestors are clean and free from dirt or grease.
Fit non-return valves on the torch.
Check for any leaks before lighting up.
Do not use oxygen/fuel gas equipment without approved flashback arrestors.
Flashback arrestors should comply with BSEN 730.
View of the BCGA, courtesy of Doug Thornton – Director and Secretary to BCGA
“It’s becoming clear that the current UK method for dealing with acetylene cylinders when they have become involved in fires is certainly safe, but excessively so.”
“Myth and legend has grown around acetylene and this was part of the reasoning behind extending the cooling period applied when the UK FRS protocol was changed in 2003. The change was made on a basis of fear and emotion more than science and the consequent disruption when 200m hazard zones are held for 24 hours, in fact often prejudices safety elsewhere because emergency service resources are needlessly tied up, ambulances are delayed and so on.”
“In 2008 and co-sponsored with HSE, DfT and Transport for London, BCGA commissioned BAM in Germany (The Federal Materials test Institute and renowned acetylene ‘Gurus’) to do some more work on acetylene. Their challenge is to answer the seemingly simple question: ‘after how many hours of realistic cooling can we be sure that any acetylene cylinder, which might have been hiding a decomposition reaction after fire exposure, is safe?’.”
We cannot detail the BAM work here but early conclusions are:-
That the question can be dependably answered through a combination of experiment and heat transfer modelling work which is applicable to real fire scenarios
That decomposition is not a risk unless the cylinder has been exposed to over 350c, meaning that cylinders which have not faced direct flame impingement are not at risk.
“Meanwhile, BCGA are very impressed with huge strides which London FRS has made over the last year, reducing their average disruption time around DA cylinder incidents from what was over 19 hours down to 2 hours. The statistics which London (FRS) have collected along the way very much support the BAM research indications.’’
Thornton would not be drawn on where he thinks the answer to the big research question above (How many hours cooling needed?)will emerge, but said, “If I was a betting man, I’d have a tenner it will be a lot less than even the old 12 hours cooling.”
Thornton expects the research to come to
a conclusion in the first half of 2009.