Comments on: What is the TNT equivalence of acetylene

By: Mark Ribbands

Mark Ribbands — Thu, 13 Jul 2017 08:47:16 +0000

In reply to Mark Ribbands. Gentlemen, thank you for the support .

By: Ian MacDonald Watson

Ian MacDonald Watson — Wed, 21 Jun 2017 20:59:14 +0000

Chris,

Some work by the UK MOD Naval Authority Explosives suggested that the most likely result of heating gas cylinders in a fire, whatever their contents, would be to cause the pillar valve to blow out, rather than an explosion (as suggested by David Marriott above). However the extent of the investigation was limited and it would need to be repeated with various sizes and types of manufacture of acetylene cylinder related to your question to confirm the answer.

By: Chris Marriott

Chris Marriott — Tue, 20 Jun 2017 13:35:16 +0000

Thank you all very much for your feedback. My initial response to our regulator was along similar lines, i.e. that an acetylene bottle would not ‘detonate’. However, they want to know what the ‘worst case’ over-pressure could be and TNT equivalence was considered to be the simplest method to determine this, albeit significantly overly conservative.

By: David Marriott

David Marriott — Fri, 16 Jun 2017 10:17:03 +0000

I would suggest keeping it simple. There is no mileage in trying to give an explosives equivalence because 1) Acetylene isn’t an explosive and 2) the data would be extremely variable depending on circumstances. Acetylene burns quiet badly in atmosphere with a ‘smoky’ yellow flame. This only improves with the introduction of additionalOxygen (more Oxygen, better burn, hotter flame). You can’t weld with Acetylene alone (not hot enough). The hazards to consider with bottled/ pressurised Acetylene would include damaging the cylinder (causing a leak – fire hazard or explosive decompression – as with any, even inert, pressurised gases) and heating the cylinder. The heating of the cylinder in a fire for instance, would increase the pressure of Acetylene possibly to the extent of causing a physical explosion. This may give a shrapnel hazard and pressure wave hazard. If an ignition source were in proximity, there would also be a combustion hazard (Acetylene burns). Consideration should be given to any Oxygen or oxidisers are also in proximity as these too present hazards and increase the hazard of the Acetylene.

By: Don Williams

Don Williams — Thu, 15 Jun 2017 23:17:40 +0000

I undertook research for a similar question some years ago. I agree with Mark, firstly gas is not HD1 and applying chemical explosive considerations to gas events is not of great value.
“A gas explosion is where an increase in pressure causes a mechanical rupture or a cloud of combustible gas ignites. Gas explosions are further defined as: Boiling Liquid Expanding Vapour Explosion (BLEVE), Vapour Cloud Explosions (VCE) and a “Jet” of burning gas. A VCE where a cloud of gas reaches a critical fuel air balance, referred to as the “stoichiometric” range concurrent with contacting an ignition source. In a VCE it is possible for the burning cloud front to achieve a “deflagration to detonation transition” resulting in an expanding high-pressure, high velocity detonation front (See, for example, CCPS 1994).” From Bomb Safety and Security – the Manager’s Guide Williams D. 2016 Collaborative Publishing
There are a number of texts on the very complex area of gas explosions, some of the best are by TNI of the Netherlands. The overpressure / distance is difficult to assess depending on the type of event. If the cylinder ruptures and the result is a VCE the pressure will depend on the volume of gas, the point at which the ignition source is contacted, the speed of the gas cloud, weather, terrain, etc. and whether DDT is achieved (I would suggest unlikely for a small cylinder).
My review found that the gas cylinders were designed to be safe to use and handle including being unlikely to BLEVE if subjected to fire. Even if acted upon by external effects such as fragment impact etc they could be expected to respond as designed and either survive, vent or ‘jet’, this being a fire rather than explosive event.
The US did some work on the use of acetylene as a ‘booster’ after the 1993 World Trade Center bombing which may be of relevance.
In summary I suggest not trying to use TNT equivalency for a gas event rather concentrate on ensuring the cylinders are manufactured, placed and used in accordance with the Standards and local requirements.

By: Martyn Sime

Martyn Sime — Thu, 15 Jun 2017 21:59:22 +0000

There are several methods for calculating TNT equivalence but most rely on some form of comparison of the energy released in the explosion process, the quantity of gas produced and the theoretical temperature of explosion. One ready reckoner is available at https://www.ime.org/content/tnt_calculator.

To a degree actual results will depend on the mechanism, which as Mark infers above, would in this case depend on whether the acetylene was explosively decomposing or, in effect, buring in air. Having been involved in investigating one serious acetylene explosion that allowed me to pick up data from others I would agree that there are lots of variables including the size of cylinder, degree of filling or potentially over filling and the nature and state of the absorbent mass which can result in some cylinders simply becoming hot or warm as the decomposition inside the cylinder is arrested by the absorbant mass or in other cases the cylinder violently rupturing.

If your deterministic model is based on blast effects related to TNT equivalence you could end up with an overestimate of the harm resulting from over pressure and an underestimate of the potential harm caused by heavy steel fragments.

By: Peter Ritchey

Peter Ritchey — Thu, 15 Jun 2017 20:53:55 +0000

Lees’ Loss Prevention in the Process Industries: Hazard Identification, Assessment and Control,Frank Lees,Butterworth-Heinemann, 25 Jan. 2005 Page 17/154-157 ( https://books.google.com.au/books?id=UDAwZQO8ZGUC&pg=SA17-PA154&lpg=SA17-PA154&dq=acetylene+versus+tnt&source=bl&ots=2n0t5CZNXO&sig=lCUBvm6V73Bw17y6Leoyk7WE7VA&hl=en&sa=X&ved=0ahUKEwjpkvzZwsDUAhUBGZQKHXKkCgUQ6AEIQjAE#v=onepage&q=acetylene%20versus%20tnt&f=false) gives some good information. In essence the gas would need to be disseminated into a cloud with optimum oxygen to combine to form an explosive mixture, which can be estimated in terms of TNT equivalence . The situation could arise from a sustained leakage.
As an aside I tested a ful 45kg gas cylinder with a 1.25lb CompB block (back in the 80’s). The result was a rupture of the cylinder, no fire and liquid gas remained in the cylinder for some time after.

By: Mark Weatherbe

Mark Weatherbe — Thu, 15 Jun 2017 19:37:46 +0000

Double true on Mark’s comment. The best that could be hopped would be yet more safety Nazi involvement.

By: Yves Pelletier

Yves Pelletier — Thu, 15 Jun 2017 19:33:46 +0000

MSHA define ACETYLENE DATA.
Personally from past experience in Bomb Disposal I finde Acetylene extremely dangerous. Very volatile compared to TNT. Easily ignited
I consider it to be extremely dangerous.

By: Andrew Pettitt

Andrew Pettitt — Thu, 15 Jun 2017 16:59:50 +0000

https://jameskennedymonash.wordpress.com/tag/acetylene

Might be useful

By: Thomas Burky

Thomas Burky — Thu, 15 Jun 2017 13:19:46 +0000

In reply to Mark Ribbands. As a chemist who has done many safety evaluations I agree with Mark's comment. The series of events that would be required to produce anything like an explosion from an acetylene bottle (if properly stored outdoors) are beyond remote. If some sort of consideration must absolutely be had for a safety analysis, then I would factor in acetylene as a fuel source for a fire event rather than in the context of an explosion.

By: Malcolm Ingry FIExpE

Malcolm Ingry FIExpE — Thu, 15 Jun 2017 09:05:49 +0000

Explosion Data [5]
Measured peak explosion pressure: 10.6 atmospheres.
Measured peak rate of pressure rise: 1415 bar-meters per second