Client Background
The client is a manufacturer of acetylene black and carbon materials. Their specific process involves the production of these materials using flammable gases in an oxygen reduced atmosphere.
Client Problem
Process conditions are a hybrid scenario, formed of other gases / vapours and the product being processed or formed. The atmosphere in question was a mixture of acetylene, oxygen, nitrogen, CO, hydrogen & CO2.
Client Objectives
Client initially wanted to establish the Limited Oxygen Concentration (LOC) of the carbon and acetylene black in a gas mix.
Strategy
Sigma-HSE advised that even though there are high levels of flammables, the oxygen content should already be low enough to render the mixture non-flammable as it is below the LOC of both flammable components.
A program was devised to test the acetylene for “dust combustibility” within specific gas mixtures submitted by the client.
The theory was that the weakly combustible acetylene will not be combustible in the atmosphere it is processed in. This could then be used as the “Basis of Safety”.
The material, at start-up is processed in air and therefore a known flammable atmosphere is present.
Although known as a weak combustible dust, acetylene is extremely conductive and therefore determination of minimum ignition energy (MIE) is not possible in a standard MIE apparatus.
The material coats the explosion vessel and creates a short circuit between the high voltage and earth electrodes used to produce a spark of known energy.
A program was devised to test the acetylene for “dust combustibility” within specific gas mixtures submitted by the client.
The theory was that the weakly combustible acetylene will not be combustible in the atmosphere it is processed in. This could then be used as the “Basis of Safety”.
The material, at start-up is processed in air and therefore a known flammable atmosphere is present.
Although known as a weak combustible dust, acetylene is extremely conductive and therefore determination of minimum ignition energy (MIE) is not possible in a standard MIE apparatus.
The material coats the explosion vessel and creates a short circuit between the high voltage and earth electrodes used to produce a spark of known energy.
Insights and results
Testing Overview
The apparatus for dust combustibility mentioned above (20L sphere) uses a 2 kJ ignition source.
Instead, various ignition energy chemical igniters were purchased (100, 250 and 500J) and trials were conducted by reducing the power of the ignition source to establish the range between the lowest ignition energy and highest no ignition energy (basically the MIE).
Instead, various ignition energy chemical igniters were purchased (100, 250 and 500J) and trials were conducted by reducing the power of the ignition source to establish the range between the lowest ignition energy and highest no ignition energy (basically the MIE).
Sigma-HSE found that
The sample of acetylene black tested (in air), formed a flammable atmosphere and was ignitable using a standard 5 kJ ignition. It was not ignitable when using a lower powered ignition source.
It was established that an energy of 250 J would initiate ignition, however 100 J did not, therefore the bespoke “MIE” of the material, in air was reported as 100 – 250 J. This is a huge energy and not one feasible in industry.
Testing the same material in special gas mixes, directly relatable to the clients process environment (highly flammable gases, with low oxygen levels) proved not to be combustible. With this special information, the client was able to prove that the basis of safety for their process could be safely stated as “no flammable atmosphere was present”.
