Minimum pressure to support a flammable atmosphere

Welcome aboard "BILLC".

You pose an interesting question that cannot be answered due to insufficient information. You are also very close to an area that could be regarded as "tabboo" until your credentials are understood.

The Chem guys would be wanting to know things like

What is the "ignitable" material? (And then whether it's self supporting in combustion/explosion. Does it need Oxygen like petrol does or is it like TNT?)

What's the residual gas? (Like "Is it in the same ratio as air or is the 100mbar pure Oxygen?")

Regardles of what anyone on this site says in relation to this, when you have the details, contact a professional in fire/explosives for them to absolutely check everything. You only get one chance.

From my limited experience the answer from others will also be "Depends absolutely on the specifics of the situation and cannot be answered in the general and public arena of this forum."

Thanks for the reply,

Oddly enough I am a proffesional in fires and explosions, the prevention of them, and hazardous area classification is what I do and I am not operating in any taboo area!.

There are two reasons for the question, one of which is drawing down a vacuum on a vessel that initially contains solvent under air, at what point of vacuum is the atmosphere inside the vessel incapable of being ignited, due to the mean molecular separation of the oxygen and solvent molecules in the mixture. I have read somewhere that below 100 mbarA ignition cannot happen, but I can't remember where, I have also read in an internet article today the figure 60 mbarA.

The second reason is an intended plant to test gas meters, using natural gas, filling them by first evacualting them and then flooding them with gas (pure natural gas no oxygen) and then letting them back up with nitrogen.

The hope is that throughout this cycle there will never be a potentially explosive atmosphere, but to smooth out the operation there is an evacuated reservoir which could contain a mixture of natural gas, air and nitrogen, but would be held at a very low pressure in normal operation. It is the consideration of the explosion risk within this vessel that concerns me.

Any pointers very gratefully received.

Billc

I think one more parameter was missed and that is Temperature. It sure affects the ignition point other than fuel density. There is critically medium pressure level where chances of ignition are comparatively higher than at high concentration level and very low concentration level of fuel. For any volatile matter, these figures must have been compiled and can be obtained from sources like www.ics.org www.epa.gov/ etc.

The US bureau of mines has done extensive testing of the flammability limits of methane in different atmospheres (different O2/N2 mixtures). They may have also done testing at different pressures. I used to have a hardcopy of this report but could not locate it. I think it was originally published CA 1963.

I don't think there is any precedent for methane in air to detonate Vs deflagrate. On the presumption this is true, you may get a practical answer to your question as follows:

For stoichiometric combustion with air in a closed system calculate the ratio of final/initial pressure. Within the adiatitic assumption, the ratio of final/initial pressure will be the same regardless of pressure where combustion was initiated. The stoichiometric assumption should represent a worst case. If the resultant pressure from combustion under vacuum is within the safe range of the device, you have identified a safe operating window in the event it does burn even though you have not answered the question whether it could burn.

There are several good answers/questions and I have another question - what ignition source are you worried about.

Self ignition for natural gas is about 1700 degrees F.

An open flame or a spark - then ignition is below 0 F.

So as asked by others - what is the gas - what is the atmosphere? O2 level? Temperature?

About 14 years ago I designed and built a gas meter test system for a UK based gas meter manufacturer which is still in operation. The gas meter manufacturer has asked for an larger version to increase production capacity. The system uses an intermediate inert gas to seperate the fuel from the oxidant

The sequence of operations is as follow:

The connections are pressure tested with air to the maxomum design pressure of the meters (75mbar).

The meters are purged of air with a slug of nitrogen (constant flow rate for a timed period to give 5 meter volumes through the meter).

Natural Gas is then used to purge the nitrogen to 100% gas (constant flow rate for a timed period to give 5 meter volumes through the meter).

Calibration tests carried out

The meters are purged of Natural Gas with a slug of nitrogen (constant flow rate for a timed period to give 5 meter volumes through the meter).

Air is then used to purge the nitrogen (constant flow rate for a timed period to give 5 meter volumes through the meter).

A different approach to your problem.

I suggest you contact NASA -I am certain they (one of their thousands of contractors) has conducted studies of this subject. The lower explosive limit pressure will depend on the substance oxidizing--the higher the heat release, the lower the self sustaining pressure.