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Compressed Air and Spontaneous Combustion / Auto-Ignition
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Can someone help me understand how compressing air can increase the risk of spontaneous combustion / auto-ignition of oils, greases, etc?
I have read HSG39 and it refers to this as a hazard but provides no detail or evidence as to why.
I'm talking here about compressed air only, I fully understand the risks associated with using pure oxygen.
Obviously increasing the oxygen content significantly increases the flammability range; but with compressed air, surely you're only changing the amount of total air (Nitrogen, Oxygen, CO2, etc) but all in the same concentrations??
I'm a chemical engineer by education, so I have a fairly good grasp on this sort of thing, but I'm struggling to understand this.
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Rank: Super forum user
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Is it not because compressed gas is hotter that gases at atmospheric conditions?
David
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Rank: Forum user
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David,
Thanks for your reply.
I don't believe this is anything to do with temperature, although the exhaust gas from a compressor would indeed be warm / hot.
Are there any chemists out there?
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Rank: Super forum user
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Im no expert, but doing a little search on-line I found this link with the following info:
http://www.aocweldingsupply.com/News/1318
Air compressors work by mechanically decreasing the volume of air increasing its pressure. During this compression the temperature of the air increases.
Since air is under pressure it has the hazards associated with compressed gasses, and by adding flammability to a compression hazard you would have an increased volatility risk.
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Rank: Super forum user
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Rich, can you point us the the paragraph in the document where it says this? Some context would make a lot of difference.
I'm a chemist, btw
Jane
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Rank: Forum user
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Guru,
Thanks for the link but I'm not sure that's it though. Compressed air isn't flammable.
Jane,
HSG39 lists this a number of times:
In "Introduction"
2 There are many ways in which compressed air can be dangerous, for example:
(d) oil-coke deposits in a system can spontaneously ignite and cause an explosion;
Explosion protection
24 If components run hot, or oil coke deposits spontaneously ignite, there may be an explosion.
The context that I'm searching for this is:
We are preparing a new structure to be installed subsea that includes hollow (tubular) sections.
The structure has to be pressurised to the seabed water pressure prior to sailing.
The installation contractor proposes to fill the structure with Nitrogen, which at a rather large sized structure and 10 bar pressure is a significant quantity of Nitrogen.
I have asked why the structure can't be pressurised with air (corrosion is not a concern, so air could be used from that standpoint) to remove the hazard associated with handling such large quantities of Nitrogen; the response that I got was that there was a risk of any residual oils or greases inside the structure igniting but the contractor couldn't give me a definitive answer.
It seems that filling these structures with Nitrogen is just what they always do.
HSG39 may not be the ideal document, as it mostly deals with air compressors themselves, rather than compressed air.
I'm thinking that their response was along the lines of pressure changing the flammability range or auto ignition temperature of the oils / grease inside.
Any ideas?
Thanks to everyone that has commented so far, btw!
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Rank: Super forum user
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They may be right - pressure does seem to influence the minimum ignition temperature. I have done some searches in Google Scholar, and there was some research with hydrocarbons used as fuels here:
http://www.jstor.org/stable/2935480
It looks as though increasing pressure reduces the minimum ignition temperature.
While the act of compressing air does heat it, the compressed air will soon cool down to the ambient temperature. However, the inrush of compressed air into the structure I think might lead to local heating.
I'm not sure that this is an experiment I would want to do!
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Rank: Super forum user
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quote=rich_bannister]Can someone help me understand how compressing air can increase the risk of spontaneous combustion / auto-ignition of oils, greases, etc?
Certainly I'm no scientist, but compressing gases produces heat, so I'd assume that if the heat generated by compression exceeds the LEL or flashpoint of a material or substance that would lead to ignition?
I'm probably wrong though....
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Rank: Super forum user
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Hi folks - interesting topic and has been a subject that has been of major concern in many industries that use compressed gases.
Whilst it is true that the compression of the gas generates heat, this isn't the major part of the problem - it's the release or escape of the gas that' normally presents the problem; and not just oxy or compressed air as pretty much all compressed gases present the similar problem of static generation and also heat generation as the pressurised gases escape through whatever exit means is provided.
In itself this is normally not a problem as design of exit points can take the projected heat generation into account and dissipate it, but if there are any adjacent flammable materials [normally oils, lubricants and paints] - especially at the gas exit point - the potential for a fire, or more likely an apparently spontaneous explosion is raised considerably. Given the original post - this appears to be the area of concern.
Nitrogen, being a natural extinguishant, is the preferred pressurisation agent where the cleanliness of the containing structure [with regard to flammable materials] cannot be assured and the heat & static generation may not capable of being controlled as it simply doesn't support combustion wheras "Air" does.
There may also be a pressurisation issue due to the proposed location but I can't answer that.
Hope this helps Frank Hallett
You might like to try asking organisations such as your local Fire Service, Diving organisations, BOC and similar for more technical explanations; but that's the problem as I understand it.
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Rank: New forum user
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Unlike you, I am not a chemist so if the following is wrong, let me down gently. It seems to me that the proportions of gases in air is irrelevant. That which is not oxygen does not act as a suppressant unless in large enough proportions to starve a fire. Compressing air increases the amount of oxygen per unit volume. To achieve the same increase per unit volume of uncompressed air would require an increase in oxygen percentage. The effect of that would be that any small ignition source would provide a fierce fire, even in substances that will not normally burn easily in air. Compressing air then, would also increase the risk and intensity of a fire, even to the point of explosion. It is possible that spontaneous combustion could occur from the strong oxidising reactions which could occur if any suitable contaminents were present and could spread even to metal pipework. Well, that's my take.
Regards,
Bob
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Rank: Forum user
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Bob Baynes wrote:Unlike you, I am not a chemist so if the following is wrong, let me down gently. It seems to me that the proportions of gases in air is irrelevant. That which is not oxygen does not act as a suppressant unless in large enough proportions to starve a fire. Compressing air increases the amount of oxygen per unit volume. To achieve the same increase per unit volume of uncompressed air would require an increase in oxygen percentage. The effect of that would be that any small ignition source would provide a fierce fire, even in substances that will not normally burn easily in air. Compressing air then, would also increase the risk and intensity of a fire, even to the point of explosion. It is possible that spontaneous combustion could occur from the strong oxidising reactions which could occur if any suitable contaminents were present and could spread even to metal pipework. Well, that's my take.
Regards,
Bob
I am (or at least was) a chemist and Bob is correct in what he says IIRC. If my memory serves me we are talking about Dalton's laws of partial pressure, which for our purposes essentially say that the chemical activity of a gas is (partly) determined by its concentration in the atmosphere completely independent of any other gas that may be present. thus increasing the air pressure increases the concentration of oxygen per unit area with a resultant increase in the likelihood of fire.
For the same reasons your partially empty bottle of fizzy drink will stay fizzy longer if you squeeze the bottle to remove most of the airspace above the drink before putting on the cap.
I now wait to be throughly trounced by a chemist who has looked at a textbook far more recently thatn i have!
Jimothy
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Rank: Super forum user
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rich_bannister wrote:Can someone help me understand how compressing air can increase the risk of spontaneous combustion / auto-ignition of oils, greases, etc?
I have read HSG39 and it refers to this as a hazard but provides no detail or evidence as to why.
I'm talking here about compressed air only, I fully understand the risks associated with using pure oxygen.
Obviously increasing the oxygen content significantly increases the flammability range; but with compressed air, surely you're only changing the amount of total air (Nitrogen, Oxygen, CO2, etc) but all in the same concentrations??
I'm a chemical engineer by education, so I have a fairly good grasp on this sort of thing, but I'm struggling to understand this.
Interesting range of opinions from a number of “safety professionals”, I thought that being a safety professional meant not dabbling in areas outside of your comfort zone. i.e. to coin a phrase, How can you tell when "you don’t know what you don’t know".
Is there a significant risk of an ignition of residuals within the subsea structure?
No, I think not, and that’s notwithstanding that the amount of residuals within the structure is unlikely to even reach lower explosive limits.
Rich, given the last sentence of your post, I am surprised you are asking the question. I would have thought that you would understand the concept of adiabatic compression and expansion?
“An ideal gas heats up during the adiabatic compression, while it cools down upon adiabatic expansion”. i.e consider a diesel engine.
Granted, ignition is likely within oil lubricated air compressors and associated compressed air systems or in the case of these compressors, ignition in the event of oil or other contamination at the pre filter, other causes could be something as simple as a failure of compressor after cooler.
The only way your sub sea structure would be subject to an ignition and over pressure scenario, would be to introduce rapid pressurisation of the internal volume of the structure and have residuals or contaminants at 50 %of LEL and above or at or below UEL. This will not be the case at regulated discharge pressure of the compressed air into the structure and particularly so in the case of adiabatic expansion and cooling.
BTW. You can verify volume of air required to pressurise your structure and the time required to reach 10 Bar(g) at a reasonable air flow rate and pressure using the ideal gas law, but you already know that ;}
Bob/Timothy, I would be very interested in seeing your calculations concerning the increase of O2 conc with pressure??
PS I am not and never have been, or even wanted to be a chemist :}
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Rank: New forum user
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bleve,
I thought the only one with a problem on this topic was rich_bannister, apparently not. I am not a chemist. I acknowledged my shortcoming in that direction upfront. I put forward something for him, with his own, undoubtedly greater chemical knowhow, to consider. You appear to have misunderstood my post. That does not matter. You do not need the answer. You also make the common assumption that all contributors are safety professionals but this is a public discussion forum. I shall make no further contribution to this topic.
Bob Baynes
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Rank: Super forum user
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rich,
I have followed this since your OP.
I am curious with regard to our concerns over nitrogen.
Also, I would be more concerned about the stored energy in the structure if tested with compressed gas of any kind rather than the spontaneous combustion of the compressed air.
There may be residual hydrocarbons within the structure, and there will almost certainly be residual hydrocarbons in almost any compressed air distribution system if manufactured in the traditional manner with screwed steel joints from the manufacturing and assembly process.
I await correction but cannot recall or even envisage, any incidents of spontaneous combustion of compressed aid distribution pipework without other failure modes contributing to the incident.
I suspect that this is a large volume structure?
There are several case studies of the explosive failure of significant apparently sound structures when under pneumatic testing.
HSE strongly recommend hydraulic testing due the staggering reductions in the stored energy.
Can you not hydraulically test your structure?
Are you not able to allow water into the interior as this would be cheap? Also much safer.
The only issues would be the differing sizes of the molecules with regard to the differing test fluids.
Nitrogen is preferred for inflating aircraft tyres and racing car tyres I am led to believe as the molecule size is consistent and larger then some of the molecules in air, which can seep through the rubber tyres which are semi-permeable.
However, would this be an issue in your applications?
If the structure is to be at STP when submerged then subjected to external pressurisation by sea water then if fresh water is used to pressurise the interior for test, I suspect that the molecule size will be smaller, thus it will ensure that the sea water cannot enter if the fresh water cannot escape.
As I understand it you can also hire nitrogen generators so that you do not have to transport or store large volumes under pressure if you wish.
Though I would still recommend hydraulic testing!
I am not sure if I have misunderstood your OP or not.
p.s. Not a chemist, but an engineer, who has worked a lot in "failure of stuff"!
Paul
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