Is it not because compressed gas is hotter that gases at atmospheric conditions?

David

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.

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!

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....

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

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 :}

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