Compression Ignition in Pneumatics 1

[jasoncwells]

A long time ago, I recall that there is a concern with compression ignition of oil or other flammables inside a pneumatic system if the rate of pressurization was too high. The place that I used to work had really high air pressures. I'm not a pneumatics guy as my day job. My google-fu gives me lots of links about diesel engines.

The air pressure in the proposed system will be 500 psig. Perhaps ignition is not a concern at lower pressures.

Can you point me to a good reference for this?

 

[btrueblood]

Find the lower ignition temperature (flash point) for your oil/flammable. Find the adiabatic temperature rise for a rapid compression event (hint: if you are filling from a pressurized tank into a lower pressure space, can the total temperature, T0 , increase?). Second hint: if a piston is moved freely by a compressed air pushing it from behind, will the opposite side increase in temperature?

 

[jasoncwells]

Somewhere in the deep dark recesses of boxes from my last move lies a thermo text. I am told by the grey beards that if it should ever see the light of day, that a great scourge will fall upon the land.

Looks like I'll have to do some work.

 

[CWB1]

The two links below are good relevant primers.
https://apps.dtic.mil/sti/tr/pdf/AD0701576.pdf
https://www.icheme.org/media/9189/paper58-hazards-23.pdf

 

[Compositepro]

In reality no one thinks about this issue because it does not cause any problems. Just like gas tanks on cars are not exploding regularly. It special cases it may need consideration. Why are you concerned?

 

[Prometheus21]

Having designed and manufactured equipment intended for both low and high pressure (industrial) gas service, including oxygen, the general risk of ignition in compressed air systems is always something to consider; however briefly.

Note that there are a multitude of possible ignition mechanisms at play; particle impact, adiabatic compression heat (gas hammer effect), electric arc and spark ect. Combine this with a highly flammable contaminant in your system and yes; you will have an ignition. There is a reason pneumatic impact test methods and standards cover air, in addition to oxygen and other gas blends containing oxygen. You will also find that ignition temperatures will be listed for both air and oxygen atmospheres. Side-note: above 3000 psia the selection of non-metals should equal for both air and oxygen systems, at 5000 psia the selection of metals should also be equal.

As you can tell a possible ignition will depend on so many variables, its impossible to say for certain if you will have one or not, and if you will get a kindling chain. At 500 psig you will (IMO) be fine as long as your system is moderately clean. Like Compositepro states; no one really thinks about this issue, accidents happens so rarely. Then again they DO happen, and those cases are the entire reason why cleaning for oxygen service (and/or oxygen blends-like air) is such a huge part of my job description. So my recommendation? Always err on the side of caution when dealing with pressurized systems, especially if they have an oxygen content of around 21% or more.

As for good references: CGA & EIGA documents are good starting points, as well as your typical text books on the topic of thermodynamics and heat transfer.

 

[kjoiner]

Not completely relevant but high power spring piston air guns can experience "dieseling" if excess oil is in the compression chamber. Some people think it's a way to get more power and intentionally put oil in the chamber but that comes at the expense of damage to seals and other components.

Kyle

 

[jasoncwells]

In reality no one thinks about this issue because it does not cause any problems. Just like gas tanks on cars are not exploding regularly. It special cases it may need consideration. Why are you concerned?

We are looking at taking a hydraulic landing gear actuator full with mil oil, draining it, and then using compressed air to unlock and retract the actuator. The pressure needed to unlock the actuator is 500 psig.

So we will certainly have a substantial amount of oil contamination and a reasonably high air pressure. It's worth considering the risk and doing the homework.

 

[jasoncwells]

The two links below are good relevant primers.
https://apps.dtic.mil/sti/tr/pdf/AD0701576.pdf
https://www.icheme.org/media/9189/paper58-hazards-23.pdf

The first link lead me to this https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1559-3584.1960.tb02385.x. The citation is found in the footnotes of your link.

Given that "The Unintentional Ignition of Hydraulic Fluids Inside High Pressure Pneumatic Systems" by White starts with a description of a fatal accident in the US Navy on a carrier, that is most likely the source for the training I got in A school and the source of my initial suspicion regarding auto-ignition.

The accident was the USS Bennington in 1954.

Adding these breadcrumbs here for posterity.

 

[CWB1]

Not related to pneumatics hence my earlier failing to mention/sidetrack, but I've done some related testing on fuel systems. On large natural gas industrial engines, packaging often requires piping a fuel/air mix several-tens of feet to the engine so autoignition, engine backfires, etc are legitimate tho unlikely concerns. We added PRVs and bungs fitted with spark plugs to a length of the fuel piping in a dyno cell, tried to ignite it with an engine running, then forced backfires at various temps, fuel system pressures (comparably low to pneumatics), and fuel mixtures. We discovered that it was actually harder to ignite in reality than CFD predicted.