Vacuum pump - flammable atmosphere

[scorer]

A bit of a basic post with little detail to start off but here goes.

With a vacuum system, how can you estimate if there is a flammable atmosphere inside the vac pump.

I imagine it's a case of calculating flow rate of flammables removed, and amount of air leakage into the system. Then looking to see if it lies between upper and lower flammability limits. Am I on the right track? Any ideas on how to proceed would be helpful.

Also, what sort of a bang do you get, and how likely is it to happen when running at say 33mbara?

 

[25362]

One source says that flames don't propagate at pressures below 50 mm Hg. You should, thus, not worry about flammabilities or explosivities at 33 mbara.

However, at pressures greater than 50 mm Hg, and up to 1 bara, LFL values are just the same as those at atmospheric pressure.

 

[Torricelli]

worst case will happen during the evacuation from atm. pressure to the operating pressure.

25362's info is of interest for multistage systems (for example roots blower + liquid ring vacuum pump): If you are able to make sure that the vacuum blower is never running at a pressure higher than the limit of flammability (50mmHg?) but starting when P(system)<50mmHg, you may save the flamme arrestors (and the related pressure drop) for the first stage.

Torricelli

http://www.vacuum-guide.com

 

[scorer]

Thank you for the info.

25362, do you know what the source is for the fact that flames don't propagate below 50 mmHga? Can anyone enlighten me on the reason behind this.

 

[25362]

Theories on gaseous diffusion and molecular collision would probably elucidate flame propagation as a result of heat transfer by convection, conduction and radiation, however I don't know of a reasonable and proved explanation.

I can tell you that it has been proved that pressures below atmospheric narrow the flammability range by rising the LFL and reducing the UFL until the two limits coincide and the mixture becomes nonflammable.

I'm not sure but I think it was the Bureau of Mines that published curves on the flammability of hydrocarbons as function of pressure. I happen to have that for methane showing that at pressures lower than 100 mm Hg, and 20^oC, there is no flame at any (0-100%) concentration of methane in air.

 

[quark]

25362,

What about above ambient pressures at discharge side of a pump?