Flash Back Protection (LEL/LFL determination) 2

[BlkFury]

I will be setting up flash back protection for a large number of processes (exhaust) and I'm having trouble finding sources that are more specific to pipe flow concentrations in regards to LEL/LFL. These sitiuations would not be a percentage "In Air". I need a source that can give me an LEL/LFL based on oxidizer/fuel ratios and inert/carrier percentages. The "Air" componet would only come from small leaks, but the process may have an oxidizer in the effluent stream.

 

[Flareman]

BlkFury
You can construct a workable flammability curve estimate with Nitrogen, Oxygen and Flammable on a triangular diagram

Horizontal axis = 0 -100% N2
Vertical line stating at N2=79% = air
Oxygen starts at 0 N2 = 100% O2 and runs to top of Air line
Gas starts at 100% N2= 0% Gas and runs to top of Air line

(3) three sets of coordinate lines
Gas is parallel to N2 axis
Oxygen is parallel to Gas axis
Nitrogen is parallel to O2 axis

All points add up to 100% total mixture

Find the stoichiometric % of gas air mixture on the air line.
Draw the stoichiometric line from 0% gas through the stoich % in air and project to the O2 line

Mark the published LEL and UEL on the air line
Draw a horizontal line through the LEL from the stoic line to the O2 line. This is the LEL of all mixtures.
From the intercept of the LEL and stoic line, draw through the UEL% in air and project to the O2 line
This is the UEL for all mixtures.

The triangular diagram to the RH side of the air line is the same diagram you would normally see in a set of LEL/UEL curves. Just the base line is different because the Nitrogen line needs a second axis which starts at the bottom of the air line (N2=79%) and runs out to the RH corner. Call this added nitrogen and scale it 0 - 100%.
If you have a mixture of flammable and inert it works in this part of the curve.
If you have oxygen, it pulls you over into the LH side of the curve.

If you have an inert other than Nitrogen, use the Nitrogen equivalent for the inert in question
N2=1
H2O=1.35 x N2
CO2=1.82 x N2 (ie 3vols CO2 counts as 5.46 vols N2)
SO2=2.1 x N2
He =0.65x N2

Points inside the envelope are flammable.
Points which are outside the envelope will move in a straight line to the bottom of the air line when they get out of the pipe into the atmosphere so you can also tell whether your mixture is permanently non-flammable, or flammable when it meets the air (because it psses through the envelope)

These are rough estimates but should give a good idea of what you need to do.
USE A GOOD FACTOR OF SAFETY
I recommend at least 1.5, and remember that the corner where the UEL line and LEL line meet the Stoic line is where you have the biggest error.

This is NOT a chemically consructed curve. if you need to think about oxidisers other than contained oxygen, you have to start from scratch and look at the heats of formation and enthalpy based on stoichiometry as if it were a chemical reaction (which of course it is).


I hope that helps you.

David

 

[BlkFury]

Thanks. That confirms what I found in a 1997 NASA document; the LEL/UELs are drawn in on the triangle graph as well. I will be running some tests in 2" SS pipe this next week to look at pressure waves and confirm flame velosity. By my calculations, the addition of an oxidizer really opens the envelope.

 

[Flareman]

BlkFury

You tests sound intriguing. It would be interesting if you could publish a link to your eventual findings.

Have you looked at the work done by some commercial manufacturers in their efforts to develop detonation arrestors to meet the USCG specifications?
Flame breaking into cells etc.

Personally, I don't see enough in the literature to allow "interpretive evaluation" of flash-back information because most work is done in pipes with uniform bore (it's a lot easier that way). The problem is that hardly any real systems are constant bore and the run-up distance to detonation depends on pressure growth, which, in part, is a function of total volume and closed or open end, but is also quite dependent on the properties of the open end of the pipe through which the exhaust has to pass.
Despite that, the standard run-up distance is always applied commercially, based on the pipe diameter containing the arrestor, with little thought to the downstream configuration.

Just trying to stimulate things for you to think about.

David.

 

[BlkFury]

I have looked at other works, but I prefer to do the testing myself. I also want to simulate an air leak in the line, test with strait pipes, several 90's, etc. My predecessor did some work, but I don't think it was very comprehensive; didn't take into account process oxidizers. It will be a few week down the road, but I'll see if I can't publish the results for you somewhere.

-Sam

 

[Flareman]

BlkFury

Sorry to be a haunt, and I hope I'm not crossing a line here but you mention "process oxidizers". I'm not sure what you are suggesting.

If you are indicating that you don't have oxygen itself but rather materials which are essentially ustable in themselves or something which is potentially reactive with the fuel, you probably need to start with first principles and look at the relative heats of formation and enthalpies of the various materials over a range of temperatures as if you are designing a chemical reaction ('cos you are).

Normal LEL is just a measure the minimum energy output from a chemical reaction (combustion) which can transmit activation energy from a reacting zone into an adjacent non-reacting zone. The use of LEL is an engineering convenience rather than a scientific basis.

You may find that the "flammability curve" for such a mixture is miles away from the oxygen curve because of the energy features of the other reactant(s). Oxygen and Nitrogen, being elements, have zero heat of formation which levels that playing field for normal combustion.

Another aspect of the reactive situation can be seen if you look at the way that acetylene decomposes in a pipe at low pressure. The decomposition is like combustion (but with no oxygen) and the fuel transitions from one molecule at a positive heat of formation into two molecules with a lower cumulative energy level. The released energy during the decomposition is enough to transfer activation energy into the adjacent layer and the process propagates explosively .. unless .. you are in a small enough pipe, in which case the heat transfer to the pipe wall tends to absorb some of the energy and the process is arrested. The less suface area to pipe area you have (as the pipe gets bigger) the easier it is to get an explosion. I mention this because your 2" pipe tests may not translate to 6" etc pipes unless you address the relative energy levels on this basis.

Please don't misunderstand my intent here but I feel a need to point this out to round out the subject for other readers.

Good luck anyway

David

 

[BlkFury]

Good points all around and I appreciate any in-depth knowledge on the subject. All my processes will be exhausting to 2" pipes with a wide range of fuels, oxidizers, and pyrophorics; including our friend ClF3. My main concern in this instance are the high volumes of H2 and possible oxidizers in the flow. I'll be sure to check out the heats of formation for the 'molecule' constituents. Thanks for your help. I'll post updates on some of my findings, but I need to be careful about posting proprietary information.