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safety factor for operating above UEL

safety factor for operating above UEL

safety factor for operating above UEL

I am trying to find some references, such as codes and standards, that give an idea of acceptable safety factors for gas concentrations when running a system above the upper explosive limit.

Does anyone know of any standards or codes that address this?


RE: safety factor for operating above UEL


    The old Bureau of Mines published lots of experimental data on the upper explosive limits of various fuel/air mixtures.  The reports are on the web at


     I am not certain if there is a standard on safety factors that address this.  The Bureau of Mines website above might be a good place to start looking.  

      I would think that a chemical engineer who works in the petroleum industry would know about this topic.


RE: safety factor for operating above UEL

As far as I know there are no specific requirements for operation above the upper flammibility limit, but it is suggested that, where possible, the composition should at least be twice the UFL.

This is in contrast with operations below LFL, where requirements are given in NFPA 69.

RE: safety factor for operating above UEL

The thing that determines the safety factor above the UEL is the system design itself. For example we operate extraction pumps down to 25% v/v flammable gas (mines gas CH4 plus other alkanes UEL 15%ish) with a 10% v/v safety factor.

This may not seem like a large safety factor, but we use liquid ring pumps and they only compress to low pressures, so the system in and of itself has not got obvious sources of ignition.

Where the gas is delivered to engines or boilers the safety factor is increased as an ignition source is introduced.

By careful measurement of the gas mix and careful assessment of the T90 times for the transducers (and pipes feeding transducers) you can stop any mix below the UEL reaching the source of ignition.

You would obviously need to design for an explosion within the system and consider flame arresters / explosion isolation, explosion pressure relief / containment, but the level of protection can be increased to suit the risk and the scale of hazard.

As far as I am aware there are no international standards for this, but in Mining, each country sets national limits.

RE: safety factor for operating above UEL

Butlen, I don't know to what pressure the mentioned liquid ring pumps discharge, however the old formula that gives the corrected UFLp under pressure is always useful:

UFLp = UFL + 20.6 (log P + 1)

Where P is in megapascals.

Thus, if the discharge pressure goes up to, say, 3 bara = 0.3 MPa, the expected increase in the UFL of 15% would be given as:

UFLp = 15 + 20.6 (log 0.3 + 1) = 24.8% leaving little margin below the adopted 25%.

Is inerting being used to reduce even more the oxygen concentration of the mix ?

RE: safety factor for operating above UEL

       25362   posted a formula for how the upper flammability limit is affected by pressure.  Is there a similar formula for the lower flammability limit?  Does anyone know where this formula came from?


RE: safety factor for operating above UEL

To j2bprometheus:

1. The expression is empirical, quoted from M.G.Zabetakis,"Fire and Explosion Hazards at Temperature and Pressure Extremes," AIChE-Inst. Chem. Engr. Symp. Ser. 2, Chem. Engr. Extreme Cond. Proc. Symp., 1965, pp 99-104.

2. Pressure has little effect on the LFL except at very low pressures (<50 mm Hg abs.), where flames do not propagate.

Besides, here are several threads that dealt with similar subjects:


No doubt there are more threads dealing with ammonia and hydrogen flammability limits.

RE: safety factor for operating above UEL

A little late with this response but ..
NFPA 86 likes -
25% of the LEL concentration for lean gases/mixtures .. and
UEL concentration of Oxygen (presumably at 25 degC) minus 3% for rich mixtures;
Coast Guard Regs (33CFR154) requires Oxygen at UEL/1.75 for rich mixtures.

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