Question 230Safety & Environmental - Assistant Engineer

The correct answer is B) that concentration above which the mixture is too rich to burn. The upper explosive limit (UEL) is the maximum concentration of a flammable vapor or gas in air above which the mixture is too rich to burn. At concentrations above the UEL, there is too much fuel (flammable vapor) compared to the available oxygen, making the mixture unable to ignite and sustain combustion. The other answer choices are incorrect: A) is incorrect because the UEL is the concentration above which the mixture is too rich to burn, not just enough to produce an explosion. C) is incorrect because the UEL is not defined as a specific percentage of flammable vapor, but rather the maximum concentration above which the mixture is too rich. D) is incorrect because the UEL is not defined in terms of the percentage of oxygen present, but rather the concentration of the flammable vapor.