Question 1142 Deck General

The correct answer is C) a flammable vapor concentration exists at the sample point, but it is too lean to burn. The explanation is as follows: 1) Gas detection systems on LNG tankers are typically set to alarm at 30% of the lower explosive limit (LEL) of the cargo vapor. This means that when the gas detector senses a vapor concentration that is 30% of the minimum concentration required for the vapor to ignite and explode, the alarm will sound. 2) At 30% of the LEL, the vapor concentration is too lean to actually burn or explode. The alarm is meant to alert the crew to the presence of flammable vapors so they can take appropriate safety measures, but an explosion is not imminent at this level. 3) The other answer choices are incorrect because they do not accurately describe the situation. An explosion is not about to take place (choice A), the atmosphere is not 30% explosive (choice B), and the detector is not sampling a space with 30% cargo vapor by volume (choice D).