Tankers carrying cryogenic cargoes, such as LNG, are fitted with gas detector systems alarmed at 30% of the lower explosive limit. If the gas detector alarm sounds, this means __________.
β’ Lower Explosive Limit (LEL): what it actually represents in terms of flammable range β’ Why gas detectors on LNG/cryogenic tankers are set to alarm at 30% of the LEL, not at 100% LEL β’ Difference between percent of LEL and percent by volume of gas in air
β’ Ask yourself: when a detector is set to alarm at 30% of the LEL, is the atmosphere already within the flammable range, or is this an early warning before it becomes flammable? β’ Consider whether "30% of the LEL" means 30% of the minimum flammable concentration, or 30% of the entire atmosphere being gas. β’ Think about what the purpose of the alarm is from a safety standpoint: to warn you that conditions are imminently explosive, or that you are approaching a dangerous level and must act before it becomes flammable?
β’ Distinguish clearly between "too lean to burn" vs. "about to explode"βwhich better matches an alarm set to 30% of the minimum flammable concentration? β’ Verify that LEL is a concentration point, not a volume fraction of the whole atmosphere like "30 percent by volume" or "30 percent of the atmosphere is explosive." β’ Check whether any option incorrectly treats 30% of LEL as if it means "30% gas in the air" instead of "30% of the minimum flammable concentration value."
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