CMC Flammable and Combustible Materials 2 — Questions and Answers
Question 1: What is the approximate lower explosive limit (LEL) of gasoline vapor in air?
- 0.1%
- 1.4% (Correct answer)
- 5.3%
- 15%
Correct answer: 1.4%
Gasoline vapor has an LEL of approximately 1.4% and a UEL of about 7.6% by volume in air.
Question 2: A flammable vapor with a density greater than air (vapor density >1) will tend to:
- Rise and dissipate at the top of the tank
- Mix evenly throughout the tank atmosphere
- Sink and accumulate in low-lying areas and bilges (Correct answer)
- React preferentially with nitrogen in the atmosphere
Correct answer: Sink and accumulate in low-lying areas and bilges
Vapors heavier than air (density >1) sink and pool in low areas such as bilges, pump rooms, and tank bottoms where they can form explosive concentrations.
Question 3: The upper explosive limit (UEL) represents the vapor concentration above which:
- The mixture is too rich in fuel to ignite (Correct answer)
- The mixture becomes most easily ignited
- The mixture is too lean in fuel to ignite
- Ignition produces the most intense explosion
Correct answer: The mixture is too rich in fuel to ignite
Above the UEL the fuel-to-air ratio is too high (too rich) for combustion to be sustained; there is insufficient oxygen to support ignition.
Question 4: Hydrogen gas has an exceptionally wide flammable range. Its approximate LEL and UEL are:
- 1.4% and 7.6%
- 4% and 44%
- 2.5% and 80%
- 4% and 75% (Correct answer)
Correct answer: 4% and 75%
Hydrogen has a LEL of 4% and a UEL of 75% by volume in air, making it one of the most easily ignitable and widely flammable gases encountered in marine environments.
Question 5: On a catalytic combustion CGI, a reading of 0% LEL in a tank recently discharged of petroleum cargo most likely indicates:
- The concentration may exceed the UEL, rendering the sensor ineffective (Correct answer)
- The tank atmosphere is safe for hot work
- The sensor requires recalibration before use
- No flammable vapors remain in the tank
Correct answer: The concentration may exceed the UEL, rendering the sensor ineffective
Catalytic bead sensors can be poisoned or saturated above the UEL and may read zero in an over-rich atmosphere that is actually above the flammable range — a potentially fatal misreading.
Question 6: The flash point of a liquid is the minimum temperature at which:
- The liquid begins to boil and generate large quantities of vapor
- The liquid produces sufficient vapor to form an ignitable mixture near its surface (Correct answer)
- The liquid ignites spontaneously without an external ignition source
- The vapor concentration reaches the UEL
Correct answer: The liquid produces sufficient vapor to form an ignitable mixture near its surface
Flash point is the lowest temperature at which a liquid generates enough vapor to flash momentarily when an ignition source is applied near the surface, as determined by standardized test methods.
Question 7: If a tank atmosphere registers 50% LEL on a combustible gas indicator, which statement is TRUE?
- The tank poses an immediate explosion hazard from any spark
- The atmosphere can be ignited directly without any heating
- The vapor concentration is between the LEL and UEL
- The vapor concentration is at half the value of the lower explosive limit (Correct answer)
Correct answer: The vapor concentration is at half the value of the lower explosive limit
A 50% LEL reading means the actual vapor concentration is half of the LEL value — still below the LEL and therefore below the explosive range, though continued rise would be hazardous.
CMC Flammable and Combustible Materials 2
What is the approximate lower explosive limit (LEL) of gasoline vapor in air?