flammable range and vapor control Practice Questions

1. What is the approximate flammable range of gasoline vapors in air?

• A.1.4% to 7.6% by volume✓
• B.0.5% to 3.0% by volume
• C.5.0% to 15.0% by volume
• D.10.0% to 25.0% by volume

Why: Gasoline vapor has a flammable (explosive) range of approximately 1.4% to 7.6% by volume in air. Below 1.4% the mixture is too lean to ignite; above 7.6% it is too rich, known as the Lower and Upper Explosive Limits (LEL/UEL).

2. A cargo tank atmosphere reading of 0% LEL on a combustible gas indicator means the atmosphere is:

• A.Free of flammable vapors — safe for hot work only after a gas-free certificate is issued✓
• B.Fully inerted and ready for entry without supplied-air breathing apparatus
• C.At the upper explosive limit and too rich to ignite
• D.Immediately dangerous to life and health (IDLH)

Why: A 0% LEL reading means no detectable flammable vapor, but a gas-free certificate signed by a Marine Chemist or USCG-authorized person is still required before hot work may begin, per 33 CFR Part 126 and OSHA standards.

3. When a tank atmosphere is above the Upper Explosive Limit (UEL), it is described as 'too rich to burn.' What is the primary hazard if this condition exists?

• A.Air ingress can bring the mixture into the flammable range during purging or ventilation✓
• B.Spontaneous combustion will occur immediately upon crew entry
• C.The cargo pump will cavitate and create a static charge
• D.The inert gas system will be overwhelmed and lose positive pressure

Why: Although a vapor concentration above the UEL will not ignite, introducing air during ventilation or purging can bring the mixture through the flammable range, creating a period of ignition risk before reaching a safe level.

4. What oxygen content in a cargo tank atmosphere is considered safe for inerted tanks and will NOT support combustion?

• A.Below 8% by volume✓
• B.Below 16% by volume
• C.Below 21% by volume
• D.Below 10.5% by volume

Why: Industry standards (ICS/OCIMF International Safety Guide for Oil Tankers and Terminals) specify that an oxygen content below 8% by volume will not support combustion of hydrocarbon vapors, providing a safety margin below the critical 11% threshold.

5. The autoignition temperature (AIT) of a petroleum product is:

• A.The minimum temperature at which the substance ignites spontaneously without an external ignition source✓
• B.The temperature at which the vapor pressure equals the lower explosive limit
• C.The highest temperature at which the substance can be stored safely
• D.The temperature required to sustain a flame after the ignition source is removed

Why: The autoignition temperature is the lowest temperature at which a substance will self-ignite without any external spark or flame, caused by self-heating and oxidation reactions — important in hot work decisions and cargo heating operations.