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National Grid Task-70: Properties of Natural Gas and Abnormal Operating Conditions —
100 Practice Questions with Answers & Rationales
Section 1: Fundamental Properties of Natural Gas (Questions 1–20)
Q1. What is the primary component of natural gas?
A) Propane
B) Methane
C) Ethane
D) Butane
Correct Answer: B
Rationale: Natural gas is predominantly methane, typically comprising 70–96% by volume,
with smaller amounts of ethane, propane, butane, and other hydrocarbons .
Q2. What is the specific gravity of natural gas relative to air?
A) 1.5
B) 0.6
C) 1.0
D) 2.0
Correct Answer: B
Rationale: Natural gas has a specific gravity of approximately 0.6 compared to air (1.0),
meaning it is lighter than air and will rise and dissipate quickly when released outdoors .
Q3. Natural gas is ______ than air.
A) Heavier
B) Lighter
C) The same weight
D) Variable
Correct Answer: B
Rationale: With a specific gravity of about 0.6, natural gas is significantly lighter than air .
Q4. What is the flammability range of natural gas in air?
A) 1–5%
B) 4–14%
C) 5–15%
D) 10–20%
Correct Answer: C
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Rationale: Natural gas will ignite only when its concentration in air is between 5% and 15%.
Below 5% (LEL) the mixture is too lean; above 15% (UEL) it is too rich to ignite .
Q5. The Lower Explosive Limit (LEL) of methane is:
A) 1%
B) 4%
C) 5%
D) 15%
Correct Answer: C
Rationale: The LEL is the minimum concentration of gas in air that will ignite. For methane,
this is 5% gas-in-air .
Q6. The Upper Explosive Limit (UEL) of methane is:
A) 5%
B) 10%
C) 15%
D) 20%
Correct Answer: C
Rationale: Methane's UEL is about 15%, above which the mixture is too rich to ignite .
Q7. What do we add to natural gas to give it its distinctive smell?
A) Chlorine
B) Carbon dioxide
C) Mercaptan
D) Hydrogen
Correct Answer: C
Rationale: Mercaptan has a strong, sulfur-like odor (often described as rotten eggs), making
otherwise odorless natural gas detectable .
Q8. After odorant is added, natural gas is said by many to smell like:
A) Fresh flowers
B) Rotten eggs
C) Bleach
D) Gasoline
Correct Answer: B
Rationale: Mercaptan gives natural gas a distinctive "rotten egg" smell, allowing leak
detection by human senses .
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Q9. Natural gas will not burn unless fuel, heat, and ____________ are present?
A) Pressure
B) Oxygen
C) Water
D) Nitrogen
Correct Answer: B
Rationale: The fire triangle requires fuel, heat, and oxygen for combustion to occur .
Q10. What is the autoignition temperature of methane?
A) 250°F (121°C)
B) 538°C (1000°F) approximately
C) 100°C (212°F)
D) 750°F (399°C)
Correct Answer: B
Rationale: Methane requires about 538°C to autoignite without an external spark. This is
why hot surfaces or engine components can ignite a leak .
Q11. Natural gas combustion produces primarily:
A) CO and soot
B) CO₂ and water vapor
C) NOx only
D) Sulfur dioxide
Correct Answer: B
Rationale: Complete combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O. Incomplete
combustion can produce CO .
Q12. What causes carbon monoxide (CO) production?
A) Complete combustion
B) Incomplete combustion
C) Excess oxygen
D) Low temperature
Correct Answer: B
Rationale: Carbon monoxide is a byproduct of incomplete combustion, which occurs when
there is insufficient oxygen for complete burning .
Q13. What color flame indicates complete combustion of natural gas?
National Grid Task-70: Properties of Natural Gas and Abnormal Operating Conditions —
100 Practice Questions with Answers & Rationales
Section 1: Fundamental Properties of Natural Gas (Questions 1–20)
Q1. What is the primary component of natural gas?
A) Propane
B) Methane
C) Ethane
D) Butane
Correct Answer: B
Rationale: Natural gas is predominantly methane, typically comprising 70–96% by volume,
with smaller amounts of ethane, propane, butane, and other hydrocarbons .
Q2. What is the specific gravity of natural gas relative to air?
A) 1.5
B) 0.6
C) 1.0
D) 2.0
Correct Answer: B
Rationale: Natural gas has a specific gravity of approximately 0.6 compared to air (1.0),
meaning it is lighter than air and will rise and dissipate quickly when released outdoors .
Q3. Natural gas is ______ than air.
A) Heavier
B) Lighter
C) The same weight
D) Variable
Correct Answer: B
Rationale: With a specific gravity of about 0.6, natural gas is significantly lighter than air .
Q4. What is the flammability range of natural gas in air?
A) 1–5%
B) 4–14%
C) 5–15%
D) 10–20%
Correct Answer: C
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Rationale: Natural gas will ignite only when its concentration in air is between 5% and 15%.
Below 5% (LEL) the mixture is too lean; above 15% (UEL) it is too rich to ignite .
Q5. The Lower Explosive Limit (LEL) of methane is:
A) 1%
B) 4%
C) 5%
D) 15%
Correct Answer: C
Rationale: The LEL is the minimum concentration of gas in air that will ignite. For methane,
this is 5% gas-in-air .
Q6. The Upper Explosive Limit (UEL) of methane is:
A) 5%
B) 10%
C) 15%
D) 20%
Correct Answer: C
Rationale: Methane's UEL is about 15%, above which the mixture is too rich to ignite .
Q7. What do we add to natural gas to give it its distinctive smell?
A) Chlorine
B) Carbon dioxide
C) Mercaptan
D) Hydrogen
Correct Answer: C
Rationale: Mercaptan has a strong, sulfur-like odor (often described as rotten eggs), making
otherwise odorless natural gas detectable .
Q8. After odorant is added, natural gas is said by many to smell like:
A) Fresh flowers
B) Rotten eggs
C) Bleach
D) Gasoline
Correct Answer: B
Rationale: Mercaptan gives natural gas a distinctive "rotten egg" smell, allowing leak
detection by human senses .
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Q9. Natural gas will not burn unless fuel, heat, and ____________ are present?
A) Pressure
B) Oxygen
C) Water
D) Nitrogen
Correct Answer: B
Rationale: The fire triangle requires fuel, heat, and oxygen for combustion to occur .
Q10. What is the autoignition temperature of methane?
A) 250°F (121°C)
B) 538°C (1000°F) approximately
C) 100°C (212°F)
D) 750°F (399°C)
Correct Answer: B
Rationale: Methane requires about 538°C to autoignite without an external spark. This is
why hot surfaces or engine components can ignite a leak .
Q11. Natural gas combustion produces primarily:
A) CO and soot
B) CO₂ and water vapor
C) NOx only
D) Sulfur dioxide
Correct Answer: B
Rationale: Complete combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O. Incomplete
combustion can produce CO .
Q12. What causes carbon monoxide (CO) production?
A) Complete combustion
B) Incomplete combustion
C) Excess oxygen
D) Low temperature
Correct Answer: B
Rationale: Carbon monoxide is a byproduct of incomplete combustion, which occurs when
there is insufficient oxygen for complete burning .
Q13. What color flame indicates complete combustion of natural gas?
