CNG Properties and Safety 3 — Questions and Answers
Question 1: What is the Upper Flammability Limit (UFL) of methane in air?
- 10%
- 15% (Correct answer)
- 20%
- 25%
Correct answer: 15%
Methane's Upper Flammability Limit is approximately 15% by volume in air, above which there is insufficient oxygen for combustion.
Question 2: Which organization sets the standards for CNG cylinder design and testing in the United States?
- EPA
- NFPA
- DOT (Correct answer)
- OSHA
Correct answer: DOT
The U.S. Department of Transportation (DOT) sets and enforces standards for CNG cylinder design, testing, and certification under 49 CFR.
Question 3: What does the PRD (Pressure Relief Device) on a CNG cylinder do?
- Prevents overfilling during fueling
- Releases gas if cylinder temperature or pressure becomes excessive (Correct answer)
- Shuts off fuel flow when the engine is off
- Measures remaining fuel quantity
Correct answer: Releases gas if cylinder temperature or pressure becomes excessive
A PRD is a thermal-activated safety device that vents gas from the cylinder if it is exposed to excessive heat or fire, preventing catastrophic rupture.
Question 4: What is the autoignition temperature of methane?
- 300°F (149°C)
- 537°C (999°F) (Correct answer)
- 200°C (392°F)
- 700°C (1292°F)
Correct answer: 537°C (999°F)
Methane has an autoignition temperature of approximately 537°C (999°F), which is higher than gasoline, making it harder to ignite without a spark.
Question 5: In a CNG fueling station, what is the purpose of a cascade storage system?
- To cool the gas before dispensing
- To store gas at multiple pressure levels for efficient sequential filling (Correct answer)
- To filter contaminants from the gas supply
- To measure dispensed fuel volume accurately
Correct answer: To store gas at multiple pressure levels for efficient sequential filling
A cascade system stores CNG in banks at different pressure levels (high, medium, low) and draws from them sequentially to fill vehicles efficiently.
Question 6: When a CNG vehicle is involved in a collision, what is the first safety concern regarding the fuel system?
- Fuel contamination of the ground
- High-pressure gas release or cylinder integrity (Correct answer)
- Fuel igniting from catalytic converter heat
- Battery sparks igniting fuel vapors
Correct answer: High-pressure gas release or cylinder integrity
The primary concern after a collision involving a CNG vehicle is the integrity of the high-pressure cylinders and the potential for pressurized gas release.
Question 7: Which gas is added to CNG to give it a detectable odor for leak identification?
- Hydrogen sulfide
- Mercaptan (odorant) (Correct answer)
- Carbon monoxide
- Ammonia
Correct answer: Mercaptan (odorant)
Mercaptan (typically ethyl mercaptan or similar thiols) is added as an odorant to natural gas so leaks can be detected by smell.
CNG Properties and Safety 3
What is the Upper Flammability Limit (UFL) of methane in air?