CNG - Compressed Natural Gas CNG Properties and Safety Questions and Answers 1 — Questions and Answers
Question 1: In the event of a leak, how will compressed natural gas (CNG) behave in an open environment?
- It will pool on the ground, creating a concentrated hazard.
- It will dissipate rapidly upwards into the atmosphere. (Correct answer)
- It will remain close to the leak source, forming a visible cloud.
- It will react with moisture in the air and become inert.
Correct answer: It will dissipate rapidly upwards into the atmosphere.
CNG is lighter than air, with a specific gravity of about 0.6 compared to air's 1.0. This property causes it to rise and disperse quickly when released, reducing the risk of forming a flammable mixture near the ground.
Question 2: A technician is inspecting a CNG vehicle involved in a minor collision. The technician notices a slight hissing sound and detects a gas leak near a cylinder valve. What should be the technician's IMMEDIATE action?
- Attempt to tighten the valve fitting to stop the leak.
- Drive the vehicle to a designated outdoor repair area.
- Close the manual shut-off valve for that specific cylinder, if accessible. (Correct answer)
- Use a portable fan to disperse the leaking gas.
Correct answer: Close the manual shut-off valve for that specific cylinder, if accessible.
In the event of a leak, the first step is to isolate the source of the fuel. Closing the cylinder's manual shut-off valve will stop the flow of high-pressure gas from that specific cylinder, mitigating the hazard. Technicians should never attempt to tighten fittings on a pressurized system.
Question 3: Which of the following represents the correct flammability range for CNG in air?
- 1% to 5%
- 15% to 25%
- 5% to 15% (Correct answer)
- 20% to 30%
Correct answer: 5% to 15%
CNG has a relatively narrow flammability range. It will only ignite when its concentration in the air is between approximately 5% and 15%. Below 5%, the mixture is too lean to burn, and above 15%, it is too rich.
Question 4: What is the primary function of a Pressure Relief Device (PRD) on a CNG cylinder?
- To regulate the flow of gas to the engine.
- To indicate the current pressure level inside the cylinder.
- To prevent overfilling during the refueling process.
- To safely vent the gas in the event of high temperature or pressure. (Correct answer)
Correct answer: To safely vent the gas in the event of high temperature or pressure.
A PRD is a critical safety feature designed to protect CNG cylinders from rupture. In the event of a fire or other situation causing excessive heat or pressure, the PRD activates to release the gas in a controlled manner.
Question 5: Compared to gasoline, the auto-ignition temperature of CNG is significantly:
- Lower, making it easier to ignite.
- Higher, making it less likely to ignite from contact with hot surfaces. (Correct answer)
- Approximately the same.
- Variable and dependent on atmospheric pressure.
Correct answer: Higher, making it less likely to ignite from contact with hot surfaces.
CNG has a high auto-ignition temperature of about 540°C (1200°F), whereas gasoline's is much lower, around 246-280°C (475-536°F). This property makes CNG less likely to ignite accidentally on a hot surface, such as an exhaust manifold.
Question 6: According to Federal Motor Vehicle Safety Standards (FMVSS) and industry best practices, how often must a detailed visual inspection of CNG cylinders be performed?
- Every 12 months or 12,000 miles.
- Only after a vehicle fire or collision.
- Every 36 months or 36,000 miles, whichever comes first. (Correct answer)
- At every scheduled oil change.
Correct answer: Every 36 months or 36,000 miles, whichever comes first.
Regulations and best practices, such as those referenced by FMVSS 304, require a detailed visual inspection of CNG cylinders and the fuel system at least every 36 months or 36,000 miles, and also after any fire or accident.
CNG - Compressed Natural Gas CNG Properties and Safety Questions and Answers 1
In the event of a leak, how will compressed natural gas (CNG) behave in an open environment?