CNG - Compressed Natural Gas Emergency Procedures and Response Questions and Answers

Question 1

A CNG vehicle is involved in a fire, and flames are impinging on the fuel cylinders. According to standard emergency response guidelines, what is the correct action to take regarding the application of water?

Accepted Answer

Do not apply water to the CNG cylinders, as it can prevent the PRDs from activating.

Answer

Applying water to CNG cylinders exposed to fire is extremely dangerous because it can cool the cylinder and the thermally-activated Pressure Relief Devices (PRDs). This cooling effect can prevent the PRDs from reaching their activation temperature (typically 212°F - 220°F), which would otherwise allow them to safely vent the gas. If the PRD fails to activate, the pressure inside the cylinder can build to a catastrophic level, leading to a rupture or explosion. The standard procedure is to let the fire burn and protect exposures.

Question 2

A technician needs to perform maintenance that requires depressurizing the low-pressure side of a CNG fuel system. After closing the manual cylinder valves, what is the next critical step in the procedure?

Accepted Answer

Start and run the engine until it stalls from lack of fuel.

Answer

After isolating the high-pressure cylinders by closing their manual valves, the correct procedure to depressurize the lines leading to the engine is to run the engine until it consumes the remaining fuel in the lines and stalls. This safely purges the low-pressure side of the system. Only after the engine has stalled and the ignition is secured should a technician proceed to bleed any minimal remaining pressure.

Question 3

A driver reports a strong 'rotten egg' smell inside the cab of a CNG vehicle while it is parked indoors. Which of the following actions should be taken IMMEDIATELY?

Accepted Answer

Ventilate the area, eliminate ignition sources, and move the vehicle outdoors if safe to do so.

Answer

The 'rotten egg' smell is from the mercaptan odorant added to natural gas for leak detection. In an enclosed space, the primary concern is the accumulation of gas to a flammable concentration. The immediate priorities are to ventilate the area by opening doors and windows, eliminate all potential ignition sources (including electrical switches and fans), and then, if it can be done safely, move the vehicle to an open area. Starting the engine or turning on electrical fans would introduce a potential ignition source, which is extremely dangerous.

Question 4

During a post-accident inspection, a technician finds that a thermally-activated PRD has vented, which is evident by a missing fusible plug and soot around the vent line. What is the correct service procedure?

Accepted Answer

Replace the entire PRD assembly with a new, approved unit.

Answer

Pressure Relief Devices (PRDs) are single-use safety devices. If a PRD activates due to temperature or pressure, it is not serviceable and must be completely replaced with a new unit approved by the cylinder manufacturer. Attempting to repair, clean, or reuse an activated PRD is a serious safety violation that compromises the integrity of the fuel system.

Question 5

A CNG vehicle has been partially submerged in flood water, with the water level reaching the bottom of the fuel cylinders. After retrieving the vehicle, which of the following is a mandatory first step before attempting to start or service the fuel system?

Accepted Answer

Disconnect the battery and perform a detailed inspection of the entire fuel system.

Answer

After a vehicle has been submerged, especially in water that could contain contaminants, the first and most critical step is to ensure the vehicle is safe. This involves disconnecting the battery to prevent any electrical shorts or sparks. A thorough inspection of all fuel system components, including cylinders, valves, regulators, and electrical connections, must be performed by a qualified technician before any attempt is made to start the vehicle or pressurize the system. Water and debris can cause corrosion or damage that may not be immediately apparent.

(CNG) Compressed Natural Gas Practice Test

(CNG) Compressed Natural Gas Practice Test

CNG - Compressed Natural Gas Emergency Procedures and Response Questions and Answers