CNG - Compressed Natural Gas Fueling Station Operations Questions and Answers 1 — Questions and Answers
Question 1: A technician is observing the fueling of a vehicle at a fast-fill CNG station that uses a three-bank cascade storage system. To achieve the most complete fill, in what sequence should the dispenser draw gas from the storage banks?
- First from the low-pressure bank, then medium, and finally topping off from the high-pressure bank. (Correct answer)
- First from the high-pressure bank, then medium, and finally from the low-pressure bank.
- Simultaneously from all three banks until the vehicle's cylinder pressure equalizes.
- Only from the high-pressure bank to ensure the fastest possible fill time.
Correct answer: First from the low-pressure bank, then medium, and finally topping off from the high-pressure bank.
A cascade storage system is designed to maximize the pressure differential between the station's storage and the vehicle's tank throughout the fill. It starts with the lowest pressure bank, which can still transfer a significant volume of gas into the nearly empty vehicle tank. As the vehicle's tank pressure increases, the system switches to the medium, and then the high-pressure bank to complete the fill, ensuring a fuller, more efficient transfer of gas.
Question 2: During a routine inspection of a CNG dispenser, an operator notices the hose has significant abrasion, exposing the underlying reinforcement braid. What is the most appropriate action?
- Wrap the abraded area with high-pressure tape as a temporary repair.
- Continue using the hose but at a reduced fueling pressure.
- Immediately remove the dispenser from service until the hose can be replaced. (Correct answer)
- Note the damage in the maintenance log for repair at the next scheduled service.
Correct answer: Immediately remove the dispenser from service until the hose can be replaced.
A damaged fueling hose, especially with exposed reinforcement, poses a significant safety risk due to the high pressures involved (up to 4,500 psi). Industry best practices and safety standards mandate that any hose showing such wear must be taken out of service immediately to prevent a potential catastrophic failure and gas leak.
Question 3: Which of the following best describes the primary operational difference between a time-fill and a fast-fill CNG station?
- Time-fill stations use cryogenic liquid, while fast-fill stations use compressed gas.
- Time-fill stations fuel vehicles directly from a smaller compressor over several hours, while fast-fill stations use large, high-pressure storage vessels for rapid fueling. (Correct answer)
- Time-fill stations are only for public retail use, whereas fast-fill stations are exclusively for private fleets.
- Time-fill stations require the vehicle's engine to be running during fueling, unlike fast-fill stations.
Correct answer: Time-fill stations fuel vehicles directly from a smaller compressor over several hours, while fast-fill stations use large, high-pressure storage vessels for rapid fueling.
Time-fill stations are typically used by fleets that refuel overnight. They use a smaller compressor to fill vehicles directly over a period of several hours. Fast-fill stations, common for public retail, use a large compressor to pressurize a cascade of storage vessels, allowing vehicles to be refueled in a manner of minutes, similar to gasoline.
Question 4: A driver pulls away from a CNG dispenser with the nozzle still connected to the vehicle. What safety device is specifically designed to mitigate this exact scenario?
- The emergency shutdown (ESD) button.
- A hose breakaway coupling. (Correct answer)
- The pressure relief device (PRD) on the storage tanks.
- The dispenser's mass flow meter.
Correct answer: A hose breakaway coupling.
A hose breakaway coupling is a safety device installed on the dispenser hose. It is designed to separate at a designated pull force, automatically sealing both the hose and dispenser sides to prevent a large, uncontrolled release of high-pressure natural gas in a drive-away incident.
Question 5: According to NFPA 52, which of the following is a mandatory requirement for the operator of a CNG fueling station?
- To recalibrate the station's compressor on a weekly basis.
- To have a documented, active maintenance plan for the station equipment. (Correct answer)
- To visually inspect every vehicle's CNG cylinders before allowing fueling.
- To provide a certified attendant for every public fueling transaction.
Correct answer: To have a documented, active maintenance plan for the station equipment.
Recent versions of NFPA 52, the Vehicular Natural Gas Fuel Systems Code, mandate that every CNG station, whether new or existing, must have a written and actively implemented maintenance plan. This plan is crucial for ensuring the ongoing safety, reliability, and proper performance of all station components.
Question 6: A fleet manager is planning a new CNG fueling station and wants to refuel a large fleet of buses overnight to take advantage of lower off-peak electricity rates. Which type of station would be most cost-effective and suitable for this operational model?
- A fast-fill station with a five-bank cascade.
- A mobile LNG (Liquefied Natural Gas) fueling tanker.
- A time-fill station. (Correct answer)
- A combination-fill station with an emphasis on the fast-fill dispensers.
Correct answer: A time-fill station.
Time-fill stations are ideal for fleets that can refuel at a central location over an extended period, such as overnight. They typically have lower initial infrastructure costs than fast-fill stations because they can use smaller compressors and do not require extensive high-pressure storage cascades. This design also allows fleet managers to schedule fueling during off-peak hours to reduce electricity costs.
CNG - Compressed Natural Gas Fueling Station Operations Questions and Answers 1
A technician is observing the fueling of a vehicle at a fast-fill CNG station that uses a three-bank cascade storage system.
To achieve the most complete fill, in what sequence should the dispenser draw gas from the storage banks?