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Module 6: CNG Depressurizing and Defueling
Lesson 6.2: Depressurizing
Depressurizing ▪ Fuel Systems without Defueling Port or Bleed Valve
High-Pressure o Always follow manufacturer’s procedures
Components o Generic description of procedure
(Excluding ✓ Close manual cylinder valves or disconnect electric solenoid valves
Cylinders, Valves or ✓ Perform rundown procedure to depressurize
PRDs) ▪ Fuel Systems with Defueling Port and/or Bleed Valve
o Generic description of procedure
✓ Close manual cylinder valves or disconnect electric solenoid valves
✓ If defueling port is present – use defueling port to depressurize the
remainder of the high-pressure fuel system
✓ If bleed valve is present – perform rundown procedure and then use bleed
valve to depressurize the remainder of the high-pressure fuel system
Rundown Procedure 1. Shut off all manual cylinder valves or disconnect electric solenoid valves
2. Start the engine
3. Run until it stalls or shuts off
4. Wait 1-2 minutes, attempt to re-start engine
5. When engine will not run, the high-pressure fuel system should have low pressure
▪ Check pressure gauges to confirm
6. Open bleed valve or secondary filter drain valve
CAUTION: Cylinders, cylinder valves, PRDs, and connecting lines are still under pressure!
Do not attempt any repair to valves, cylinders, PRDs or their connections.
Depressurizing Low- ▪ Always follow manufacturer’s procedures
Pressure ▪ Generic description of procedure
Components o Close all manual cylinder valves or disconnect electric solenoid valve
o Using defueling port or bleed valve, depressurize the high-pressure fuel system
o Use bleed valve on low-pressure fuel system (if present) to depressurize
Lesson 6.3: Reasons for Defueling CNG Vehicles
Planned Defueling ▪ Cylinder out of date
▪ Final disposal of vehicle
▪ Service related
o Component replacement (warranty, recall or failure)
Emergency ▪ Cylinder inspection reveals level 2 or 3 damage
Defueling ▪ Any damage that causes concern for failure of any
high-pressure CNG fuel system component
NGV Essentials and Safety Practices (CNG & LNG) © 2024 AFVi All Rights Reserved Page 6-3
