Page 50 - CNG/ LNG ESP Participant Guide 2024
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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
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