CNG Fueling Receptacles, Defueling, and Leak Detection
The fill path
A CNG vehicle is filled through a receptacle (commonly the SAE/ANSI J2600-style nozzle interface) that connects to a fill line, a check valve, and the cylinder(s). Key concepts:
- The receptacle must be clean, undamaged, and free of debris; a damaged receptacle leaks or fails to seal.
- A check valve prevents backflow from the cylinders to the receptacle.
- Filling is a high-pressure, high-flow operation; the station controls fill rate and temperature compensation.
Defueling
Defueling (draining cylinders for service) must follow a defined procedure: connect an approved defueling hose/vent to a safe location, open the cylinder valve, and vent slowly to a safe, well-ventilated area away from ignition sources and occupied spaces. Never vent into an enclosed space; methane rises and accumulates at ceilings. Confirm the system is at zero pressure before opening any high-pressure joint.
Leak detection
- Bubble solution on joints is the classic positive-confirmation method for a known suspect area.
- Electronic combustible-gas detectors (calibrated) are used for general leak surveys; they must be zeroed in clean air and used per manufacturer instructions.
- Pressure decay testing confirms system integrity when a leak is suspected but not localized.
Traps
- Venting to a "safe area" means open, ventilated, ignition-free - not just "outside."
- A check valve failure can show as a leak at the receptacle or slow pressure loss after fill.
- Detector alarms can be false from other combustibles or sensor drift; confirm with a second method.
Rule of thumb
Zero pressure before you break a joint. Confirm with a gauge and a bleed - never assume a closed valve means an empty line.