CNG Fuel System Inspection, Safety, and Service Procedures
What you must know
Natural gas is flammable over roughly 5–15% by volume in air and is lighter than air, so it disperses upward rather than pooling. That single fact drives most bus shop practice: ventilation at the ceiling, no ignition sources above the vehicle, and leak detection by smell (odorant), by electronic combustible-gas detector, and by soap solution - never by open flame. Know the system layout: fill receptacle, check valve, high-pressure lines, cylinder valves, manual shutoff, pressure regulator, low-pressure lines, and the engine fuel shutoff solenoid.
Traps
The exam likes the difference between the manual cylinder valve (isolates each cylinder) and the emergency fuel shutoff (stops flow to the engine). Also know that a full cylinder is about 3,600 psi at 70°F, and that pressure falls as temperature drops - a low gauge reading on a cold morning is not automatically a leak. Defueling must follow the manufacturer's procedure; venting gas to atmosphere in a closed shop is a safety violation. PPE includes face shield and insulated gloves when working on high-pressure fittings.
Rule of thumb
Any time you open a high-pressure line, close the cylinder valves, run the engine until it stalls to bleed down the lines, then verify zero pressure at the gauge before loosening a fitting. If you can't prove it's at zero, treat it as full pressure.
Worked example
A bus smells of gas after fueling. You find a slight leak at the fill receptacle check valve. Correct action: isolate cylinders, bleed the line, replace the check valve, leak-test with soap solution at operating pressure, and document. Do not simply tighten and release.