CNG Fuel Lines, Fittings, and High-Pressure Components
High-pressure plumbing
CNG lines are typically rigid stainless steel tubing in the high-pressure section, with flexible braided stainless hoses used where movement or vibration requires it (engine, cylinder connections). Fittings are commonly cone-and-thread or flareless compression designs rated for the system pressure. Candidates must know that:
- Tubing must be routed with adequate bend radius, supported, and protected from abrasion, heat, and vibration.
- Only components rated for CNG service pressure may be used - never substitute general hydraulic or air fittings.
- Thread sealants and lubricants must be compatible; some sealants are prohibited because they can contaminate or degrade.
Leak-tightness concepts
Gas-tight joints depend on correct assembly torque and clean, undamaged sealing surfaces. Over-torquing deforms ferrules and can cause leaks; under-torquing leaks immediately. A proper leak check uses a compatible leak-detection solution or an electronic detector - never an open flame.
Component identification
Know the high-pressure filter (protects downstream components from debris), manual service valves, check valves (prevent backflow during fill), and pressure sensors/transducers that feed the controller.
Traps
- Questions may show a joint that "looks tight" but weeps - pressure decay testing and bubble testing catch what eyes cannot.
- Confusing a check valve with a shutoff valve is a classic distractor: a check valve is one-directional and not a service isolation point.
- Flexible hose assemblies have a defined service life and inspection criteria; they are not "install and forget."
Rule of thumb
Torque to manufacturer specification with a calibrated wrench, then verify with a leak test - assembly torque alone is never proof of a gas-tight joint.