CNG Storage Cylinders and Fuel Lines
What you must know
CNG buses carry Type 3 (metal liner, composite hoop wrap) or Type 4 (plastic liner, full composite) cylinders, typically rated 3,600 psi service pressure. Cylinders have a limited service life set by the manufacturer and an inspection interval mandated by regulation (commonly every 36,000 miles or 36 months, whichever comes first, on U.S. transit buses - verify against the current rule and the cylinder label). Inspections look for abrasion, cuts, gouges, impact damage, heat damage, chemical attack, loose or corroded mounts, and damaged valves, PRDs, and piping.
Traps
Questions test PRD (pressure relief device) function: a temperature-activated device that vents gas if the cylinder is engulfed in fire - it is not a normal pressure regulator and must never be plugged or replaced with a different rating. Also know that composite cylinders must not be painted, solvent-washed, or repaired with filler; damage beyond the manufacturer's criteria means removal from service, not patch. High-pressure lines are stainless steel or reinforced; bends must respect minimum radius, and fittings are specific to CNG - never substitute hydraulic or plumbing fittings.
Rule of thumb
If damage depth or length exceeds the manufacturer's condemnation limit, or the cylinder is past its life date, it is out of service - no exceptions for "it holds pressure fine."
Worked example
A cylinder shows a gouge in the composite overwrap exposing fibers. It holds pressure and passes a leak test. Correct call: remove from service. Exposed fiber is structural damage; pressure-holding proves nothing about burst margin.