CNG Storage Cylinders and Valves
Cylinder types
Candidates must be able to distinguish the four common construction types:
- Type 1: all metal (steel or aluminum). Heavy, robust, common in older and commercial fleets.
- Type 2: metal liner with hoop-wrapped composite.
- Type 3: metal liner with fully wrapped composite.
- Type 4: non-metal (plastic) liner with fully wrapped composite. Lightest, most sensitive to impact and permeation.
Each type has its own inspection criteria and damage tolerances. Composite damage (cuts, abrasion, impact, UV/chemical exposure) is judged differently than metal corrosion or gouging.
Cylinder valves and safety devices
The cylinder valve is the primary isolation point. Most CNG cylinder valves integrate:
- A manual shutoff (usually quarter-turn) for service and emergency isolation.
- An excess-flow valve that closes on a sudden high flow (e.g., a severed line) but allows normal fill/draw flow.
- A pressure relief device (PRD), typically a temperature-activated device (fusible or frangible) that vents cylinder contents if surrounding temperature rises, protecting against overpressure from fire.
- Some designs include an internal solenoid for electronic shutoff.
Traps
- A PRD is temperature-activated, not a pressure-relief valve in the ordinary sense; it does not open to regulate normal pressure.
- Excess-flow devices can nuisance-trip on rapid fills; questions may test whether a no-start after a fast fill is a valve trip.
- Never judge a composite cylinder by metal rules - impact and abrasion are the killers.
Rule of thumb
Cylinder work is a qualified-person task: if you are not trained and authorized to inspect or service the specific cylinder type, your job is to identify, document, and refer - not to "fix."