The US Department of Transportation requires that CNG tanks used on motor vehicles must be inspected periodically. As the structural integrity of CNG tanks degrade over time, all tanks are required to have an expiration label. Do not continue to use CNG expired cylinders! These cylinders must be inspected at least every 36 months or 36,000 miles, whichever comes first for damage and deterioration and after a motor vehicle has an accident or fire. At Natural Gas Vehicles Texas Inc. has qualified CNG tank inspectors on site to perform necessary cylinder inspections, maintenance and repairs. For a larger fleets we can even bring our inspectors to your location.
CNG Tank types include:
Type 1: This is an all metal – cylinder made of steel. There is no covering, other than paint, on the outside of the cylinder. This is the most common type of CNG Tanks.
- Type 2: This is a metal cylinder (steel or aluminum) with a partial wrapping that goes around the cylinder. The wrapping is usually made of glass, aramid or carbon, contained in an epoxy or polyester resin.
- Type 3: This type of cylinder is fully wrapped with the same kind of material used for the partial wrapping of a Type 2 cylinder. This type of CNG Tank has a metal liner, usually aluminum.
- Type 4: This type of cylinder is fully wrapped with the same kind of material used for the partial wrapping of a Type 2 CNG tank. This type of cylinder has a plastic liner.
CNG Vehicle Fuel Tank Standards – All CNG vehicle fuel containers must meet the federal government’s Federal Motor Vehicle Safety Standard (FMVSS) 304 (49 CFR 571.304), Compressed Natural Gas Fuel Container Integrity. All CNG vehicle fuel containers should meet American National Standards Institute (ANSI)/Canadian Standards Association (CSA) NGV2, Basic Requirements for Compressed Natural Gas Vehicle Fuel Containers. This industry standard is more comprehensive and up-to-date than FMVSS 304.
Label Requirements (S7.4, FMVSS 304)
“Each CNG fuel container shall be permanently labeled with the information specified in paragraphs (a) through (h) of this section.” ….“(g) The statement: ‘This container should be visually inspected after a motor vehicle accident or fire and at least every 36 months or 36,000 miles, whichever comes first, for damage and deterioration.’”
Periodic In-Service Inspection Requirements (Sec. 4.1.4, NGV2)
“Each container shall be visually inspected at least every 36 months / 36,000 miles, or at the time of any re-installation, for external damage and deterioration….The inspection shall be performed by a qualified container inspector in accordance with (1) the manufacturer’s recommendations and (2) the inspection procedures provided in Compressed Gas Association (CGA) pamphlet C-6.4”
Pressure Relief Device (PRD)
All CNG tanks must have one or more pressure relief devices which have been bonfire tested and approved by the tank manufacturer so as to rapidly vent gas away to the atmosphere in case of excess temperatures. When a natural gas vehicle is on fire, rapidly-expanding CNG in the tank can create internal pressures to the point of tank failure and catastrophic explosion. The PRD is usually located at the tank valve and is designed to activate at a specified temperature so as to rapidly empty the tank via the PRD port on the valve. It is especially critical that CNG tanks in enclosed areas (i.e. car trunk, SUV cargo area) be equipped with secured PRD tubing to vent away the fuel in case of fire. [Ref. NFPA-52 Sections 6.4 & 4.5]
Securing CNG Tanks
CNG tanks must be secured to the vehicle body, bed, or frame to withstand a force of eight times the weight in all directions of a fully pressurized container (8x G force impact). Only accept the use of brackets and mounting designs which have been approved by the tank manufacturer. Never accept the use of ratchet straps, aluminum bands, etc. on your vehicle. [NFPA-52 Section 6.3]
Shielding CNG Tanks
Tanks must be protected from road hazards, loading, unloading, direct sunlight, exhaust heat, accidental cargo leakage. Shield must not directly contact tank nor trap solid materials or liquids. CNG tanks & valves must be shielded with minimum 9 inches road clearance when tires are deflated and cannot be located ahead of the front axle or behind the rear bumper attachment. Must be transversely mounted if behind rear axle. [NFPA-52 Section 6.3]
Corrosion-resistant fuel lines (generally stainless steel) must be mounted, braced, and supported to minimize vibration and protected against damage, corrosion, or breakage due to strain or wear. No use of cast iron, plastic, galvanized pipe, aluminum or copper. Fueling connection must be ANSI listed. [Ref. NFPA-52 Sections 4.8 & 4.10]
Venting All Fittings
The neck of the tank and all fittings located in a vehicle compartment (i.e. trunk or SUV cargo area) must be enclosed in a gas-tight polyethylene enclosure that is vented directly to the outside of the vehicle but not into a wheel well. [NFPA-52 Section 6.4]
A valve that automatically prevents the follow of CNG to the engine when the engine is not running, even if the ignition is switched on must be provided. [NFPA-52 Section 6.6.3]
Fuel Line One-Way Valves
Fuel lines must have two backflow check valves that prevent the return flow of gas from the tank to the fill connector, mounted to withstand breakaway force of 150 lb (68 kg) when applied in any direction that the vehicle would move. [NFPA-52 Sections 6.6 & 188.8.131.52]