B51-97, Part 2

1. Scope 1.1 This Standard sets out minimum requirements for serially produced light-weight refillable gas cylinders, original equipment and conversions, having a water capacity exceeding 20 L (1220 in³) but not exceeding 1000 L(35.5 ft³). The cylinders are intended only for the onboard storage of high-pressure compressed natural gas as a fuel for automotive vehicles to which the cylinders are to be fixed. Cylinders may be of any steel, aluminum, or nonmetallic material, design, or method of manufacture suitable for the specified service conditions. 1.2 Cylinders covered by this Standard are designated as follows: (a) CNG-1 - Metal; (b) CNG-2 - Metal liner reinforced with resin-impregnated continuous filament (hoop-wrapped); (c) CNG-3 - Metal liner reinforced with resin-impregnated continuous filament (fully wrapped); and (d) CNG-4 - Nonmetallic liner with resin-impregnated continuous filament (all-composite). 1.3 Service conditions to which the cylinders will be subjected are detailed in Clause 4. This Standard is based upon a working pressure for natural gas as a fuel of 200 bar (2900 psi) settled at 15°C (59°F), with a maximum filling pressure of 260 bar (3770 psi). Other working pressures can be accommodated by adjusting the pressure by the appropriate factor (ratio). For example, a 250 bar working pressure system will require pressures to be multiplied by 1.25. The service life of the cylinder is to be defined by the manufacturer and may vary with applications. Definition of service life is based upon filling the cylinders 1000 times a year. The maximum service life of types CNG-2, CNG-3, and CNG-4 cylinders shall be 20 years. For all cylinders a "safe life" design principle is used. For metal and metal-lined cylinders, the cylinder life is based upon the rate of fatigue crack growth. The ultrasonic inspection, or equivalent, of each cylinder or liner is required to ensure the absence of flaws that exceed the maximum allowable size determined by fracture mechanics. This approach permits the optimized design and manufacture of light-weight cylinders for natural gas vehicle service. For all-composite cylinders with nonmetallic non-load-bearing liners, the "safe life" is ensured by appropriate design methods, design qualification testing, and manufacturing controls. 1.4 This Standard does not cover the use of stainless steel or welded metallic cylinders or liners.