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Advantage/Disadvantage? Plastic Cylinder Liners

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  • Advantage/Disadvantage? Plastic Cylinder Liners

    I see that some (or all?) CNG cylinders have a plastic liner inside. It doesn't make sense to me. Although I see the advantage because the cylinder would be lighter -- wouldn't the gas permeate through the pourous plastic lining? How can I be sure that the gas isn't leaking? Seems to me metal would work better.... What are your thoughts on this?

  • #2
    Re: Advantage/Disadvantage? Plastic Cylinder Liners


    There are plastic lined CNG cylinders (referred as Type IV). These, in theory, are lighter than metal lined CNG cylinders (referred as Type III), however, at larger volumes (>150 Liters) the weight comparison of Type IV vs. Type III are very similiar.

    In answer to your question about permeation. Yes, Type IV cylinders have natural gas permeating through the plastic (usually High Density Polyethylene - HDPE). NGV2 (Cylinder specification for CNG cylinders) allows some permeation through the sidewall. The permeation must be less than 0.25 cc of natural gas per hour per liter of water volume of the container. Or in English units it is 0.432 cubic inches per hour per cubic foot of water volume of the container. In fact every Type IV cylinder made is leak tested prior to being sold.

    Type III cylinders do not have permeation. Steel and aluminum have superior permeation resistance especially with natural gas.
    Mountain Green, Utah
    2003 CNG Cavalier
    2003 CNG Silverado 2500HD


    • #3
      Re: Advantage/Disadvantage? Plastic Cylinder Liners

      The resin/ graphite fiber wrapping the plastic gives strength to the thin plastic bottle underneath the fibers, the fibers alone cannot seal gases under pressure. They take this approach to save weight.