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Procedures for safely bleeding CNG valves

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  • Procedures for safely bleeding CNG valves

    One risk which natural gas vehicle service technicians occasionally face is dealing with the need to safely remove fuel from a cylinder when the valve will no longer open. Generally this scenario occurs with remotely actuated solenoid valves. Solenoid valves provide a useful safety aspect to the vehicle as the flow of CNG is stopped when the vehicle is keyed off, or when the airbags deploy. On the other hand, when they fail closed the cylinder can no longer be used and the CNG could be stuck in there forever unless an alternate method is provided to safely remove the fuel.

    From NFPA-52 (2010)
    6.6.1 Every cylinder shall be equipped with either of the following:
    (1) A manual valve
    (2) A normally closed, remotely actuated shut-off valve connected directly to the cylinder and equipped to bleed the cylinder manually

    The recent fatality in Oklahoma underscores the need for natural gas vehicle technicians to understand how each valve manufacturer provides for the manual bleeding of a cylinder. Attached here are sample procedures for GFI and OMB as reference, but you should always contact the valve manufacturer directly for specific guidance as to the valve in question.

    Be also aware that some valves (many OMB types for example) have built-in excess flow limiters. While this is also a useful safety feature (i.e. stops flow if a high pressure line breaks), on the other hand it can fool the technician into believing the tank is empty when gas flow stops during the venting process.

    Finally, venting a CNG cylinder must be done via grounding the cylinder to avoid static buildup and also via use of a vent stack. This could be the topic of another discussion thread here in the Safety Forum if someone would like to start it?

    If others have information as to safe valve bleed procedures, please share them with the community.

    Thanks, and be safe.
    Attached Files
    Last edited by John Mitton; 08-02-2012, 04:47 PM.