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Thread: gas pipe load calculations

  1. #1
    Writing, constructionDIY Member Yersmay's Avatar
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    Sep 2005
    Los Angeles

    Default gas pipe load calculations

    I'm curious about how gas pipe load calculations are derived. Can someone explain the variables assigned and how these are set up in formula(s)? Thanks in advance and apologies if this is much too complicated for this type of 'classroom'.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Aug 2004
    Cave Creek, Arizona

    Default gas pipe

    There are formulas but most of the tables are drawn up from empirical data. The way the size is calculated has several flaws but they are not addressed in the real world. For example, I once asked an inspector, "If I have a big boiler near the gas meter and it calls for a 3" gas line, why, if I add a barbecue 100' away does the table suddenly tell me the boiler now needs a 4" supply line?"

  3. #3
    Plumber jimbo's Avatar
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    Aug 2004
    San Diego


    I believe most installers, and for that matter most designers, probably use the tables in the National Fuel Gas Code book.

    The tables are derived using formulas of fluid dynamics. I don't know if I have ever even seen such a formula, but suspect it is fairly complex, involving the calculus.

    The diameter of a pipe, and to a lesser extent, elbows and fittings, cause friction in a pipe as any fluid....gaseous or liquid....flows through it. The friction causes a drop in pressure.

    A gas appliance needs a certain minumum pressure at the inlet, and in full operation will need "X" many cubic feet per minute. The tables allow you to select a pipe size that for the distance in question will deliver at least the minumum number of CFM to satisfy all the appliances connected to it.

    The project then boils down to laying out the appliance loads and the pipe runs and adding up cfms.

  4. #4
    Master Plumber master plumber mark's Avatar
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    Feb 2005
    indianapolis indiana - land of the free, home of the brave....
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    Talking what are you doing????

    It really depends on what you want to do....

    I ran a gas line out to my gas grill in 3/8 copper
    for about 30 feet and it cooks steaks just fine....

    But it is wiser just to oversize something with one inch black gas pipe if you are doing a home...

    I once did a water heater where some stupid furnace
    man ran only a only a 1/2 gas line into the furnace. from the meter...

    it ran 10 feet from the meter and they did not
    care about any future additions to the line....

    so I had to tear out all his work and run in a
    1 inch line to the meter .....to provide gas for both appliances...

    I suppose I could have squeaked by and run 3/4 if I looked at the book,
    but then you got to think of the next guy who might want to add a laundry or stove some day.

    I have not even looked at the book in years,

    I just know its best to go bigger or stay home...

    the cost is not even a factor

    give the next guy a fighting chance


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