Help! Natural Gas pipe sizing question.

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by TFastle, Jun 24, 2014.

  1. TFastle

    TFastle New Member

    Messages:
    5
    Location:
    New Mexico
    I am having a pool put in. I told the pool company I would supply the Natural gas to the equipment pad. It's a 250' run. It will be high pressure (2psi and then a regulator at the pad) and go directly from the meter to the pad. I also hope to put a grill and natural gas torches at the pool off this line later, a total of about 440,00 btu's. My plumber is running 1" plastic (like pex) and says it will be plenty. After asking him about the calculations I am losing confidence. Can someone tell me how many btu's I will have available with this set up and, most importantly, will it be enough???? There are no other legs to the system, just a straight run right out of the meter. I tired calculating it myself but am not sure I am doing it right. Any help would be GREATLY appreciated! Thanks!!!
  2. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,265
    Location:
    Maine
    The tables are available within the national mechanical code
  3. TFastle

    TFastle New Member

    Messages:
    5
    Location:
    New Mexico
    I've seen and looked at dozens of Tables. Not being familiar with them, to be honest, they are not clear. One looks to say that 200 ft run at 2 psi with 1" line (and 1 psi pressure drop?) will deliver 1,174 cfn which would be 1,174,00 btu's. But does not go to 250' and another seems to read 240,000 BTU's at 300 feet with same specs. I was hoping someone that knew how to calculate it or read the charts based on my situation described above could indeed confirm what the plumber is telling me, ... that there will be plenty of gas. Thanks
  4. Reach4

    Reach4 Well-Known Member

    Messages:
    2,656
    Location:
    IL
    No experience, but the table at the top of http://www.engineeringtoolbox.com/natural-gas-pipe-sizing-d_826.html seems to say you can carry 100,000 BTU/hr in 1 inch pipe for 300 feet before compensating for fittings or whatever. That would be at about 0.25 PSI. So as a sanity check thing, it would seem to me that you could approach roughly 800,000 BTU/hr with 2 PSI.
  5. TFastle

    TFastle New Member

    Messages:
    5
    Location:
    New Mexico
    Thanks Reach but how do you know that particular chart is at .25 PSI? It probably says so somewhere but I can't find it. Also, I guess a case could be made that at 8 times the pressure you would get 8 times the CFH and BTUs but, I am not sure that is the case. Been a while since my college physics days but I don't think pressure and velocity have a linear relationship. Ideally I'd find a table that says, @ 2psi, 1" platic pipe, 250' to Tim's pool = X BTU's max. :eek:)
  6. Reach4

    Reach4 Well-Known Member

    Messages:
    2,656
    Location:
    IL
    Higher velocities could cause turbulance, but I expect that to be small at 2 PSI.

    http://www.engineeringtoolbox.com/natural-gas-pipe-calculator-d_1042.html

    See calculator at the bottom.

    Standard house gas is 6 inch water column. Figure 12 inches into regulator should be more than enough to get 6 inch WC out, I presume. 2 PSI is about 48 inch water column. Figure 36 inch WC drop is OK.

    SG=0.6... seems like that's the number to use for natural gas, but I don't know why. Density WRT air is my guess.

    100,000 BTU per 100 cubic feet, or 1,000 BTU per cubic ft.

    300 feet of 1 inch pipe. (there are tables of how many feet a fitting counts as for this purpose)
    738 CF/hr -> 738,000 BTU/hr

    Incidentally, the pipes out of the regulator would be larger due to lower pressure.
    Last edited: Jun 24, 2014
  7. TFastle

    TFastle New Member

    Messages:
    5
    Location:
    New Mexico
    Thanks very much Reach, very helpful and makes sense. The one thing I don't fully understand is the Water Column "Drop". I guess I see water column a fine way to measure pressure and, after looking online, since 1psi = about 27" of water column then 2 psi would = about 54". What is the pressure "drop" then. Using the calculator, why it not just use 54, the water column of 2 psi natural gas? Clearly there is a very good reason but I don't understand it. I suspect it relates to your statement , "Figure 12 inches into regulator should be more than enough to get 6 inch WC out, I presume." but I'm not sure what you mean by that either. I guess I see it as the regulator out will be equal to 6 as long as the supply is 6 WC or more. I think it's clear I will have enough gas for my needs but would like to understand it a bit more if you don't mind explaining more. If not, I understand and thanks for your input. (Is the WC drop maybe the difference between what you have (54") and what you need at the regulator at the other end to push the 6 in WC? Hence you suggested that 12" is going to be needed (or used) on the supply side of the regulator when it gets there and I only get the difference to generate CFH?). Interesting stuff.
    '
  8. Reach4

    Reach4 Well-Known Member

    Messages:
    2,656
    Location:
    IL
    Pretty much. I just took what I thought would be an overly-conservative guess that the regulator would be able to maintain 6 inch WC out with 12 inch WC in. I am sure your regulator will have an actual spec.
  9. TFastle

    TFastle New Member

    Messages:
    5
    Location:
    New Mexico
    Thanks for the input. Went for a long ride on my mountain bike and saw the error in my ways. The drop is the difference between what I have to start with and what I NEED at the other end and the calculator is telling me the CFH after taking away the loss due to pipe size and distance. Mucho thanks for your input and explanations! I'm now pretty comfortable I'm good with the 1" pipe!
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