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Thread: Natural gas supply pipe size general theory.

  1. #1

    Smile Natural gas supply pipe size general theory.

    In considering an upgrade to some of my natural gas piping, I noticed something I would like to understand better. I just bought a new kitchen range,60,000 btu input, but did not hook it up yet as I need to have to have the gas supply moved a couple of feet. It's the end of the run so normally no big deal but in checking charts, it seems some of my piping is undersized so I'm considering the upgrade at the same time. Inside my crawlspace, I have about 20' of 1" pipe, then a 1" x 1" x 1/2" tee supplying a clothes dryer, and continuing on to a reducing tee 1" x 1/2" x 1/2". The tap supplies a water heater but then the trunk (as I call it) or continued piping which is 1/2" goes on to supply a 100,000 btu heater, then another 10' where it used to supply a built in wall oven, and another 8' for and old cooktop. The new range will replace these 2. I'm considering going back to the reducer before the house heater and upgrading to 3/4" or 1" to properly meet demand downstream but here's my concern. **I thought I had 1" service coming from my gas meter as it comes through the wall from outside as 1". Looking at the meter today, I noticed 3/4" in (which is fine because it's probably higher pressure before the meter, but then also 3/4" out of the meter. A couple of elbows and a few inches later it goes to a reducer (or in this case, it would be an enlarger) which brings this from 3/4" to 1" and then enters the house. This boggled me! Some thoughts came to mind like "a chain is only as strong as it's weakest link" or in this case, "a piping system can only supply as much as it's smallest diameter allows through". Is this normal?? Is it ok because it's only a short run at the beginning? I would think my capacity is limited to 3/4" no matter what they added to it afterward?? Sorry so long and technical but I'm big on theory and like to understand as much as possible before doing something or having something done for me. Thanks for reading! I'll check back later as I have to go to work now...

  2. #2
    DIY Senior Member jastori's Avatar
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    "a piping system can only supply as much as it's smallest diameter allows through"

    No, it is not so simple. How much gas will flow through a given run of piping depends on lots of details. All piping adds 'resistance' to the flow of a gas or liquid. Pressure pushes the gas or liquid through the pipe, and overcomes the resistance. Smaller pipe has higher resistance to flow than larger pipe. Longer pipe has more resistance to flow than shorter pipe. A 90 deg. bend has higher resistance to flow than a 45 deg. bend, etc.

    In the end, it is the total resistance between the source and the appliance that matters (as well as the available pressure). A short section of 3/4" pipe mixed in with a long run of 1" pipe will have significantly less resistance (and higher resulting flow volume) than the same total length of 3/4" pipe.

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    In the Trades Gary Swart's Avatar
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    Your questions are good, but they point out one reason why you should get a professional plumber to examine and evaluate your gas pipe, and make whatever changes are called for. The other reason is, gas work is best left to professionals for safety reasons. Water leaks can be a PITA, but gas leaks can ruin you whole day.

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    Whats the pressure of your gas service. Your gas supplier will be able to answer that. Its very important.

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    Plumber jimbo's Avatar
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    Yes, a fitting will affect flow, but at least as important is the friction loss that is a function of flow rate ( cubic feet per hour) X size of pipe X LENGTH of pipe.

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    Quote Originally Posted by shorecat99 View Post
    In considering an upgrade to some of my natural gas piping, I noticed something I would like to understand better. I just bought a new kitchen range,60,000 btu input, but did not hook it up yet as I need to have to have the gas supply moved a couple of feet. It's the end of the run so normally no big deal but in checking charts, it seems some of my piping is undersized so I'm considering the upgrade at the same time. Inside my crawlspace, I have about 20' of 1" pipe, then a 1" x 1" x 1/2" tee supplying a clothes dryer, and continuing on to a reducing tee 1" x 1/2" x 1/2". The tap supplies a water heater but then the trunk (as I call it) or continued piping which is 1/2" goes on to supply a 100,000 btu heater, then another 10' where it used to supply a built in wall oven, and another 8' for and old cooktop. The new range will replace these 2. I'm considering going back to the reducer before the house heater and upgrading to 3/4" or 1" to properly meet demand downstream but here's my concern. **I thought I had 1" service coming from my gas meter as it comes through the wall from outside as 1". Looking at the meter today, I noticed 3/4" in (which is fine because it's probably higher pressure before the meter, but then also 3/4" out of the meter. A couple of elbows and a few inches later it goes to a reducer (or in this case, it would be an enlarger) which brings this from 3/4" to 1" and then enters the house. This boggled me! Some thoughts came to mind like "a chain is only as strong as it's weakest link" or in this case, "a piping system can only supply as much as it's smallest diameter allows through". Is this normal?? Is it ok because it's only a short run at the beginning? I would think my capacity is limited to 3/4" no matter what they added to it afterward?? Sorry so long and technical but I'm big on theory and like to understand as much as possible before doing something or having something done for me. Thanks for reading! I'll check back later as I have to go to work now...
    Your exactly correct. "A piping system can only supply as much as its smallest diameter allows through"

    How could that statement not be true. You can only fit so much gas through a size and lenght of pipe that the pressure will allow. Higher the pressure the more flow you can get through the smaller pipe and it will flow at a higher velocity

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    Thanks everybody! I learned a lot.

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    The two keys are the street pressure and the size of the pipe coming out of the regulator...what happens before it is somewhat important, but up until the regulator, the pressure is likely MUCH higher (depends on where you live), so a smaller pipe could still supply what you need. Once it is in the house, though, it is all a function of size and distance and flow requested...the longer the pipe, the more fittings, and the relevant sizes will limit how much gas can flow.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9
    Moderator & Master Plumber hj's Avatar
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    When you refer to capacity of a given pipe, you have to take into consideration its length also. A smaller size does not necessarily compromise the rest of the system, because for a short section, the gas will increase its velocity to compensate for the reduced area. The majority of residential gas meters are 3/4", regardless of whether the rest of the system is 1/2" or 2". Unless you have a medium pressure. or propane, system, (and that would require additional considerations), your pressure is in the neighborhood of 6" w.c. or about 1/4 psi.
    Licensed residential and commercial plumber

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    Like HJ says, a short section acts more like the Venturi in an old carburator...the narrowing increases velocity so the gas mixes better with the air then the intake opens up to distribute it well. This may help http://www.engineeringtoolbox.com/na...ing-d_826.html
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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