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Help Tim!!!!

JamieP
JamieP Member Posts: 26
There has been much debate in our shop regarding this topic lately!



We all know if you need "more" gas because of multiply appliances in a location you need more VOLUME!







The debate is where do you need the volume from! Some guys think that

you need the largest pipe to carry the gas as soon as it comes through

the wall and into the house!







Others think that you can add the "volume" section close to the

appliances and that will be fine. Argument being that a lot of the low

pressure gas lines in our area come into the homes 3/4" anyway so why

does it mater where we add the volume?







Any well knowledge gas pipe sizing guys out there?



Jamie

Comments

  • icesailor
    icesailor Member Posts: 7,265
    Chips:

    Isn't that sort of like asking if one can get 10# of cow chips in a 5# bag? The first 5# goes easy. After that, it gets harder and harder.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Interesting question.

    I wonder about that too. I am not a gas expert, but I did take high school and college physics. But that was all 50 years ago, so I am sometimes rusty about these things. At my church, a gas line comes out of the ground, goes through a regulator, then through the meter then along an outside wall and into the building. It makes a couple of 90 degree turns as it goes around corners.



    The pipe out of the ground looks about an inch, as do the connections to the meter. But after the first 90 degree turn, it goes up noticably in size to perhaps 1 1/4 inch or 1 1/2 inch. It then drops back down to one inch once it enters the building, down to 3/4 inch to the gas valve (the on-off ones outside the furnaces, and I am not sure if it drops down to 1/2 inch or not (each furnace is 125,000 BTU/hour) for the last foot or so. So it looks to me as though the bigger pipe is just a tiny buffer tank that might smoothe out surges as a furnace turns on and off, and it might have slightly less resistance to flow than the smaller size pipes in there. But it seems to me that every piece of pipe, no matter what the diameter, adds to the resistance to flow. It is just that the bigger pipes have less resistance.



    In my house, all the pipe is 1 inch until right near the boiler where it goes down to 3/4" to the on-off gas valve, and then down to 1/2 inch to enter the boiler (that has a 1/2 inch fitting). But that boiler has an input rating of only 80,000 BTU/hour.



    It seems to me this stuff calculates just like electricity, and I know how to do that. And therefore, the resistance at the tightest point controls the flow the most. When I had gas installed in my house that never had gas in it, an agent of the gas company came out, sized the house, asked what the gas would be used for (heating boiler with indirect hot water), and decided their smallest meter would be OK and they ran 1 inch plastic tubing at "high" pressure (15 psi, if I remember correctly) from the street to the regulator at the meter where it was reduced to, I suppose, about 7 inches water gauge.



    I just looked up a gas piping chart and it tells me my own gas pipe is too small. It says for 30 feet of 1 inch schedule 40 pipe, I can get 37,500 BTU/hour through it. Maybe less, because I did not allow for the elbows and that some of it is CSST that I assume has more resistance. And my boiler is reated at 80,000 BTU/hour. That is with some pressure drop. So If I actually draw 80,000 BTU/hour, I imagine I get double that pressure drop. Now my heat loss says I should expect to need only 30,000 BTU/hour when it is 14F below design day temperature, I guess I have nothing to worry about, but I am surprised the inspector approved that.
  • Tim McElwain
    Tim McElwain Member Posts: 4,480
    edited January 2012
    Jamie

    There has been much debate in our shop regarding this topic lately!



    We all know if you need "more" gas because of multiple appliances in a location you need more VOLUME!





    The debate is where do you need the volume from! Some guys think that

    you need the largest pipe to carry the gas as soon as it comes through

    the wall and into the house!



    THERE ARE SEVERAL DIFFERENT METHODS OF RUNNING PIPE ALLOWED IN THE CODES THERE IS THE "LONGEST LENGTH METHOD" OR THERE IS THE "BRANCH LENGTH METHOD" AND THEN THERE IS THE "HYBRID PRESSURE METHOD" EXAMPLES OF THESE ARE IN THE CODE BOOKS.





    LONGEST LENGTH IS THE SIMPLEST AND IS EFFECTIVE FOR ALL PIPING SYSTEMS, BUT IT CAN RESULT IN LARGER PIPE SIZES THAN NEEDED.







    BRANCH LENGTH IS AN ALTERNATE SIZING METHOD THAT COULD PERMIT SLIGHTLY SMALLER PIPE DIAMETERS IN SOME SEGMENTS OF A PIPING SYSTEM WHEN COMPARED WITH THE LONGEST LENGTH METHOD.





    A LENGTHY DISCUSSION WOULD BE REQUIRED TO EXPLAIN THE HYBRID PRESSURE AND IF YOU ARE INTERESTED IT CAN BE FOUND IN NFPA 54 NATIONAL FUEL GAS CODE.





    Others think that you can add the "volume" section close to the

    appliances and that will be fine. Argument being that a lot of the low

    pressure gas lines in our area come into the homes 3/4" anyway so why

    does it matter where we add the volume?





    WHEN DISCUSSING THIS TOPIC WE FIRST HAVE TO BE CLEAR ON THE TYPE OF GAS PRESSURE BEING DELIVERED TO THE DWELLING IS IT "LOW PRESSURE" 7" TO 10" W.C. OR HIGH PRESSURE UP TO SOMETIMES 100 POUNDS PRESSURE WHICH IS THEN REDUCED TO 6" TO 7" COMING OUT OF THE POUNDS TO INCHES REGULATOR. WITH HIGH PRESSURE THE PIPE COMING THROUGH THE WALL FROM THE GAS MAIN IN THE STREET MAY BE ONLY 3/4" BUT IT IS AT VERY HIGH PRESSURE SO SIZE IS NOT IMPORTANT.







    HERE IS THE DEAL WHEN SIZING PIPE CHOOSE THE METHOD YOU ARE GOING TO USE, MAKE A PIPING PLAN, ACCOUNT FOR FITTINGS ACCORDING TO TABLES GIVEN, AND DEPENDING ON THE METHOD SIZE TO THE LOAD (BTU OF EQUIPMENT) AND YOU WILL NOT BE WRONG. THE PROOF WHEN YOU ARE DONE IS AT FULL LOAD ALL THE EQUIPMENT RUNNING THERE IS NO PRESSURE DROP AT ANY PIECE OF EQUIPMENT.







    SO YOU SEE BOTH ARGUMENT COULD BE CORRECT DEPENDING ON THE PIPING METHOD CHOSEN.



     
  • Tim McElwain
    Tim McElwain Member Posts: 4,480
    JDB I am not sure how you came to

    your sizing on your Ultra. According to NFPA 54 pipe sizing charts 30 feet of 1" pipe with an allowable pressure drop of .3" W.C. will give you 284,000 BTU's. If a allowable loss of .5" W.C. it will give 374,000 BTU's.



    As for CSST it is measured in Equivalent Hydraulic Diameter there are numbers on the CSST example an EHD 23 is roughly the same as black pipe 3/4", and EHD 30 is the same as 1" pipe. What is the EHD number on your CSST?



    Correct operation of an Natural gas or LP system is that with all equipment operating with correct pipe sizing there should be no measurable pressure drop at the appliances.
  • lchmb
    lchmb Member Posts: 2,997
    I like

    the longest run method. I may make it a little larger than needed but i wont have to redo it if they add something else down the road..:)
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