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Gas drip leg/reservoir

Harvey Ramer
Harvey Ramer Member Posts: 2,221
I am installing a standby 20kw genset. The gas piping will run underground 150' to the genset. The gas pressure in the underground line will be 2 psi. I need to install a regulator on the line by the generator to bring the pressure down to 10" wc. This will not allow sufficient piping between the regulator and the genset gas valve for buffering. I was wondering if anybody knows of any inline reservoir that would be available for this situation? It would be ideal if it could serve as a drip leg as well.

While on the subject, anybody have pipe sizing tables for SDR11 IPS. Specifically for 2 psi. but I would like the charts for low pressure and 10 psi as well?




  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    sufficient piping between the regulator and the genset gas valve for buffering

    I have a 14Kw Kohler generator (natural gas fueled) outside my house, and it is a little different from yours. And I am not a professional, so I will not give any advice here.

    There is a 50 psi line down my street that comes through a 1 inch plastic pipe to my house, where a regulator takes it down to about 7 inches water gauge pressure, and it goes through the meter.

    On the way out of the meter, it comes to a T and a 1 inch black pipe goes into my house I guess 20 feet or so to my heating boiler.

    Out the other leg of the T the installer ran a 1 1/4 pipe about 2 feet to some 1 1/4 inch plastic slightly flexible gas pipe in the ground and where it came up it stepped down to about 1/2 inch as it entered the generator set. There are shutoff valves and stuff there too. Inside the generator there is a small regulator that goes to the gas valve and carburettor. It is only a foot or so from the regulator in the generator to the carburettor.

    The total distance from the gas company regulator to the meter to the generator is probably only 8 to 10 feet. The installation manual says that distance can run 3/4 inch pipe, but the contractor says it works better with the larger pipe, and most of the plumbing cost was labor, not the pipe itself.

    Could it be that 8 to 10 feet is enough, or could it be that runnning 1 1/4 inch pipe instead of 3/4 is reservoir enough?
  • kcopp
    kcopp Member Posts: 3,990
    is this....

    what you need? this stuff is great underground.

  • Harvey Ramer
    Harvey Ramer Member Posts: 2,221

    That is precisely what I was looking for.


  • Harvey Ramer
    Harvey Ramer Member Posts: 2,221

    The genset I am installing needs a minimum of 7"wc. It is 150 feet away from the gas meter. Hence I chose to switch to a 2psi system. Now I will have the underground riser coming up right at the genset with a regulator to bring the 2psi down to 10" wc. There will only be room for a foot or 2 of pipe from that point to the genset valve. I need a minimum of 10' of pipe betweek the regulator and the genset or, as I was hoping, a small canister like buffer that could be placed in line.

  • SWEI
    SWEI Member Posts: 7,356
    Small canister-like buffer

    would no doubt have to be listed.  Any reason a 4" x 24" nipple with some reducers on each end wouldn't work? 
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Any reason a 4" x 24" nipple with some reducers on each end wouldn't work?

    I suspect that when my contractor ran 1 1/4" pipe between the meter and the generator, when 3/4" was all that was required, that that may have taken the place of a reservoir.

    1" between the gas company regulator and the underground part (about two feet),  1 1/4" for 8 feet or so of underground part, then 1/2" from where it came out of the ground to the generator (about 2 feet or a little less). And right inside the generator is their regulator. Then a regular reciprocating engine to spin the alternator. The controls vary the gas-air delivery to the generator to keep the right generator speed independent of the electrical load.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I think SWEI is correct, ...

    ... but I am not a professional. True, the pressure reduction from 2 psi to 10" is more than mine from 7" to whatever the carburettor regulator in my generator requires, but the issue is the same, you do not want to have the two regulators fighting one another setting up a resonance.

    SWEI is suggesting an explicit "buffer tank" with the oversize section of pipe. And I may have the same thing with the 8 foot run in the underground part of my system.
  • icesailor
    icesailor Member Posts: 7,265
    2# Vs 10"# gas:

    I'm not there, and no expert, But it sounds like from what you write, that the issue is the transition between the 2# gas and a separation between the two regulators.

    A properly sixed "larger" 10"# gas line would eliminate all your problems. Or so it might seem. 20KW output takes a large motor to drive the alternator.

    A 2# regulator at the point where the 10" gas goes into the ground and runs 150' will give you all the buffer space you could ever need. Or so or seems to me.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,221

    True, I could do that. It would mean switching to 1-1/4" pipe versus 3/4" . I'll have to see how much difference in cost between the 2.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Pipe sizes for 20 KW generator.

    My generator is 14 KW, but the installation manual also specifies the gas pipe line size for their 20 KW unit

    They require 5 to 11 inch pressure at the generator, and its input pipe is 1/2 inch. These are all assuming natural gas.

    Minimum pipe sizes:

    25 to 50 feet: 1-inch.

    100 to 200 feet: 1 1/4 inch.


    Full load  281 cu-ft/hour

    75% load 243 cu-ft/hr

    50% load 161 du-ft/hr

    25% load 127 cu-ft/hr

    I assume these are minimum pipe sizes.
  • Bob Harper
    Bob Harper Member Posts: 914
    gas codes

    You'll need a listed shutoff then sediment trap immediately upstream of the MP regulator then another trap or pressure tap immediately downstream. The sediment trap not only protects the regulator but provides a means of attaching a pressure gauge on the 2-10 psi side or a manometer on the low pressure downstream side. Start with a capped tee off the appliance valve and work your way back for the minimum offset.
  • icesailor
    icesailor Member Posts: 7,265
    edited August 2014
    Larger Service Size:

    Remember that the larger pipe size has less restriction than the smaller one. And the larger size will be acting as an accumulator and protecting you (as best it can" from initial pressure drops at start up or if something else comes on line and uses a large initial start load.

    If you look, often with PE pipe, the difference between the smaller pipe and the larger pipe is stupid money.

    Something to think about.

    3/4" Poly pipe is closer to 1/2" ID, and 1 1/4" is closer to 1" ID. Bigger is always better.