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Thread: Fire Dpt. Says Nat. Gas. Engine Generator Not Emergency

  1. #21
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    Alternatively, OP it may be worth getting natural gas outage history from utility. Are you in a seismically active area? Another point is do you really have an Emergency System or a Legally Required Standby System? Review Article 700 vs 701 Handbook commentary. AHJ may likely be more open to considering acceptance if it's the latter with low or no outage history.

  2. #22
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    Quote Originally Posted by smoothops10 View Post
    I'll admit I'm suprised, in the example you provided that looks to be true but that is a residential generator in 14kW range. I doubt OP is questioning about a generator this size. It is not the case for the larger sizes.

    The project I worked on where we looked at the Dual Fuel NG/LPG (Automatic Changeover) option was a 350kW NG generator. The generator had a 355kW rating on Rich-Burn Natural Gas and only a 240kW rating on Rich-Burn LP Gas. Data sheet also indicated " For dual fuel engines, use the LP gas ratings for both the primary and secondary fuels."
    http://www.kohlerpower.sg/onlinecatalog/pdf/g4191.pdf
    Very interesting. What puzzles me is that at several points they indicate that NG has a fuel value of about 1,000 BTU/ft3 while LP is 2,500 BTU/ft3, clearly much higher. Maybe carburation is a limiting factor? Can't get enough air into the combustion chamber to take advantage of the difference?

  3. #23
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    Quote Originally Posted by gadfly56 View Post
    Very interesting. What puzzles me is that at several points they indicate that NG has a fuel value of about 1,000 BTU/ft3 while LP is 2,500 BTU/ft3, clearly much higher. Maybe carburation is a limiting factor? Can't get enough air into the combustion chamber to take advantage of the difference?
    LPG is primarily composed of propane, butane, isobutane and mixtures of these gases.

    NG is predominately methane.

  4. #24
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    Quote Originally Posted by Chamuit View Post
    LPG is primarily composed of propane, butane, isobutane and mixtures of these gases.

    NG is predominately methane.
    All very interesting, but it still doesn't answer the question. If you look here, there's a Kohler 20 kW unit that it only 20 kW on LP, but 18 kW on natural gas, so it's not a brand specific difference.

  5. #25
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    Quote Originally Posted by gadfly56 View Post
    All very interesting, but it still doesn't answer the question. If you look here, there's a Kohler 20 kW unit that it only 20 kW on LP, but 18 kW on natural gas, so it's not a brand specific difference.
    For proper combustion LP needs an air to gas ratio of 25:1 - NG needs 10:1 ratio. So you have to change orifices on the Kohler 20kW or turn the selector on Generacs (which changes the orifice size).

    LP has a higher BTU value 2572 Btu/ft3, NG 1011 Btu/ft3.

    You can only fill that cylinder up with so much air/gas.

  6. #26
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    Quote Originally Posted by Chamuit View Post
    For proper combustion LP needs an air to gas ratio of 25:1 - NG needs 10:1 ratio. So you have to change orifices on the Kohler 20kW or turn the selector on Generacs (which changes the orifice size).

    LP has a higher BTU value 2572 Btu/ft3, NG 1011 Btu/ft3.

    You can only fill that cylinder up with so much air/gas.
    That was the information missing from your previous post, which directly answered the question in my post. It appears, then, that for smaller generators, the combustion chamber is sized for LP and you use a larger orifice for NG, while for larger units the chamber is sized for NG and is therefore combustion air limited for LP.

    Further investigation at Kohler indicates that the 40 - 100 kW units are rated as equal on NG or LP, and the tip-over starts at 125 kW.
    Last edited by gadfly56; 10-15-18 at 05:30 PM.

  7. #27
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  8. #28
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    Remember what happened in Massachusetts. They're right.
    I'm in over my head...

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