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Thread: Dry hydrant

  1. #1
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    Default Dry hydrant

    Hi,

    Do any of you guys know a formula that relates: lift, pump rating and pipe size on dry hydrant.

    We want to maximise a new dry hydrant installation and increasing pipe size seams just to simple. There has to be a relation with the capacity of the engine and the lift at the site.

    Ex; 1250 pump, 15 ft of lift What is the optimal pipe size for drafting operations on that dry hydrant?

    Regards,

    Sly


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    MembersZone Subscriber Heretic's Avatar
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    You would want to keep the dry hydrant the same size as your hard suction, i.e. 3" or 6". If you need more water, add a second dry hydrant, and hookup two seperate hard suctions, one to each hydrant.

    -H
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    RDL 210, Check Fire Rescue mag. In the last few month they have run a series on fire flows with much dedicated to rural water supply. I know they did a story on drafting operations on dry hydrants, dry hydrant installation, pipe size and lift. As well as Friction Loss in hard suction hoses.


    I will try to find what issue it was in I have a wonderful filing system

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    Quote Originally Posted by Heretic
    You would want to keep the dry hydrant the same size as your hard suction, i.e. 3" or 6". If you need more water, add a second dry hydrant, and hookup two seperate hard suctions, one to each hydrant.

    -H
    Well, not exactly! You want to be able to rely on the pipe to supply all the water you can possible want to draft from it. What will you be drafting with? In our area,most pumpers at draft are using 6 inch hard suction. So every dry hydrant head is a 6 inch female. Saves farting around with adapters.

    Now, what's the pumper rated for. Assuming 6 inch hard suction, depending on the degree of list, you're looking at 1,000 to 1,500 GPM. There will be some friction loss even in PVC pipe. We run 8 inch schedule 40 PVC for the entire underground. Convert it to 6 inch at ground level. Never had a problem.

    When in douby, oversize the pipe. You can always use a reducer at the head. It's a real pain to dig it up to enlarge
    Remember, it IS as bad as you think and they ARE out to get you!

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    Hi Jim917,

    I agree with you 100% about wanting to get the most out of the dry hydrant.

    There must be a way to calculate the optimal size of pipe accordind to pump capacity and lift on site. You can't oversize your pipe indefinilty...

    Regards,

    Sly

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    Quote Originally Posted by RDL210
    Hi Jim917,

    I agree with you 100% about wanting to get the most out of the dry hydrant.

    There must be a way to calculate the optimal size of pipe accordind to pump capacity and lift on site. You can't oversize your pipe indefinilty...

    Regards,

    Sly
    Well, actually yes you can. Think of the pond or stream you are using as an unlimited supply of water. Now, how the heck are you going to get to the water when there's 30 inches of ice? Ah! Dry hydrant.

    Properly installed, the dry hydrant is undergrond and under the ice and has water all the way back to the final elbow where you rise above ground. So your engine is only drafting it the last 8, 10 or 15 feet up. Would you rather be drafting from a pipe where that residual water is in a chamber 4 inches in diamtere, 6 inches in diameter or 12 inches in diameter. Remember why we use large diamter hose! The volume doubles very quickly with each additional inch of size.

    PVC pipe is very cheap. The labor and equipment cost to do the excavation is the major expense of any dry hydrant. What do you need today? If you need a 6 inch, I'd install a 10 inch. That way 10 years from now, you won't be kicking yourself in the butt.
    Remember, it IS as bad as you think and they ARE out to get you!

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    Last edited by MG3610; 09-04-2005 at 10:11 AM.

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    Sly,

    Remember that the only thing you have working for you to move the water from the source (river, pond, cistern) to the pump is atmospheric pressure...about 14.7 psia(101 kPa). I f you have 15 feet of lift from your source to your outlet, you only have 14.7 psia minus 15 ft x 0.434 psi/ft (elevation head) = 8.2 psia. In a perfect world, you could use all of that to push water into your pump. Reality is that you can probably use less than half, or about 3 psi, and have it actually work, since you cannot measure or account for all the variables (friction loss through strainers, fittings, hard suction, and pump inefficiencies). As long as you keep the total friction loss through the piping under 3 psi, you should be okay. I wouldn't go any smaller than the barrel of the hydrant, and I wouldn't go larger unless I had long pipe runs. Lift is far more important. At 20 feet of lift, you only have, at most, 1 psi available to overcome friction loss in the piping.

    You will need friction loss tables for the pipe you are using; or you can calculate it using either the Hazen-Williams, or Darcy-Weisback (sp?) equations. You'll find the Hazen-Williams equation in NFPA 13. Darcy-Weisback is a little more complicated.
    An ounce of prevention is worth a pound of suppression! Sometimes even more!

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