1. #1
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    Question LDH & low pressure hydrants

    Need your advice and thoughts on hydrant setup on 4 inch LDH. First of all we are a rural volunteer department with few hydrants. Most hydrants are low pressure, all have two 2.5 outlets. We do have a few hydrants that will flow over 1,000 GPM plus but most are less. Where we do have hydrants long lays are required, each engine has 1,000 ft of 4 inch storz coupled hose. The question is the setup, we currently use two 3 inch lines with hydrant valves to feed a our 4 inch through a clappered siamese connection. Would we be as effective on low pressure hydrants to hook the 4 inch to one 2.5 outlet? This would expedite hookup and be less work for the hydrant operator. My concerns are maintaining the available flow and keeping the hydrant operations standardized regardles of flow capability. Just for info we do train on relay pumping on these long lays. Your thoughts are appreciated.

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    arkff
    Not exactly sure on your set-ups, can you give us a little more detail? Also, do you have static and hydrant flow test data?
    Give us the specifics and will do some hydraulics calcs and see what we come up with.

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    Thanks for the interest cilfd. Typical readings on the hydrants are 90-95 static, 45-55 residual, 35 to 45 pitot. The hydrant connections are two 50 foot three inch lines , typical double jacketed, with 2.5 couplings. These lines run from the 2.5 outlets of the hydrants to a clappered siamese that has two 2.5 inlets feeding the 4 inch storz outlet. 4 inch LDH supply is Snaptite rubber jacket. We have hydrants with readings below the numbers above but the majority fall in that category. The question is could we get the same flow from a single 2.5 hydrant outlet by adapting the 4 inch to the 2.5 outlet. Thanks again for the interest and help.
    Last edited by ArkFirFtr; 10-06-2002 at 11:43 AM.

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    arkff,
    I assume your pitot was for a single 2 1/2" hydrant outlet so I used an avg pitot of 40 psi for just about 1000 gpm.
    Here's what I got with a water supply of 92 psi static, 47 psi residual flowing 1000 gpm. I used a total supply line length of 250 ft. (200-4" + 50 of dual 3's) or 250' of 4" for straight 4".

    500 gpm flow
    4" direct to single 2 1/2" outlet: 500 gpm available at 250' at 70 psi
    4" w/dual 3's: 500 gpm available at 250' at 65 psi

    1000 gpm flow
    4" direct to single 2 1/2" outlet: 1000 gpm available @ 250' at 15 psi
    4" w/dual 3's: 1000 gpm available at 250' at 10 psi

    So the 4" straight to the hydrant gives an extra 5 psi for a given flow and distance. As you probably know, 4" beats dual 3's in friction loss in the 10% range, plus you've got some extra loss in the siamese, so as far as the total supply line goes - the straight 4" would beat the dual 3's to siamese by a small margin.

    The other issue is with regard to whether the water supply changes when you open both 2 1/2" discharges vs. one. This may have a small impact. Remember though, that the water supply in the main supplying that hydrant can have only 1 water supply curve and that curve will be a single supply throught the hydrant main valve, up through the barrel and just before it leaves whichever discharges are open and then once it leaves the hydrant , totally dependent on the supply line(s). Will 2 discharges give you a better water supply curve vs. one? The only real way to figure that out it is to flow test a hydrant both ways and plot the two supply curves. You'll get two different flows and two different residuals. Plot both water supplies. Pick a flow you are interested in, let's say 750 gpm. See what residual pressure is available on both curves. The two discharge flow test would have to beat the single discharge flow test at 750 gpm by greater than the 5 psi penaltly you are getting from using the dual 3's for 50'.

    The other factor is whether you want that extra flow from two discharges. Your single discharge test is giving you around 1000 gpm. If 1000 gpm or less is Ok, then I would say use 4" striaght to the hydrant. If your 2 discharge flow test gets you let's say 1100 gpm. Then if you want that extra 100 gpm, you'll need to use both discharges. But there is that 5 psi penalty for using both discharges with dual 3's for 50'. So you can get up to the 1100 gpm with using both discharges, but the penalty is less distance in your supply line since you'll have less pressure available to "push" that water to your engine. Or if you want the same length of line, then you will not get the entire extra 100 gpm ,just some percentage.

    So I would say that if you're satisfied with 1000 gpm, then go with the 4" direct and gain some pressure (or supply line distance). If not, flow test the hydrant with both butts open and see what you get. If the extra flow is worth something, stick with the dual 3's, but realize there is a pressure (or distance) penalty.

    Or, if you really wanted to max out the hydrant, use dual 4's to feed a LDH siamese to your single 4". You probably can see though, that this starts getting into the law of diminishing returns - a lot of extra hose, a big LDH siamese, etc. for really what is probably going to be a small increase in flow/pressure. But it does somewhat depend on what your goal is.

    I've used just what you are considering with 5". Spin on a single 2 1/2" NH x 5" Storz adapter and hook the 5" up direct. Simple and quick.

    Hope this helps.

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    Thanks loads for the work! Yes, the pitot was for a single outlet. That was exactly the info I needed. Most of our hydrants flow 1,000 GPM or a little less. The direct to the hydrant with the 4 inch is a simpler connection and we definitely operate on the K.I.S.S. principle. We are a very small department so buying 4 inch LDH siamese would not be worth the small gain. Tanker shuttles and drop tanks are the more common water supply for us. Thanks again!

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    Have you considered asking your water authority to replace the hydrants with mor emodern heads? From what I understand, the replacement of the old barrel with one that has a 4.5 inch outlet as well as one or two 2.5 outlets is not very expensive. Certainly, it would be worth asking as well as telling them that is what you want on all new or replacement hydrants.
    Remember, it IS as bad as you think and they ARE out to get you!

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    You might also want to consider making the jump to 5" While you do not have the flow to support the 1500gpm 5" is capable of, at 1000gpm 5" friction loss is very small and would allow you to flow capacity on a much longer lay. You would not have to replace all your 4" right away, just do one truck at a time and carry 4" to 5" adaptors on all trucks.

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    Try a Hydrant assit valve,I heard there expensive but maydo the trick.

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