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  1. #1
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    Default How many GPMs from a Hydrant ?

    We are specing out a new truck,and some of us are split on getting a 1500gpm pump or a 2000gpm pump.We currently have a 1500 gpm pump,and we have hydrants in 95% of our district,but we also have access to a few drafting areas. The water mains vary from 4 - 24 inches with static pressure from 35 -100 psi, so I'm wondering if there is a formula to figure how many GPMs could be obtained from each hydrant knowing the water main diameter and static pressure.


  2. #2
    Forum Member FWDbuff's Avatar
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    Do you guys have a Pitot Guage? If not, try to borrow one. See if a mutual aid company has one, or maybe a fire training academy, or even someone that tests pumps. See if they have one that is built into a 3" manifold (as opposed to a hand-held one) that will thread onto one of the "ears" of the hydrant. I know this will only give you the rough flow of one of the 3" ears, but there is a formula out there somewhere, when fed in with the given flow of the ear, it will give you a rough calculation of GPM.

    Now all we need is someone to cough up that formula. Chiefengineer11?????? T.C. ????? Rescue 101??????
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    From www.firehydrant.org/info/ftest4.html

    Q FORMULA

    The Q formula produces a value in GPM based on the nozzle diameter and pitot pressure (solving for "Q".)

    Q=29.83cd2√p

    Where Q=observed flow, c=coefficient, d=outlet diameter, p=pitot pressure.

    That is d squared, the superscript 2 won't copy over but I hope the site can explain it better.

    Good Luck!

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    You won't get any pump to create water. On a pressurized system the pump will most often pump well over the rated pump capacity as long as you have more than enough discharges of the right sizes. We've flowed over 2800 gpm from a single 1500 gpm pumper on a decent hydrant.

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    Quote Originally Posted by wconlon53 View Post
    We are specing out a new truck,and some of us are split on getting a 1500gpm pump or a 2000gpm pump.We currently have a 1500 gpm pump,and we have hydrants in 95% of our district,but we also have access to a few drafting areas. The water mains vary from 4 - 24 inches with static pressure from 35 -100 psi, so I'm wondering if there is a formula to figure how many GPMs could be obtained from each hydrant knowing the water main diameter and static pressure.
    How often do you draft? What size supply hose do you carry? What is the longest lay of hose youd likely use at draft? How much suction hose will you carry?

  6. #6
    Forum Member SFD_E73_RET's Avatar
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    If you have hydrants design your truck around big power and big brakes. From hydrants you can get out what you can get in.

    PS. Static pressures won't tell you much on a hydrant, it has nothing to do with volume, well almost nothing.

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    Quote Originally Posted by MG3610 View Post
    How often do you draft? What size supply hose do you carry? What is the longest lay of hose youd likely use at draft? How much suction hose will you carry?
    We have a few pre-plans that are setup with drafting,but I don't think we've drafted more then 5 times in the last 20 years.We currently carry 900' of 3",1500' of 5" LDH , and 3 10' lengths of hard suction.

  8. #8
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    Lightbulb

    Don't forget, your coefficient changes depending on the type of edge you have on the orrifice too. .7, .8 and .9 in the metric system going from the least efficient edge to the smoother, rounded edge. This reflects a greater gpm available from more efficient waterflow.

    I might have it backwards- but I'm pretty sure those were the numbers.
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    Just tossing in my $0.02. I know that 2000 gpm sounds great but are you prepared to buy a bigger motor to keep it pumping easily into the future. Also I have to ask, have you ever had a fire where your 1500 gpm pumper wasn't enough? It is my opinion that, unless you have a aerial attached to your apparatus, 1500 gpm will serve you well and cost you less. Unless you are one of those small fire departments that has 20 people show up and only one truck 2000 gpm is overkill. Think about it, 1 officer and 1 operator leaves 4-6 people to man the hose lines. Unless it is a surround and drown, 4-6 people cannot man enough hoses for 2000 gpm.

    These are my opinions and mine only.
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    Its this simple......

    If you get a 2000 GPM pumper, and draft to supply streams.....2000 GPM is produced at 150 PSI from your pump (at the conditions of the UL test). Subtract 20 PSI for residual and you are left with being able to move the capacity of the pump about 500 feet through 5" hose. 2000 GPM is one wagon pipe and one aerial master stream. Not uncommon for a major fire.

    Don't take advice from people that have opinions....go based on facts.

    Based on your information, 1500 GPM should serve you well, but dont forget you can increase about another few hundred GPM in most cases by adding a second suction line to your pump. Its as simple as adding an additional rig to the box alarm to report to the draft site and drop off their hard suction hose to let you borrow for more water flow capability. For the rare times you draft, put the money into the rigs other features. Spec enough discharges to get the water out though when you do need it most. Don't go with the minimum, get several discharges on the pump panels, not just a 2 1/2 and an LDH connection like so many places do.
    Last edited by MG3610; 04-30-2008 at 09:52 PM.

  11. #11
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    I agree with MG 100%... but if you draft a bit, spec drafting specific options as well. MIV valves and seperate primers are a godsend if you're heavy into drafting. A couple of 2000GPM pumpers in Bladensburg Maryland even have the passenger side fitted with dual steamer intakes so that they can draft at capacity off one side of the rig. Just my $.02
    FTM-PTB DTRT

    Everything I state on here is to support and aid my fellow firefighters. Everything I post is my opinion only, and in no way should be taken as an official opinion of any Company, Department, or Municipality I represent... oh and this includes Pierce Mfg, as so their legal department has advised me; since they apparently also invented the right to control "Free Speech".

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    Quote Originally Posted by MG3610 View Post
    Its this simple......

    If you get a 2000 GPM pumper, and draft to supply streams.....2000 GPM is produced at 150 PSI from your pump (at the conditions of the UL test). Subtract 20 PSI for residual and you are left with being able to move the capacity of the pump about 500 feet through 5" hose. 2000 GPM is one wagon pipe and one aerial master stream. Not uncommon for a major fire.

    Don't take advice from people that have opinions....go based on facts.

    Based on your information, 1500 GPM should serve you well, but dont forget you can increase about another few hundred GPM in most cases by adding a second suction line to your pump. Its as simple as adding an additional rig to the box alarm to report to the draft site and drop off their hard suction hose to let you borrow for more water flow capability. For the rare times you draft, put the money into the rigs other features. Spec enough discharges to get the water out though when you do need it most. Don't go with the minimum, get several discharges on the pump panels, not just a 2 1/2 and an LDH connection like so many places do.
    We have speced out plenty of discharges,which is why I think some members think we need the 2000GPM vs 1500GPM pump.We have speced a top mount pump panel with 3 crosslays,a 3" front bumper line with 1 1/2" gated wye, deck gun, 2 LDH discharges,and 4 - 2 1/2" discharges.

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    Quote Originally Posted by wconlon53 View Post
    We have speced out plenty of discharges,which is why I think some members think we need the 2000GPM vs 1500GPM pump.We have speced a top mount pump panel with 3 crosslays,a 3" front bumper line with 1 1/2" gated wye, deck gun, 2 LDH discharges,and 4 - 2 1/2" discharges.
    Our 1250 Single Stage has (2) bumper 1 1/2", (2) 2 1/2" on the officer side panel, (1) 3" officer side panel, (1) 2 1/2" drivers side panel and (2) 2 1/2" rear tailboard as well as 3" deluge gun riser.

    Look at pumps, and how many discharge ports are availible, versus how many are typically used (spec'd with valves). Theres usually alot left over.

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    Along the lines that MG3610 speaks, it's good to think about he anticipated uses of the discharges. With two LDH discharges and a deck gun you should be able to flow most of what you can get in, at draft or from a hydrant. How many lines will ever be stretched from the same engine? Here's how we layed out our new rescue pumper:
    (1) front bumper 2.5" -wyed for two 200' 1.75" preconnects, but could be used as a front 2.5" discharge as needed
    (1) 2" discharge direct to 200' preconnect in left rear compartment. Pretty singular use.
    (1) 2.5" main bed discharge off the rear- preconnected to 300' of 2.5" with SB nozzle. Another lay of 300' is adjacent for extending this line
    (1) 3" main bed discharge off the rear- preconnected to 300' of 3" and a RAM 500 gpm monitor. Can also be used as a "flying standpipe" or rear lot line
    (2) 5"LDH discharge with 5" to 2.5" reducers- these are for supplying the ground mount monitor, sprinklers/standpipes, the tower gun or another engine. We found it hard to imagine the scenario where more than 2 of these would ever be done from the same engine at once.

    That's it, keeping the pump area fairly well compact, reducing the panel size and cost of multiple unused valves. We beleive in trying not to put all our eggs in one basket, so generally not more than 2 to 3 offensive discharges would be used at once.

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