1. #1
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    Feb 2005

    Default Tender filling Calculations

    I'm currently taking a hydraulics class, and while out at lunch the other day some of my class mates and I came up with a question, but we are on spring break so haven't had a chance to ask our instructor. However, the question is really bugging me, so I'm going to throw it out here for you guys

    Ok, Here goes: When pumping to fill tenders, such as at a drafting fill site, how do you know how many GPM you are flowing?

    The scenario we came up with: You are drafting with an engine with a 1000gpm pump, with a single 50' 3" line going into a direct tank fill on a tender. How would you calculate friction loss to know what your pump pressure should be?

    To use the formula FL=CQ^2L, you have to know your GPM, which our feeble minds haven't been able to deduce.

    Obviously, if you were pumping a nozzle, you would know what it was rated for, but here you just have a 3" line flowing into a tank. One thought that was thrown into the ring was to use the smoothbore GPM formula, but we weren't sure if that would work or not.

    Any thoughts? I'm sure there is probably a simple answer to this, and we're just not seeing it.


  2. #2
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    skvfd5's Avatar
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    Apr 2005
    Kalispell, Mt.


    When using only 1 line to supply the tender you gpm is going to be limited to how much that line can flow. When working with new firefighters I still use the old rule of thumb, 1 1/2"=150gpm, 2 1/2"=250gpm and 3"=400gpm. If your only working this out to come up with an approx fill time this should get you a ball park figure. I'm sure some of the more savy engineers will be able to get you the exact calc. but as I stated this will be ballpark.


  3. #3
    55 Years & Still Rolling
    hwoods's Avatar
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    Jun 2002
    Glenn Dale Md, Heart of the P.G. County Fire Belt....

    Question And.............

    Why worry?....... We do this excercise by pumping a flat 75 PSI Discharge on the pump.
    Never use Force! Get a Bigger Hammer.
    In memory of
    Chief Earle W. Woods, 1912 - 1997
    Asst. Chief John R. Woods Sr. 1937 - 2006

    IACOJ Budget Analyst

    I Refuse to be a Spectator. If I come to the Game, I'm Playing.


  4. #4
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    Dec 2002
    Central NJ


    I think the Chief here has the simplest answer. Most tankers are limited to 100 PSI fill pressure....so FYI

    50 PSI through a 2 1/2" outlet (hose coupling) is about 1300 GPM.

    100 PSI through a 2 1/2" outlet is about 1800 GPM.

  5. #5
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    Jan 2007
    Pa Wilds

    Smile Old Guy - See Def. of Kuh Shise in German Dictionary

    The determining factor in this problem will be which of several physical limitations occurs first. Since you mentioned draft, we will assume this will be the factor that bites you first. 1000 gpm pumper = 5" suction sleeve and will therefore run away from water (cavitate) at about 1100 gpm. (10'lift through 20 ft. suction sleeve). So this will be the limiting factor for flow. There are several things you can do to help, but we will suggest them later.
    Filling with one 50 ft. section of 3" line (assume 2 1/2" couplings) Friction loss will be (gpm/100) squared or 11 x 11 = 121 psi for 100 ft line so FL will be 60 psi. There is another problem... you will need to get your head inside the pump panel to look at elbows and valves. If internal piping is 2 1/2" then every 90 degree bend will cause increased loss. One way to estimate this loss is to assume that a 2 1/2" elbow creates 4 psi drop at 250 gpm, but Friction loss is a square function.... That 4 lb loss at 250 gpm becomes 16 psi at a flow of 500 gpm continuing the square expansion .. to 1000 gpm - each elbow will produce 64 psi of loss. Lets say that there are 2 - 90 degree bends then to fill at 1000 gpm you will need to pump the master discharge gauge at 188 psi. (64 psi per elbow + 60 psi for the 50' of 3" hose) You won't be able to believe the individual line gauge, because the water will be flowing so rapidly past the gauge connection that the venturi action will cause some drop in the gauge below the master discharge.
    More problems... If this engine is equipped with 2 1/2" valves or worse not have "full flow valves" you will get some more losses internal to the pump panel. A gasoline motor can cause additional problems if the flow is large and the pressure is relatively low .. lets say a front mount with 1 elbow, then 120 psi at 1000 gpm the gasser probably won't be able to develop suffient torque and Horse Power at the low rpm. What low rpm, you ask... well pump discharge pressure is directly related to pump rpm so if you need lower pressure (below 150 psi), you must turn the pump slower. But gasoline engines need high rpm to develop the HP.

    How can you help to increase fill rate... use 2 fill lines from the engine..it will cut the friction losses by a factor of 4 simply because of 1/2 speed of water in the lines. Set-up a porta tank next to the engine and draft from both the porta tank and the stream, but you will need to have a butterfly valve on either the stream side or the porta tank side so that you can regulate the flow of water coming out of the porta tank. I would also consider putting one or more portable pumps into the pony suctions on the engine from the stream.
    When it is time to fill, increase rpm until you start to cavitate (rpm increases, but pressure does not) slowly open the butterfly and then your tank to pump lines and again increase rpm until you get the desired discharge pressure. A properly sized engine matched to a good pump (Watrous or Hale) will probably run out of throttle at around 1400 gpm. That is you can continue to try increasing throttle rpm but it will not increase, because all of the available engine power is being used to deliver water to the tanker. Be absolutely certain that there is suffient openings in the top of the tank to prevent pressure build up inside the tank.
    DO NOT allow your water tank to go completely empty and do not allow the strainer in the porta tank to become uncovered. any loss of water on the suction side will immediately result in a loss of prime. Many times in the northern climate, tank to pump valves have been compromised due to freeze - thaw that can happen when you are not operating in cold weather and the tank to pump valves will not seal completely. Air can fly through a small space between the ball and seat so fast that you won't be able to pull a prime in the suction sleeve, but if you can keep the valve covered with water, it will effectively keep out the air and thus allow you to get a prime.

    Let everyone know how you make out in the practical application. Tanker fill rates should be done carefully, but you should be able to fill 2000 gal tankers in under 2 minutes.
    Kuh Shise

  6. #6
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    Feb 2005

    Default Thanks

    Thanks for the help everybody - you have given me a lot of good information.

    Hopefully in the next couple of weeks we will get a chance to go out and give this a try and see how we do. We will be using my department's equipment - our engine is a front mount 1000gpm pump driven by a CAT 3208 diesel, so I wouldn't think that torque would be too big of a problem. (But I could be wrong). We will experiment with using one and two 3" lines to fill.

    Several guys from my department ran some timed tests on this evolution recently, but I haven't had a chance to look at their results, so I will be interested to see how my numbers end up comparing to theirs.

    And like Chief Woods said - "Why worry?" In my department most of drivers will pump between 50-75psi, I have just been really digging into the hydraulics side of things and wanted to look at this from that perspective.

    Thanks again everybody (especially KuhShise for the excellent post) you have given me a much better understanding of this operation.

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