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  1. #1
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    Default Draft questions...

    How long does it usually take to get a prime with a 1250GPM pump? our 1000GPM can get a prime in about HALF the time of the 1250.Does the inch difference in the suction line (5in 1000 6in 1250)make that much difference? We also had a new operator park just far enough away from the dry hydrant that the line had very little sag in it and it seemed to pick up faster.Anyone else have this problem?


  2. #2
    Senior Member Dalmatian90's Avatar
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    Interesting questions!

    The suction hose I can answer easily -- 6" has 30% more volume for the same length than 5". So that would account for a third of the difference alone, all else equal.

    Second one, I'm going out on a bit of a limb (hooking up my harness now...). I haven't thought about it before, but here it goes:
    -- 750/1000gpm pumps usually use same housings. The 1000gpm simply has a wider (?) impeller to move the larger volume.
    -- Similiarly 1250/1500gpm pumps usually use the same housings.
    -- Since water is incompressible, and we pump at the same pressure (pressure is essentially velocity) we need a larger volume pump casing to allow more water to go through at a given speed. Since water doesn't compress, a 1500gpm housing should need to be 50% larger in volume.

    30% for suction + 50% for pump casing volume -- that's 80% and right about "twice as long to prime" your seeing.

    Then there are Other Factors...
    -- Is there an air leak in the priming pump's line? Pump still holds prime after since the leak only affects the primer...but still takes longer to prime.

    -- Is the primer pump smaller capacity? For that matter, could something be holding the primer back (smaller plumbing, insufficient electrical wiring).

    Just some thoughts...
    IACOJ Canine Officer
    20/50

  3. #3
    Senior Member Dalmatian90's Avatar
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    Also, re-reading your post,
    We also had a new operator park just far enough away from the dry hydrant that the line had very little sag in it and it seemed to pick up faster

    My other thought is...could it be some seals/valves that where dry or unexercised, and the longer they were wet and being used during the drill, the better they performed? So by the time you moved the farther away, the a minor leak somewhere had fixed itself?
    IACOJ Canine Officer
    20/50

  4. #4
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    Good question about priming when drafting. This is the guideline used when you service test your pumps annually. Prime should be reached in within 30 second for fire pumps rated less than 1500 gpm, 45 seconds for 1500 gpm pumps and greater. I hope this guide line helps out. Also remember your appliances connected to your intakes (ex. gated intake valve) will give air another place to enter the hose. Resulting in longer priming time. This is if you use these type of appliances.

  5. #5
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    You said it takes about twice as long to get prime. How much time for each pump are you actually talking about? Also how long of a lift, and how long suction hose are you trying to draft from? If the 1250 takes 15 seconds & the 1500 30 seconds than you are doing good. The times the marinefireman said are what NFPA requires, so if you are within these then there isn't a problem. Another way to get faster prime times in large pumps is to run dual primer pumps. Hope tihs sheds some light.

  6. #6
    Senior Member Dalmatian90's Avatar
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    Just remember the NFPA figures don't take into account the dry hydrant, or long hard suctions. I think (not positive) that's 30 or 45 seconds with a 20' length of hard suction at a 10' lift -- obviously longer suctions (including dry hydrants) & higher lifts will take longer. But you still should be able to make the time when you're using the conditions specified by NFPA.
    IACOJ Canine Officer
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  7. #7
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    Hi There,
    A lot of things affect priming time so it is hard to compare two different trucks. The larger suction size and pump size do have a lot to do with it. Check to be sure something on the pump is not left open, like drain, auxiliary coolers, any number of things.
    One trick to try, hook up all your suctions and then put the cap on the end, using the primer pull a "dry prime" by operating the primer until you compound guage drops as low as it will go, like around 20". Then shut the truck off and listen, if you have any leaks you will hear it. The gauge should not loose more than 2" vacumm in ten minutes. Note if you have an electric primer, then the pump does not have to be turning for this test so you don't hav eto worry about "running it dry". Things such as size of priming pump, are both trucks same style pumps? both single stage or two stage, if two stage are you drafting in volume mode? You should be. You said you are working off a dry hydrant, this adds to the effective length of the suction hose (actual hose plus the pipe length to the water). The change you noted when you lifted the hose is because of an air pocket created when the hose leaves the pumper, drops to the ground and has to go back up to meet the dry hydrant. It is difficult to clear this air pocket out when you are drafting, idealy your suction hose should slope down from the pump to the water, this allows air to rise up to the top of the pump where the primer is connected. Without the slope the air bubble tries to remain at the top of the DH. I would suggest you support that hose between the truck and the DH, when it fills with water it is going to be pretty heavy and could pull out or break at the coupling, or even break the DH.
    I hope this helps a little.
    Our Department covers a 50 Sq. mile Town with very few hydrants, drafting is an almost weekly operation.

    John C.
    Captain, Hope Jackson Fire Co.

  8. #8
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    Hello again,

    Vacuum test is a good way to test for leaks. I have a question about the vacuum test requirements. The amount of time (10 min) I believe was lowered to 5 min when the standard was last revised. Not sure but I will look into that.

  9. #9
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    WHEN WE SPEC'D OUT OUR NEW ENGINE DUAL PRIMER PUMPS WERE A NECESSITY 1250 GPM HALE W/ 10' LENGTHS OF 6" FLEX SUCTION HOSE ABOUT 12 SECONDS W/ 1 LENGTH 15 SECONDS W/ 2 LENGTHS IF YOU NEED IT REAL FAST USE YOUR TANK TO PUMP VALVE TO FLOOD THE PUMP IT DOES HELP!

  10. #10
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    WHEN WE SPEC'D OUT OUR NEW ENGINE DUAL PRIMER PUMPS WERE A NECESSITY 1250 GPM HALE W/ 10' LENGTHS OF 6" FLEX SUCTION HOSE ABOUT 12 SECONDS W/ 1 LENGTH 15 SECONDS W/ 2 LENGTHS IF YOU NEED IT REAL FAST USE YOUR TANK TO PUMP VALVE TO FLOOD THE PUMP IT DOES HELP!

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