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Flow is tied to engine speed???

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  • Flow is tied to engine speed???

    Another jetter question...I understand pumps make flow and the restriction(nozzle) makes pressure. What I'm not sure of is that the flow directly tied to the speed of the engine? If I put the throttle of the engine to half and I had a 7 gpm pump...would I now get 3.5 gpm out of it?

    2 part question...if this is true, why not have a jetter with high flow and pressure and just increase or decrease the nozzle size to change pressure? This way the throttle changes the gpm and the nozzle takes care of the pressure(such as going through orangeburg). Or is this just a crazy thought?
    Buy cheap, buy twice.

  • #2
    Re: Flow is tied to engine speed???

    Doesn't the PSI also change with the RPM though? So, I guess it could be done if you factor that into your nozzle changes.



    • #3
      Re: Flow is tied to engine speed???

      pumps are rated at a certain gpm at a given/ maximum rpm. since the pump is basically a 3 piston pump on average, each piston stroke displaces that much water. multiplied by the amount of times/ rpm per minute it strokes.

      so if you size you nozzles for the max gpm, that also equates to the max rpm in theory.

      by placing a nozzle that is smaller on the hose, then realistically you don't need the rpm's as your nozzle will only flow so much gpm. the pressure is dictated by the back pressure of the nozzle and hoses.

      my 4000 psi 18 gpm machine develop those numbers at approx 2500 rpm on the engine. as i step down to 1/4'' or even 1/8'' hose, i drop the rpm to idle and even have to open up the bypass valve on the machine. ever since i added an unloader, i've managed to blow less burst disc on the pumps over pressure safety valve disc.

      now on a cart style jetter, with no storage tank, running a small nozzle on a large pump will overheat the water via the unloader returns into the pump and heats up. but by lowering the rpm's then less gpm will be produced. keep in mind at a certain rpm, less air flow will be produced to cool the engine. but then again less load is on the engine too.

      ben, just build the jetter and you'll learn from your mistakes

      phoebe it is


      • #4
        Re: Flow is tied to engine speed???

        If you put too small a nozzle on a high Gallon per minute pump it will surge back and forth.
        Seattle Drain Service


        • #5
          Re: Flow is tied to engine speed???

          yes, with the kind of pumps we use, gpm is directly proportional to rpm

          1/2 rpm = 1/2 flow rate

          MOST small engine are governed,so when you select a 'throttle' position, you are actually asking the engine for an rpm, and the governer will open and close the actual throttle with changes in load to maintain that rpm.

          so, yes, typically 1/2 throttle = 1/2 flow rate

          part deux:

          the nozzle creates pressure because it is a restriction to flow. you set the rpm. the pump pushes however much water it pushes at that rpm. the pressure results from pushing that much gpm through whatever restriction youve put downstream of the pump.

          so, to answer someone else's question, pressure does vary with rpm, but only indirectly. pressure to push fluid past a restriction increases with flow rate. as you increase rpm, the pump makes more flow. pushing that additional gpm through the same restriction (hose and nozzle) creates more pressure.

          You can use a high gpm machine to make lower flows by running it at part speed. but a nozzle sized for the higher gpm will not provide enough restriction to develop the pressure you want at lower gpm. you'll need different sets of nozzles to run at different gpm. its not a crazy idea. it works. I do it sometimes.

          people in areas where there is a lot of orangeburg no doubt have ways of dealing with it. I see only a little of it, and my opinion is that it is junk and needs to be replaced or lined. I'm not going to try to develop techniques to work on stuff I don't see much of that is junk.
          Last edited by Ace Sewer; 09-21-2011, 12:21 AM.
          This is my reminder to myself that no good will ever come from discussing politics or religion with anyone, ever.