Jet Drive gpm/gph flow rates?

Discussion in 'Jet Drives' started by oragonads, Apr 7, 2011.

  1. oragonads
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    oragonads Junior Member

    Anyone know what the flow rate of water from a jet drive is? Gpm or gph? And what HP of engine it takes to get that flow?
     
  2. baeckmo
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    baeckmo Hydrodynamics

    Depends on the power and type of unit. As an example, a jet with an inlet diameter of 230 mm and with a power input of 200 hp would swallow something like 0.3 m3/sec (=18000 liters/minute). The power required for pumping is "pressure increase x volume flow/(pump hydromechanical efficiency)". Use consistent dimensions (N/m^2 and m^3/s), and you will get power in Watt.
     
    Last edited: Apr 14, 2011
  3. oragonads
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    oragonads Junior Member

    Thank you baeckmo. So if I'm correct then a 200 HP engine with about a 9 inch intake would move close to 5000 gallons per minute depending on specifics. That's a lot of water.

    I'm not the greatest mathematician so those theorems or what not don't make a lot of sense to me :).

    So a small pump with an intake of 3-4" is possible to move at least 600-800 gpm. Any idea of HP needed to run a small pump like that.
     
  4. oragonads
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    oragonads Junior Member

    So I'm guessing a 30 HP engine could move about 750 gpm. How high is the pressure from a jet pump compared to a centrifugal pump?
     
  5. Frosty

    Frosty Previous Member

    Your questions are not relative, thats like asking a car sales person how fast the wheels go round. What are you trying to work out and why.

    To every motion there is an equal and opposite motion, that how a jet works, not pressure or gallons but weight of water thrown backwards which is thrust. The boat is pushed forwards equal to the weight thrown backwards
     
  6. anthony goodson
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    anthony goodson Senior Member

    And the velocity at which it is thrown
     
  7. oragonads
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    oragonads Junior Member

    Ok, so this is what I'm trying to figure out.

    Let's say you had twin 50 HP engines. Both engines are exactly the same. One engine is on a stand with a centrifugal pump, the other with a jet pump. They are both hooked up to a tank of water.

    Which pump will move the water quicker? Which pumps output will have greater pressure?

    From my understanding, a centrifugal pump spins an impellar that has an intake in the center of it. The water is moved from the center of the impellar to the outside by water traveling up the vanes caused by the inertia (i think thats the word I'm looking for :/) of the spinning impellar.

    It seems to me that a jet pump would move water more efficiently then a centrifugal pump because its impellar actually pushes the water rather then flinging it. It also seems it would have greater discharge pressure for the same reason.
     
  8. anthony goodson
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    anthony goodson Senior Member

    A jet pump can use a centrifugal ,mixed flow, or axial impeller ,depending on the intended application. Research the different makes and their uses.
     
  9. baeckmo
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    baeckmo Hydrodynamics

    Ok, that is where you have to understand the simple equation I mentioned above, namely the power equals flow times pressure. So you can design the pump to give a high flow at a low pressure or a low flow at a high pressure for a given power.

    The physical configuration of the pump will reflect this balance. For a fixed rotational speed, the high-flow / low pressure pump will have a mainly axial flow direction with piping dimensions to suite the high flow. This configuration is what you normally find in pumps for jet propulsion.

    At the other end of the operational scale, you have the high-pressure / low flow machine. Here the pipe dimensions are designed for low flow, but since high pressure requires high peripheral speeds, the impeller outlet diameter has to be larger than for the axial pump; here you have the radial outlet of the centrifugal pump.

    This brings us to your tank experiment. You have to specify either how high the recipient of the flow is placed in relation to the tank you are emptying (that is called the static head), or what velocity the outflow must have (say for a fire-fighting pump in order to "throw" a given distance). Any combination of flow-pressure may be specified, as long as you stick to the power limit (P=Q*p).

    Now, if you look at industrial pumps, you will find that each supplier has a variety of configurations, from pure radial to pure axial pumps, all in a variety of pipe connection diameters and power levels. All in order to suit the demands for specific operational requirements.

    What you see in terms of pumps for waterjet propulsion are pump configurations for high flow / low pressure which is essential for good propulsion efficiency. A specific propulsive thrust at a given speed is produced with lower power input for this configuration, than for a high pressure / low flow pump.

    A common open propeller is the ultimate high-flow device, which explains its superiority for marine propulsion. But it would be a disaster if used to feed a fifi monitor....

    To summarize: any pumping operation, be it the emptying of a tank or propulsion of a boat, is characterized by two requirements; the required flow and the required pressure increase.
     
  10. oragonads
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    oragonads Junior Member

    Thanks again baeckmo, that's alot of good information.

    The reason why I brought this up is because I like to dredge for gold. Every gold dredge manufacturer uses centrifugal pumps and I was wondering why no one has designed an open propeller style jet pump. The 3 major manufacturers of dredges all build there own centrifugal pumps and they are almost all identical.

    As an example, a 6" dredge usually requires a pump that delivers 500-600 gpm. Keene engineering ( one of the major manufacturers) manufactures a 600gpm centrifugal pump with a 4" intake, 3" discharge, 185 feet of head lift, and requires at minimum a 16hp engine. This pump costs $625.

    I have been trying to figure out if a jet pump can perform as good if not better.
     
  11. oragonads
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    oragonads Junior Member

    Also, the largest pump they produce is a 1500 gpm centrifugal pump with a 5" intake, 4" discharge, 230 feet of head lift, and requires at minimum a 35hp engine. That is big enough for a 8" - 10" gold dredge, and is my dream :). Unfortunately that pump costs $1650 :eek:
     
  12. oragonads
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    oragonads Junior Member

    Here is an example of how a gold dredge works for those who don't know.

    [​IMG]
     
  13. baeckmo
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    baeckmo Hydrodynamics

    Well, in that case, the ejector ("Power jet" in your sketch) is the determining factor. It is in effect a "hydraulic transformer", changing a high-pressure / low flow into a high flow / low-pressure operation. Its operating point is determined by the nozzle area for the supply ("driving") flow from the pump, meaning that for each ejector there is an optimum supply pump.

    The reason for this setup, which is used for various gravel transport duties, is that the rotodynamic pump does not tolerate the grinding from the slurry. The conclusion regarding your opening question is thus that a high flow pump, like those for waterjet propulsion is not suitable for your purpose. On the other hand, there are thousands of surplus industrial centrifugal pumps around for scrap money. The trick is to understand the matching of flow and pressure for the ejector.

    This matching has a major influence upon the overall running costs for your rig; with wrong matching there will not be nuggets enough to pay your fuel bills!
     
  14. oragonads
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    oragonads Junior Member

    Dang, something I thought was simple turned out to be very complex, lol. Everyone seems to think that only a centrifugal pump would work because it can handle the slurry. But in actuality the pump shouldn't be sucking up any solids from the water at all if the dredge is set up right. If it sucks from near the surface then it wouldn't be any different then a jet boat in a lake. Also dredges are required to have a screen on there pump intake to keep from sucking fish, so I don't see why a rotodynamic pump wouldn't work. To me it seems they would be more efficient, simpler, and dependable. I'm going to see if I can pick up a cheap jets and do some testing.
     

  15. oragonads
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    oragonads Junior Member

    Any idea how well a jet ski engine and pump might work for a 6" dredge?
     
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