Jet Drive gpm/gph flow rates?

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

  1. baeckmo
    Joined: Jun 2009
    Posts: 1,294
    Likes: 262, Points: 83, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Sorry pal, you're missing my point, or simply not reading? The ejector is there in order to handle the slurry that would ruin the centrifugal pump. The centrifugal is providing the "powering water" and is working with "clean" water in order to survive. In the mining industry special slurry pumps with rubber coating are used, but they only survive within a small range of velocities.

    There is one type of rotodynamic pump, that might work in the slurry (at least for hobby use), and that is the vortex pump, which is a centrifugal with the impeller recessed backwards into the housing. Try to Google "Turo pump" or "Egger pump", and you will get the idea. This gives you more water per hp than the ejector combo.

    If a waterjet pump could handle the slurry, it would be much more efficient, but believe me, the impeller would simply disappear within minutes in a gravel soup!! And it does not have the pressure/flow characteristics to power an ejector if you used it for a pump+ejector setup.

    So, back to basics; the key element is a good design of ejector. A good one will have an efficiency of ~25-30 %, a bad one ~<5 %. That boils down to a difference of a factor 5:1 when it comes to your pump and engine investment and running costs. Spend your money on a recognized and tested ejector, don't mess with surplus bitsandpieces. The ejector will have a design pressure and flow stamped on it.

    The ratio (pressure/flow^2) is a constant, so once you have your ejector, you go looking for a pump with similar ratio. Dimensioning factor will be the ejectors connection dia for the "power water"; find a pump with a corresponding pressure side. Its suction will probably be one pipe size bigger than its outlet. Then its just to wind it all up with an engine giving the required power and rpm.
  2. baeckmo
    Joined: Jun 2009
    Posts: 1,294
    Likes: 262, Points: 83, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Maybe important to add that there is a critical flow velocity in the piping when pumping slurry. Too low speed lets the gravel plug the system; too high speed will erode the material. This is the reason that the pump pressure is important; the velocity depends upon the pressure differential in the ejector.
  3. Brian@BNE
    Joined: Jan 2010
    Posts: 262
    Likes: 13, Points: 18, Legacy Rep: 151
    Location: Brisbane, Australia

    Brian@BNE Senior Member

    I had a Keene 3" unit a few (quite a few!) years ago. Good gear, well balanced system overall. If I recall correctly, the centrifugal pump had a 4.5HP motor, 2" suction and 1.5" outlet. But the outlet then went down to a 5/8" nozzle at the point it inserted into the ejector (at a critical angle), and immediately after the injection point the ejector flared out to a larger diameter also. The suction strength and flow volume of slurry have a practical range for good working efficiency, and that was what the pump, nozzle configuration and flare was all about.

    I always felt a 4" would be optimum size for a portable unit to use with a diver. The guy I bought my 3" unit off went up to a 6" unit, and everything just scaled up in size so it was a real handful to move and operate. He needed that scale for a few months of operation in a particular location but it became a bit much for he and his girlfriend to manage alone. A friend has a 1.5" unit as well as a 3" unit. The former can be back-packed into remote locations, for streams with low flow rates but lots of cracks/crevices in the stream bed to clean out, so size of system will depend on your project location and gravel characteristics.

    If your location requires 8-10" because of gravel size and volume, then trust me, it is beyond the hobby scale and is in the small industrial/mining activity range. Keene can cater for it in their range of stock units, but they aren't cheap. But remember that with their decades of in-field R&D I doubt you could improve it. There isn't a cheap solution with better efficiency - go with one of the established manufacturers. Try to find a used 3" or 4" unit and field test it yourself before committing to many $$

    I've contemplated buying one of their new systems, but getting a permit to use one here would be very difficult and the places in NZ they are legal aren't that accessible to me at present.
  4. SKR
    Joined: Nov 2010
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: RU

    SKR Alexander

    0.3 m3/sec (=18000 liters/second)

    You're wrong, we in Russia would be correct as follows:

    0.3 m3/sec (=300 liters/second)

    In general, the calculation of water jet is not confined to the selection of GPM / GPH.

  5. baeckmo
    Joined: Jun 2009
    Posts: 1,294
    Likes: 262, Points: 83, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Oops, 0.3 m3/s = 18000 l/minute, of course not only in Russia....... Original post is corrected.

    "In general, the calculation of water jet is not confined to the selection of GPM / GPH"

    Not clear what you mean; in the design of the pump module for waterjets the flow and pressure are key parameters for the definition of operating point. If you mean the slurry ejector, it is the necessary driving momentum that counts; again flow and pressure, so what is your point?
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.