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Pumpable slurries What concentration?

  1. Mar 13, 2015 #1

    rollingstein

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    I have an application where a slurry of salt (NaCl) in water needs to be pumped a short distance; approx. 30 ft. Flow rates are modest approx. 5000 Litres per hour. The head requirement is negligible i.e. not much of a height change other than the frictional head.

    What's the maximum concentration of solids we should shoot for? In typical slurry pumping applications what conc. do designers restrict themselves to? The proposed pump is centrifugal.

    Also, would it be feasable to gravity flow such a system? The available vertical height head isn't much. Approx. 10 feet. Would it flow freely under gravity or choke.

    Piping is currently 3 inch diameter steel. That's roughly a velocity of 0.3 m/sec which doesn't seem much.
     
  2. jcsd
  3. Mar 15, 2015 #2

    Astronuc

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    Is this a slurry or solution? The term 'slurry' usually connotes suspended particulates in a liquid medium, e.g., water.

    Is the solution a brine? See - http://en.wikipedia.org/wiki/Brine

    One would consider the end product, and how much energy is required to move the product. There would also be considerations of corrosion, and if a slurry, erosion of the piping and pump.
     
  4. Mar 15, 2015 #3

    rollingstein

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    No it is not a solution. Pumping a solution is a problem I understand & know how to design.

    But this is a slurry. There are salt crystals in a saturated solution coming from an evaporator.
     
  5. Mar 15, 2015 #4

    Astronuc

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    OK - just checking.

    It seems there are numerous handbooks from various pump manufacturers, e.g., Weir/Warman, Gould, KSB, Metso and so on. Some can be downloaded, but others may have to be purchased.

    Some examples - (links may eventually become inactive).

    http://www.weirminerals.com/products/centrifugal_slurry_pumps/warman.aspx
    http://www.pumpfundamentals.com/slurry/WeirSlurryPumpingHandbook.pdf (2002)
    http://www.weirminerals.com/pdf/Slurry Pumping Handbook - 2009.pdf (2009)

    http://turbolab.tamu.edu/proc/pumpproc/P4/P419-25.pdf
    http://turbolab.tamu.edu/proc/pumpproc/P13/P13193-211.pdf

    http://www.metso.com/miningandconst...AF6002ECEAB/$File/The_Slurry_Pump_Program.pdf

    It would seem to be a matter of characterizing the slurry, since apparently the flow rate has already been determined. Knowing the viscosity and flow rate, one should be able to size the pump. One consideration would seem to be whether or not there might be an accumulation or precipitation of salt as some location.
     
  6. Mar 16, 2015 #5
    I'd get in touch with local distributers/manufacturers that deal with pumps like Warman, Goulds, Toyo, etc. Salt is tricky stuff and can be hell on pump internals.

    With that said, there's plenty of literature out there regarding the pumping of slurries. The main issues are:
    - settling of suspended solids due to low fluid velocity
    - additional power required due to increased density of conveyed liquid
    - derating of equipment to account for suspended solids particles
    - wear and scaling in pumping system due to sedimentation

    Definitely get to reading that weir minerals slurry pumping handbook. It does a pretty good job at outlining the theory with decent examples.

    Some additional points:
    1) at 5000 L/hr (roughly 22 gpm) in a 3 inch steel pipe, the velocity is as you said ~.3 m/s (.95 fps). This does not fall into the generally accepted range of economic pipe velocities (~4-8 fps for slurries) (though I've seen plenty of systems that have flows this low due to utilizing existing pipes for new services). Usually that's not an issue, but with slurries, you have to strike a balance between keeping velocities minimal so that the power required to pump against the head loss is not crazy high, and maintaining a minimum velocity to avoid significant sedimentation in the pipes (which will lead to scaling and potentially blocked flow at elbows and fittings).

    2) Solids concentration varies wildly, between fractions of a percent to >50% solids by weight. Read through the handbooks for the whole view of how this affects pumping. As it goes up, though, things get a lot trickier very quickly.

    3) Gravity flow is a potential solution, and a 3 inch pipe could be suitable for that (looking only at % full, and fluid velocity) depending on the slope at which you can install your pipe (can it be installed with a >5 degree decline?).
     
  7. Mar 18, 2015 #6

    rollingstein

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    Thank you! I'm reading up on those resources now.
     
  8. Mar 19, 2015 #7

    rollingstein

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    Exactly like you guys predicted plugging has been a big problem when we've tried this in practice.

    I think we made a design mistake by using pipe sizes too big in the hope that this would prevent plugging. But this intuition seems wrong. In hindsight.

    Smaller pipe dia would give larger velocities & hence lower settling & plugging.
     
  9. Mar 19, 2015 #8
    I helped work on a project to replace a bunch of salt slurry lines (KCl not NaCl) a few years ago.
    I realize this isn't helpful but the process engineers had some pipe flow modeling tool in which you could specify the slurry specific gravity and model the flow rates. I can't for the life of me remember what the name of the program is.

    However a couple other things to think about as well.
    Plugging will happen. No way around it. It will happen regardless of what you do. So what we did instead of trying to solve a seemingly unsolvable problem was to add in entry points along the pipe, so that when there was a plug the pipe fitters wouldn't need to cut open the pipe to get water lines in there working to unplug.
    We simply added joints like:

    ----\ \------
    -> ->->->
    ------------
    For such a short line might not be necessary, or maybe add one at the halfway point.

    The slurry will eat away at the inside of the pipe rather quickly. Not talking just about corrosion. You are basically pumping liquid sandpaper through that pipe. Every few months think about rolling the pipe by 90degrees. This will help the pipe wear more evenly and you'll get a longer life out of it.

    Hope that helps, even though it doesn't answer your original question :D
     
  10. Mar 23, 2015 #9

    rollingstein

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    @cpsdave

    Thanks! That anecdotal experience does help. The program wasn't ASPEN was it?

    Yes, we've added generous flanged joints in there & that's helping. We've been facing about a choke every 24 hrs so not so happy. Especially the pump suction plugs up. They have dropped that spool piece very often.

    I'm almost thinking of getting rid of the pump entirely & switching to a large dia sloping pipe & use a gravity transfer. I'll keep the developments posted on here.
     
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