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  • 1.  Pumping Downhill

    Posted 02-03-2017 10:17 AM
    I have designed pumping systems in the past which pumped downhill.  However, I am always interested in avoiding unusual problems in unique situations.  Has anyone come across a highly unexpected condition which added complexity to a system design for pumping downhill?  How did you address the challenge?

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    Cecil Whitaker P.E., F.ASCE
    PRESIDENT
    Whitaker Engineering Pc
    Indianapolis IN
    (317) 823-9050
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  • 2.  RE: Pumping Downhill

    Posted 02-06-2017 06:25 PM
    Just to kid you at first, I thought *@it flows down hill on it's own!  Now seriously, the biggest challenge in pumping downhill is when there is a large high spot between the start and end of the force main-how do you deal with all the forces when the velocities get moving on the downhill side?  Does a simple vacuum release device serve the purpose in all cases?  Curious to read other comments.

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    Harold Dungan P.E., M.ASCE
    PRESIDENT-H2Oaks Consulting LLC
    H2Oaks Consulting LLC
    Maxwell IN
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    Original Message


  • 3.  RE: Pumping Downhill

    Posted 02-07-2017 09:58 AM
    We occasionally experience this issue when applying a flow rating equation to a pump unit within a pumping station. Through dimensional analysis, one can demonstrate for a constant-speed pump unit pumping against a positive static head (i.e. uphill) that the discharge rate Q can be expressed as Q = A + BHC, where H is the static head while A, B, and C are parameters that are typically determined by fitting this equation to measured flow vs. static head data. For a positive static head, B is inherently < 0. However, what if the static head is negative as the writer inquired about? Intuitively, under these circumstances it appears that the same equation could be used to compute discharge if the sign of B was reversed and H was set to the absolute value of the static head. However, I could not find anything in the literature to substantiate this. So, I suppose I'm in a similar situation. Is there any literature dealing with this subject?

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    Mark Wilsnack P.E., D.WRE, F.EWRI, F.ASCE
    Principal Engineer
    Hydrology & Hydraulics Bureau
    South Florida Water Management District
    West Palm Beach FL
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    Original Message


  • 4.  RE: Pumping Downhill

    Posted 02-07-2017 09:58 AM
    To add to your thoughts, once a vacuum break occurs, water hammer might follow as a column of water is let go.

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    Cecil Whitaker P.E., F.ASCE
    PRESIDENT
    Whitaker Engineering Pc
    Indianapolis IN
    (317) 823-9050
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    Original Message


  • 5.  RE: Pumping Downhill

    Posted 02-07-2017 10:28 AM
    We designed a 36 MGD pump station exactly like that a few years ago.  The pump station elevation was several ft higher than the discharge point but there was an intermediate high point.  

    There is no real concern if the friction loss at design flow in the downhill section of the pipe is higher than the the static head (in the downhill section of the pipe between the intermediate high point and discharge point). If the friction head is higher, the flow in the downhill section occurs due to the pump head, if the friction head is lower (than static) the flow occurs due to gravity.

    The client wanted a flow range of 10 MGD to 36 MGD. At 36 MGD the firction loss was higher than the static, however, at 10 MGD friction head was lower than the static.  We installed an isolation valve near the discharge point and tied it to the pump operation so that the valve closed when the pumps stopped.  This kept the pipe full at all times and kept the pump on its curve. We also installed a 10" bypass at this valve.  At lower flows Valve 1 closes and Valve 2 opens, the smaller pipe created enough headloss to keep the pump on the curve and keep friction loss  higher than static.  At higher flow Valve 2 closes and Valve 1 opens, again, the pump stays on the curve and the friction loss is kept higher than the static.

    The pump station has been in operation since 2013.


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    Abhishek Gupta P.E, A.M.ASCE
    Project Engineer
    AECOM
    Greenville SC
    (864) 234-3584
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    Original Message


  • 6.  RE: Pumping Downhill

    Posted 02-07-2017 10:30 AM
    A related question. I am designing two 102" pipelines that will exchange water into a canal that is about 250' above and 8 miles from the bottom of the hill. One pipe will pump 300 cfs of desalinated water uphill while the other pipeline will convey 300 cfs of brackish water downhill for processing. I recognize that some electrical pumping will be needed to offset friction losses in the two pipes. I can put an electric pump/generator on the two pipes but the conversion from mechanical to electric energy and back results in an efficiency loss between the two systems. Has anyone seen a system where the mechanical hydraulic energy from the "down" pipe can be mechanically linked to provide some of the mechanical pumping energy for the "up" pipe? 

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    Michael Clinton P.E., F.ASCE
    President, Michael Clinton Consulting, LLC
    Las Vegas NV
    (702) 255-1536
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    Original Message


  • 7.  RE: Pumping Downhill

    Posted 02-07-2017 11:03 AM
    The first question to ask is why are we pumping downhill?  If at all possible, pump to the ridge line and grade the downhill line as an open channel gravity line.   The worst case scenarios are where the highest point is near the beginning of the line, and there are one or more subsequent, but lower, high points.  In those cases use dual combination air/vacuum relief valves at all high points with each sized to protect the line if acting alone.  Add additional dual vacuum release valves on any long descending legs at appropriate intervals. 

    For clear water applications, and if flow is high enough to justify the economics, consider adding a small hydroturbine at the discharge to keep the hydraulic gradeline above the pipeline profile, and recover the energy through the turbine.

    We do a lot of mining slurry pipelines, and this is a common problem.  First choice it to pump to any highpoint and break the grade with surge tanks avoiding any sags where solids would deposit.   Another option is to use a choke station at the discharge, but again only if velocities, when choked, are still above critical deposition velocity.  Last choice is to accept hydraulic jumps in the descending legs and provide atmospheric air vents to ensure adequate air above the jumps.  The Bureau of Reclamation has some good guidance on aerating pipelines.  The pipeline liner has to be resistant to the increased turbulence.  I don't particularly like/trust air/vacuum valves on slurry pipelines.

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    Raymond Hamilton P.E., M.ASCE, BCEE
    Principal Engineer
    Amec Foster Wheeler
    Denver CO
    (303)975-2195
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    Original Message


  • 8.  RE: Pumping Downhill

    Posted 02-07-2017 12:56 PM

    You may need a combination air release/vacuum breaker at the high point, and a buffered check valve at the pump.  Otherwise, the high point may fill with air when the vacuum breaker opens, and you will get water hammer when the column of water tries to keep moving when the pump shuts down, and then reverses back down the hill.  Also, if the high point isn't very high, you may get a siphon effect which may result in the pump losing prime.



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    William Forbes D.Eng., P.E., M.ASCE
    Virginia Bch VA
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    Original Message


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