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CHALLENGES FACED WHEN CHOOSING THE CORRECT PUMP SIZE DUE TO NO FRICTION HEAD LOSS BEEN CALCULATED ON THE RISING MAIN IN THE SYSTEM(SOURCE TO TANK) 2.)NEED FOR CUMULATIVEFRICTION HEADLOSS

ALLAN MULONGO
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Hello Bentley Communities,

I am facing challenges when it comes to choosing the correct pump size for my water system.WaterCad is not able to calculate for me friction head loss in the pipes and I will assume this is because of zero-flow in the pipe, yet I have entered the demand at the end of the pipeline.Can WaterCad be able to calculate head loss due to friction or in other words allows flow in the rising pipeline from the source to a tank on the upstream side of the network? ; as it is possible when designing a rising main from any source to a node located at a higher elevation?Is it because of the tank that the system will not allow any flow?In the case where I replace the tank with a junction node then losses will be calculated in the pipe.

How will I be able to choose the correct pump size for my network since I need to know the total amount of pressure of water needed at the end of the network(which is a function of friction losses)/Is there a way that I can allow friction losses to be calculated without removing "TANK 5"

Please have a look at the attached file.SITIKHO buyofu ANALYSIS.rar

The second question is that I would address the need for Cumulative friction losses along the pipeline.Can you guys please introduce it the pipe flex table?

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  • 0 Offline

    WaterCAD has all the tools you need for the hydraulic design of your pump and associated piping.  is this a wastewater rising main or a water distribution system? I'll assume for now it is wastewater.

    Lay out the piping.

    Make the wet well a tank element and have the load be a negative demand on the tank. Make the receiving manhole a reservoir  element with the water level the level of the typical depth in the manhole.

    Insert a likely candidate pump(s) in the pump station.

    Right click on the System head curve option on the pump can view the system head curve. This gives you the head you'll need to provide at various flows. If it is very steep, upsize your piping. If it is flat, you can downsize it.

    Now try different pumps. Pick a model and impeller size such that the operating point, desired flow and best efficiency point coincide. If you want to pump 40 L/s, pick a pump with a BEP of 40 and make sure it will operate at 40.

    Next set up an extended period simulation run and adjust the controls so that it will operate as desired.

    The steps are slightly different for a water distribution system primarily in terms of how you handle the suction side piping and the fact that you place demands on the outlet side.

    Best wishes,

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  • 0 Offline

    WaterCAD has all the tools you need for the hydraulic design of your pump and associated piping.  is this a wastewater rising main or a water distribution system? I'll assume for now it is wastewater.

    Lay out the piping.

    Make the wet well a tank element and have the load be a negative demand on the tank. Make the receiving manhole a reservoir  element with the water level the level of the typical depth in the manhole.

    Insert a likely candidate pump(s) in the pump station.

    Right click on the System head curve option on the pump can view the system head curve. This gives you the head you'll need to provide at various flows. If it is very steep, upsize your piping. If it is flat, you can downsize it.

    Now try different pumps. Pick a model and impeller size such that the operating point, desired flow and best efficiency point coincide. If you want to pump 40 L/s, pick a pump with a BEP of 40 and make sure it will operate at 40.

    Next set up an extended period simulation run and adjust the controls so that it will operate as desired.

    The steps are slightly different for a water distribution system primarily in terms of how you handle the suction side piping and the fact that you place demands on the outlet side.

    Best wishes,

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  • 0 Offline in reply to Tom Walski

    Thank you Tom for the reply,

    The network is a rising water distribution system but not a wastewater system.Can you explain this in relation to WaterCad v8i Series 6?I expect higher negative pressures at the end of the pipeline than the one shown that is when headlosses are calculated along the rising main.As you can see on the attached file above,no headlosses have been computed by the engine.

    Or what is the best approach that I should use to choose an appropriate pump for the system?

    And What is the recommended range of velocity on the rising main when using a pump?

  • +2 Offline in reply to ALLAN MULONGO

    For a water system, you need to understand the suction side source to model the suction HGL. Are you pumping out of a tank or well or from a more complex system? Then pick the design flow for each pump. Run the system head cure and see what head the pump must provide at that flow.

    If there is no tank on the discharge side of the pump, you will need to use pressure dependent demand to generate the system head curve. See this wiki article on pressure dependent demand. https://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/2671/using-pressure-dependent-demands

    Answer Verified By: ALLAN MULONGO  

  • +1 Online in reply to ALLAN MULONGO

    Hello Allan,

    I looked at the model you had attached to your initial post. It does not have any pumps, and the tanks are set as top-fill. WaterCAD will solve flow in the pipes based on energy balance from the known HGLs if the tanks and reservoirs, demands, pumps and other control devices.

    I do see a length of pipe with zero flow, between T-2 and T-4 and T-5. Based on the Inlet Pipe designation for these tanks, T-4 and T-5 are downstream of T-2. However, the tank inlet elevation for T-4 is 1260.75 m and for T-5 is 1289.17 m, yet the initial elevation of T-2 is 1,234.98 m. Since the only source for these tanks is at a lower elevation than their inlet inverts and since water cannot flow in reverse from the tank inlet pipe, water does not flow in the pipes. Please check the "inlet pipe" designation, inlet invert level and base/initial elevations of your tanks and correct as needed.

    You can read more about the setup of top-fill tanks here: Modeling top fill tanks and throttling inlet valves

    As Tom mentioned, if you are trying to select a pump to add energy from T-2 up to T-4/T-5, you'll need to place a candidate pump and use the system head curve and other tools to help with pump selection. See more here: General Pump Selection Process


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

    Answer Verified By: Sushma Choure 

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