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tie in to an existing network

shams
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hi

we have an existing irrigation network with multiple pump stations feeding the same network in many scenarios , due to newly developed areas the network had extensions and  there was new demands required.

we have no data about this network.

the client requires the construction of a new Pump station and a discharge main to connect this new PS to the existing network .

this main will tie in with the network piping (not reservoirs )in many locations and the demand is given at each of these locations.

the question is  : is it correct to create the model adding demands at tie in points and run  the model to get pump head , i found the head around 17 m which looks very low as other connected PS's to that network have heads as 83 , 55 , 60 m.

note: we can't model the whole network (the existing and the new main) so i only modeld the new line with  the new PS.

thanks

  • 0 Offline
    If you don't provide sufficient suction head at the new pump, it will not work correctly and cavitation will damage the pump.The key is determining what the suction head at the tie-in point will be. Read about Net Positive Suction Head in any hydraulics book.

    You can get an appreciation of this suction head by putting a pressure recorder or data logger at the tie-in point and observing pressure fluctuations. Look at the lowest pressure.

    The problem is that the new pump will drop the pressure even further. Is the new flow large or small relative to the existing flow?

    With a water distribution system, you can open a hydrant and observe the drop in pressure in the pipe. You can make reasonable decisions based on that. I'm not sure if you can run such a test in your system.

    Of course, the correct way to solve this is to construct a model of the existing system. This can be used for this project and all future projects. Yes, it will cost money to do this but the cost of a possible poor design would be much larger. Modeling will pay for itself.
  • 0 Offline in reply to Tom Walski
    hi Tom
    thanks for your reply

    yes we have available NPSH much greater than required.
    but my new PS discharge head was chosen to 17 m based on my new line loses , other PS's heads are greater.
    I know we have no data but I need an approach to choose a higher head for the new pump , some value that is reasonable although not exact.

    best regards
  • 0 Offline in reply to shams
    How much pressure to you need for the sprinklers (I'm assuming spray irrigation)?

    Are you pumping uphill or over flat terrain? If it's flat, you may not need much head.

    You need to model the expansion and determine how much head you need to overcome the elevation and friction. You also want to determine the operating point of the pump. You want it to match up with the best efficiency point of the pump. If the head is too high, this can result in an inefficient operating point and excessive power draw. This can burn up the motor fairly quickly.
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