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Pump Head of a High Service Pump at Water Treatment Plant is Higher than Actual

Dear Community

I have a model for a City of 30,000 with a Water Treatment Plant. For High Service (HSP) Pump Elevations, there are elevations for suction side and discharge side and I use the suction side.

HSP discharge elevation: 958.50 ft

HSP suction pipe elevation: 940.00 ft

The reservoir elevation can be between 843-852 ft based on the control and I use 843 ft as the worst case scenario.

When I compared the pump head of actual and modeled HSP pumps during hydrant flushing tests with known water elevations at towers, the model results show always about 8-9 psi higher pump head than the actual.

The pump curve are correct so the HSPs pumps higher flow of water in the model.

What can be the cause of the 8-9 psi discrepancy? 

Also, I actually requested the City to provide the pump discharge pressure. However, they provided me the pump head instead. What is the disadvantage of comparing the pump head instead of pump discharge pressure?

Yoko

  • First thing you want to do is compare your static HGLs between the field and model. The HGLs should very gradually decrease from the pump station moving into the system. At points near the tank, you should be vey close. If they don't agree, then there is something wrong with boundary conditions (tank HGL, pump discharge HGL).

    If you are good under static conditions but off during flow tests, look for trends. Are the discrepancies the same for all tests? Are they good near the tank and get worse as you move away? Are some good but some are outliers?

    When you say "pump head", are you talking about change in head across the pump, discharge pressure converted into head units, HGL at discharge?

    Remember to work in HGL units, not pressure. Working with HGL makes it easy to spot inconsistent data.

  • Tom,

    Thank you so much for your reply.

    The field pressure data match with the discharge rate (gpm) along the pump curve. So I guess this is discharge pressure?

    The field data is about 64 psi and the model 'pump head' is about 171 ft.

    When I look at the model 'pump discharge head', it is about 32 psi. 

    I am confused with the definition of the pump discharge head and pump head in the model.

    Could you please help me understand?

    Are you saying that looking at overall HGL in the system is important and I should not worry too much on this one point data at the Water Treatment Plant?

    I thought I posted two different questions, but received similar answers so I am trying to understand.

    Thank you for your advice in advance.

    Yoko

  • We can be more efficient if you send me your phone number and we can talk. (tom.walski@bentley.com)