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Pump Head & Flow WaterGems

I need technical assistance for the attached cooling water model

KSP-Cooling Water System.wtg

Process Brief: The cold water from the elevated tank (actually a cooling tower) is pumped (fixed speed pump) through heat exchanger (chiller) and back to the tank. There are 7 cooling towers. 7 pumps and multiple heat exchangers all connected through common header arrangement. The model is attached. Now the task is to evaluate the head developed by the pump for this process.

Please provide your inputs for following:

  1. I understand that Water gems-Pump element does not have the feature to evaluate the head based on flow only.  I have to give an arbitrary pump-head co-relation in-order to finalize the pump head. Please confirm
  2. The required pump flow is 14,300 usgpm, so I had set up flow demand in the tank as 14,300 usgpm and the flow through FCV as 14,300 usgpm. The pump head is approx 35 m, which I think is not correct, because the fixed pressure drop across heat exchangers, suction diffusers (modelled as GPV) = 1.22+2.23+2.59+1.22 = 7.26 m and the static head difference between the tank initial elevation and tank top pipe inlet is 16.93-7.68=9.25 m. Therefore the total head required by the pump = 7.26+9.25+losses in the pipe = 16.51+piping pressure loss. I did a quick calculation in excel to evaluate the frictional pressure loss which is 5.5 m. Therefore the total head of the pump should not be more than 16.51+5.5= 22.01 m. Can you help in finding the error in the model.
  3. I had provided the demand for the tank as 14,300 usgpm, however in the tank result sheet I see that the flow in & flow out does not match to 14,300 usgpm

Thanks

Pratik

  • Hello Pratik, 

    Please note that only

    KSP-Cooling Water System.wtg file is upload, .KSP-Cooling Water System.wtg,sqlite file is also required to open the model which is database file, please upload it

    1. If you only know the pump flow, then you can determine the head required by pump using process mentioned in below wiki, Estimating a pump curve for a model You can use system head curve for more accurate representation of pump which is mentioned in wiki as well. General Pump Selection Process 
    2. Check if there are headlosses in pipes using flex table or by color coding pipes, have you specified minor losses as well? Head required by pump will differ as per model conditions as well, you mentioned you are using GPV in the model, check GPV headloss curve, results of GPV. 
    3. Check the calculation summary, check flow supplied, flow demanded and flow stored. If the the flow supplied by pump is more then, rest of the water is stored in tank, in addition to demand of the tank.  See below example. 

    Regards,

    Sushma Choure

    Bentley Technical Suppport

  • This model require wtg,sqlite to open! can you provide it??

  • I would like to add a few more general comments in addition to Sushma's response:

    • Pumps can operate both with downstream boundary conditions (tanks or reservoirs, in which case the pump flow corresponds to the head necessary to overcome static and dynamic lift) or with only demands (in which case the pump head corresponds to the flow from the sum of the downstream demands). If there is storage in the system, then the tank element could be used for the boundary condition. See more here: How are pumps handled in each OpenFlows product?
    • If this is a closed loop system without storage, see: Modeling a closed loop system
    • You mentioned you placed a demand on the tank - this does not control the inflow or outflow of the tank, but rather models an outflow leaving the tank, in addition to other inflow and outflow from connected pipes based on the hydraulics of the system. You would not use a tank demand if you are trying to force the model to have a certain flow. Instead it may be more of a calibration exercise if you are trying to match the model pump flow to a measured pump flow in the real system. Model the network as it is, then adjust uncertain parameters until the results are in line with expectations.


    Regards,

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

  • There is an error while uploading the sqlite file, it says contact admin

  • There is an error while uploading the sqlite file, it says contact admin

    1. I did try calculating the head by deleting the pump and taking note of HL at junction nodes. The head required is 44.19-6.27=37.92 m. However this not desired outcome.

    2. The headloss defined by me for the GPV's are true as required by the vendor.