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Flow Control Valve and Inlet throttle valve at tanks

Dear Bentley

My model is proposed ground water TSE tanks with Main Pump Station which shall be used to fill a proposed 23 irrigation tanks. The 23 irrigation tank is sized based on two day storage, have of each tank capacity shall be used within 4 hours for landscape irrigation and the other have kept for the second day. The main pump station is designed based on one day irrigation demand and 16 hr pumping time.

In order to make the 23 tanks are filling and emptying on the same time, I added Flow control valve at each tank to control the flow rate entering the tank. Everything on the model wen well but the problem of negative pressure at some junction appears at time of Pump Stop at 16 hr. my questions as follows:

  1. Is the negative pressure at the time of 16 hr is due to pump stop (water hammer) and the pumps need surge protection?

  2. My client is asking me not to use FCV and instead of this I should use “Inlet Valve Throttles?”, the new option add under tank symbol in series 4, so kindly if you can explain to me exactly how to use this option this will be great help.

I have attached the model and the figure for your review.

 

Thanks in advance dear Bentley

Eng. Sayed Elhagar

Final With FCV Valves.rar
  • Hello Sayed,

    The negative pressures are occurring mainly because the downstream HGL is below the ground surface elevation. When the pump is on, the headloss required to reduce the flow to the FCV setting is causing the hydraulic grade to drop below the ground elevation. When the pump is off, the hydraulic grade throughout the system is lower and below the elevation in many cases.

    As for needing surge protection, the negative pressures are relatively small, but you would need to use your engineering judgment to ascertain if something is required. Note that you may be able to calibrate and check your input to make sure that everything is entered correctly. If some of the data is incorrect, you may be able to get rid of the negative pressures that way.

    For the use of the "Inlet Valve Thottles" option, this can be used instead of a valve. You would need to have the discharge coefficient of the valve, since this option doesn't allow for flow settings or other similar settings. This option is typically used for modeling throttling valves, such as a float valve. With this option, you define the level that the valve will start to close and become fully closed. Along with the discharge coefficient, these levels will need to be defined as well.

    Please let us know if you have any questions.

    Regards,

    Scott Kampa

    Bentley Technical Support

    Answer Verified By: Sushma Choure 

  • Dear Scott

    Thanks a lot, I appreciate this excellent response. actually the problem that I have to fill all tanks within 16 hours, some tanks filled very rapidly while others can not be filled within assigned pumping hours also some tanks are empty on the other direction despite I have check valve on inlet pipe which was very strange to me , this the reason I used flow Control valve. so kindly if you have any suggestion how to model this , it will be great help to me.

    regards;

    Eng. Sayed Elhagar

  • Hello Sayed,

    In the latest version of WaterGEMS, the tanks appear to fill simultaneously. I opened the tank FlexTable and added the Percent Full column. I then opened the Time Browser (Analysis > Time Browser) and step through each time step. It appeared the the Percent Full for all tanks was the same at all time steps. There are some small deviations (at 11 hours, some were 84.38% full, some 84.37%, but very close), but they were basically identical.

    Give this a try on your end. If that doesn't help, try upgrading to the latest version of WaterGEMS. I am using WaterGEMS V8i SELECTseries 5 to test this.

    Regards,

    Scott

    Answer Verified By: Sushma Choure