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water hammer Scenario mismatch

Hello,
I am working on the model on water Hammer consist of Source Reservoir, Pumps, Hydro-pneumatic tank, Air valve at high points (Double acting), Flow control valve at termination and the termination reservoir.
I have made the Scenarios as follows:
1. Scenario with protection (Active topology  Source Reservoir, Pumps, Flow control valve at termination and the termination reservoir)
2. Scenario without protection (Active topology  Source Reservoir, Pumps, Hydro-pneumatic tank, Air valve at high points (Double acting), Flow control valve at termination and the termination reservoir)
For Scenario 1  I run the validation (OK), initial conditions (OK), and then finally compute. In this result shows that the Pumping station is delivering 130 MGD to termination reservoir.
But
For Scenario 2  I run the validation (OK), initial conditions, In this result shows that the Pumping station is delivering 5 MGD to termination reservoir. Although the conditions are same.
Kindly guide why this was happening.

Also guide which valve is better to use in the model for the given condition
Condition: I need 130 MGD flow at termination reservoir. And also want HGL 10m above the High point.

Regards
Alvi

  • Hello Daniel,

    You mentioned that the scenario without protection has a hydropneumatic tank and air valve, did you mean to reverse these? (was that supposed to be the scenario with protection?)

    For Scenario 2  I run the validation (OK), initial conditions, In this result shows that the Pumping station is delivering 5 MGD to termination reservoir. Although the conditions are same.

    If the hydropneumatic tank is indeed used in "scenario 2", it could be that the "treat as junction" property is set to "False", and the initial HGL of the tank is set very high, so that the pump shifts on its curve and produces a high head and thus a low flow. Check the tank initial HGL and consider choosing "true" for "treat as junction" if the tank actually floats on the system. See more about this here: Modeling Reference - Hydropneumatic Tanks

    Also guide which valve is better to use in the model for the given condition
    Condition: I need 130 MGD flow at termination reservoir. And also want HGL 10m above the High point.

    Which valve are you referring to? Are you asking which type of valve could be used to control the flow at the downstream end of the system? Generally speaking it is best to model what is actually in the system. Meaning, what device in the actual built system is used to control the flow? If there is actually a flow control valve that throttles its position to limit the flow to 130 MGD, you could model it as an FCV with downstream reservoir set based on the downstream water level. However, keep in mind the information mentioned here:

    Other options to model the end of the system include a demand of 130 MGD at a junction, a reservoir with HGL set so that the pump delivers 130 MGD, or a D2A element configured to model 130 MGD. If you are designing the system, you will need to consider what will actually be present on the downstream side and consider how the transient wave reflection will differ between any assumed boundary conditions. See: HAMMER Transient wave reflection behavior for different boundary conditions

    If your high point is much higher than the downstream boundary hydraulic grade, it may not be possible to maintain 10 m of pressure at the high point. Also, does that requirement refer to the initial conditions (normal operating conditions with the pump on) or does it refer to the transient conditions? (during an emergency pump shutdown?) It may not be possible to maintain a positive pressure at the high point if the pump remains off during the transient and you may need to size your storage to maintain the pressure for a certain duration, and model a pump shutdown followed by startup. See: Negative pressures occur during a transient simulation no matter what protection is used


    Regards,

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