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PRESSURE DEPENDENT FLOW IN A GRAVITY SYSTEM

HELLO BENTLEY FAMILY,

I am trying to model a complex gravity flow system where water flows naturally by gravity from the reservoir (a river upstream) to several tanks downstream. Is there the best way to run the model and allow the system to deliver a reasonable amount of flow by automatically adjusting itself in accordance to the existing conditions(active topology/base physical elevations e,t,c) without the need to introduce Flow Control Valves before each tank?

My point is that in any real gravity flow system water tends to flow up to the maximum capacity the pipeline will hold/allow depending on the terrain surface and amount of energy to be overcome along that pipeline route without the need of introducing a flow control valve. We all understand that more rugged terrain surfaces will require more energy to be overcomed in the pipelines than smoother terrains which will enable the pipe to carry more water. In the real scenario, the natural low will apply and flow delivered will automatically adjust unlike in watergems where negative and even larger flows are experienced without the introduction of FCVs.

Is there a way to allow the software to automatically adjust the flow delivered in an open gravity system(reservoir to tank downstream) to be reasonable and match the existing ground conditions without the need of a FCV. (Lets maybe say: A PRESSURE DEPENDENT FLOW) which will maintain a positive residual head in the downstream side even if it is 0.000001m of water or soo.

Or is there any way to tackle this issue? I was thinking maybe the Pressure Dependent Function will sort me but I have tried and realized it won't. Can anyone please guide me by using the attached file.Let's say, pick a pipeline from the Reservoir to BPT1-Check the selection sets set they will help. With this pipeline, without the use of a FCV, can the software readjust itself into the normal conditions available and allow water to flow without any negative pressures experienced. Please find attached.

CHEPTAIS ANALYSIS.rar

  • Hello Mulongo,

    I don't know that PDD would help in this case. The only demands are in pipe networks that have low flows already. The high flows (and thus the areas with the lowest pressures) are areas with flow between boundaries, such as reservoir to tank or tank to tank. PDD will not help with the flows between boundaries where there are not demands in between. 

    What governs the flow there is the headloss in the pipes and the difference in the hydraulic grade between sources. The elevation differences in these cases are such that the flows are higher than might otherwise be expected. Reviewing the elevation data for the reservoirs and tanks would be recommended in this case.

    Regards,

    Scott

  • Hello Scott,

    We are all right to say that the flows between two tanks/reservoir & tank will always depend on the physical characteristics of the existing system(normally the HGL difference{elevation difference-total headloss).

    Lets put this discussion to an end with the two summarized questions:-

    1. Can I simulate a PDD between two tanks / between a reservoir and a tank, in whichever way possible, in order to avoid negative pressure heads to be experienced at the junctions which I believe are a result of very high insignificant flows when the model is run in the normal Volume based Demand?

    2. Without the use of a control valve or pipe flow controls(which in that scenario they don't work), can we have the water software readjust itself and allow only the required/limited amount of flow such that negative pressures won't be experienced in the junctions in between the tanks unlike now where negative pressures(a sign of no flow in the real world) are experienced yet insignificant large flows are delivered

    The physical data used was extracted from the survey data. Changing any values will tamper with the integrity of the output results.

  • In an Open system, what controls the junction demands and the mass balance and pipe flow rates?

  • Hello Mulongo,

    1. Pressure Dependent Demands only related to demands as applied to elements, and will not apply to flow between boundary elements.

    2. Without using elements like valves or pumps, only element and pipe headloss play into the flow between boundary elements, like reservoirs and tanks. You can try adjusting the roughness coefficients in the pipes or including minor losses. I would be concerned that this may only make the negative pressure issues more pronounced. 

    Node elevations are not the issue so much as the elevations of the reservoirs and tanks. Do you have some sort of SCADA system or other field results that you can compare to the model data? Some sort of confirmation on flows and tank/reservoir elevations may be needed. 

    Regarding open systems, do you mean a system with only one boundary element and a series of nodes downstream? If so, the flow is dependent on the demands, since WaterGEMS and WaterCAD will always satisfy demands in the system. If there are boundary elements as well as demands, the flow will depend on both the demands and the mass balanced between boundary elements.

    Regards,

    Scott