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Thread: Perched Reservoir Control

  1. #1

    Perched Reservoir Control

    I've modeled a distribution system using two perched reservoirs on opposite sides of the City to simulate their constant head pump stations. I'd like to limit the total flow out of one of the reservoirs (to simulate to capacity of the pump station) but when I use a FCV there is significant headloss across it that lowers the pressure coming out of that reservoir, even with large diameter pipes/valves. If I raise to head of the reservoir there is just more headloss across the FCV. Is there any way to control the flow out of a reservoir without a FCV? Is there any way to force a FCV to have zero headloss?

    Thanks!
    Scott

  2. #2
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    Scott,

    Thanks for your post. You ask a series of interesting questions. Let's start with you specific questions regarding FCV's first. Our responses are in BOLD Blue.

    I'd like to limit the total flow out of one of the reservoirs (to simulate to capacity of the pump station) but when I use a FCV there is significant headloss across it that lowers the pressure coming out of that reservoir, even with large diameter pipes/valves. If I raise to head of the reservoir there is just more headloss across the FCV. Is there any way to control the flow out of a reservoir without a FCV?
    Answer: FCV's that are adding headloss are doing so to limit the flow out of the valve. If they imparted zero headloss they would be acting only as an open pipe. The reason that changing the valve size and the pipe size did not work was that unless the FCV imparted headloss, the flow out of the reservoir would exceed the flow limit setting that the FCV uses. Thus, when you increase the reservoir head the valve will add additional headloss so that the flow through the valve matches the setting. Let's say the valve has a setting of 1000 gpm, in essence the model is calculates what the downstream head of the valve would need to be to push exactly 1000 gpm through the valve. If that head is lower than the upstream head, the valve will exert headloss on the FCV so that the head downstream will equal the head necessary to get exactly 1000 gpm through he valve If the head upstream is not sufficient to push the 1000 gpm, the model will issue a warning that the valve is open but cannot deliver flow. The flow through the valve will depend on the head upstream and how much water the available head can push into the system. This is why in your specific case the model is imparting headloss as the downstream head necessary to match your desired FCV setting is lower than the head of the reservoir.

    One way to specify a specific flow at a point without using a FCV is to use a negative demand. When used, the model will determine the head needed to push the value of the negative demand into the system. In essence this is very similar to what a FCV does, except that a negative demand has no connection to any source. However, the issue with using a negative demand is that you then have essentially zero control regarding the head at the junction. The head will be set based on the head required to impart the negative demand as a point of supply in the system. If you ever wish to test this you can close the valve and put a negative demand at a node downstream of the FCV and run the model. You will see that the head for your desired negative demand flow is less than the head of your reservoir, which is why the FCV is inducing headloss.

    The only other method of controlling flow is to use a Variable speed pump (VSP) set to use flow control. However, the mathematics of how this works would have the similar limitations to a negative demand or a FCV. In order for a flow to be specified a specific head is necessary. The VSP would adjust the speed setting of the pump so that the downstream head would match what was necessary to get the desired specific flow setting. As you can imagine, this forces the pump to operate at a very specific point and have everything else work with the head value it specifies to get the flow. This can create model instability at times as it is not always 100% possible to achieve a specified flow in each and every system especially if multiple sources are trying to control the HGL.

    The key point in all this is that to get a specific flow requires a very specific head. This is why the FCV is inducing headloss.



    Is there any way to force a FCV to have zero headloss? Answer: Unfortunately there is no way to do this. The headloss is a necessary parameter as explained above.

    Further thoughts:
    Now to get to your underlying issue, How do I get each source to match the head and flow they had in the real world? You may want to consider adding in the specific booster pumps at both facilities if you want to match a specific head and flow. Flow at a pump station is fairly tightly controlled in a model for pumps that use accurate pump curves as the flow for the pump is associated with where the pump curve intersects the system curve.

    What you might want to check is the following:
    1) Is the discharge head and flow of both stations tightly controlled or is there a slight variance that is observed in the real actual data. If there is a slight variance, how far off is the discharge head of the FCV from actual? Perhaps the induced headloss you observe is pretty close to the discharge pressure observed at that pump station and this is close enough.
    a) Note: remember that pressure at a node or valve is based on the valve or node elevation. Pressure (psi) = (Head (ft) - elevation (ft) )/2.3 ft/psi So make sure you are using an elevation for your point of comparison in the model to the point where the pressure is measured.
    2) You might want to also review your demand distribution and the pipe c-factors used. If either of these are off significantly, this could impact the flow balance between the two stations.
    3) If you are considering using a FCV after this, I would suspect the other reservoir is left uncontrolled (just a pipe connecting to the reservoir.) This is important in an EPS run where the demand will vary and the flow from both stations may change. If you used FCV's on both stations and the demand exceeded the combined FCV settings the model could crash as it would not have enough supply to meet system demand. Make sure you leave flexibility in your model to handle flow that varies.
    4) This is one advantage of using pumps and pump curves. The pump curves will control the flow based on the capacity of each pumps, and the flows supplied will vary to meet demands. If the stations are set up to try and maintain a constant head (as noted) you could use the model VSP option to match the head (i.e pressure) setting was used. You can't fix both head and flow with a VSP but if the head is correct and the elevations are correct, it is likely the system demand distribution, demand diurnal, or the system pipe roughness is causing too much supply to come from a given station and too little to come from the other. Fix those and your flow balance should correct itself.

    Lastly, if you would like to discuss this in more detail please send us an email at support@innovyze.com and we would be glad to provide further more detailed assistance.

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