Hello,
I'm modeling a combined supply/distribution system (in WATERCAD) with a well and (well) pump delivering water to 5 distribution tanks, some with two additional booster pumps online to get water uphill to the higher tanks. The system is combined supply/distribution, and I've modeled demand as unit demands at nodes, counting the number of unit demands (homes) at each node and applying that. Also, I have a diurnal pattern for the system to vary demand throughout the day. I used the control wizard to define pump on/off based on tank levels. The controls below are indicating pumps are on when associated tank gets to 5ft level or less, and pumps are off when the level nears overflow (within about 1/2 ft).
My first question is, should the above control settings work? If I start with all the tanks near full, the pumps never turn on and the tanks remain full all the time. If I start with tanks near empty, the pumps run all the time but the tanks never fill up. To me, this means the model isn't converging or something, even though it says it is. If I start with the tanks partially full (at about 10', the well pump does actually seem to cycle on and off. And the first tanks on line actually go up and down in level, alsmost as expected - although I feel the tanks (#4 and #5 not shown here) are running empty more often than I expected.
What still really makes me think something is wrong, Tank #1, which follows a booster pump is shown to be filling and emptying if you look at flow, however the level goes to 33% full and doesn't move. This is impossible given the flows reported during the EPS. Any thoughts on why this is happening? I'm trying to get better data on my booster pump, but even if I don't know the most current pumping conditions, the model shouldn't do what it is doing I think.
First, are you using a Control Set? If so, make sure that the correct controls are applied to the control set and that the correct control set is applied to the Operational alternative.
If that isn't the case, I find that graphing the results together in the same graph can be useful in troubleshooting issues. Graph the tank HGL and pump flow in the same graph and compare this to the control for those elements. This can sometimes help find flaws in the controls.
Also check if there are any user notifications, particularly related to the pump, such as "Pump cannot deliver head or flow". If the pump is not flowing because it is at the shutoff head, that will obviously impact the results.
If that doesn't help, we may need to see a copy of the model to look into this. There are two options for sharing your model files on BE Communities. If you would like the files to be visible to other members, compress the files into a zip file and upload them as an attachment using the ‘Advanced Reply editor’ before posting. If your data is confidential, you can follow the instructions in the link below to send it to us via Bentley Sharefile. Files uploaded to Sharefile can only be viewed by Bentley.
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Regards,
Scott
I am not using a control set. I think this is ok, since all my scenarios are for the existing system, and the existing pump controls are constant.
The image above shows flow into the tank varying with no change in level over time. I've looked at a graph of the booster pump too. It indicates the pump is running all the time (head/flow varies with demand), but never shuts off. Once again, I'm not seeing how demand can go up and down, with water flowing in and out of tank without the tank level changing.
Since I have three pumps in the system, I inactivated a portion of the system to isolate the well pump to the storage tank #4 it fills. The level of tank #4 varies as expected between 5ft level and full as the well pump turns on and off as directed by the controls. When I hook up the rest of the system though (including booster pumps) is when things stop working and I get the constant tank levels downstream with pumps running and demand varying. This makes me think my booster pump definition is way off or my base demand/pattern is incorrect (or both).. I'm going to analyze the pump head needed in a spreadsheet and see if that helps. If not, I will upload the model, as I don't see anything wrong with my demand setup.
I have narrowed down the issue.
The system includes two tanks, #1 & #2 with the same base and overflow elevation, but #2 is 2.5 miles farther than #1 from the booster station through a different branch of the distribution system.
The extra pipe means, depending on flowrate, tank #2 is hydraulically 30-50ft 'higher' than tank #1 when considering headloss.
I read that it is better to model tanks that 'float' on one another as a single larger tank. I have done this for a different tank site where the two tanks are right next to eachother, but this is not reasonable for tanks #1 and #2, since they are too far apart hydraulically and should operate as separate tanks.
I can get the system to model with either tank online but not both. If I keep either tank active and the other inactive (with pump controls tied to the active one), the model cycles as expected in ESP with tank filling and emptying as expected, pump switching on and off. I varied the pump curve, optimizing for one tank and then the other, and even if I set up the pump for the 'wrong' tank, the model still performs well, filling/emptying tank and meeting demands.
If both tanks are active/online, the pump runs constantly, with water entering and leaving both tanks, but the tank levels flatline to a partially full level.
Do you have any tips for how to set the booster pump controls / define these two tanks so that the system can be modeled accurately with both tanks in operation??
Ignoring one of the tanks isn't an option because one of the objectives of the project is to consider improvements to the supply/distribution lines between these tanks.