EPS PROBLEM

Hello,

When you compute any Extended Period Simulation (EPS) scenario you would be getting a result summary which shows the total flow supplied and the total flow demanded in the network. You can access this by going to the Home Tab and find "Summary" under the Calculation Tab. It would look something like this

Here you can see that the results are given per hour for a total duration of 24 hours in this case. You can also graph these results for better visualization. See the article below for details;

Graphing results from the Calculation Summary

For your tank becoming completely full I would suggest you graph the outflow from the tank as well as percentage full which would show you how the capacity of the tank is being utilized in your scenario. I am sharing some reference articles which can be useful in this case;

Graphing flow coming out of a tank

What happens when a tank becomes empty or full? (altitude valve)

Let me know if this helps.

Why is this happening?

"When using alternative "EPS" the Tanks are always full. When using alternative "EPS Hour Demand" the Forest Ave and Western Ave tanks empty and shows pump 15895 and 15896 cannot supply flow at specified head."

Did you receive model?

Hello Dick,

I see that you also created a service request for this issue. I will post here the response here, in case other people can benefit as well. I will leave the service request open for a few days, just in case.

The tanks are always full because the hydraulic grade in the system is generally higher than the maximum elevation in the tanks (when using the EPS demand alternative). Since the hydraulic grade is higher than the tank, flow will not come out of the tank. So when the tanks do become full, they stay full and do not drain. In addition, the initial elevation of the tanks is currently equal to the maximum elevation, meaning the tank is initially full. If you set the initial level to a lower value, the tanks will fill for a few time steps, but still become full since the water is flow from high hydraulic grade to the relatively low hydraulic grade in the tanks.

(In addition, you have entered an inactive volume for the tanks of 2,000,000 MG. This appears to be throwing off hydraulic grade and percent full calculations for the tanks, making them appear to be higher than the actual top of the tank. I set the inactive volume for the three tanks to zero and this issue didn't occur any longer, though the tanks still became full.)

If you expect the tanks to drain, you may need to review the elevations in the tanks. I would also review the controls you have on the elements. For instance, there is a control for the Lead PMP to turn off when the Western Ave Standpipe tank is greater than 816 feet. But since that is the maximum elevation for the tank, the hydraulic grade will never get higher, thus the pump would not turn off. You also have pumps with duplicate labels, so I would review the controls to make sure the correct pumps are assigned to the right tanks. This link has information about troubleshooting controls: Troubleshooting Controls in WaterCAD and WaterGEMS.

Regarding the results using the "EPS Hour Demand" demand alternative, the demands for that scenario are many times higher than in the alternative "EPS." This is likely accounting for the issue. The tanks are emptying to try to satisfy the demands. Once the tanks are empty, the demand is being met by flow from the pumps. The message about the pump not being able to deliver head or flow is because the pump is operating far down on the pump curve. If you look at the results, you will see that there is flow, but the pump head is zero. Since the tanks are empty and the pump is not adding head, this will impact the pressure results as well.

For this issue, I would make sure the demands are entered correctly. If they are correct, this may be an indication that the supply system is undersized for that amount of demand. The Scenario Comparison tool can be useful to identify issues like this. I created a new child scenario to your scenario "EPS" and set the demand alternative for the new scenario to "EPS Hour Demand," while leaving everything else the same. I then used Scenario Comparison to confirm the demands were different and by how much.

Regards,

Scott

Thanks Scott. This was very helpful

Hi Scott-

I am having trouble understanding the correlation between patterns and demand for EPS. If the demand is expressed in daily average rate (gpm) i.e. total 24 hour demand divided by 1440 minutes and pattern time step is 1hr. over 24 hr. period is each hour demand calculated as average rate (gpm) x pattern multiplier or average rate (gpm) x 60 minutes x pattern multiplier?

Thanks