I am modeling a small water system that has a booster station to increase pressure for the northern half of the distribution system. The booster pumps draw from a wholesale water supply at relatively constant pressure. They discharge to a set of four small hydropneumatic tanks. The hydro tanks then regulate pressure in the north half of the distribution system. I'm getting strange results where pumped flow into the hydro tanks is much higher than customer demand drawing out of the hydro tanks, sometimes for sustained periods of time (such as the booster pump discharging at 100 gpm for over an hour whereas customer demand drawing from the hydro tanks is only 3 gpm during that period). The calculated pressure in the hydro tanks then swings wildly all over the place, rather than the model following the controls that should tell the pumps to shut off when the hydro tanks reach the real-life pump cut-out pressure. I'm at a loss as to what is causing this issue, as I have calibrated models for similar small systems with nearly identical booster pump/hydro tank configurations with no problems.
This is what the hydropneumatic tank pressure looks like. The booster pump controls tell it to turn on when hydro tank pressure drops below 53 psi and to turn off when hydro tank pressure exceeds 67 psi. These are the pump cut-in/cut-out pressure settings in the field.
The liquid volume in the hydropneumatic tank swings up and down, which doesn't equate at all with the relatively constant 1-3 gpm demand on the tank or the periodic inflow from the booster pump (100+ gpm for short periods).
Hello Kristin,
Are you using hydropneumatic tanks in water supply system to supply/store water in the distribution network in WaterGEMS / WaterCAD instead of tanks?
What user notifications you receive on computation? It could be possible that controls are not followed as defined, which can be understood by user notifications.
How do you model a booster pump in WaterGEMS/WaterCAD?
You may upload model files for our review to comment further.
Sharing model files.
Regards,
Sushma Choure
Bentley Technical Suppport
To add to Sushma's reply - when using hydropneumatic tanks in an EPS in WaterCAD or WaterGEMS, you will often need a much smaller calculation timestep to avoid problems like this, as large changes in pressure can occur within a relatively small period of time. Check your controls as Sushma mentioned but also try using a much smaller timestep, perhaps with a shorter run duration to test. This is mentioned in the following related article from our Wiki: Modeling hydropneumatic tanks in an EPS in WaterGEMS and WaterCAD
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Answer Verified By: Sushma Choure
Thank you both for your replies. The reporting time step was set at 0.25 and I changed it to 0.025. That captured the extremely short booster pump runs needed to fill the hydropneumatic tanks and allowed the pumps to follow the on/off controls based on hydro tank pressure. All set now.