In generating system heads curves for an in-line booster PS I'm getting results that show a odd curve where the head actually reduces at higher flows under certain time steps in my EPS model. The elevated storage tanks in this system are all at the same overflow, on both the suction and discharge of the PS, but we are pumping to an area further at the extreme ends of the system. There are several elevated storage tanks on both sides of the PS. What I have found is that the time steps when the system curve has this odd behavior is at times when the tank on the discharge side of the PS, closest to the PS, is completely (does the model automatically assume an altitude valve in this instance?). I still cannot explain why the system curve behaves this way even with the downstream tank full. I've attached a picture of the system head curves, I've generated several for various time steps, so some of the them look typical, but seven or so that don't are all in a period of the EPS with that tank completely full. I appreciate any help on these system curves.
Usually a kink in the curve like this indicates that there has been some discrete change in the system. It may be a PRV or other valve changing position of a different pump is kicking off.
You're right in suspecting the tank filling and then shutting off as the cause.That tank may fill at an HGL of say 100 ft. When it fills and shuts off, the next highest tank controlling the HGL may be at 90 ft.
One test you can run is to change those tanks into reservoirs and see what happens.
The theory behind system head curves is that you have the same conditions for the full range of flows. If the system changes, that assumption is violated.
Answer Verified By: Brian White
Just to close the loop on this discussion - I have documented some of Tom's advice in the "troubleshooting" section of the System Head Curves article
Jesse DringoliTechnical Support Manager, OpenFlows ProductsBentley Communities Site AdministratorBentley Systems, Inc.