My summary shows demand and flow in balance, but I am clearly losing volume out of storage over the duration of the simulation. Time steps of the demand pattern and the calculation options match and there appears to be no flow into reservoir elements
Ken,
Flow Supplied is flow to/from reservoirs and negative demands (inflows), Flow Stored is flow to/from tanks and Flow Demanded is flow leaving from demands. See: Calculation summary: Flow Supplied, Flow Demanded and Flow Stored
If your volume balance in the calculation summary is not making sense based on the results at reservoirs, tanks and demands, make sure your reporting options are set so that you are not missing out on intermediate timesteps. If tanks/reservoirs are rapidly oscillating, graphs can miss out on this detail. See this: Tank level change not corresponding to net outflow
If this still does not help, please provide a screenshot of your calculation summary and the specific results seen at one of the specific timesteps that does not look right, or provide a copy of the model for review along with steps to reproduce.
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
I uploaded a copy of the model. The scenario I noticed the issue in is "2030 Min Day". If you trend the tanks for all time steps the loss of volume is pretty clear.
ken, the flow supplied, demanded and stored in the Calculation Summary do not show a balance across all timesteps but rather one timestep at a time. Maybe by "balanced" you're referring to convergence? (the network being balanced vs. unbalanced?)
Can you explain in more detail what you mean by "loss of volume"? Tanks can drain to supply demands, or to fill other tanks or reservoirs at lower elevations if the hydraulics warrant. I graphed percent full for all tanks together (see below) and I see most are filling and draining, while a few are dropping and not recovering. This does not necessarily indicate something is wrong with the program but rather that the hydraulics are warranting that these tanks drain and not refill.
Also, you have set to use a constant reporting time of 0.5 hours, which means you are only seeing results in the graphs and other places, at every 0.5 hours. If you set this to "<All>" and re-compute, you will see there are numerous intermediate timesteps being inserted (which is also why it takes a while to compute). In particular T-18 rapidly oscillates between full and empty. The extra timesteps are inserted due to this, and the impact of a full or empty tank might be causing the problem you're experiencing. See: What happens when a tank becomes empty, full, overflows?
You will need to set controls to prevent tanks from becoming full or empty. As an example to illustrate, if you morph T-18 into a junction, the first ~40 hours of the simulation will run much faster, until tank "Stryker 1" becomes empty and the same problem occurs with the intermediate timesteps.
Here is the screenshot of tank percent full with your 0.5 hr constant reporting step:
My flow supplied and demanded for the all time steps line appear to be balanced, implying I should have minimal change in the total volume stored within tanks. Does this mean something else?
In the screenshot where you highlight the "all timesteps" row, the overall flow supplied equals the flow demanded because reservoirs are the only source of water, along with any small volume of initial water stored in the tank (hence the small negative flow stored). Meaning, over the course of a longer period of time, the flow in equals the flow out. Notice you do have a small negative value shown in the flow stored, which reflects that there is a smaller volume of water in the tanks at the end of the simulation vs. at the beginning. This minimal reduction makes sense to me as most tanks seem to be filling and draining while only a few continue to drain and become empty. (see my previous screenshot)
The other, individual values seen in the Calculation summary are on a per-timestep basis. For each timestep there is a mass balance shown for the flow rate (not volume) supplied, demanded and stored. Let's take a small network as an example, where a single tank connects to a single demand with a single pipe. For an EPS, the flow stored will be a negative value equal to the flow demanded so there will be a mass balance for every timestep, yet the tank will continue to drain and become empty. If there was a reservoir that supplied the tank with controls and the tank filled and drained but ended up at the same water surface elevation at the end of the simulation, the net flow stored in the "all timesteps" row would be zero and the flow supplied would be equal to the flow demanded.
Also keep in mind that, as previously mentioned, tanks becoming full or empty will skew the results due to the rapid oscillation that occurs and potential for unbalanced timesteps and hydraulic disconnections. In some cases if a tank is empty and there are demands only connected to that tank, the tank will continue to supply the assumed demand, which will skew the results. (see this). Those results will further be skewed by ignoring intermediate timesteps.
If you modify the model to show all timesteps, add controls to prevent tanks from becoming full or empty and ensure there are no unbalanced timesteps, the flow balance results should make more sense.
Note that I have added a new section at the bottom of the following article, with information on the "All Time Steps" row of the Calculation Summary: Calculation summary: Flow Supplied, Flow Demanded and Flow Stored
That T-18 element seemed to be the problem. Reworked the flow control valve arrangement to eliminate the need for that tank and the volume changes across the tanks looks to be in balance now.