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.
This is an interesting situation; the problem is that the actual working tank volume becomes "drowned out" by the overall tank volume in the floating point calculations. There is a way to fix this, but it's not trivial in that it touches a lot of the computational engine. In the meantime the suggested work around should absolutely fix the issue.
Am I right in assuming that the inactive volume in this case is an error?
Wow! Thank you for reviewing. Yes, it was my mistake to enter the inactive volume overestimated, units, units, units.
I adjusted all the inactive volumes to 0, after reading it is only for water quality. I may consider water quality later. If I do, considering I have controls set up between the pump and tanks, am I right to guess the model will automatically consider the inactive volume that occurs if a tank always starts refilling when it reaches, for instance, 10 ft of depth?
I'm just thinking the inactive volume value can be ignored/left at zero in my case even when considering water quality.
Thanks for the excellent support offered through this forum,
Ryan
>> am I right to guess the model will automatically consider the inactive volume that occurs if a tank always starts refilling when it reaches, for instance, 10 ft of depth? <<
Yes, any volume between the minimum level and maximum level is automatically considered, even if the tank is constrained by way of controls (or other reason) to operate within those limits. This is still considered to be "active" volume even if the model doesn't necessarily exercise the full range of the tank in any run.
Anything outside the "active" volume (outside the modeled limits) is what the program requires the user to input, e.g., to cater for varying tank geometries outside the normal operating range, but which may mix with the active volume. Several different mixing models are available for the WQ analysis, that consider different types of tank construction. The effect on WQ results of any inactive volume will depend on the mixing model chosen.