| Product(s): | WaterCAD, WaterGEMS, HAMMER |
| Version(s): | CONNECT Edition, V8i |
| Area: | Layout and Data Input |
Problem
How can I model a tank in WaterCAD or WaterGEMS whose cross sectional area is not circular or rectangular and/or has its shape vary with respect to depth?
When setting up a variable area tank, how does the cross section curve setup work? What is the relative depth/volume based on?
Solution
The Variable Area option can be used to model a tank whose cross sectional area is not circular or rectangular and/or has its shape vary with respect to depth. This is available by selecting "variable area" from the "Section" dropdown in the tank properties:
The variable area tank cross section curve is based on the total tank depth/volume; "Volume Full (Input)". Be careful to check the units of the depth/volume columns in the cross section curve - they default to percent, meaning it will expect values between 0 and 100, unless you change the units to be unitless (right click the column header >unit and formatting) in which case it would be expecting between 0 and 1.0.
Values between 0 and 100% in the depth ratio column represent depths between the minimum tank elevation and maximum tank elevation. Values between 0 and 100% in the volume ratio column represent volumes between a volume of zero and the total tank volume ("Volume Full (Input)").
For example, say we have the following tank:
- Minimum elevation of 100 ft
- Maximum elevation of 200 ft
- Total volume is 500,000 gallons (entire tank volume, between min and max elevations.)
- The volume below 150 ft is 100,000 gallons
Assuming the default % units, we would develop the cross section curve based on a total depth of 100 ft (200 - 100 = 100) and total volume of 500,000 gallons. 500,000 would be entered as the "Volume full (input)",
100 ft as the "elevation (minimum)" and 200 ft as the "elevation (maximum)". The cross section curve would be like this:
0,0
50, 20
100,100
Meaning, at 0% of the depth (100 ft) there would be 0% of the full volume (0 gallons). At 50% of the depth (150 ft) there would be 20% of the total volume (100,000 gallons) and at 100% of the depth (200 ft) there would be 100% of the volume (500,000 gallons.)
Of course, you should use more points than this for greater accuracy. Linear interpolation is used for compute volumes for depths between the points you enter in the curve.
Inactive Volume and water quality considerations
When using the variable area option for a tank, along with the Inactive Volume field, you may find that water age results are not accounting for the inactive volume. This is because the inactive volume field is for reference purposes only for the variable area option. Inactive volume only applies to constant area / cylindrical tanks. To address this limitation you could either:
- Approximate the tank as a constant area / cylinder
- Explicitly model the varying tank area including the area below the tank operating range. Note that with this approach, because the tank’s minimum elevation corresponds to the bottom of the volume curve, it means that the tank's minimum elevation field will have to be artificially set as equal to the base (i.e., at the bottom of the inactive volume). This may not be a problem if the simulation runs in such a way that the minimum of the actual allowable operating range (what would have been the minimum elevation if not for the workaround) is never reached.