# Storage volume of wetwell and pipe

 Applies To Product(s): SewerCAD, SewerGEMS Version(s): 08.11.XX.XX Area: Modeling Original Author: Jesse Dringoli, Bentley Technical Support Group

# Problem

When modeling a wetwell with significant storage in the upstream pipe:

1) How can I ensure that the volume of the pipe is accounted for in the storage capacity of the wetwell?

2) How can I report the storage volume of the wetwell plus the pipe? For example the volume between the pump on and off elevation:

# Solution

The storage capacity of the upstream pipe is accounted for automatically with the dynamic numerical solvers (Implicit Dynamic or Explicit SWMM) available in Bentley SewerGEMS.

In Bentley SewerCAD, the Convex numerical solver is used to route flow through conduits, so significant storage volume in a conduit would not be directly accounted for.

If a significant pipe storage volume needs to be accounted for in SewerCAD (or with the GVF Convex solver in SewerGEMS), you can use the "Variable Volume"  option in the "Section" field in the wetwell properties. Essentially you would model the wetwell chamber and pipe volume as one single storage unit, by way of adjusting the cross section curve and full volume. You'll need to calculate this externally based on the geometry of the wetwell and pipes. For example you could calculate the elevation-volume relationship for the component pieces separately (the pipes and the wetwell), then add/combine them together for an overall elevation-volume curve. If you don't have the equations, you might be able to find something online, for example www.lmnoeng.com/.../InclinedCyl.php

The volume of storage between the pump on and off elevations (per the above illustrations) is not directly displayed or reported anywhere, as there could be multiple parallel pumps with different control elevations, among other factors. The "Volume" result field found in the properties or graph of a wetwell can be used to see the current volume of water inside the wetwell, so you could for instance locate a time step (using the Time Browser) when the depth in the wetwell is equal to the off control, locate a time step when it's equal to the on elevation, then subtract to get the volume between (assuming the on and off elevations are not equal to the bottom and top of the wetwell). You could also calculate this manually (which should be fairly simple for a wetwell with a constant area for example) or simply estimate the control elevations.

Note the following:

- If you're not including the pipe storage volume with the variable volume option per above, the storage volume of the connected upstream conduit(s) would not be accounted for in the "volume" result in the wetwell. You would need to separately calculate this volume based on the size and slope of the pipe(s), or use the above workaround to incorporate the conduit volume in the wetwell as a single unit.

- Enhancement #183224 has been filed for considering for a future release the ability to compute pump on/off elevations automatically based on various criteria.

-  Regardless of the above, you will still be able to visualize the water surface in the upstream pipe along in profile view. For example: