SewerCAD graph data table missing timesteps (hydraulic vs. hydrologic timestep)

  Applies To 
  Product(s): SewerCAD, SewerGEMS (GVF-Convex solver)
  Version(s): V8i, CONNECT Edition
  Area:  Output and Reporting
  Original Author: Jesse Dringoli, Bentley Technical Support Group

Problem

This article explores the differences between the Hydrologic Timestep (gravity network), Hydraulic Timestep (pressure network) and Intermediate timesteps (pressure network). Some situations that can arise because of the differences between these include:

  • Graph data table only shows data in increments of the calculation timestep (even though there may be intermediate timesteps from the pressure solver and even though the graph itself may show them) 
  • Pump flow is seen in the receiving conduit at a timestep before it occurs in the pressure network..

Solution

By default, the data tab of a graph will be shown at every hydraulic/reporting timestep. If you'd like to see results at intermediate timesteps, this is controlled by the Time Browser (formerly known as the EPS results browser). Go to Analysis > Calculations > Times to open the Time Browser (or Analysis > Time Browser in V8i), then select "All / Pressure" in the "Increment" dropdown. This will cause the data table to show results at every pressure timestep, including intermediate times.

This can produce some brief inconsistencies in results between the pressure and gravity networks, though. For example, a pump flow pulse may be seen in a conduit downstream of a pressure pipe at a timestep before it occurs in the pressure network, if the pump turns on at an intermediate timestep (which is common because intermediate timesteps are inserted at the exact time that the wetwell level triggers the control). In the example below, the timestep is set to 0.1 hours and the pump turns on at an intermediate time step of 2.22 hours. The gravity network starts to "see" the pulse at the closest timestep, 2.20 hours, and "N/A" is reported for the conduit flow in the data table for the 2.22 hour row (which is shown because the Time Browser increment is set to "All / Pressure".)

The graph of flow in the two pipes may look like this:

With the hydraulic and hydrologic timestep set smaller, the graph and data table results would typically look better. For example: 

See Also

Wetwell level drops when pumps are off, or other unexpected results in graphs

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