Using EPS Snapshots to apply demand patterns to a steady state simulation

Applies To 
Product(s): WaterGEMS, WaterCAD
Version(s): CONNECT Edition, V8i
Area:  Calculations
Original Author: Jesse Dringoli, Bentley Technical Support Group


When using the EPS Snapshot calculation option, why is the calculated demand not equal to the base demand times the multiplier for that same time step in that demand pattern?

Problem number: 33025


Below is an explanation of the fields in the calculation options when "Is EPS Snapshot?" is set to "True", which is followed by an example for a better understanding:

With a continuous pattern the calculated demand at a particular time step is equal to the average of the calculated demand for that time step and the next one. The "Equivalent Hydraulic Time Step" field is used in order to make sure that the pattern multipliers used in an EPS Snapshot run exactly match those in an equivalent EPS run. You need to specify the hydraulic time step of the EPS run that you wish to match. (The hydraulic time step is the time interval used between hydraulic calculations assuming that no intermediate time steps are required for controls.) The "Start Time" field in the calculation options is where you set the time that you want to run the EPS snapshot for.

For example, assume the base demand on a given junction is 500gpm, the equivalent hydraulic time step in the calculation options is set to 1 hour, the pattern multiplier for time step 8 is 1.0 and for time step 9 is 1.945, and the Start Time field would be 8 am (assuming the simulation was started at 12 am). The total calculated demand at the junction shown for time step 8 will be (500 * 1) + (500 * 1.945) / 2 = 736.25.

For the continuous pattern, if you want to achieve a demand that is very close to the one specified in the pattern you could set the "Equivalent hydraulic Time Step" field to a very small number like 0.5 minutes or 0.1 minutes. This should result in a calculated demand that is very close to 100% of the demand for a given junction for the specific start time that you selected.

For Demands with Stepwise Patterns:

If the Start Time defined does not coincide with the intervals created with the "Equivalent Hydraulic Time Step", then the demand multiplier will be interpolated. For example, if a stepwise demand pattern has a multiplier of 1.00 at 9:00 am and a multiplier of 2.00 at 10:00 am, then the multiplier at 9:15 am would be 1.25 (see image below).

See Also

What is the difference between a continuous versus a stepwise pattern?