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Modeling an inflow (negative demand) in WaterCAD or WaterGEMS

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

Problem

How can you force an inflow into a system without the use of a pump and reservoir?

How do negative demands work?

Solution

To force an inflow into a system in WaterCAD or WaterGEMS, use a negative demand. A negative demand is like having a known flow or injected flow into the system. If you have -60 gpm on a junction that junction will force 60 gpm into your network.

Note that you'll need to ensure that the forced inflow is physical possible given other downstream boundary conditions. For example this will not work if there is no storage (tank or reservoir) downstream, as the downstream demand will need to be exactly equal to the forced inflow. In another example, a forced inflow can conflict with a Flow Control Valve (FCV) flow setting. 

See articles further below for more on other options for modeling inflow.

Note: to model an inflow during a transient simulation in HAMMER, use the Periodic Head-Flow element.


See Also

Modeling Fixed Inflow from a Marginal Source

Modeling a Connection to an Existing System

Ill-conditioning User Notification for valves in a hydraulic model

Modeling a change in HGL or flow during a transient simulation

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