Modeling a Constant Headloss

  Applies To 
  Product(s): WaterGEMS, WaterCAD, HAMMER
  Version(s): V8i, CONNECT Edition
  Area:  Modeling
  Original Author: Akshaya Niraula, Bentley Technical Support Group

Problem

How can I simulate a constant headloss across a valve?

Solution

This can be done via a GPV (General Purpose Valve) or a PBV (Pressure Breaker Valve).

PBV

A Pressure breaker valve (PBV) creates a specified headloss across the valve and is often used to model components that cannot be easily modeled using standard minor loss elements.

To induce a specific headloss, select "pressure" as the "Setting Type", change the units of the "Pressure Setting (Initial)" to linear units like m H2O, then enter the desired headloss. Note the calculate headloss matching the headloss setting in the screenshot below.

GPV

A GPV defines a rating table of flow versus headloss. For a constant headloss, enter a constant headloss value for the expected range of flow: 

Transient Simulation Considerations

The PBV and GPV elements will not work the same during a transient simulation in HAMMER. HAMMER always uses the pair of initial flow and initial headloss through each valve element, to calculate a discharge coefficient which it uses during the transient simulation. This means that even if you use a PBV for the initial conditions, the headloss will vary during the transient simulation. To model a boundary condition at a constant hydraulic grade during the transient simulation, use the Periodic Head-Flow element.

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