How can I model a backflow preventer?

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

Problem

How can I model a backflow preventer or Reduced Pressure Zone (RPZ) Valve? 

Solution

Option 1

If a single headloss value can be assumed, this can be modeled using a PBV that creates a specified head loss across the valve followed in line with a check valve. See: Modeling a Constant Headloss. Make sure the orientation of the adjacent pipe is correct and then choose "True" for "has check valve?" in the pipe properties. These are often used to model components that cannot easily be modeled using a standard minor loss element. This valve does not automatically check flow and will actually boost the pressure in the direction of reverse flow to achieve a downstream hydraulic grade that is lower than the upstream grade by the set amount. 

Option 2

If the headloss characteristics are more complex, you can model this with a General Purpose Valve (GPV) and a check valve on the adjacent pipe. In the GPV properties, enter a table of flow versus headloss. As with the PBV configuration above, make sure the orientation of the adjacent pipe is correct and then choose "True" for "Has check valve?" in the pipe properties. If a certain amount of head is required to push open a closed check valve, it may be desirable to include some special entries in the GPV curve: 0,0 and 0.001,X, with ‘X' being the head value above which the valve can reopen. See image below.

Troubleshooting

If the model is unbalanced when using one or more GPVs as backflow preventers, you may need to add additional points to the GPV headloss curve to "smooth" the transitions, reduce the value of the first positive headloss value at the zero flow point (second row in the screenshot above), consider simplifying (for example use a PBV instead) or adjust the advanced calculation options per this article: Troubleshooting the Network Unbalanced or Cannot solve network hydraulic equations user notification

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