Modeling Air Valves At High Points in WaterCAD or WaterGEMS

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

 

Overview

The Air Valve element in WaterCAD and WaterGEMS allows users to accurately model the effects of intermediate high points in a water network. This TechNote describes the effect of using air valves at high points in WaterCAD or WaterGEMS and also compares the implementation of the current capability with modeling approaches used in previous versions.

 

The Trouble with High Points

In the past (before the introduction of the air valve element), high points in a pipeline in WaterCAD or WaterGEMS would not be considered for the pump operating point. Basically they would only consider the boundary conditions (reservoirs and tank elevations) in your system. So, the pump will add enough head to lift the water to the downstream known hydraulic grade. It does not consider junction elevations in between and simply calculates a pressure at the junction locations based on the difference between the hydraulic grade and the physical elevation. So, this sometimes resulted in a negative pressure being calculated in the vicinity of the high point. This situation can be seen in the profile below, in which the hydraulic grate line (in red) is lower than the pipe elevation (in green) at the high points.

This approach basically simulated the effect of water being siphoned over the high point, which is usually not the case in real systems, since most utilities place air release vacuum breaker valves at the high points, and since the vapor pressure of water limits the height of a potential siphon. Since neither of these factors was accounted for in most pressure pipe models, including older versions of WaterCAD and WaterGEMS, the results typically overestimated pump flow and underestimated head (i.e., the pump ran too far to the right on its curve). Basically the head added by the pump was lower than what would really be necessary to 'lift' the water to the high point

In earlier versions of WaterCAD and WaterGEMS (V8 XM and below), it was difficult to implement a workaround to this problem. Some possible solutions included placing a small tank at the high point, controlling flow with a FCV (flow control valve) or using a PSV (pressure sustaining valve) to set the pressure to zero. In many cases, the modeler would simply ignore the negative pressure and accept the pump operating point.

However, even with a workaround, modelers sometimes found that the high point may be pressurized for higher flows, as shown below. This situation could prove especially problematic in the case of an extended period simulation demonstrating both flow regimes.

 

The Air Valve Solution

WaterCAD and WaterGEMS V8i provide an answer by incorporating a new air valve element. By placing an air valve at the high point, the pump sees the air valve elevation as its downstream boundary condition for instances in which pressure would have otherwise been negative at the high point:

  

Note: The Air Valve element was actually added to WaterCAD and WaterGEMS in the V8 XM release, with version number 08.09.400.34. However, the air valve in this version did not include the special behavior described in this technote; it always acts as a junction during the EPS or steady state simulation. It only operates during a transient simulation, when opening the model in Bentley HAMMER.

For instances in which the pipeline functions under pressure/full flow for its entire length (e.g., during the high-flow condition), the pump operating point is correctly based on the downstream boundary condition, similar to the behavior in older versions.

When the air valve is open, the hydraulic grade on the downstream side may be less than the pipe elevation. This can be displayed as the hydraulic grade line drawn below the pipe. This should be interpreted as a pressure pipe that is not flowing full. Full flow resumes at the point where the hydraulic grade line crosses back above the pipe. To accurately observe this phenomenon in a profile, you should ensure that the elevation of the junction immediately downstream of the air valve is above the point where full flow resumes. For example if your next-downstream junction is far away and at a low elevation, you may not observe the part-full phenomenon. This is because WaterCAD/WaterGEMS can only report/compute hydraulic grade at nodes, so it can only draw the HGL between them.

 
Transient Simulation Implications in HAMMER

If this situation occurs in the initial conditions (steady state or EPS) in HAMMER, note that it may be best to end the system at the high point using the Discharge To Atmosphere element, since the transient waves would not travel past the air gap / part-full flow, and since the discharge at that points acts like free discharge:

Troubleshooting

Because air valves have the possibility to switch status, they can lead to instability in the model especially if there are many air valves in the system. This may result in a Network Unbalanced message, with timesteps shown in red in the calculation summary. To improve the stability of the model, it is best to only enable the air valve that will be open during the simulation. This can be done by setting the property "Treat air valve as junction?" to True for those valves that are expected to be closed anyway. (pressure always positive).

For any air valve that you expect to be open and that should behave as described in this article, ensure that you choose "false" for the "Treat air valve as junction?" attribute. Start by setting all air valves to the default of "true" for "treat as junction", run the model, then note the upstream-most point in profile view that shows negative pressure (the first high point). Use an air valve there, and choose "false" for "treat as junction" (allow it to open). Run the model again, then observe if there are any other local high points further downstream that have negative pressure, then enable the air valve there as well, and so on.

 

If all of the pumps upstream of an air valve are off, the pressure network is disconnected in that area and the model will issue warning messages for all nodes in that vicinity indicating that they are disconnected.

In addition, the profile between the air valve and the pumps that are Off will be inaccurate. To make the profile view accurate, you can place an imaginary tank or reservoir on a short branch with a tiny diameter pipe at an Elevation (Initial) equal to the air valve elevation. This tank (which will not contribute significant flow) can eliminate the disconnected system message and correctly represent the fluid in the upstream pipe when the pump is off.

 

See Also

AWWA Book: M51 Air Valves: Air Release, Air/Vacuum, and Combination, Second Edition

WaterGEMS V8 Modeling FAQ 

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