Preparing an existing model for a transient analysis in HAMMER

  Product(s): HAMMER
  Version(s): CONNECT Edition, V8i
  Area: Layout and Data Input

How to:

Prepare an existing water model for a transient analysis in Bentley HAMMER.

Background

The purpose of this article is to clarify how to minimize the number of pipes and extra detail in the model while still maintaining accurate transient results. This is done to reduce run duration, make models easier to troubleshoot, and make results easier to analyze.  

Basic Preparation

HAMMER uses the same file type as WaterGEMS and WaterCAD, and for that reason, a user can directly open a model created in WaterGEMS or WaterCAD in Bentley HAMMER. This makes for a convenient analysis of an existing hydraulic model.

1) It is generally recommended that the HAMMER model is of a well-calibrated system. HAMMER will run on a model that has not been calibrated, but the results may not be as accurate since the initial conditions calculation will not be as accurate.

2) The initial conditions calculation should have the expected results for a given time step. For instance, the pressure and flow results should be reflective of the system.

3) Element elevations should be entered and accurate as well, and should represent pipe elevations, not ground. Since elevation and pressure are related, accurate input is required for good analysis. Information on elevations in HAMMER can be found at the following link: What does node elevation represent?

4) If there are any unexpected user notifications, these should be resolved. If you receive a red user notification, such as a "Network Unbalanced" message, this will need to be resolved before you compute the transient run to assure the most accurate results possible. You can find that many of the notifications that you could receive are addressed in a wiki or forum post, which will allow you to locate the content you're looking for. This wiki article will explain how you can search our Communities site for that information.

5) If you have any modeling artifacts or "tricks", those should be removed as they can skew the transient results. For example a connection to an existing system represented by a "fake" reservoir and pump based on hydrant flow tests. Transient waves would not reflect off these conditions in the same manner as the actual system, which could skew the transient results.

Simplification and Skeletonization

If you have a large system, you may want to consider simplifying or skeletonizing it. HAMMER will compute large, full-scale models, however the run time will be quite long and you may spend excessive time cleaning up the model due to issues such as pipe roughness (due to the way that HAMMER must compute an appropriate Darcy-Weisbach friction factor - example). Transients are less likely be impactful with smaller pipes leading to individual customers compared to the results in a larger main after a pump station for example. This is especially the case in highly looped networks, which tend to dampen out transient effects. So, it may not be worth the potential extra effort to maintain a full-city transient model (and wait for it to compute), if you only need to focus on a small area.

The Skelebrator tool can help with simplifying a model. You can find information on using Skelebrator in the Help documentation.

Note: in recent versions of HAMMER, Skelebrator is included. For older versions, you may need to use Skelebrator in WaterGEMS.

There is no rule of thumb when it comes to how far to skeletonize a transient model, as every system is different. Typically, you would want to focus on removal of smaller, lower-flow pipes, farther away from transient events where the transient energy tends to dampen out. You could also select a key series of pipes and nodes in the model and export these as a submodel. You could then import the submodel into a new model. Note that with the submodel approach, some adjustment may be needed in order to assure that the imported submodel computes and gives the correct initial conditions results.

Finally, you can use active topology selection to make only the elements you are interested in active in the model. As with the submodel method, some adjustment may be needed to assure the model computes correctly. You could use a separate demand alternative if you need to lump demands onto the main pipeline)

If in doubt, try a sensitivity study - compute a transient event (such as a pump shutdown) for a large version of the model (for example 10 inch pipes and higher), and observe the transient envelope in a profile from the transient event (pump station) to a downstream boundary such as a tank. You could also color code on the max or min transient pressure statistical result field to visualize how far into the network the transient has an impact. Then, perform a heavy level of skeletonization on the model, re-run the same transient event and compare the transient envelope in that same profile. if you see that the differences are very small, then you may have confidence to proceed with the highly skeletonized version of the model. If there are significant differences, then try a less-extreme level of skeletonization. When you are comfortable with the level of skeletonization, you can then proceed with your transient simulation studies, with improved performance (run time) compared to the full-pipe model. 

For more discussion on the subject of simplifying models for transient purpose, see the course called "Skeletonization for transients" in the HAMMER Learning path on Bentley Learn. Also see the paper "Effects of Skeletonization on Transient Results" by Walski, Daviau, Coran.

Boundary Considerations

In particular, it is important to consider how the termination point of the network (where you "cut it off") behaves during a transient event. Meaning, the assumption you make at the point where you stop (cut off) the network. Here are the primary options:

  1. Junction with demand - may represent numerous downstream demands in the full system. Pressure waves will reflect.
  2. Reservoir - set to the hydraulic grade at that point in the real system. Pressure waves will reflect.
  3. Endless pipe - fictitious long pipe with tiny wave speed, such that a pressure wave does not bounce back. 

A transient wave will reflect differently against an open boundary such as a reservoir or tank, versus a closed boundary such as a dead end or demand.

The third approach would not reflect the wave, so it may be appropriate if the un-modeled downstream system is long and the pressure waves would mostly dissipate downstream. This is sometimes known as an "endless pipe", "pipe to eternity" or "infinite pipe". In HAMMER this can be modeled with a pipe connected to a junction, with a very long user-defined length and/or a very small wave speed. If you need to have flow out of this pipe (into the unmodeled system), enter a demand or reservoir at the terminating junction (depending on how you would like the hydraulics to behave with the initial conditions solver) and adjust the wave speed to a very small number, so that the wave never returns.

If unsure, try modeling it all three ways (scenarios and alternatives can be used for this, namely with the active topology alternative) and compare the transient results. Animate profile paths to understand how the waves interact.

Ideally, it is best to model a simplified version of the system all the way back the source and all the way to the farthest point in the network, even if it is a simplified version of it.

Identifying Main Pathways

To help identify main pathways in the model (which could also be your profiles to create and view in the Transient Results Viewer), consider the below:

1)  Use Network Navigator and choose the ‘Network Trace’ option from the pull down menu. From there choose the “Path to the Nearest Downstream Element of Specified Type…”  or the “Path to the Nearest Upstream Element of Specified Type” depending on which end you choose to start from. This will locate the most direct path to those elements.

2)  Choose the Start Element in the query parameters dialog box by clicking on the drop down arrow next to <None> and choosing the “Select…” option to select the element from the drawing pane. Once it’s selected choose the ‘Element Type’ from the drop down list below that. Finally, click the compute button in the upper right corner of the window to execute the query.

3)   Click the selection arrow icon to the immediate left of the pencil icon that looks like an arrow with dots in the upper left to select the elements in the drawing. Once the elements are selected right click on one of the selected pipes in the drawing pane and choose “Create Selection Set”. Label the selection set appropriately. 

4)  With the selection set highlighted (red by default) click Edit > Invert Selection. This will choose all the other elements in your model.  Hold down the control button and left click on the branches that you’d like to remain in the model. This will remove them from the selection set you’re going to create. Right click on one of the pipes that’s still highlighted and choose “Create Selection Set” then name it appropriately.   

5)  Go to Tools > Active Topology Selection and click the drop down arrow next to the third icon to the left and choose the selection set you created in step 4. This will highlight those pipes. Click the green check mark icon to inactivate those pipes. 

See Also

Can Water Hammer do transient analysis for one zone in the model?

General HAMMER V8i FAQ

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