How To

How to model demand patterns in Bentley HAMMER

Background

Hydraulic pattern are often used in WaterGEMS and WaterCAD to adjust the demands. However, this pattern is not used by HAMMER. It is only used for the initial conditions, particularly for EPS runs. In the transient solver, the demands used by the system will be that derived from the time step used for the initial conditions calculation.

Steps to Accomplish

The way you would need to model this will depend on how the system is expected to work. If the demand increases before the transient event, the simplest way to model this would be to simply start with an initial demand expected at that point and then adjust the operating rule at the appropriate time. In other words, what happens to the demands before the transient event will not really matter.

However, if the demands change (either gradually or rapidly) during the transient event, you will need to set up your model a little differently. HAMMER comes with an element called Periodic Head-Flow. This is used when you need to control the flow or head in the system. You can find some additional in formation on this element in the Help documentation. It basically allow you to define how the flow or head changes with time.

For this to work, you would replace the demand node with the Periodic Head-Flow element. Open the properties for the element and set Transient Parameter to "Flow". Click the ellipsis (...) button in the Flow Pattern field to open the table. Enter the flow expected at this node; a positive value represents outflow. When you compute the model, the outflow will follow the pattern you entered.

Note that the Period Head-Flow elements only allows one pipe connection. In other words, it needs to be at the end of the system. If there are nodes and demands downstream of the demand node, you could leave the current set up as is, then add a pipe at a tee with the Period Head-Flow element attached to it. Make the pipe a large diameter with a smooth friction factor, so that addition losses are not introduced. This type of setup will allow you to model demands downstream of the pattern demand node. Of course, if the pattern demand node is at the end of the system as it is currently modeled, you can simply replace the node with a Periodic Head-Flow element.

See Also

How are demands treated during the transient simulation in HAMMER?

Modeling a change in HGL or flow during a transient simulation