Product(s):	WaterGEMS, WaterCAD
Version(s):	V8i, CONNECT Edition
Area:	Modeling

Problem

Is it possible to model two or more separate Variable Speed Pumps (VSPs) that discharge to the same system but only one turns on at a time?

When trying to do this, the VSPs may appear to not respect the controls, or various errors may occur including:

"Unable to solve network hydraulic equations"
"Variable speed pumps not in parallel cannot be controlled by the same control node."

Solution

By default, if multiple VSPs are considered in parallel (based on proximity of a common downstream node), then special logic takes over which pump turns on; a "lead" pump turns on first, then a second "lag" pump only turns on if the lead pump cannot achieve the desired target within the specified maximum relative speed factor. If you have a system where separate variable speed pumps control the same downstream system yet only one turns on at a time, the following tips may help you model this:

To ensure that the special logic does not take control of the pumps, insert several extra junctions upstream and downstream of each pump (see screenshot below.) This will cause the program to not see the pumps as being in parallel.
Use a control node along the same "path" directly downstream of each separate VSP. Meaning, a node upstream of the first common junction of the two separate paths.
If you are only switching between these pumps between scenarios, you could also try setting the one that is off as a non-VSP ("Is Variable Speed Pump?" = False), or even making it inactive (with active topology). However if you will have a need to run an Extended Period Simulation (EPS) and switch between one pump running vs. another, you'll need to use the workaround.
In order to have two pumps operate at a relative speed factor of 0.95, you will need to change the control to turn the pump on to use the Pump Setting option and set a value of 0.95. Do the same thing for both pumps. You do not need to change the controls that turn the pump off.

Modeling multiple VSPs where only one turns on at a time

Problem

Solution

See Also