# Controls for alternating or cycling between multiple pumps

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

# Problem

Is there a way to set up the pump controls so that a pair of pumps are controlled by the hydraulic grade of the tank, but only one pump will be on at a time?

In other words, the pumps will cycle back and forth. When the tank reaches a minimum elevation, the first pump will turn on. However, the next time the tank reaches the minimum elevation, the second one will turn on instead.

# Solution

## Time-Based Controls

One option is to set up controls based on clock time or time from start. With this approach, you won't be able to have conditions based on other things like tank HGL. For example:

The two simple controls take care of the initial (time zero) conditions and then the other two controls with composite control conditions swap the pump status every 24 hours.

Here is the graph of flow through pumps with, one pump running for 24 hrs, then the other.

## Controls based on hydraulic conditions

Another way of modeling cycling pumps using logical controls is to use a variable based on pipe status. Please review the attached model, which was created in WaterGEMS/WaterCAD SELECTseries 2 (build 08.11.02.31). You will need this build or later for the model to run.

In order for the cycling to work, you will need a dummy pipe. This pipe will open and closed based on the controls and will help govern which pump is open to supply the demands and fill the tank downstream of the pumps. Assume that the tank is called T-1, the pumps are called PMP-1 and PMP-2, and the dummy pipes are called Control Pipe and Fake Pipe. Fake Pipe is only there to help clean up user notifications; Fake Pipe will be left open at all times. See the image below for the general setup:

If you want the Pump-1 to supply the tank first, Pump-2 will be off and Control Pipe will be closed. Then set up the controls to say:

IF (T-1 HGL<xxx) and (Control Pipe Status = Closed), THEN (PMP-1 status = On)

To turn off Pump-1:

IF (T-1 HGL>xxx) and (PMP-1 Status = On), THEN (PMP-1 status = Off) and (Control Pipe Status = Open)

Then for Pump-2 to supply the tank:

IF (T-1 HGL<xxx) and (Control Pipe Status = Open), THEN (PMP-2 status = On)

To turn off Pump-2:

IF (T-1 HGL>xxx) and (PMP-2 status = On), THEN (PMP-2 status = Off) and (control Pipe Status = Closed)

This workflow could be used with the SewerCAD solver as well. The sample model below is not compatible with SewerCAD.

# Model Files

Cycling between Pumps - WaterGEMS model