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Pump Station Control: On/Off Control Strategy (Part 1)

By David W Spitzer and Walt Boyes

E-Zine May 2015

Click here to read “Pump Station Control: Introduction”

A common pump station control strategy is on/off pump control. This strategy is often selected when operating a single pump in a pump station with one pump fed from a sump or tank. The pump is turned on when the level in the tank or sump is high, and turned off when the level is low. This can be implemented using two limit switches --- one to detect high level and another to detect low level. The outputs of the level switches are used to operate the pump motor starter, where appropriate provisions are made for the installation of a hand-off-auto (HOA) switch that allows for manual or automatic pump operation.

Note that on/off pump operation results in zero flow when the pump is off, and full flow when the pump is on. This operation might have significant effects on the process. For example, using this control strategy in the wet well of a wastewater treatment plant can cause the flow through the entire plant to stop. If the control strategies in the plant do not take this into account, chemical additives like chlorine could continue to flow while the pump is off. This can cause concentrations of additives that not only cause instability, but also waste chemicals. The additional cost of the chemicals could be significant, especially as the amount of time at zero flow increases. If chlorine is being added while there is no flow, a significant hazard can be created, as well.

This control strategy can also be used when a spare pump is installed. In this case, a sequencer may be used to alternate the pumps to distribute wear between the two pumps. Should one pump fail to start, the other pump can automatically be started to handle the load. In addition, a high-high level switch could be added to operate both pumps under high flow conditions. It is useful to take another contact from a current sensor to make sure that the pump started and is running at load.

Click here to read “Pump Station Control: On/Off Control Strategy (Part 2)”

From Flow Control (June 2002)

ISSN 1538-5280

Spitzer and Boyes, LLC
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