E-Zine July 2015
Click here to read “Pump Station Control: Introduction”
Click here to read “Pump Station Control: On/Off Control Strategy (Part 1)”
Click here to read “Pump Station Control: On/Off Control Strategy (Part 2)”
The pump station can also be controlled such that flow is modulated. When operating a single pump in a pump station with one pump fed from a sump or tank, the modulation could be based upon a signal from a liquid level transmitter. This modulation can be achieved using a control valve on the discharge of the pump, but issues associated with operating high on the pump curve (dead-heading) should be considered. In part for this reason, it is more common to vary the speed of the motor driving the pump. In addition, varying pump speed is more energy efficient than using a control valve and can reduce pump wear and reduce pump maintenance.
Modulating control allows flow to be varied continuously from near zero to maximum flow. When the incoming flow into the sump or tank is relatively steady, the pumped flow can be manipulated to match the incoming flow. When this occurs, the level can be controlled to remain relatively stable. If the level is relatively steady, the incoming flow is approximately equal to the pumped flow, and therefore the pumped flow will be relatively steady.
In contrast, the on/off control might cause the pumped flow to jump from zero to one pump flow several times each hour, as the level is allowed to rise and fall significantly (from 20 to 60 percent in the example). These flow changes can have significant adverse effects on the process, as outlined above, when appropriate compensation is not included in the control strategy. Using a modulating strategy to stabilize the pumped flow can make the process operate better with less complex control strategies.
Click here to read “Pump Station Control: Modulating Control Strategy (Part 2)”
From Flow Control (June 2002)