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Pump Station Control: Modulating Control Strategy (Part 2)

By David W Spitzer and Walt Boyes

E-Zine August 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)”
Click here to read “Pump Station Control: Modulating Control Strategy (Part 1)”

As previously stated, modulating flow using a variable speed drive to vary pump speed is more energy efficient than installing a control valve on the pump discharge. In a control valve installation, the pump generates hydraulic pressure, some of which is dissipated across a restriction (the control valve) to manipulate flow. In a variable speed drive installation, pump speed is manipulated to generate only the hydraulic energy needed to pump the desired flow without any restriction.

In some applications, the difference in electrical energy consumption between the two installations can be sufficiently large to justify replacing a control valve installation with a variable speed drive installation. Potential energy savings are especially large when the pump is relatively large as compared to the operating load. For example, this can occur when a pumping system is sized for the maximum possible flow (such as during start-up), but normal operating flow is much smaller.

In some applications, one operating pump cannot handle the design load. In this case, variable speed drives should be installed on all of the operating pumps to vary their speeds in unison. When the pumps are identical, operating the pumps at the same speed will distribute the flow equally between the pumps. This strategy should avoid potential backflow situations that can occur when the pressure produced by one pump operating at high speed overcomes the pressure produced by another pump operating at a lower speed.

As in on/off applications, the pumps can be sequenced to distribute wear and change the number of operating pumps as the load changes. Control strategies used to determine the number of operating pumps include the use of pump speed and/or pumping flow rate information.

Modulating control as described above can be implemented using a single control loop consisting of a level transmitter, level controller, and variable speed drive(s). In most applications, superior control can be achieved by using a cascade control strategy that includes information from a flow transmitter that measures the pumped flow. In some applications, further improvement can be obtained using feedforward control techniques that incorporate other process measurements.

Click here to read “Pump Station Control: Package and Custom Installations”

From Flow Control (June 2002)

ISSN 1538-5280

Spitzer and Boyes, LLC
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