Back to
BASICS:
CONTROL
SYSTEMS
THE SUMMER 2007 ISSUE OF VALVE MAGAZINE CONTAINED A
“BACK TO BASICS” ARTICLE ON CONTROL VALVES. HERE, THE
AUTHOR EXPANDS ON THIS THEME BY ADDRESSING CONTROL
SYSTEMS. BY JIM CASEY
While the control valve is the final
control element of a control
system that directly affects the
process, there may be other devices
for final control, depending upon the
variable that is controlled. Examples
include a variable speed pump or a
roller in a rolling mill. The other elements of a control loop are the
process sensor, the process controller
and the setpoint input (see Figure 1,
page 14).
The Basic Feedback
Control Loop
Figure 1 represents a system intended
to keep a constant downstream pressure regardless of variable load on the
pump. The example illustrates that if
the system is not demanding flow,
product is recirculated back to the
tank to keep the pump from running
back on its curve.
The pressure is measured by the
pressure transmitter (PT). The controller (PC) compares the measured
value to the setpoint, and sends a
position signal to the final control element (PCV) so that the process value
and the setpoint value become the
same. As engineers, we can observe
many control loops in our plant environment. Sometimes the components
of the control loop are scattered
throughout the plant, making it difficult to experience how they all work
together.
One control loop that can be experienced without much difficulty is the
