A process is broadly defined as an operation that uses
resources to transform inputs into outputs. It is the resource that provides
the energy into the process for the transformation to occur.
Below figure shows a hot water
generation process commonly found in plants. The input to this process is cold water and the
output of the process is hot water. Steam is the resource that provides energy for the
transformation to occur within the heat exchanger plates.
Process
control
is the act of controlling a final control element to change the manipulated
variable to maintain the process variable at a desired Set Point.
Below figure shows a block
diagram of a process with a final control element and sensors to measure the
manipulated variable and process variable. In single loop control systems the
actual value of the manipulated variable
is often not measured, the value of the process variable is the only concern.
Below figure shows a heat
exchanger. We see that the manipulated variable (MV) is steam pressure. The final control
element is the valve, by changing the valve opening we are changing the flow of steam which we can
measure by its pressure. The process variable (PV) is the temperature of the water exiting
the heat exchanger; this is the measure of the process output that responds to changes in the flow
of steam.
This is a controllable process
because opening the valve will always lead to an increase in temperature, conversely closing
the valve will always lead to a decrease in temperature. If this were not true, if sometimes on
closing the valve we had an increase in temperature, the process would not be controllable.
Following are definitions of some
terms used above:
·
The manipulated
variable (MV) is a measure of resource being fed into the process, for example,
how much thermal energy.
·
A final
control element (FCE) is the device that changes the value of the
manipulated variable.
·
The controller
output (CO) is the signal from the controller to the final control element.
·
The process
variable (PV) is a measure of the process output that changes in response
to changes in the manipulated variable.
·
The Set
Point (SP) is the value at which we wish to maintain the process variable
at.