Basic Terms of Process Control
Process
It is a grouping of equipment in a sequential order required for the production of a material or part of a production.
Process Control
The term process control describes the control of variables associated with a process in order to maintain them at the desired value. Example: Liquid level, the flow of fluids, temperature of fluids.
Process variable
The term process control describes the control of variables associated with a process in order to maintain them at the desired value. Example: Liquid level, the flow of fluids, temperature of fluids.
Process variable
The variable that changes with respect to time in the process.
Controlled variable
The variable that is being controlled in the process
Manipulated variable
The variable which is changed in order to reduce error and thereby to bring the controlled variable closest to set point.
Setpoint
It is the desired value of the variable being controlled. It is an input variable of a controller.
Error
The difference between the measured value and desired value of a variable.
Disturbance
It is any variable that causes undesired variations in the controlled variable.
Example: Temperature Control in a Furnace
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| Temperature Control in a Furnace |
The furnace heats the process fluid from its inlet temperature to the desired outlet temperature. The fuel gas is used for heating. The fluid outlet temperature is controlled by manipulating the flow rate of fuel gas. The control strategy used in this example is feedback control. Now, we shall apply the basic terms to this temperature control system.
Process
The inlet process fluid pipe, the fuel gas pipeline, the furnace, the fluid tubes inside the furnace, the outlet process fluid pipe together form the process.
Process variable
The
Controlled variable
The outlet temperature of the process fluid.
Manipulated variable
The flow rate of fuel gas.
Setpoint
The desired value of outlet process fluid temperature.
Disturbance
The temperature of the inlet process fluid, the flow of the inlet process fluid are the major disturbances. They have a strong potential to cause variation in the outlet fluid temperature.
Components of Process Control Systems
Controller
It is a device or an algorithm that works to maintain the value of the controlled variable at set point.
Final Control Element
It is the device that transfers the controller’s output to the process.
Measurement System
It consists of a sensor and transmitter. The sensor measures the process variable and produces mechanical, electrical or other related phenomena. the transmitter converts the phenomenon into a signal that is suitable for transmission.
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