Study of Process Control must begin by understanding the basic concepts of Process Control. This article explains the Basic Concepts in Process Control, so let's begin...
What is a process?
In the context of the Chemical Process Industry, Process is an activity that involves raw materials and most often energy coming together to produce a desired product.
From a Process Control standpoint, the term (Process) also includes the Equipment where the process takes place, for example, the reactor with associated accessories such as the jacket, the agitators, etc.
Also a Unit Operation such as drying or distillation, which does not involve any chemical changes, would also be called Process in a discussion of process control.
Definition of Process
A useful definition of Process in the context of process control is any activity that has one or more processes Variables associated with it that are important enough to be controlled and whose values need to be known.
Example of a Process
Manufacture of Ammonia by combining H2 and N2 at a specified temperature and pressure is an example of a process in the normal sense. However from the Process Control standpoint, not only the ammonia reactor but equipment for a unit operation in the Ammonia plant such as a heat exchanger would also be designated as a process while considering control of parameters associated with it.
What is Dynamic Variable?
Even though we are familiar with the term Variable, it is necessary to remember in the current context that we are referring to Dynamic or continuously changing variables. Any parameter that changes either spontaneously or because of external influence is a Dynamic Variable.
Example of A Dynamic Variable
All variables associated with a process or equipment in a process plant are dynamic whether it is the temperature of a batch reactor or temperature of feed to a continuous distillation column or level of a tank.
Process Control and Its Primary Objective
Process Control refers to a system of managing and regulating different variables within a specific process to maintain desired outcomes or conditions. So the process control therefore means regulating one or more dynamic variables associated with the process.
The primary objective of process control is to cause some dynamic variable to remain fixed at or near some desired specific value. As the variable itself is dynamic, we must continuously provide corrective action to keep the variable at the fixed value.
Regulation is the name given to this operation of maintaining the constant value of a dynamic variable. As defined earlier, process control therefore consists of regulating one or more dynamic variables.
Dynamic variables can be of three primary types: Controlled variables, Manipulated variables and Load variables:
1- Controlled Variable
As the name suggests, any variable that is being controlled is called a controlled variable. Controlled variables indicate product quality or operating condition of the process.
Example: Temperature, Pressure, Level or Composition could be controlled variables in a given process. For a binary distillation column, the top column temperature could be a controlled variable.
2- Manipulated Variable
A variable which is manipulated in order that the controlled variable gets maintained at the desired value is called the manipulated variable.
Example: Flow rate of an incoming or outgoing stream is a very common manipulated variable in a vast majority of processes. A control loop itself can be a manipulated variable for controlling another variable in more complicated systems.
3- Load Variable
A variable which affects the regulation i.e. maintainability of a controlled variable at the desired value and is itself not manipulated, is called a load variable.
Set Point SP
The desired, fixed value at which the controlled variable is to be maintained is called the set point usually abbreviated as SP.
Process Variable PV
The controlled variable is also called the process variable and is abbreviated as PV. For example, even though the desired reactor temperature may be 85 C, the instantaneous PV value may be 83, 84 or 86 C at three different time instants.
Example: For a shell and tube heat exchanger used for preheating the raw material stream, the exit temperature is the controlled variable and the process variable, steam flow to the shell side is the manipulated variable and temperature of the stream at the inlet to the heat exchanger could be a load variable. The desired exit temperature of 85 C is the Set Point. Instantaneous exit temperature is called PV and may assume values physically achievable, say 83, 84.
Output OP
The value of the electrical signal produced by a controller after comparing the SP and PV values and performing a special internal calculation is called the output and is abbreviated as OP. Output is mostly expressed as a percentage of the full scale, e.g. 67%.
Control Loop (Loop)
An entity consisting of a process, a measurement device, a comparator, a controller and a final control element that acts on the process is called a control loop or simply a loop.
Closed and Open Loop
Depending upon whether all these elements of a control loop are connected to each other with the controller working in automatic mode, the loop may be called either a closed loop or an open loop.
A closed loop becomes open when an operator presses the Auto/Manual selection button on its front panel to select the manual mode. Other reasons for opening of a loop are:
- Failure of the sensor or transmitter which ends the ability of the controller to receive/observe Instantaneous values of the controlled variable
- The saturation of controller output at 0 or 100%, This ends the ability of a controller to influence the process
- Failure of the valve actuator due to friction or debris in the valve.
What is A Process Control System?
A process control system is a set of components working together to achieve the objectives of monitoring, controlling and optimizing a process. Such a system consists of electro-mechanical, analog and digital electronic devices, computer systems with associated hardware and special interfaces for human operators.
In the system, there is a continuous interaction taking place between these different components. Modern process control systems perform many more tasks apart from regulating dynamic variables such as supervision and planning.
Plant Area Classification
Generally, different processes require different levels of control. As a result, plants are divided into three basic areas based on the complexity of the process control systems needed:-
- The first area is the basic regulatory control, which is used in simple processes with minimal variability. Here, the control loop maintains the process variables within a narrow range, ensuring stable operation and preventing deviations from the set point.
- The second area is advanced regulatory control, which is used in more complex processes that involve multiple variables. These systems have multiple control loops working in tandem to regulate different factors, with each having its own set point and control device. These loops interact with each other to maintain overall process stability and optimal performance.
- Lastly, we have the third area of process control, which is known as multi variable control. It is used in highly complex processes that require precise manipulation of various variables to achieve the desired outcome. These systems have advanced algorithms and software that analyze vast amounts of data from multiple sensors to provide optimal control and ensure the process's overall efficiency.
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