Process controls is a mixture between the statistics and engineering discipline that deals with the mechanism, architectures, and algorithms for controlling a process. Some examples of controlled processes are:
- Controlling the temperature of a water stream by controlling the amount of steam added to the shell of a heat exchanger.
- Operating a jacketed reactor isothermally by controlling the mixture of cold water and steam that flows through the jacket of a jacketed reactor.
- Maintaining a set ratio of reactants to be added to a reactor by controlling their flow rates.
- Controlling the height of fluid in a tank to ensure that it does not overflow.
To truly understand or solve a design problem it is necessary to understand the key concepts and general terminology. The paragraphs below provide a brief introduction to process controls as well as some terminology that will be useful in studying controls. As you begin to look at specific examples contained here, as well as elsewhere on the wiki, you will begin to gain a better grasp on how controls operate and function as well as their uses in industry.
1.2 Process Control Background
The role of process control has changed throughout the years and is continuously shaped by technology. The traditional role of process control in industrial operations was to contribute to safety, minimized environmental impact, and optimize processes by maintaining process variable near the desired values (1). Generally, anything that requires continuous monitoring of an operation involve the role of a process engineer. In years past the monitoring of these processes was done at the unit and were maintained locally by operator and engineers. Today many chemical plants have gone to full automation, which means that engineers and operators are helped by DCS that communicates with the instruments in the field.
What are the benefits of Process Control?
The benefits of controlling or automating process are in a number of distinct area in the operation of a unit or chemical plant. Safety of workers and the community around a plant is probably concern number one or should be for most engineers as they begin to design their processes. Chemical plants have a great potential to do severe damage if something goes wrong and it is inherent the setup of process control to set boundaries on specific unit so that they don’t injure or kill workers or individuals in the community.
1.3 The Objectives of Control
A control system is required to perform either one or both task:
1. Maintain the process at the operational conditions and set points
Many processes should work at steady state conditions or in a state in which it satisfies all the benefits for a company such as budget, yield, safety, and other quality objectives. In many real-life situations, a process may not always remain static under these conditions and therefore can cause substantial losses to the process. One of the ways a process can wander away from these conditions is by the system becoming unstable, meaning process variables oscillate from its physical boundaries over a limited time span. An example of this would be a water tank in a heating and cooling process without any drainage and is being constantly filled with water. The water level in the tank will continue to rise and eventually overflow. This uncontrolled system can be controlled simply by adding control valves and level sensors in the tank that can tell the engineer or technician the level of water in the tank. Another way a process can stray away from steady state conditions can be due to various changes in the environmental conditions, such as composition of a feed, temperature conditions, or flow rate.
2. Transition the process from one operational condition to another
In real-life situations, engineers may change the process operational conditions for a variety of different reasons, such as customer specifications or environment specifications. Although, transitioning a process from one operational condition to another can be detrimental to a process, it also can be beneficial depending on the company and consumer demands.
Examples of why a process may be moved from one operational set point to another:
- Product specifications
- Operational constraints
- Environmental regulations
- Consumer/Customer specifications
- Environmental regulations
- Safety precautions
1.4 Definitions and Terminology
In controlling a process there exist two types of classes of variables.
1. Input Variable – This variable shows the effect of the surroundings on the process. It normally refers to those factors that influence the process. An example of this would be the flow rate of the steam through a heat exchanger that would change the amount of energy put into the process. There are effects of the surrounding that are controllable and some that are not. These are broken down into two types of inputs.
- Manipulated inputs: variable in the surroundings can be control by an operator or the control system in place.
- Disturbances: inputs that can not be controlled by an operator or control system. There exist both measurable and immeasurable disturbances.
2. Output variable- Also known as the control variable These are the variables that are process outputs that effect the surroundings. An example of this would be the amount of CO2 gas that comes out of a combustion reaction. These variables may or may not be measured.
As we consider a controls problem. We are able to look at two major control structures.
- Single input-Single Output (SISO)- for one control(output) variable there exist one manipulate (input) variable that is used to affect the process
- Multiple input-multiple output(MIMO)- There are several control (output) variable that are affected by several manipulated (input) variables used in a given process.
- Cascade: A control system with 2 or more controllers, a "Master" and "Slave" loop. The output of the "Master" controller is the setpoint for the "Slave" controller.
- Dead Time: The amount of time it takes for a process to start changing after a disturbance in the system.
- Derivative Control: The "D" part of a PID controller. With derivative action the controller output is proportional to the rate of change of the process variable or error.*
- Error: In process controls, error is defined as: Error = setpoint - process variable.
- Integral Control: The "I" part of a PID controller. With integral action the controller output is proportional to the amount and duration of the error signal.
- PID Controller: PID controllers are designed to eliminate the need for continuous operator attention. They are used to automatically adjust system variables to hold a process variable at a setpoint. Error is defined above as the difference between setpoint and process variable.
- Proportional Control: The "P" part of a PID controller. With proportional action the controller output is proportional to the amount of the error signal.
- Setpoint: The setpoint is where you would like a controlled process variable to be.
1.5 Design Methodology for Process Control
- Understand the process: Before attempting to control a process it is necessary to understand how the process works and what it does.
- Identify the operating parameters: Once the process is well understood, operating parameters such as temperatures, pressures, flow rates, and other variables specific to the process must be identified for its control.
- Identify the hazardous conditions: In order to maintain a safe and hazard-free facility, variables that may cause safety concerns must be identified and may require additional control.
- Identify the measurables: It is important to identify the measurables that correspond with the operating parameters in order to control the process.
Measurables for process systems include:
- Flow rate
- Electrical behavior
5. Identify the points of measurement: Once the measurables are identified, it is important locate where they will be measured so that the system can be accurately controlled.
6. Select measurement methods: Selecting the proper type of measurement device specific to the process will ensure that the most accurate, stable, and cost-effective method is chosen. There are several different signal types that can detect different things.
These signal types include:
- Infrared (IR)
7. Select control method: In order to control the operating parameters, the proper control method is vital to control the process effectively. On/off is one control method and the other is continuous control. Continuous control involves Proportional (P), Integral (I), and Derivative (D) methods or some combination of those three.
8. Select control system: Choosing between a local or distributed control system that fits well with the process effects both the cost and efficacy of the overall control.
9. Set control limits: Understanding the operating parameters allows the ability to define the limits of the measurable parameters in the control system.
10. Define control logic: Choosing between feed-forward, feed-backward, cascade, ratio, or other control logic is a necessary decision based on the specific design and safety parameters of the system.
11. Create a redundancy system: Even the best control system will have failure points; therefore it is important to design a redundancy system to avoid catastrophic failures by having back-up controls in place.
12. Define a fail-safe: Fail-safes allow a system to return to a safe state after a breakdown of the control. This fail-safe allows the process to avoid hazardous conditions that may otherwise occur.
13. Set lead/lag criteria: Depending on the control logic used in the process, there may be lag times associated with the measurement of the operating parameters. Setting lead/lag times compensates for this effect and allow for accurate control.
14. Investigate effects of changes before/after: By investigating changes made by implementing the control system, unforeseen problems can be identified and corrected before they create hazardous conditions in the facility.
15. Integrate and test with other systems: The proper integration of a new control system with existing process systems avoids conflicts between multiple systems.
1. Romagnoli, Jose A. Introduction to Process Control. s.l. : CRC press, 2006.