Process Control and Automation in Industrial Plants

Posted on Oct 27, 2024 in Electronics

Process Control and Automation

Control Variables

Perturbations: These are the variables that affect the system’s excitation and can be manipulated.

Feedback Control

Control Elements:

  • Sensors/Transmitters: Measure the output and/or disturbance and send a correlated signal.
  • Controller: Modifies the system dynamics to achieve the desired output.
  • Actuator: Changes the system’s output (e.g., amplifier).

Controller Types

  • Two-Position Controller: Moves the final control element between two fixed positions based on a single controlled variable value. Often used with a differential band or neutral zone.
  • Proportional Controller:
  • Proportional-Integral Controller:
  • Proportional-Derivative Controller:
  • Proportional-Integral-Derivative Controller:

Plant Instrumentation

Key Aspects: Instruments, control system, control philosophy, P&ID.

Industrial Processes

  • Manufacturing: Mass production of identical units (e.g., automotive, cookie production).
  • Continuous Process Industries: Bulk production involving movement and storage of liquids, gases, pastes, and/or solids, often with physicochemical changes.

Instrumentation Types

  • Pneumatic Instrumentation: Uses compressed air (typically 3-15 psig) to transmit signals proportional to the measured variable.
  • Electronic Instrumentation: Employs electronic transmitters and controllers, offering advantages like faster transmission speed and more complex control strategies. Standardized signal is 4-20 mA DC.

Electronic Controllers

Relay boards have been largely replaced by computer systems, including Programmable Logic Controllers (PLCs), for reliable discrete signal processing.

Sequential Control

Manages the starting and stopping of plant equipment and their functional relationships.

Operation Modes

  • Local: Equipment operation from the field station.
  • Remote: Equipment operation from the control room, either in automatic or manual mode.
  • Automatic: Plant controlled from the control room with minimal operator intervention.
  • Manual: Individual operation of each piece of equipment based on its specific conditions.

Operation Commands and Indications

These facilitate equipment operation in various modes from both field and control room stations.

Field Station

  • Commands: Start (momentary), Stop (momentary), Local-Off-Remote selector.

Control Room Station

  • Commands: Start, Stop, Auto/Manual selector push buttons and lights.
  • Dedicated Keyboard Keys: Alarm acknowledgment, display page selection.
  • Screen Displays: Mimic diagrams, operation status, alarm indications, process variable values.

Interconnected Signals

The PLC implements sequential and simultaneous control algorithms, receiving and sending signals from the field, motor control center, and instruments.

Control Philosophy

Defines how the plant or process control system operates.

Automation and Monitoring Structure

Central control room for command and monitoring of plant equipment and process variables.

Levels of Operation

Operators in the control room have primary responsibility for control and supervision.