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Chemical Engineering has shared a new FAYF. This FAYF is related Choosing Control System.

Process engineer - Why you need to know Control System
Choosing a control system is not a process engineer task. It is typically Instrumentation and Control engineer task. However, understand what is control system, how many type of control system, how a control system works, differences between control system, etc may be important to a process engineer as process engineer define main control objective in Piping & Instrumentation Diagram (P&ID) to achieve the control target and preparation of System Control Narrative so that Control engineer can further develop the control logic.

Typical Control System
Two main type : Distributed Control System (DCS) and Programmable Logic Control System (PLC). A DCS is designed for process control with refinery control origins whilst a PLC is designed for machine or motion control with car factory relay panel origins. While PLCs are sometimes used for process-control applications, there are some trade-offs in terms of degree of programming, robustness and operational suitability.

Control System Characteristic
Distributed control system (DCS)
  • Continuous control
  • Parameters change frequently
  • Sequence capability in batch process
  • Operators intervention
  • Integration with other applications
  • Robust control
  • Multiple controllers interaction
  • High availability
  • Historical data collection
Programmable Logic Control System (PLC)
  • High speed processing
  • Discrete application (digital inputs/outputs)
  • Application predominantly in machine/motion control
  • Application do not change frequently
  • SCADA architectures with PLCs
  • Capability of operators intervention & decision
  • Ladder logic is effective way to handle logic
  • Separated application with small amount of I/O
Ref :
1. Gregg, J., Control System Selection, Chem. Eng., August 2002, pp. 62–66.
2. Bohan, J., Industry Solutions Manager, Honeywell Process Solutions (Phoenix, Ariz.), personal communication, Apr. 6, 2009.

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