How to Interpret DCS and PLC Symbols on a P&ID

Summary

This video explains how to interpret PLC and DCS symbols on P&ID diagrams. It covers the evolution of P&ID symbols due to the advent of computerized control systems, highlighting the differences between standalone instruments and shared control systems like PLCs and DCSs. The video provides a step-by-step look at symbols used to represent these systems, helping engineers and technicians understand the interaction between field instruments and controllers. It also introduces viewers to RealPars courses on PLC programming and industrial automation, offering further learning opportunities.

Takeaways

• 😀 A P&ID (Piping and Instrumentation Diagram) is a crucial tool for Instrumentation & Control Engineers, technicians, and maintenance personnel to understand plant control systems.
• 😀 P&ID symbols were created before PLCs (Programmable Logic Controllers) and DCS (Distributed Control Systems) existed, with control systems initially being standalone and physical.
• 😀 Modern industrial processes use PLCs and DCS to control systems through computerized virtual controllers, replacing physical control panels.
• 😀 New P&ID symbols were developed to represent PLC and DCS systems, reflecting the shift from physical to digital control systems.
• 😀 A P&ID symbol for a standalone instrument is represented by a circle with a tag number, while adding a box around the circle indicates that the instrument is part of a DCS.
• 😀 PLC and DCS symbols help connect field instruments to the control system and display the status of devices on the Operator HMI (Human-Machine Interface).
• 😀 A flow control loop, such as the one involving Flow Transmitter 'FT501' and Flow Indicating Controller 'FIC501', uses electrical signals to control processes and is visible on the HMI screen for operators to adjust and monitor in real-time.
• 😀 The PLC symbol in P&IDs is typically a diamond inside a box, representing a software instruction that performs a function in the plant, such as controlling a valve with emergency shutdown software.
• 😀 A square box with a circle in it on a P&ID indicates a DCS-controlled process, while a square with a diamond inside it shows a PLC-controlled process.
• 😀 RealPars offers a free online course on PLC Hardware, providing easy-to-follow lessons that are high-quality, with ongoing access to an exclusive library of industrial automation content for members.

Q & A

What is the main purpose of a P&ID in an industrial plant?

A P&ID (Piping and Instrumentation Diagram) serves as a roadmap, providing details about the instruments and devices in a plant, their locations, and how they interact with other systems.

How were process control loops handled before PLCs and DCS systems?

Before PLCs and DCS systems, process control loops were standalone, meaning each process had its own individual controller, with large control rooms filled with physical devices like controllers and strip chart recorders.

What is the role of PLCs and DCS in modern industrial processes?

PLCs and DCSs are used to control modern industrial processes through computerized systems, replacing the need for manual controllers and enabling more efficient and integrated control systems.

How are stand-alone instruments represented on a P&ID?

Stand-alone instruments are represented by a circle with a tag number on a P&ID, and a horizontal bar across the middle of the circle indicates that the instrument is physically located on the main control panel.

What does it mean when a circle symbol is surrounded by a box on a P&ID?

When a circle symbol is surrounded by a box on a P&ID, it indicates that the instrument is part of a shared display and control in a DCS, rather than being a physical, stand-alone instrument.

How are PLC and DCS symbols different on a P&ID?

DCS symbols are typically a circle with a box around it, indicating shared control in a DCS system, while PLC symbols are drawn as a diamond inside a box, representing software instructions for a specific function.

What does the DCS symbol on a P&ID represent for field instruments like 'LT501' and 'FT501'?

The DCS symbol for field instruments like 'LT501' and 'FT501' indicates that these instruments send electrical signals to the DCS controllers, which in turn control the processes and display real-time data on the HMI.

What is the significance of the HMI in modern control systems?

The HMI (Human-Machine Interface) allows operators to interact with the DCS or PLC systems, adjust control functions, and monitor real-time inputs and outputs from various devices in the plant.

What does a PLC symbol on a P&ID indicate about the 'SDV503' valve?

The PLC symbol on a P&ID for the 'SDV503' valve indicates that the valve is operated by an 'Emergency Shutdown' software instruction, written in the PLC program.

How does the computerization of process control systems impact P&ID symbols?

With the computerization of process control systems, new P&ID symbols were developed to represent the integration of DCS and PLC systems, replacing the traditional stand-alone instruments with virtual controllers and software instructions.