P&ID

Reading and Interpreting of Piping and Instrumentation Diagrams

In this article, we will see how to read and interpret P&ID.

How to read the Piping and Instrumentation Diagram?

The key for understanding Piping and instrumentation diagrams (P&IDs) is

  1. We must be familiar with every ISA standard P&ID instrument symbol and standard letter code of process plant instruments and equipment.
  2. Try to read some (P&IDs) Piping and instrumentation diagrams it becomes easy to read, trace, visualize and understand P&ID drawings. 
  3. It is always preferred for project engineers to go through the P&ID symbols repeatedly.

Let us see about reading the P&ID

 Consider the tag number FT 304.

Reading and Interpreting of Piping and Instrumentation Diagrams 1
  • The first letter F in the above tag number FT 304 indicates the process parameter that is being monitored and controlled. The process parameters such as Flow, Level, Pressure, & Temperature.
  • The Second letter T in the above tag number FT 304 indicates the type of controlling device used to control a process such as a Transmitter, Indicator, Controller, and so on.
  • The number 304 in the above tag number FT 304 indicates a logical numerator.

What is a Piping and Instrumentation Diagram?

  • Piping and Instrumentation Diagram in short is abbreviated as P&ID.
  • P&ID is defined as the diagrammatic representation of a process plant.
  • It is the most commonly used document in the engineering field.
  • It is a fundamental engineering document that serves various purposes
  • This describes the piping and related components of a physical process flow along with machinery equipment and instrumentation.
  • This P&ID is similar to the process flow diagram.
  • It is also known as the Process Engineering Flow Scheme (PEFS).

What do the Tag numbers on a P&ID mean?

Reading and Interpreting of Piping and Instrumentation Diagrams 2
  • Tag numbers on a P&ID are a series of letters followed by numbers that are used to identify a device type and its function being used for a particular process or a control loop.
  • The device tag in P&ID is represented by a circle followed by tag numbers such as FV304.
  • The P&ID diagram tells us where the device is located exactly.
  • The presence or absence of a line inside a circle indicates the device’s location it may be in the field or the control room.
    1. No Line inside a circle indicates that the instrument is located near the operator in the process field.
    2. A solid Line inside a circle indicates that the instrument is located in a control room which can be easily accessible by the operator.
    3. The dotted line inside a circle indicates that the instrument cannot be accessed directly.
  • The connection of an instrument to the control system and identification of devices is represented by Symbols, circles, and lines
  • The piping and connection lines on P&ID also tell about the instrument.
Reading and Interpreting of Piping and Instrumentation Diagrams 3
  1. A Dotted line A indicates an electrical connection.
  2. A Solid line B indicates an interconnection via pipe work.
  3. The Solid line C with a cross mark indicates a pneumatic connection.

How to Read and Interpret the P&ID?

Reading and Interpreting of Piping and Instrumentation Diagrams 4
  • The flow transmitter FT 304 is a field-mounted flow transmitter connected to a flow indicator and controller through an electrical connection.
  • The flow indicator and controller FIC 304 is a control room-mounted instrument.
  • A square root extraction of the input signal is given to the flow indicator and controller FIC 304 as part of the functionality.
  • Here the flow is in proportion to the square root of DP that is measured by flow transmitter FT 304.
  • The square root must be extracted to make the flow and DP proportional to each other.
  • The flow indicator and controller FIC 304 produce an electrical output and are given to a field-mounted current to a pneumatic (I-P) converter.
  • The current to pneumatic (I-P) converter TY 304 produces a pneumatic signal (3-15PSI) as an output signal and is sent to the control valve connected to it. 
  • TT 304 is the temperature transmitter and TIC 304 is the temperature indicator and controller to measure and control the temperature.
  • TIC 304 is a temperature indicator and controller output is given to a data link or through internal software represented by lines with bubbles to the flow indicator and controller FIC 304as a set point.
  • A typical on and off valve YIC 304is controlled by a solenoid valve fixed with a limit switch ZSH & ZSL.
  • ZSH indicates the open position and ZSL indicates the closed position of the valve.
  • Each input and output signal are cabled to the Programming Logic controller to make the operator accessible, indicated as a diamond in a square with a continuous horizontal line.

How to understand the P&ID sheet?

Reading and Interpreting of Piping and Instrumentation Diagrams 5

The P&ID Sheet is divided into different areas.

1. Revision history:

  • The revision history table in the P&ID sheet contains information on the revision number, Revision Description, and details of authorities such as prepared checked, and approved.
  • The first sheet is named revision 0 followed by 1,2..3 …
  • This revision history table must be kept updated periodically during every change made in the process area.

2. Title block:

  • The title block is the first thing that must be noted before reading the P&ID sheet.
  • This title block indicates the section of the plant P&ID
  • This title block includes
  • Title of Project: The name of the new or existing project. 
  • Client name: Name of the client such as factory name
  • Consultant name: Name of project developer

3. Drawing number and revision:

  • This drawing number is the most important area of the P&ID sheet.
  • It is a numerical text that indicates the number of the document.
  • This drawing number helps us to track the P&ID sheet of the required part projects in the process industry.

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