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How to Read Piping and Instrumentation Diagram

Akiko
Akiko Originally published Dec 28, 22, updated Sep 30, 24
In this article
  1. What is the Piping and Instrumentation Diagram?
  2. How to Read Piping and Instrumentation Diagrams
    1. Know the Letter and Number Combinations of a P&ID
    2. Identify and Understand Standard Equipment -Instruments
    3. Learn to Use Graphical Elements and Connecting Lines
  3. Piping and Instrumentation Diagram Examples

What is a Piping and Instrumentation Diagram?

A piping and instrumentation diagram, also called P&ID, is a diagram used to show a graphical display of a complete system. It includes all piping, instruments, valves, and equipment the system consists of. Through a P&ID, you can get the following information:

  • The mechanical and electrical details of a given system or process,
  • Piping information such as sizes and material specifications,
  • Continuation flags leading to the connecting P&IDs,
  • Flow arrows, DCS alarm types
  • The connection types from the controllers to the field devices,

How to Read P&IDs?

Here are a few aspects you should consider while reading a piping and instrumentation diagram. You need to know what P&ID symbols mean and how each symbol is constructed using graphical elements and connecting lines. If you can do this, reading a piping and instrumentation diagram won't be difficult.

image / video 16:9

#1 Know the Letter and Number Combinations of a P&ID

Letter and number combinations appear inside each graphical element, and the ISA standard defines letter combinations. Numbers are user-assigned. The first letter represents the measured or initiating variables, such as Analysis (A), Flow (F), Temperature (T), etc., with succeeding letters defining readout, passive, or output functions, such as Indicator (I), Record (R), Transmit (T), and so forth.

factory P&ID Example

Tag numbers are a combination of alphabets and numbers with varying lengths that aid in identifying the device, what it controls, the type of device, and the number with which it is registered on the P&ID. Physical instruments are indicated by a tag number and a circle around them. These numbers are also used to locate the position of equipment.

The order and letters in tag numbers are highly specified and unique for each device. The first digit/letter specifies the factors controlled and monitored by the device, while the second digit indicates the type of device. The letters 3,4 and 5 elaborate on the device’s function and hint about the following few numbers in the series.

Standard Abbreviations Used in P&ID

Abbreviations are introduced in a P&ID to keep the diagrams concise and straightforward. The table below constitutes some commonly used abbreviations.

Common Abbreviations Used in P&ID
AC Analysis Controller PB Push-button
AE Analyzer Element P/I Pneumatic To Current Converter
AAH Analysis Alarm [High] POV Pneumatically Operated Valve
BE Burner Element pHT pH Transmitter
BDV Blowdown Valve PSE Rupture Disk
CR Conductivity Recorder PR Pressure Recorder
ES Electrical Switch PZV Pressure Relief Valve
FIT Flow Indicator Transmitter RC Ratio Controller
FY Computer, Relay, Amplifier or I/P Converter, etc. in Flow Loop RO Restriction Orifice
I/I Current Indicator RTD Resistance Temperature Detector
KI Clock/Timer SA Speed Alarm
LCV LCV Level Control Valve SD Shutdown Panel
LIT Level Indicator Transmitter SV Solenoid Valve
LO Lock Open TA Temperature Alarm
MCC Motor Control Center TCV Temperature Control Valve
MT Motion Transmitter TH Thermostat
MTU Master Terminal Unit US Utility Station
NC Normally Closed WE Weight Measuring Element
NO Normally Open WS Water Supply
PA Pressure Alarm ZV Safety Shut-down (Pilot) valve
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#2 Identify and Understand Standard Equipment -Instruments

Knowing how to read the symbols and shapes in the drawing is also essential. EdrawMax provides all kinds of symbols required in P&IDs.

Equipment Instrument

All the instruments in a P&ID are marked by a circle with an assigned tag number and line inside. The tag number helps in identifying the device, whereas the line is the location indicator. Lines are classified into different types, each describing the device's location with respect to the field. The types of lines used in a P&ID are given below.

Instruments Location

  • No line: A simple circle indicates that the device is in the field and is a locally mounted instrument. The device is observable in the field and is accessed by the operator.
  • Solid Line: A solid line in the center signifies that the instrument is placed in a primary location in the control room. The instrument is visible on the front of the panel or video display.
  • Dashed Line: The dashed line specifies that the instrument is in a secondary position in the control room and inaccessible to the operator. It is not visible on the front of the panel or video display.
  • Double Solid Line: A double solid line tells that it is in a primary position in the local control center and is operator accessible at the panel front or console.
  • Double Dashed Line: A double dashed line describes that the instrument is in a secondary position in the local control center and is not operator accessible. The device is located in the field cabinet and is not visible on the front of the panel or the video display.

Shared Display and Shared Control

Shared display means when the same information can be viewed and accessed from different locations in a network. Shared control indicates that the user can interact with the system and modify the operational conditions remotely. To represent an instrument with a shared control and display system, take the tag number and draw a square around it. This indicates that the instrument can now be accessed remotely in a Distributed Control System (DCS).

Shared Display and Shared Control

According to ISA S5.1, there are mainly 4 four graphical elements: discrete instruments, shared control/display, computer function, and programmable logic controller. Circular elements indicate discrete instruments. Shared control/display elements are circles surrounded by a square. Computer functions are indicated by a hexagon, and programmable logic controller (PLC) functions are shown as a triangle inside a square. These shapes represent both the physical and the software devices. In either case, it implies all the shapes needed in a piping and instrumentation diagram. You will see these shapes anytime a system is shown on the P&ID.

    • Discrete instruments
Discrete Instruments
    • Shared control/display
Shared control/display
    • Computer function
Computer function
    • Programmable logic controller
Computer function

#3 Learn to Use Graphical Elements and Connecting Lines

The connections between elements help engineers identify a particular pipe in a standardized way. Different colors indicate different pipes to avoid confusion. Adding a single horizontal bar across any of the four graphical elements suggests that the function resides in the primary location category. A double line indicates an auxiliary location; devices behind a panel board in some other inaccessible location are shown with a dashed horizontal line.

Piping and Connection Symbols

The connection lines on a P&ID elaborate on the associations between instruments: how they connect to each other and the type of signal being transmitted. For example, a solid line signifies the connection is via pipes, while a dashed line indicates electrical transmission. Knowing the symbols is essential to understanding the P&ID charts correctly. Here are the primary line types used to connect different devices/instruments.

Major Process Piping Major Process Piping
Minor Process Piping Minor Process Piping
Electrical Signal Electrical Signal
Mechanical Link Mechanical Link
Electromagnetic (Guided) Wave Electromagnetic (Guided) Wave
Electromagnetic (Unguided) Wave Electromagnetic (Unguided) Wave
Pneumatic Signal Pneumatic Signal
Software or Data Link Software or Data Link
Capillary Tubing Capillary Tubing
Hydraulic Tubing Hydraulic Tubing

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Piping and Instrumentation Diagram Examples

Here are some famous P&ID examples created by EdrawMax, which you can download and customize for free. To get more P&ID examples and templates, visit our Template Community.

    • Plant P&ID
Plant P&ID
    • Water Boiling Process P&ID
Water Boiling Process P&ID
    • System P&ID
System P&ID
    • Thermal System P&ID
Thermal System P&ID
    • Power Generation P&ID
Power Generation P&ID
    • Air Condition Process P&ID
Air Condition Process P&ID
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Akiko
Akiko Sep 30, 24
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