How to read a P&ID?

In this article, we will learn how to read a Piping & Instrumentation Diagram which is something that engineers encounter on a daily basis.

In many of our RealPars articles, we discuss control system and instrumentation devices in great detail so you can understand exactly how they work. In this article, we will learn how to read P&ID drawings on a daily basis.

P&IDs, or Piping and Instrumentation Diagrams to give them their full name, are schematic representations of pipelines, equipment, instrumentation, and control systems found in process environments such as Oil Refineries, Chemical Plants, Paper Mills, and Cement Plants, etc.

P&ID Symbols and Codes 

The symbols contained in P&IDs represent the equipment in the process such as actuators, sensors, and controllers.

Process equipment such as valves, instruments, and pipelines are identified by codes and symbols.

As well as devices and pipelines, a P&ID will commonly contain information on vents, drains, and sampling lines as well as flow directions, control I/O and Interconnection References.

P&ID Code Format

The Instrumentation codes listed in P&IDs follow a standard format, after some practice in reading P&IDs you will know these codes by heart, but in the meantime, there are many resources on the web where you can download these tables for reference.

The first letter of the code identifies the parameters that are being controlled or monitored for example Flow, Temperature, Level or Pressure.

The next letter is used to define the type of control device being used, for example, Transmitter, Valve or Controller.

The number refers to the logical numerator.

For example, we may have a system with 4 temperature transmitters, it makes sense to identify these as  TT01,  TT02,  TT03, and  TT04.

Let’s consider for a moment an instrument on a P&ID named  FV01.

Looking up the code for this we could tell this is a Flow Valve numbered 01.

P&ID drawing symbols, circles, and lines are used to represent instruments and to show how they are connected to the rest of the system.

P&ID Instruments Location

Now that we know our device FV01 is represented by a circle, we can also tell from the P&ID where the instrument or device is located.

The presence or absence of a line in the circle determines the location of the physical device. Let’s take a look at how these are commonly represented.

NO LINE: The instrument is located in the field near the process & operator.

DOTTED LINE: The instrument is located in a control room (accessible to the operator).

SOLID LINE: The instrument is not directly accessible.

P&ID Piping and Connections

The piping or connection lines on the P&ID also tell us about the instrument, for example, a solid line would indicate the interconnection is via pipework whereas a dotted line would indicate an electrical connection.

It is worth familiarising yourself with the different types of connection symbols as this can give you an insight into the function of an instrument even before you know its code.


In this article, we have looked at the basic interpretation of a P&ID or Piping and Instrumentation Diagram.

We learned that each instrument involved in the process appears on the P&ID and is represented by a symbol and a code.

We also learned that using this code and symbol we can understand what type of device it is, its location and how it is connected to the rest of the process.

To read a P&ID effectively one must be familiar with standard instrument symbols and standard identification letter codes.

We hope you enjoyed this short article on reading P&IDs, here at RealPars our team of experts is on hand to answer your questions and respond to your feedback.

We’d love to hear your suggestions for topics you want our team to cover.

Want to Learn More?

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The RealPars Team
By Daniel Crimmins

By Daniel Crimmins

Automation Engineer

Posted on Jan 27, 2020

By Daniel Crimmins

Automation Engineer

Posted on Jan 27, 2020

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