How to Read P&IDs Correctly

Part 1

In this article, you’ll learn what Piping and Instrumentation Diagrams are and how to read them correctly.

After exploring lots of videos and articles about P&IDs we concluded that reading the valve symbols, lines, codes, etc. one by one and memorizing them, is the most tedious and wrong method of learning how to read P&IDs!

So in this article, we’ll begin a new series about how to design and read piping and instrumentation diagrams correctly.

Before we dig into the subject, let’s clarify some common mistakes and ambiguities, mostly among students or junior engineers.

Common Mistakes about P&IDs

1. P&ID versus PID

PID and P&ID are totally different things! PID stands for Proportional, Integral, and Derivative which are parameters used in algorithms for controlling the process loops.


2. What P&ID Stands for

You might hear different extended formats of P&ID like the followings, and all of them represent the same document.

— Piping and Instrumentation Diagram

— Process and Instrument Diagram

— Piping and Instrument Drawing

What P&ID Stands for

3. Number of Pages in a P&ID Document

Similar to other documents like Electrical Wiring Diagrams, P&IDs are typically more than a single page.

Four, five, ten, or more pages depending on the process size, and you only see a specific part of the process on each page. These sheets are usually interconnected with each other.

Number of Pages in P&ID

4. Relevant Documents to Piping and Instrumentation Drawings

There are relevant or maybe close documents to P&IDs that have their own usage and you should not confuse them with each other.

Relevant Documents to P&ID

For instance, a Piping Isometric Drawing.

It is a sort of shop drawing that is mostly used by piping specialists, and in addition to P&ID information, they include lots of data about fabricating pipelines such as type of fittings, flanges, direction and elevation of the pipes and instruments, and other characteristics.

Piping Isometric Drawing

Where to Start Reading a P&ID

Another fundamental, yet related document that P&IDs are actually designed based on that, is PFD or Process Flow Diagram.

In a Process Function Description you mostly see the major equipment plus some pieces of piping.

Process Flow Diagram

In a Piping and Instrumentation Diagram we have quite a lot more information, in comparison with the Process Flow Diagram, don’t we?

Piping and Instrumentation Diagram

You can think of a PFD as a preliminary version of a Piping and Instrumentation Diagram.

So, to avoid confusion, a good starting point to read a new P&ID is the Process Flow Diagram, if available.

What is P&ID and PFD

Piping & Instrumentation Diagram Example

Now, let’s explore some P&ID examples.

I have elected a part of a P&ID from a chemical process and manipulated it to essentially emphasize the automation and instrumentation aspects of P&IDs.


So, you can consider it more of an Instrument Diagram than a Piping and Instrumentation Diagram!

Instrument Diagram

1. Which Documents We Need to Read a P&ID

To investigate a P&ID, first, you should know that every company has its own way of illustrating and naming the symbols.

They show their symbology methods in detail, in some lead sheets called, ‘Legend and Abbreviation’ and usually it’s merely called Legend.

Legend in P&ID

Inside a legend, you’ll see a lot of symbols for:

– Primary equipment

– Instrumentation

– Numbering and identification methods

– Piping arrangement

– Typical details for different instruments

and a lot more.

Legend and Abbreviation

Although the fundamentals of these data are mainly derived from a standard document, known as ISA5.1, and its latest revision was released in 2009.

ISA5.1 is a standard document from the American National Standard Institution for Instrumentation Symbols and Identification.

You can download a pdf format of ISA5.1 from here.

Instrumentation Symbols and Identification

So, to interpret a piping and instrumentation diagram we have two helpful documents!

1. P&ID’s Legend

2. The ISA5.1 Standard

What is P&ID

All right! Let’s get back to the example.

2. P&ID Step by Step!

The material comes inside the Ball Mill using a screw conveyor and gets out of that in the form of powder. It has a very powerful motor and gearbox to rotate it.

Piping and Instrumentation Diagram Example

There are some utility units like gearbox lubrication and grease unit as well.

How to Read P&ID Tutorial

In large-scale projects, normally, utilities have separate sheets of P&ID and we will refer to them using some arrows.

There are the name and the drawing number of that P&ID inside the arrow.

How to Read P&ID

On page 36 of ISA5.1, we have 20 different types of graphical symbols for illustrating the instrumentation on the P&IDs and I will explain them one by one in this part and next parts of this article series.

P&ID Symbols

The first and most used instrument symbol is the simple circle or bubble.

It is used for physical devices that are installed locally in the field and directly on the main equipment like the variety of sensors or transmitters.

Instrument Symbols

For instance, we have two vibration sensors on bearing number 1 of the Ball Mill.

To identify that it is a vibration sensor I put the letters VE inside the circle.

P&ID Example

These letters are corresponding to page 30 of the standard.

The first letter is ‘V’ which means ‘Vibration’ and the succeeding letter is ‘E’ which means ‘Element’ or ‘Sensor’.

ISA Standard for P&ID

We have other sensors mounted directly on different parts of the Ball Mill.

Generally, these symbols represent a physical instrument or device – and not a computer-based or HMI object.

What are P&ID Symbols

Among the P&ID symbols we have five instrument symbols for local mounted devices.

If the physical instrument is visible and accessible to us, we use solid lines inside the circles and if they are not accessible, we will use dashed lines.

Physical Instrumentation Symbols

You might ask how a local instrument could be invisible to us.

Let me explain with an example.

You might already know that almost all electric motors have temperature sensors on their windings. As in a normal situation, these temperature sensors are inaccessible and invisible to us, we have to consider a dashed line in the middle of the circles.

Piping and Instrumentation Diagram symbols

You might argue that these sensors are not mounted on or inside a ‘panel’, so why would we consider a circle for them?

I would say that sometimes it depends on the designer how to interpret this symbology and if they deviate from the standard, they usually mention that in the legend, or they might write a note just beside the page and explain that.

Discrete Instrumentation Symbol in P&ID

According to page 43 of the ISA standard, we can also specify the type of sensor adjacent to its symbol.

Measurement Symbols in P&IDs

Of course, this note refers us to table 5.2.2.

If we look at that table, under the temperature section, we can see the abbreviation for different sorts of temperature elements, including RTD for Resistance Temperature Detector.

Measurement Symbols Notation in Piping and Instrumentation Diagrams

So we can put an RTD beside the temperature element symbols, inside the P&ID.

Temperature Sensor P&ID Symbol

These RTD sensors could be directly connected to the PLC cards, but in this case, they have been connected to some transducers within a local box that stuck to the motor housing.

How to Read P&ID pdf

So, in the P&ID, for illustrating the Temperature Transducers that are located within a local box, we can use a circle with two parallel dashed lines.

Piping and Instrumentation Diagram (P&ID)

About connecting the P&ID objects to each other, there is a specific tabular guide on page 46 of the standard.

Line Symbols in P&IDs

As shown on page 46 of ISA5.1, we should use a dashed line for electronic signals or electric connections.

P&ID Diagram

Thus, I connect all the sensors and their transducers, or let’s say transmitters, using a dashed line.

 I will complete this P&ID in the next video with lots of crucial key points.

Until then please download the ISA5.1 using the description links and have a look at that.

As always, we hope you enjoyed this article. Please spread the word by sharing this article with your friends and colleagues.

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  1. Bonnie

    I am so impressed by the information, drawings, diagrams, instruments etc as I can directly relate to where I am working. I work for a mine and we have a mill and the setup is exactly as the examples shown above. This is good for the people who are working as we will learn with real scenarios.

    Keep the great work and continue sharing and educating us.


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