What is a Legend?

Welcome to another informative blog post from RealPars. Today you will learn all about a Legend. I referenced a legend in the P&ID video. Hopefully this will help clear that up a little bit.

A legend simply put is a chart with all of the symbols used in an individual diagram. It could be a mechanical, P&ID, electrical symbol legend, or other types of diagram. This is an invaluable tool because it simply is not possible to remember every single symbol used in our diagrams. Let’s look at some different types of legend diagrams below.

There are hundreds if not thousands of components that can be used in an industrial setting. A legend chart gives each of those components a standardized symbol and labels that symbol.

Almost everyone has had to look in the owner’s manual of their car to see what the different warning lights and gauges mean. The legend for a diagram works the exact same way. It gives you a word and meaning for each of those symbols.

You probably already know what the part or component is, but you might not have known the schematic symbol for it.

Let’s look back at the P&ID diagram from the previous blog post. Now, starting with the compressor on the left, use this legend to identify the different symbols. First, you will see that this is a rotary compressor.

If you look at the legend, you will see that the lines represent pipes.

Follow the pipes to the next component and find it on the legend. Did you find the Pressure Transmitter?

Let’s keep going, the pipes split off after this component. Look at the symbol on the top line. Did you find it on the legend? It is a gate valve.

Do you see another gate valve on the diagram? Yes, it is directly below that gate valve.

The symbol left of the gate valve appears to be a valve but can you find the type on your legend. Good, it is a control valve.

Now look at the large cylinder. Find it on the legend. You will find that it is a tank. Following the tank is one more gate valve.

There is one more component here that I would like you to find. What you will find is a pressure indicator. Both lines will now flow to process.

Many diagrams will be very large with several symbols on it. The example P&ID is a very small and simple diagram.

A well thought out and drawn diagram will have a legend with each symbol used in that diagram attached with the documentation.

Take a look at the legend on the image below. You will see several symbols. Underneath the symbol is the label of what this symbol represents physically in the workplace.

Without a legend, it could be hard for you to figure out exactly what you are looking for.

This is important for maintenance workers repairing a system, operators to know which valve to turn, or engineers designing a system. If they didn’t know what all of these symbols were, it would make their job extremely hard.

Today you have learned that a legend is a chart of symbols. You also learned how to take the symbols from a diagram and find them on a legend. Those symbols are the schematic symbols that you can make or read P&ID, electrical, mechanical, or other diagrams with.

The symbols are labeled so that you know what you are actually looking at.

It is hard to draw or read a detailed diagram of any kind without a legend to accompany the diagram.

I hope this blog post has been helpful in clearing up how a Legend can assist you in reading or writing diagrams.

Thank you so much for reading, watching and adding your perspective to the conversation!

With so much love,

The RealPars Team

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