Written by Steve Hudgik
If you have a significant number of pipes in your facility, there will probably be piping and instrument diagrams (P&ID). P&IDs show the actual sequences of the valves, reducers, branches, instrumentation, and equipment in a piping system. As defined by the Institute of Instrumentation and Control a P&ID is:
To install or maintain a piping system you'll need to be able to read a P&ID. To do this you'll need to understand piping symbols.
P&IDs do not show the physical locations of the piping system or its components, it shows their sequence. Pipes are shown using a simple line. The devices and equipment installed in the piping system are shown using piping symbols.
There are more than 200 piping symbols that are used on P&IDs. That's many more than we can cover here individually, but we'll cover some of the basics.
Each piping symbol will show the method used to make the joints. The illustration below shows the five piping symbols used to show a 90 degree bend in the pipe. The pipe is represented by the line with a 90 degree bend in it. The symbols on the line indicate the method used to make the joint between the elbow (the bend) and the straight sections of pipe.
From left to right the piping symbols identify the following types of joints.
The piping symbols that identify the type of joint are the same without regard to what the piping system or device it is showing. See if you can identify the type of joint in the following piping symbols:
The “bow tie” symbol is the piping symbol for a gate valve. In symbol #1 we have two vertical lines, one of which is part of the “bow tie” symbol, on each end of the valve. This tells us that the valve is installed using a flange.
The #2 piping symbol is similar, but the vertical lines are missing. This piping symbol is showing a screwed connection. In many cases the symbol looks like the actual device. The screwed connection has the flat surface (vertical line) as a part of the valve symbol. The pipe screws into the valve, looking somewhat like the symbol.
The #3 piping symbol shows a valve again, but with an “X” connecting the valve to the pipe. This shows a welded connection.
The next piping symbol shows a check valve. The symbol for the check valve is similar to that of a gate valve, except that one of the lines that made it look like a “bow tie” is missing. The circles on either end of the check valve tell us that this valve is soldered into place.
The final symbol is for a cock valve, and the curved lines show that this value is installed using a bell and spigot joint.
This next illustration has five piping symbols that show changes to the direction of flow.
The first (#1) shows a cap, which stops the flow through the pipe. The cap is joined to the pipe using a bell and spigot connection, indicated by the curved line.
The second piping symbol shows a flanged 45 degree bend.
Piping symbol #3 is a welded cross. A cross is also called a four-way fitting. Flow may enter the cross through one connection and leave through any or all of three connections.
The next piping symbol appears to be two circles joined by a line. The smaller circle on the right indicates a soldered joint. The larger circle identifies an elbow (90 degree turn) that is pointing up. The black dot in the circle is what identifies this pipe as turning upward. If there was no black dot in the larger circle, then it represents an elbow that is pointing down.
The piping symbols for valves show the type of valve, how the valve is actuated, and how it is controlled. For example, you saw the “bow tie” symbol for a gate valve in the previous examples. The dashed line going to a box drawn in a dashed line indicates this is a float valve. Here are more valve symbols:
In the above illustration the first piping symbol represents a flanged gate valve.
The dot in the middle of the #2 piping symbol designates this as a globe valve. The left and right ends are curved to show this valve uses a bell and spigot joint.
Piping symbol #3 is a welded safety valve.
The fourth symbol represents a diaphragm valve that has a screw joint.
The last piping symbol in this illustration is a welded globe valve (indicated by the dot) that is motor operated.
There are many other types of piping symbols. However, this introduction should give you a good start.
It is good to know what the various piping symbols mean, but how can you positively identify a pipe and its components when you are physically looking at them?
ANSI A13.1 requires that all piping systems be marked with labels. This includes marking all valves. With a properly marked piping system, you should be able to identify any pipe you can see from any accessible point in the facility.
Piping & instrumentation diagram. (n.d.). Retrieved October 7, 2013, from Wikipedia.org: http://en.wikipedia.org/wiki/Piping_and_instrumentation_diagram