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Instrumentation Documents - Process and Instrument Diagrams

The next level of detail is the Process and Instrument Diagram1, or P&ID. Here, we see a “zooming in” of scope from the whole evaporator process to the compressor as a unit. The evaporator and knockout vessels almost fade into the background, with their associated instruments absent from view:

Now we see there is more instrumentation associated with the compressor than just a flow transmitter. There is also a differential pressure transmitter (PDT), a flow indicating controller (FIC), and a “recycle” control valve that allows some of the vapor coming out of the compressor’s discharge line to go back around into the compressor’s suction line. Additionally, we have a pair of temperature transmitters that report suction and discharge line temperatures to an indicating recorder.

Some other noteworthy details emerge in the P&ID as well. We see that the flow transmitter, flow controller, pressure transmitter, and flow valve all bear a common number: 42. This common “loop number” indicates these four instruments are all part of the same control system. An instrument with any other loop number is part of a different control system, measuring and/or controlling some other function in the process. Examples of this include the two temperature transmitters and their respective recorders, bearing the loop numbers 41 and 43.

1Sometimes P&ID stands for Piping and Instrument Diagram. Either way, it means the same thing.

Please note the differences in the instrument “bubbles” as shown on this P&ID. Some of the bubbles are just open circles, where others have lines going through the middle. Each of these symbols has meaning according to the ISA (Instrumentation, Systems, and Automation society) standard:




The type of “bubble” used for each instrument tells us something about its location. This, obviously, is quite important when working in a facility with many thousands of instruments scattered over acres of facility area, structures, and buildings.

The rectangular box enclosing both temperature recorders shows they are part of the same physical instrument. In other words, this indicates there is really only one temperature recorder instrument, and that it plots both suction and discharge temperatures (most likely on the same trend graph). This suggests that each bubble may not necessarily represent a discrete, physical instrument, but rather an instrument function that may reside in a multi-function device.

Details we do not see on this P&ID include cable types, wire numbers, terminal blocks, junction boxes, instrument calibration ranges, failure modes, power sources, and the like. To examine this level of detail, we must go to the loop diagram we are interested in.


• Click here for the front page of this Instrumentation Documents article

Click here for Process Flow Diagrams (PFDs)

• Click here for Loop diagrams (“loop sheets”)

Click here for SAMA diagrams

• Click here for Instrument and Process Equipment Symbols

• Click here for Instrumentation Identification Tags


Go Back to Lessons in Instrumentation Table of Contents

Comments (2)Add Comment
written by Enrique J. Rodriguez, February 12, 2013
I am teaching for Houston Community College in Process Tech and Instrumentation. ANy material that you can provide to me, would be greatly appreciated.
Thank you,
Enrique J. Rodriguez
written by Enrique J. Rodriguez, February 12, 2013
I am teaching for Houston Community COllege in Process Tech and Instrumentation. Any material you can send to me is greatly appreciated.
Thank you,
Enrique J. ROdriguez

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