What is an Instrumentation Loop Diagram?

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Instrumentation diagrams are used for understanding the process system.

P&ID that is Piping and instrument diagrams or Process instrumentation diagrams show you each loop in the system, the instruments that comprise loop and identify all process variables. A P&ID gives you an overall picture of how the process functions but sometimes you need more specific information about how instruments and devices are connected for power or communication. Instrument Loop Diagrams are used for this purpose. Instruments loop diagrams will provide more specific information about the loop.

Think of the P&ID as a road map. It shows you all the process loop and how they function together just the way a map shows you all the main roads in an area. Using the P&ID you can trace the operation of a process from the beginning to end,the way you trace a route on a map. But if you needed to go to a specific street in a city on your map you would look for an insert to give you an enlarged view of the area and see the local street,a loop drawing is similar to that.

Loop diagrams can be used in conjunction with P and IIDs . Each loop diagram gives you a more detailed view of loop in a process system. The loop diagram shows you the pneumatic or electric signal path. It gives a better understanding of how the loop operate. A loop diagram has a logical method to help you read and interpret information. ISA standard discusses the preparation and use of diagrams for instrument loops.

A loop diagram shows you how the instruments are arranged in the loop.From the loop diagram you can identify the connections between devices and the communication path. This is helpful when installing,troubleshooting or maintaining a loop.

How to read an Instrument Loop Diagram ?

First look at P&ID to identify the loop. Check the title block on both the P&ID and loop diagram to be sure they match.

The loop diagram is divided into sections that organize information from the field to the control room.Looking from left to right from the loop diagram shown above these are

Field Process Area
Field Junction/ Cable Spreading
Panel Rear/ I/O Cabinet
Panel Front / Control Panel

Different facilities may use other title headings but however they are titled each section shows you specific information about a portion of the loop and the instruments which are part of it.

The field process area shows you the field mounted instruments.

Field Junction/ Cable spreading area shows you junction boxes.Junction boxes are the connection points for the signal lines. The junction box symbol and the information it provides isn’t usually seen on the P&ID

Panel Rear / I/O cabinet area will show you the location of connection behind the panel or I/O cabinet(PLC/DCS/Stand alone).

Board mounted instruments (Panel/DCS/PLC) like the indicators seen on the P&ID will be shown in Control Panel area on the loop diagram.

The signal line indicators you find on the P&ID are also shown on the loop diagram. The P&ID shows only the signal path. On the loop diagram you will also see the connections where energy is supplied the loop. A loop diagram provides the closeup or detailed view of a loop. Understanding its design can make many task easier and safer.

Instrument Loop Diagram Symbols and References

Most of the Symbols you will see on an instrument loop diagram are the same as you have seen on P&IDs. But as you look at the loop diagram you will also notice other symbols you haven’t seen used before. These symbols represent connection points and junction boxes.They give specific information about how the instruments in the loop are connected.