Purpose of Loop Diagrams

The purpose of loop diagrams are to represent associated electrical and piping connections of a instrument loop.

Instrumentation Loop diagrams serve many purposes. Several of these stated below are in the chronology of project development.

Loop Diagrams

Loop Diagram


The loop diagrams are helpful during the design stage as listed below.

1) Illustrate control philosophy and confirm the completeness of submitted data

2) An extension of P&IDs, which show the components and accessories of the instrument loop, connections between devices, and identification of component action

3) The specification of instrument hardware items and a means of communicating requirements to vendors.


The loop diagrams are helpful during the plant construction as listed below.

1) Panel instrumentation interconnections and checkout diagram

2) Instrumentation installation references and special requirements

3) Instrumentation interconnections

4) Instrumentation loop checkout

5) Inspection and documentation

Start Up

The loop diagrams are helpful during the start-up as listed below.

1) Pre-start-up commissioning and calibration

2) Training tool and aid


The loop diagrams are helpful during the operation as listed below.

1) Communication medium between operations, maintenance, and engineering personnel

2) Training device for operations


The loop diagrams are helpful during the maintenance as listed below.

1) Troubleshooting

2) Routine calibration

3) Preventative and corrective maintenance tool


The loop diagrams are helpful during the modification as listed below.

1) Rearrangement

2) Reconstruction

3) Enhancement



The instrument loop diagram is a composite representation of instrument loop information. It contains all associated electrical and piping connections and should contain all of the information needed to accommodate the intended uses.

Classified below are minimum requirements and some established options that can be used to match the desired uses.

Minimum Requirements

1) Identification of the loop and loop components shown on the P&IDs.

2) Word description of loop functions within the title. If not adequate, use a supplemental note. Identify any special features or functions of shutdown and safety circuits.

3) Indication of the interrelation to other instrumentation loops, including overrides, interlocks, cascaded set points, shutdowns and safety circuits.

4) All point-to-point interconnections with identifying numbers or colors of electrical cables, conductors, pneumatic multi-tubes, and individual pneumatic and hydraulic tubing. This identification of interconnections includes junction boxes, terminals, bulkheads, ports, and grounding connections.

5) General location of devices such as field, panel, auxiliary equipment, rack, termination cabinet, cable spreading room, I/O cabinet, etc.

6) Energy sources of devices, such as electrical power, air supply, and hydraulic fluid supply. Identify voltage, pressure, and other applicable requirements. For electrical sources, identify circuit or disconnect numbers.

7) Process lines and equipment sufficient to describe the process side of the loop and provide clarity of control action. Include what is being measured and what is being controlled.

8) Actions or fail-safe positions (electronic, pneumatic, or both) of control devices such as controllers, switches, control valves, solenoid valves, and transmitters (if reverse-acting).

Additional Information

Additional information needs to be considered for its effectiveness in accommodating the intended uses.

Stated below are typical examples of items for inclusion at the user’s discretion.

1) Process equipment, lines, and their identification numbers, source, designation, or flow direction.

2) Reference to supplementary records and drawings, such as installation details, P&IDs, location drawings, wiring diagrams or drawings, and instrument specifications.

3) Specific location of each device, such as elevation, area, panel subdivision, rack or cabinet number and location, I/O location, etc.

4) Cross reference between loops that share a common discrete component, such as multi-pen recorders, dual indicators, etc.

5) References to equipment descriptions, manufacturers, model numbers, hardware types, specifications or data sheets, purchase order numbers, etc.

6) Signal ranges and calibration information, including setpoint values for switches, and alarm and shutdown devices.

7) Software reference numbers, such as I/O addresses, control block types and names, network interfaces, point names, etc.

8) Engraving or legend information that helps identify the instrument or accessory.

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1 thought on “Purpose of Loop Diagrams”

  1. A loop diagram for an instrumentation technician or engineer is very useful and important as a maintenance tool. It can be used to follow the connection line of an instrument in the field, hence easily locate the error or the malfunction.

    You will know through which cabinet and JBs the instrument is connected prior to enter the PLC or the DCS marshalling panel. Without the loop diagram it will be very difficult to have complete knowledge about the complete connection drawing.


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Purpose of Loop Diagrams

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