Instrument loop diagrams are also called instrument loop drawings or loop sheets. These sets of drawings are more detailed than process and instrument diagrams (P&IDs). Loop diagrams are the most detailed form of diagrams for a control system and thus it must contain all details omitted by PFDs and P&IDs alike. Loop drawings can be customized per customer taste although certain minimum standard information is required to be included in the loop sheet.

 

Typical information you would see on an instrument loop diagram or sheet are:
  • They show all the instruments in a control loop
  • Each instrument bubble in a loop diagram represent an individual device with its own terminals for connecting wires
  • Dashed lines in instrument drawings represent individual copper wires rather than whole cables
  • Terminal blocks where wires connect are represented by squares with numbers in them
  • Cable numbers, wire colours, junction block numbers, panel identification, and grounding points are all shown in loop diagrams
  • Every instrument in a loop drawing has an input calibration and an output calibration specified for the instrument
  • Loop drawings specify the action of an instrument. That is whether the instrument is reverse- acting or direct-acting. An arrow pointing up or down close to the instrument is used to denote either of direct-acting or reverse-acting. An up arrow represents a direct-acting instrument – one whose output signal increases as the input stimulus increases. A down arrow close to the instrument represents a reverse-acting instrument – one whose output signal decreases as the input stimulus increases.

 

How to read an Instrument Loop Diagram

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

example

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 supplying the loop. A loop diagram provides the closeup or detailed view of the loop. Understanding its design can make many tasks safer and more efficient.

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