Understanding Loop Diagrams and Process Loop Sheets

Put succinctly, a loop diagram is a type of detail drawing that shows—on an instrumentby-instrument and device-by-device basis—how input/output (I/O) signals from equipment are to be wired to the control system. Loop diagrams show the specific wiring details down to the level of terminal screws, junction boxes, cable labels and wire labels.

Most loop drawings (see Figure 1) are produced using a computer-aided design (CAD) package or auto-generated by an advanced plant design software package. A loop diagram is usually focused on a specific instrument or device but may include multiple sensors/actuators if they are related to each other process-wise. The use of “typicals” (a generic diagram that applies to multiple instruments/devices) is not recommended, as it defeats the purpose of the loop diagram containing instrument/device-specific I/O addresses and wiring details. Loop diagrams are generally the same size as other drawings in the drawing package for the plant. For example, the use of B-size (11” × 17”), D-size (22” × 34”), or E-size (34” × 44”) drawings sizes are relatively common in North America.