Understanding Loop Diagrams and Process Loop Sheets

The example process loop sheet in Figure 4 shows that the influent flow meter sends a 4–20 mA signal via analog output to all devices throughout the diagram. In addition to providing a flow rate signal to the control system, the meter also records the average daily flow and keeps a 24-hour record, a process known as totalizing (denoted by the “Q” in the ISA-style identifier tag). The A and B labels on the FIRQ symbols indicate that the same information is transmitted to both devices, meaning that there are devices that show the totalized flow in both the control room and the administration center.

The line types used on a process loop sheet are the same as those used on the process part of a loop diagram. That is, dashed lines for electrical signals and solid lines for piping. Figure 5 provides an overview of the line types commonly used to process loop sheets. The intent of using process loop sheets is to provide enough detailed information so that an installation electrician can know how to wire a piece of equipment and how it is used.

Further examples

Both loop diagrams and process loop sheets can range from simple to complex depending on the equipment (instrumentation, devices, control elements, etc.) they describe. The example of a complex loop diagram in Figure 6 is from ISA-5.4. It shows an orifice-plate-based flow transmitter (which uses a differential pressure transmitter to read the flow based on the pressure drop across an orifice plate) and a flow control valve on a pumped fill line to a tank.

The field I/O wiring passes through a junction box in a cable spreading room; there is another set of junction terminals at the rear of a panel; then, there is a flow controller/PLC shown in a front-facing panel. Though showing older technology, the diagram in Figure 6 shows the amount of wiring/installation details that can be included on a loop diagram.

Summary

Loop diagrams and process loop sheets are two types of detail drawings that, though they take time to prepare, can result in considerable savings in the construction, installation and commissioning phases of a project. These savings result from the installation electrician having clear instructions on how to wire and test the system instead of doing the detailed wiring design on-the-fly in the field as part of the construction process.

The production of loop diagrams and process flow sheets also makes it possible for many design issues to be seen and rectified during the design phase rather than during construction. Though there is a cost to prepare these drawings, the benefits of having them greatly outweigh the preparation cost. Hence, many instrumentation professionals refer to loop diagrams and process loop sheets as the “instrumentation engineer’s goldmine”—they are worth their weight in gold when undertaking a project.