Understanding Loop Diagrams and Process Loop Sheets

From an installation and maintenance electrician’s point of view, two of the most useful types of drawings that can be included in a contract drawing set are loop diagrams and process loop sheets. These detail drawings show how each piece of equipment (e.g., instrument, motor starter, valve actuator, etc.) is to be wired to the control system. This article provides an overview of loop diagrams/process loop sheets and explains why creating and maintaining them is worth the effort.

From P&IDs to process loop sheets and diagrams

Piping and instrument diagrams (P&IDs) are developed at the beginning of a project to define the equipment, piping and automation and control components needed to implement a process, including the control loops. P&ID drawings provide an overall summary of the process. Together with facility layout drawings, the P&ID drawings provide an overall view of a facility, what it does and where the equipment is located. For large facilities, an additional set of drawings called process flow diagrams (PFDs) are sometimes created to summarize the P&IDs even further.

With the P&ID drawings in hand, the various design disciplines can create individual drawing sets to show how different aspects of the plant are to be built. These will include structural drawings, civil engineering drawings, architectural drawings, mechanical drawings, electrical drawings and so on. From an electrical and instrumentation/contro perspective, these drawing sets will also include electrical power distribution (lighting panels, MCCs) drawings, control panel drawings (e.g., PLC panels) and loop diagrams and/or process loop sheets.

The instrumentation/control designer determines whether to use loop diagrams or process loop sheets. They both provide similar guidance for installation and maintenance electricians but, depending on the project type, one may be a better fit than the other.

A process loop sheet (PLS) provides the details for loops by illustrating all the devices and equipment in an instrumentation loop, how the various pieces interact and how the process data is transmitted to the control room. It only provides summary information about the individual wiring details. Alternatively, this information can be arranged in a loop diagram (LD) format, where the same information is shown but in the form of more detailed wiring instructions on a wire-by-wire basis down to the terminal screw.

Depending on the contractor or maintenance team, either process loop sheets or loop diagrams may be more suitable. The chief takeaway is that it is better to work out the wiring details of instrumentation and control devices during design rather than during construction. It is much easier (and cheaper) to make design decisions than to have an electrician trying to sort out the design while the construction clock is ticking.

A water and wastewater example

For example, water and wastewater plants are typically designed to accommodate influent flow rates that are not consistent from one hour to the next. These flow rates are generally expressed as low flow rate, average daily flow rate, and maximum flow rate, and are indicated in the description blocks on the process loop sheet. In a typical water/ wastewater plant, there are generally pump stations, screenings and grit removal, clarifier filters, odor control, various types of chemical injections and storage tanks in the process loop, all of which may affect the flow rate. Flow, pressure and temperature measuring devices transmit data to local PLC/ DCS control panels that interface through software with operator workstations typically located in the central control room. The loop diagram shows the processes and equipment in the control loop, and the process loop sheet provides explanations and other essential information. Sometimes the loop diagram and process loop sheet are combined as a single drawing.

Throughout the life of the instrumentation system, including its construction, installation, commissioning and operations, process loop sheets are accessed by personnel in multiple disciplines in the enterprise, as well as vendors and contractors. It is a resource used to document and explain how the control system and its instrumentation, motor starters and valve actuators have been designed to operate and how they have been installed.

Due to the number of people using loop diagrams and process loop sheets, it is recommended to use a standardized symbol and nomenclature structure so everyone involved in the project can understand the components and the process. The best practice is to use the symbols and terminology developed by the International Society of Automation (ISA) and documented in ANSI/ISA-5.1-2022, Instrumentation Symbols and Identification. It is equally important to ensure that the information on the process loop sheets is complete. Also, as with any drawings, the sheet should be assigned a revision date and include the name of the designer who last updated and checked it.

Loop diagrams explained