A diagrammatic representation of a process. Also known as an engineering flow diagram, it features all process equipment and piping that is required for start-up and shutdown, emergency, and normal operation of the plant.

It also includes identification numbers, identifiers for the materials of construction, diameter and insulation requirements, the direction of flows, identification of the main process and start-up lines, all instrumentation, control, and interlock facilities, key dimensions and duties of all equipment, operating, and design pressure and temperatures for vessels, equipment elevations, set pressures for relief valves, and drainage requirements.