P&ID symbols

About p&ID symbols

A P&ID, a piping and instrumentation diagram, is the drawing of a process plant’s piping, equipment and instruments and how they connect. The US Department of Energy’s handbook on engineering prints says P&IDs present functional information about a system: the piping layout, flow paths, pumps, valves, instruments, signal modifiers and controllers. Tony Kuphaldt’s textbook Lessons In Industrial Instrumentation describes a P&ID as the process vessels, pipes and machinery with the instruments drawn over them, showing what gets measured and what gets controlled. Each symbol below has its own page.

The letters are spelled out in more than one way. The DOE’s handbook says piping and instrument drawings, and course notes by Peter Woolf and others say piping and instrumentation diagrams. Kuphaldt writes process and instrument diagram, and adds that P&ID sometimes stands for piping and instrument diagram, which means the same thing.

What a P&ID shows, and what it leaves out

A P&ID is not drawn to scale. The DOE’s handbook says it shows only the relationship or sequence between components, so two pieces of equipment drawn side by side may not even be in the same building. Woolf and others add that it doesn’t show the elevation of equipment, relative sizes, where valves sit on the equipment or how close the pieces are. A course text by Jonathan Verret and others leaves out operating conditions, stream flows, equipment locations, supports and pipe routing as well. Kuphaldt notes that a full P&ID typically runs over several sheets, each showing one section of the process.

Valves are drawn in their normal position unless the diagram says otherwise. The DOE’s handbook reads that as the system’s normal flow path, with safety systems shown in their standby condition.

P&ID or PFD

A process flow diagram, or PFD, is the simpler drawing of the same process. Kuphaldt describes it as the big picture: vessels, pipes and the flow of the process, with individual instruments sparsely shown. The P&ID adds the instruments and what they control, and a loop diagram goes further still, down to the wire and terminal numbers of each instrument loop.

Woolf and others put the difference in terms of content. A P&ID shows all the controllers, the valve types and the materials of construction, along with minor piping a PFD leaves out. A PFD carries tabulated design values, such as normal, minimum and maximum operating conditions, that a P&ID does not. They add that the dividing line varies between institutions, and Verret and others describe the P&ID as based on the PFD with more specific information.

How to read the symbols

Most valves are variations on two triangles meeting point to point across the line. A filled dot where they meet is a globe valve and a hollow circle a ball valve; in the DOE’s chart the plain pair is a gate valve, though the handbook warns that globe and gate valves are often drawn the same. A valve with an actuator on its stem is a control valve. Equipment is drawn as simple outlines: a circle with a tangent outlet for a centrifugal pump, a circle crossed by a zigzag for a heat exchanger, a rectangle with rounded ends for a Vessel or tank.

Instruments are circles, called bubbles or balloons, with a tag inside. The tag’s first letter is the variable the loop measures or controls, the letters after it are the instrument’s functions, and the number is its loop, so FIC-42 is a flow indicating controller in loop 42. Lines across the bubble say where the instrument is: none for a field-mounted instrument, one for the main control room panel, and a square around the circle for a shared display. The thin lines between instruments are signals: dashed for an electric signal, crossed by pairs of slashes for a pneumatic signal.

Who sets the symbols

The instrument bubbles and tag letters come from the International Society of Automation’s standard ISA 5.1. The edition the ISA lists today is ANSI/ISA-5.1-2024, Instrumentation and Control – Symbols and Identification, and the ISA committee behind it is also working on adopting ISO 10628-2, the international standard for graphical symbols on chemical and petrochemical diagrams. For valves and equipment, the DOE’s handbook presents its symbols as the ones most commonly used rather than a complete set, and refers readers to the drafting standards of their own facility. Woolf and others note that most companies keep their own standards for these drawings, so a diagram’s legend sheet is the last word on what its symbols mean.

Where P&IDs are required

In the United States, OSHA’s process safety management standard for highly hazardous chemicals, 29 CFR 1910.119, lists piping and instrument diagrams among the information an employer compiles about the equipment in a process, with materials of construction and relief system design. Among the information about the process technology, it lists a block flow diagram or simplified process flow diagram.