What Is a P&ID?

A P&ID (Process and Instrumentation Diagram, also called Piping and Instrumentation Diagram) is a detailed engineering drawing that shows the piping, equipment, instrumentation, and control systems of a process plant. It is the single most referenced document during EPC project engineering, procurement, construction, and operations.

P&IDs are governed by standards including ANSI/ISA-5.1 (instrumentation symbols), ISO 14617 (graphical symbols), and PIP PIC001 (piping and instrumentation diagram documentation criteria).

What a P&ID Contains

Element	Information Shown
Process equipment	Vessels, heat exchangers, pumps, compressors, tanks—with tag numbers
Piping	Line number, size, pipe class, insulation type, flow direction
Valves	Type (gate, globe, ball, check), tag number, fail position
Instruments	Transmitters, indicators, controllers—with ISA tag numbers (e.g., FT-101, PIC-205)
Control loops	Setpoints, control actions, interlocks, emergency shutdowns
Special items	Inline strainers, steam traps, orifice plates, spectacle blinds
Line designations	Size-fluid code-line number-pipe class (e.g., 6”-HC-1001-A1A)

What a P&ID Does Not Show

P&IDs are schematic—they do not represent physical layout, spatial routing, or scale. They exclude:

• Pipe routing and exact lengths
• Equipment elevations or physical location
• Support and structural steel details
• Weld details or fabrication information

Physical routing is shown on piping isometrics and plant layout drawings. The P&ID provides the logical process design that these downstream documents implement.

Where P&IDs Fit in the Document Hierarchy

P&IDs sit between the high-level process flow diagrams (PFDs) and the detailed construction drawings:

Document	Level of Detail	Purpose
Block Flow Diagram (BFD)	Highest level	Overall plant concept
Process Flow Diagram (PFD)	Process-level	Material and energy balances, major equipment
P&ID	Detailed	All piping, instrumentation, and control logic
Piping isometric	Construction-level	Fabrication dimensions, weld locations
Piping GA / layout	Construction-level	Physical routing in 3D space

How P&IDs Are Used Across Disciplines

Every engineering discipline reads P&IDs, but for different purposes:

Discipline	Uses P&ID For
Process engineering	Verifying design intent, control philosophy, operating conditions
Piping engineering	Line sizes, pipe classes, valve types, special items
Instrumentation	Instrument locations, control loops, ISA tag assignments
Procurement	Material take-off (MTO), valve and instrument counts
Construction	Identifying tie-in points, isolation boundaries
Operations	Operating procedures, troubleshooting, isolation plans

Pro Tip

During design reviews, walk each P&ID line-by-line from source to destination. Verify that every line has a pipe class assignment, every valve has a tag number, and every instrument is connected to a control loop. Missing items at the P&ID stage become expensive field changes during construction.

P&ID Symbols

P&IDs use standardized symbols per ANSI/ISA-5.1. Valves, instruments, equipment, and piping elements each have distinct graphical representations. A gate valve is shown as two triangles meeting at a point; a control valve adds a diaphragm actuator symbol. Instrument bubbles contain ISA-standard letter codes: “F” for flow, “P” for pressure, “T” for temperature, “L” for level.

Understanding these symbols is non-negotiable for anyone working in EPC. Misreading a P&ID symbol can lead to incorrect material orders, wrong valve types, or missed safety interlocks.

P&IDs drive procurement documents including MTOs, RFQs, and purchase orders. Accuracy at this stage directly affects project cost and schedule.

Read the full guide to pipe classes and specifications