The Piping and Instrumentation Diagram (P&ID) is the definitive engineering document for a process plant. Every instrument, control loop, safety function, and equipment item is represented on the P&ID. The ability to read P&IDs fluently is a core skill for any automation or instrumentation engineer — and it is learnable. This guide walks through the conventions, symbols, and logic you need to interpret any P&ID with confidence.

What a P&ID Shows

A P&ID is not a flow diagram and not a physical layout drawing. It is a schematic that shows:

  • All process equipment (vessels, exchangers, pumps, compressors, columns)
  • All pipe lines connecting equipment, with line sizes, material specifications, and insulation codes
  • All instruments measuring process variables (pressure, temperature, flow, level, analysis)
  • All control loops connecting instruments to controllers and control valves
  • All isolation valves, manual valves, and spectacle blinds
  • All safety devices (relief valves, rupture discs, safety shutoff valves)
  • Utility connections (steam, cooling water, instrument air, nitrogen)

P&IDs do not show exact pipe routings, physical dimensions, or accurate scale. They are schematic representations of the process and control system.

Instrument Tag Numbering

Every instrument on a P&ID has a tag number that follows a defined structure. The ISA 5.1 standard defines the most common convention:

Tag format: [Loop Number]-[Letter Code][Suffix]

The letter code uses the ISA 5.1 table, where the first letter indicates the measured variable and subsequent letters indicate the instrument function:

  • P = Pressure. PT = Pressure Transmitter, PIC = Pressure Indicating Controller, PSV = Pressure Safety Valve, PSH = Pressure Switch High
  • T = Temperature. TE = Temperature Element (thermocouple/RTD), TT = Temperature Transmitter, TIC = Temperature Indicating Controller
  • F = Flow. FT = Flow Transmitter, FIC = Flow Indicating Controller, FCV = Flow Control Valve
  • L = Level. LT = Level Transmitter, LIC = Level Indicating Controller, LSH = Level Switch High, LSHH = Level Switch High High
  • A = Analysis. AT = Analyser Transmitter, AI = Analyser Indicator
  • S = Speed or Safety (context-dependent). SIS = Safety Instrumented System, SDV = Shutdown Valve

The suffix (H, HH, L, LL) indicates alarm thresholds: H = High, HH = High-High, L = Low, LL = Low-Low. A PSHH is a Pressure Switch High-High — a device that trips on a high-high pressure condition, typically connected to a shutdown function.

Control Loop Symbols

Control loops on a P&ID consist of a measurement element, transmitter, controller, and final element (control valve or motor), connected by signal lines. Standard ISA 5.1 symbols:

  • Circle: Any instrument or function. A circle with a horizontal line through the middle indicates a device located on the main control panel (shared display). A circle without a line indicates a field-mounted instrument. A circle with a double line indicates an instrument on an auxiliary or local panel.
  • Diamond: A computer function or software block (DCS controller). A diamond inside a circle means the function is implemented in a distributed control system.
  • Solid triangle on a pipe: A control valve. The triangle points toward the pipe for a globe valve body. An actuator symbol (circle above the triangle) indicates the actuator type (pneumatic, electric).
  • Dashed lines: Signal lines carrying instrument signals (electrical, pneumatic, or digital). Different line styles may distinguish signal types: solid line = pneumatic, dashed = electrical, dotted = digital data link.

Reading a Control Loop Step by Step

Take a simple flow control loop as an example. FT-201 is a flow transmitter on a process line. It sends its 4-20 mA signal (or digital HART signal) to FIC-201, a Flow Indicating Controller in the DCS. FIC-201 compares the measured flow to the setpoint and sends an output signal to FCV-201, the flow control valve on the process line.

To trace this loop on the P&ID:

  1. Find FT-201 (flow transmitter) on the process line — usually a circle connected to a measurement tap on the pipe
  2. Follow the dashed signal line from FT-201 to the controller — FIC-201 (circle with diamond) usually located in the DCS or control panel area of the drawing
  3. Follow the output signal line from FIC-201 to FCV-201 (control valve symbol on the process line)
  4. Note the valve fail position marked on the control valve — FO (fail open), FC (fail closed), or FL (fail locked). This is critical for understanding emergency behaviour.

Safety Instrumented System Symbols

Safety instruments on a P&ID are typically distinguished from basic process control instruments by visual markers defined in the site standard. Common approaches:

  • An additional circle inside or outside the main instrument circle (diamond or double-line) indicating SIS affiliation
  • A different tag prefix (e.g., XS for SIS instruments vs standard ISA tagging) per site convention
  • A text note or leader line stating “SIS” or “Safety” adjacent to the instrument
  • The SIS logic solver tagged as a separate unit (e.g., SIS-001 or a dedicated SDV tag)

Always refer to the P&ID legend and the site instrument numbering specification to understand the exact convention used, as it varies between companies and EPC contractors.

Relief Valves and Rupture Discs

Pressure relief devices protect equipment from overpressure. On the P&ID:

  • Pressure Safety Valve (PSV): An angled valve symbol on the line with the tag PSV-xxx. A PSV opens automatically when inlet pressure exceeds its set pressure and reseats when pressure drops. The set pressure and relief capacity are noted in the instrument datasheet, not on the P&ID itself.
  • Rupture Disc (RD): A crescent or disc symbol on the line. Rupture discs are non-reclosing — they burst at the rated pressure and must be replaced. Often installed in series upstream of a PSV for corrosive services or where a PSV might leak.

Reading Valve Specifications

Control valve symbols include the fail position (FO, FC, FL) and may include the valve type (globe, rotary, butterfly). Isolation valves show the valve type via standardised symbols — a solid diamond for a gate valve, an open diamond for a ball valve, a bowtie for a butterfly valve. Block valve positions (normally open NO, or normally closed NC) are noted adjacent to the symbol.

Building Your P&ID Reading Speed

Reading P&IDs fluently comes with practice, not memorisation. The most effective approach is to take a P&ID for a process you understand (a heat exchanger, a pump, a distillation column) and trace every control loop end to end. Write out the tag of each instrument in the loop, its function, and its signal path. After tracing 20-30 loops, the conventions become automatic.