What Is a P&ID?
A Piping and Instrumentation Diagram (P&ID) is a detailed schematic drawing that shows the physical sequence of equipment, piping, instrumentation, and control devices in a process plant. P&IDs are the authoritative reference document for process safety analysis (HAZOP), instrument engineering, operations, and maintenance.
P&IDs are used by:
- Process engineers — to define the control philosophy and process safety requirements
- Instrument and controls engineers — to specify instruments, control loops, and interlocks
- Operations technicians — to understand system flow paths, valve positions, and normal operating modes
- Maintenance technicians — to identify isolation points, instrument locations, and bypass paths
- Safety engineers — to conduct HAZOPs, determine safety integrity level (SIL) requirements, and verify protection layers
P&ID vs PFD vs Isometric Drawing
These three document types are often confused by newcomers:
| Document Type | Purpose | Level of Detail | Shows Control Loops? |
|---|---|---|---|
| PFD (Process Flow Diagram) | Shows overall process flow, mass balance, and major equipment | Low — conceptual | No |
| P&ID (Piping and Instrumentation Diagram) | Shows all pipes, instruments, control loops, and interlocks | High — engineering | Yes |
| Isometric Drawing | 3D representation of pipe routing, support locations, and dimensions | Very high — fabrication | No |
The PFD gives you the "what" — what is produced and consumed. The P&ID gives you the "how" — how the process is controlled, isolated, and protected. Isometrics give the construction team exact measurements for pipe fabrication.
ISA 5.1 Standard for Instrumentation Symbols
The standard governing P&ID symbols is ISA 5.1 (Instrumentation Symbols and Identification), published by the International Society of Automation. All instrument symbols on a P&ID should conform to ISA 5.1, though some companies maintain their own symbol libraries that extend or modify the standard.
Instrument symbols in ISA 5.1 use specific shapes to indicate instrument location and type:
- Circle — Instrument mounted in the field or locally
- Circle with horizontal line through center — Instrument mounted on the main control panel (front of panel, accessible to operators)
- Circle with two horizontal lines — Instrument in a shared display / DCS (distributed control system) — not locally accessible
- Square with circle inside — Computer function / PLC logic
- Hexagon — Safety instrumented system (SIS) function
Instrument Tag Format
Every instrument on a P&ID has a unique tag number that identifies its measurement type, function, and loop number. The format follows ISA 5.1:
[First Letter(s)] [Modifier] [Function Letter(s)] – [Loop Number]
Example: FIC-101 = Flow (F) Indicating (I) Controller (C), Loop 101
Example: PT-204 = Pressure (P) Transmitter (T), Loop 204
Example: LCV-305 = Level (L) Control (C) Valve (V), Loop 305
Common First Letter Codes (Measured Variable)
| Letter | Measured Variable |
|---|---|
| A | Analysis (composition, pH, dissolved oxygen, etc.) |
| F | Flow |
| L | Level |
| P | Pressure |
| T | Temperature |
| V | Vibration or mechanical analysis |
| S | Speed or frequency |
| D | Differential (used as modifier — DP = differential pressure) |
| H | Hand (manually initiated — HS = hand switch) |
Common Function Letter Codes (Instrument Function)
| Letter | Function |
|---|---|
| I | Indicator (displays the measured value) |
| T | Transmitter (converts process signal to 4–20 mA or digital signal) |
| C | Controller (compares measured value to setpoint and outputs a control signal) |
| V | Valve (final control element in a control loop) |
| S | Switch (changes state at a setpoint — used for alarms and interlocks) |
| A | Alarm (alerts operators when a variable goes outside acceptable limits) |
| E | Element (primary element that senses the process variable — e.g., thermowell, orifice plate) |
| Y | Relay or compute (signal conversion, calculation, or logic) |
Multiple function letters can be combined: FICAH-101 = Flow Indicating Controller with Alarm High, Loop 101.
P&ID Symbol Library: Key Equipment and Devices
Process Equipment
- Vessels/Tanks — Shown as circles (pressure vessels) or rectangles (atmospheric tanks) with nozzle connections indicated
- Heat Exchangers — Shown with crossed lines inside a box (shell and tube) or other specific symbols
- Pumps — Shown as a circle with a triangle pointing toward the discharge; the triangle indicates the impeller direction
- Compressors — Similar to pumps but with double-triangle symbol or specific manufacturer notation
- Columns / Towers — Tall vertical vessels with trays or packing indicated by horizontal lines inside
Valve Symbols
- Globe valve — Two triangles meeting at their points (bow-tie shape) on the pipe line
- Ball valve — Globe valve symbol with a filled circle in the center
- Butterfly valve — Globe valve symbol with a straight line through the center
- Check valve — Arrow symbol indicating one-directional flow only
- Control valve — Globe valve symbol with the ISA diaphragm or actuator symbol on top, and the instrument tag attached
- Safety relief valve (PSV) — Globe valve symbol with a spring symbol and directional arrow for discharge
P&ID Line Types and Signal Lines
P&IDs use different line types to distinguish between process piping and instrument signal lines:
| Line Type | Meaning |
|---|---|
| Solid thick line | Main process pipe (largest flows, highest importance) |
| Solid thin line | Secondary process or utility pipe |
| Dashed line (- - -) | Instrument signal line (pneumatic or electric — check legend) |
| Dotted line (. . .) | Capillary tube or hydraulic signal |
| Line with circle markers | Wireless instrument signal |
| Dashed line with E markers | Electrical signal |
| Dashed line with P markers | Pneumatic signal (3–15 psi or 20–100 kPa) |
Valve Failure Positions
Every control valve on a P&ID must specify its failure position — what position the valve moves to if instrument air, power, or signal is lost. This is a critical process safety requirement:
- FC (Fail Closed) — Valve closes on air/power failure. Used when closing the valve is the safe state (e.g., fuel feed to a burner).
- FO (Fail Open) — Valve opens on air/power failure. Used when an open valve is the safe state (e.g., cooling water to a reactor).
- FL (Fail Last) — Valve stays in last position on signal failure. Used when either open or closed could cause a hazard (requires a volume tank to hold air pressure).
The failure position is noted on the P&ID adjacent to the control valve tag, or in the valve data table. During a HAZOP, the question "what happens if the control valve fails open/closed?" is evaluated for every control valve in the system.
How to Trace a Control Loop on a P&ID
A basic control loop consists of three elements visible on the P&ID:
- Transmitter (sensor) — Measures the process variable (flow, pressure, temperature, level)
- Controller — Compares the measured variable to the setpoint and calculates an output signal (usually in the DCS)
- Final control element — Control valve (most common) or variable speed drive that changes the process
Signal flow: Transmitter to signal line to Controller to signal line to Control Valve to process change and back to Transmitter (closed loop).
Step-by-Step: Reading a Level Control Loop
Here is how to read a typical vessel level control loop:
- Step 1 — Find the transmitter. Look for LT-101 (Level Transmitter, Loop 101). It will be shown on the side of the vessel with a connection to the process, and a dashed signal line leaving it.
- Step 2 — Follow the signal line. The dashed line from LT-101 leads to the controller: LIC-101 (Level Indicating Controller, Loop 101). This symbol will be a circle with a horizontal line (panel-mounted) or a double-line (DCS).
- Step 3 — Follow the output signal. A second dashed line leaves LIC-101 and goes to the control valve: LCV-101 (Level Control Valve, Loop 101), shown on the outlet pipe of the vessel.
- Step 4 — Check the valve failure position. Look for "FC" or "FO" near LCV-101. If it says FC (Fail Closed), the outlet valve closes on air failure — level in the vessel rises. If FO, the valve opens — level drops.
- Step 5 — Identify alarms and overrides. Look for LAH-101 (Level Alarm High) or LAHH-101 (Level Alarm High High — a safety interlock). LAHH may connect to a safety instrumented system (hexagon symbol) that closes a different valve or trips a pump on high-high level.
- Step 6 — Trace the bypass. Look for a manual valve (hand valve) in parallel with LCV-101. This is the operator bypass used when the control valve is taken out for maintenance.
Common P&ID Reading Mistakes to Avoid
- Confusing transmitters with indicators — LT = transmitter (outputs a signal); LI = indicator (displays locally only). Transmitters feed the control system; indicators do not.
- Ignoring the instrument location convention — The line through the instrument bubble matters. No line = field-mounted; one line = main panel; two lines = DCS. This tells you where to go to find the instrument in the real plant.
- Missing safety instrumented system elements — Hexagon symbols indicate SIS functions (pressure safety valves, emergency shut-off valves). These are separate from the regulatory control system and often have separate instrument databases.
- Assuming all dashed lines are the same — Check the legend. Pneumatic signal lines and electrical signal lines look similar but connect to very different equipment and fail in different ways.