P&ID Reading and Drawing Guide —
Symbols, Tags and
Control Loops

The Piping and Instrumentation Diagram is the single most information-dense document on a process engineering project. It shows every piece of process equipment, every pipe, every valve, every instrument, every control loop, and every safety device — and it encodes the operating intent of the whole plant in a notation that any process engineer anywhere in the world should be able to read. In practice, not everyone can, because P&IDs are read from experience as much as from knowledge of the standard, and the standard itself is not always consistently applied.

This article covers what a P&ID is, how it differs from a Process Flow Diagram, the ISA 5.1 symbology that governs instrument representation, how instrument tags are constructed and decoded, how control loops are drawn and read, what safety systems look like on a P&ID, and how the document evolves through the project lifecycle. It is a working reference for engineers who need to read, mark up, or produce P&IDs.

P&ID vs PFD — The Fundamental Distinction

The Process Flow Diagram (PFD) is the concept document. It shows the major equipment items, the main process flows between them, the stream compositions and conditions (temperature, pressure, flow rate) at key points, and the overall material and energy balance. It does not show instruments, individual valves, bypass arrangements, or piping in any detail. The PFD is produced early in design to establish the process and is used for communication with clients, management, and non-process disciplines. It is not a construction document.

The Piping and Instrumentation Diagram (P&ID) is the engineering document. It shows every piece of equipment, every process and utility pipe with its line designation, every valve (manual and automatic), every instrument and analyser, every control loop, every safety relief device, and every connection to other P&IDs. It does not show accurate geometry, spatial relationships, elevations, pipe routing, or structural details — these are covered by piping general arrangement drawings and isometrics. The P&ID is the reference document for design, procurement, construction, commissioning, operation, and maintenance. It is the most important single document in a process plant's technical file.

Equipment Representation

Equipment is shown on P&IDs using standardised symbols defined in ISO 10628-2 and broadly consistent with ISA 5.1 conventions. The major symbols an engineer encounters:

• Vessels and tanks — vertical and horizontal cylinders for pressure vessels and storage tanks. Internals (trays, packing, dip pipes, agitators) are shown schematically where they affect the process.
• Pumps — a circle with a triangle indicating the impeller direction. Centrifugal pumps (the default) use one symbol; positive displacement pumps use another. The driver (motor, turbine) is shown attached.
• Compressors and fans — similar to pumps but with specific symbols distinguishing centrifugal from reciprocating types and from fans and blowers.
• Heat exchangers — shell and tube shown as two overlapping rectangles or the standard TEMA symbol; plate exchangers as a stylised plate stack. The hot and cold sides are labelled.
• Fired heaters and furnaces — a rectangle with a flame symbol.
• Reactors — vessel symbols with internal details (agitator, heating coil, packed bed) as appropriate.
• Filters and strainers — Y-strainers shown with the specific Y symbol; filter housings as a vessel with filter element notation.

Each equipment item carries a unique equipment tag — a code that identifies its type and number within the plant: P-101 (Pump 101), V-201 (Vessel 201), E-301 (Exchanger 301), HX-401, TK-501 and so on. The numbering convention is set at the project level and applied consistently. Equipment tags cross-reference to the equipment list, datasheets, and purchase orders.

Line Designation

Every process pipe on a P&ID carries a line designation — a coded label that defines the pipe's contents, size, material specification, and insulation requirement. A typical line designation takes the form:

4"-PW-1023-CS2-H
4" = nominal pipe size (NPS 4)
PW = fluid service code (process water)
1023 = sequential line number
CS2 = piping class (carbon steel, Class 2 — defines schedule, fittings, flanges, gaskets)
H = insulation type (H = heat traced; I = insulated; N = no insulation; T = traced)

The line designation connects the P&ID to the piping specification (the document that defines every component in that piping class) and to the piping isometric drawings that show the actual physical routing. Every change to fluid service, pipe size, or piping class breaks a line — the new designation appears on the drawing at that break point.

Valve Symbols

Valves are represented by standardised symbols on the pipeline. The most commonly encountered:

• Gate valve — two triangles meeting at their points, representing the flat gate. The standard isolation valve for on-off service.
• Globe valve — a circle on the line with an arrow or angled element. Throttling and control service.
• Ball valve — a circle with a square element inside. Quarter-turn, on-off or control.
• Butterfly valve — a circle with a line through it representing the disc. Large bore isolation, low-pressure throttling.
• Check valve — a half-arrow indicating flow direction. Non-return function.
• Safety relief valve (PSV/PRV) — a specific symbol combining the body and the spring-loaded bonnet. Always shown with the inlet and outlet directions and the relieving path (to flare, to atmosphere, to containment).
• Control valve — a globe or other body symbol with a diaphragm actuator symbol above it, plus the instrument tag linking to the controller. Fail mode (FC, FO, FL) shown on the actuator.

Hand-operated valves are shown with a handwheel symbol or a simple symbol with no actuator. Actuated valves show the actuator type: diaphragm (pneumatic), cylinder (pneumatic or hydraulic), motor (electric).

ISA 5.1 Instrument Symbols — The Bubble System

Instruments are represented on P&IDs using the ISA 5.1 bubble notation. The bubble is a circle (or a square, or a circle with a line through it) containing an identification tag. The shape of the bubble encodes the instrument's physical location:

• Plain circle — field-mounted instrument, locally accessible
• Circle with horizontal line through it — panel-mounted instrument, accessible from the main control panel or DCS operator station
• Square with circle inside — instrument in a remote panel, sub-panel, or local panel, not the main panel
• Dashed circle — computer function or software element (DCS logic, PLC function)

The instrument tag inside the bubble identifies the instrument. The tag is structured as a combination of functional letters followed by a loop number:

FIC-201
F = measured variable (Flow)
I = readout function (Indicating — has a display)
C = output function (Controller — outputs a signal)
201 = loop number (unique identifier for this control loop)

ISA 5.1 Functional Letters — Decoding the Tag

The functional letters in an instrument tag are read left to right. The first letter is always the measured variable. Subsequent letters define what the instrument does with that measurement:

Common instrument tag examples decoded:

• TT-101 — Temperature Transmitter, loop 101. A sensor that outputs a 4–20mA or digital signal.
• TIC-101 — Temperature Indicating Controller, loop 101. Has a display and a controller output — likely driving a control valve or heater.
• TAH-101 — Temperature Alarm High, loop 101. Triggers an alarm when temperature exceeds a setpoint.
• TAHH-101 — Temperature Alarm High High, loop 101. A second high alarm at a higher setpoint, typically initiating an automatic shutdown.
• TSH-101 — Temperature Switch High. A discrete on/off device that trips at a setpoint rather than transmitting a continuous signal.
• FE-201 — Flow Element, loop 201. The primary sensing element (orifice plate, flow nozzle, Coriolis tube) in the flow measurement.
• FT-201 — Flow Transmitter. Converts the primary element signal to a standardised output signal.
• FIC-201 — Flow Indicating Controller. Displays flow and outputs a control signal — typically to a control valve FV-201.
• FCV-201 — Flow Control Valve, loop 201. The final control element driven by FIC-201. (Note: some companies use FV-201; conventions vary.)
• PSV-301 — Pressure Safety Valve (relief valve), loop 301.
• LSL-401 — Level Switch Low, loop 401. Trips when level falls below a minimum.
• HS-501 — Hand Switch, loop 501. An operator-initiated switch — a pushbutton, keyswitch, or selector on a control panel.

Control Loops — How They Are Drawn

A control loop on a P&ID consists of a measurement element, a transmitter, a controller, and a final control element (usually a valve), all connected by signal lines and carrying the same loop number. Signal lines are distinguished from process lines by their line style:

• Process pipe — solid line, heavier weight
• Pneumatic signal — dashed line (line with evenly spaced dashes)
• Electrical signal — solid line with forward slashes at intervals
• Data/digital communication — solid line with backward slashes, or a specific bus notation
• Hydraulic signal — solid line with circles at intervals

A basic temperature control loop controlling a heat exchanger outlet temperature works as follows on the P&ID: the temperature element (TE-101) is shown connected to the process pipe at the exchanger outlet. A signal line runs from the TE to a temperature transmitter (TT-101), which is shown as a bubble either at the element or at the instrument location. A signal line runs from TT-101 to the temperature indicating controller (TIC-101), shown as a panel-mounted bubble (circle with horizontal line). A signal line from TIC-101 runs to the temperature control valve (TCV-101) on the heating medium supply to the exchanger. The whole assembly — TE, TT, TIC, TCV — carries the number 101, identifying them as parts of the same loop. The valve shows its fail mode: FC means on loss of signal the valve closes, cutting off heat — the correct fail mode for a heating application where overheating on instrument failure is the hazard.

Safety Instrumented Systems — SIL and SIS on P&IDs

Safety Instrumented Systems (SIS) — automatic shutdown systems, emergency shutdown systems (ESD), and fire and gas systems — are shown on P&IDs using specific notation to distinguish them from basic process control. The most common representation:

• SIS-associated instruments are shown with a different bubble style — often a diamond or square rather than a circle, or a circle with a specific annotation indicating SIL-rated equipment
• SIS logic solvers (Safety PLCs) are shown as a separate function block, distinct from the DCS
• Shutdown valves (SDV) — solenoid-operated valves that fail closed on loss of power or signal — are identified by their tag and their fail mode clearly marked
• The SIL (Safety Integrity Level) rating of each safety function is sometimes annotated on the P&ID, though detailed SIL documentation is held in the SIL study and Safety Requirements Specification (SRS) rather than on the drawing itself

A high-high pressure trip might appear as: PSHH-301 (Pressure Switch High High, loop 301) with a signal line to a shutdown logic block, which outputs a signal to SDV-301 (Shutdown Valve 301) — shown with fail-closed notation. The PSHH instrument is in a diamond bubble, identifying it as part of the safety system, not the process control system. The same measurement may have a parallel PTI-301 (Pressure Transmitter Indicating) in a round bubble for operator display — the process control and safety chains are deliberately separate and shown as such on the P&ID.

The P&ID in the Project Lifecycle

The P&ID is not a static document — it evolves through defined stages throughout the project, and the revision status should always be checked before using it as a reference:

• Conceptual / Basis of Design P&ID — major equipment and principal flows only. Produced to define the process scope and agree the basis of design with the client. Equivalent to a developed PFD.
• Issued for Design (IFD) / Approved for Design (AFD) — the P&ID used by all disciplines (piping, civil, electrical, instrumentation) to develop their designs. Must be formally approved and change-controlled from this point.
• Issued for Construction (IFC) — the P&ID that goes to site for construction. All design development completed. Further changes require formal change notices and P&ID revisions.
• As-Built — the P&ID updated to reflect the installation as actually built, including site modifications. The permanent record of the plant as constructed. Held in the Health and Safety file under CDM and the Operating and Maintenance manuals.

HAZOP and the P&ID

The P&ID is the primary input document to a HAZOP (Hazard and Operability) study. The HAZOP team works systematically through the P&ID, applying guidewords (More, Less, None, Reverse, As Well As, Other Than) to each process parameter at each node to identify potential deviations and their consequences. A P&ID that is incomplete, inconsistent, or at the wrong revision at the time of HAZOP will produce an incomplete HAZOP — deviations that should have been studied will be missed. The HAZOP output (action items, safeguarding additions, instrument requirements) feeds back into P&ID revisions. The final IFC P&ID should reflect all HAZOP actions that were implemented.

Common P&ID Errors to Watch For

• Missing line breaks at piping class changes — when the fluid service or pipe spec changes within a line, the line designation must change and the break must be shown. An unbroken line implies the same specification throughout, which is frequently wrong.
• Unlabelled manual valves — all manually operated valves in a controlled or safety-critical system should carry a valve tag for isolation permit and maintenance reference. Unlabelled valves cannot be positively identified in an isolation procedure.
• Missing vent and drain connections — every pressure system requires means of depressurisation and draining for maintenance. These are frequently left off early P&ID revisions and added late, sometimes after HAZOP.
• Instrument signal type not indicated — if signal line styles are not consistently applied, it is impossible to determine from the P&ID whether a connection is a 4–20mA analogue signal, a HART digital overlay, a Foundation Fieldbus segment, or a pneumatic signal — each of which has different engineering and hazard implications.
• Safety and process control signals in the same loop — SIS and DCS functions must be independent. A P&ID that shows the same transmitter feeding both the DCS controller and the SIS shutdown logic without clear separation is either showing a design that violates IEC 61511, or has not been drawn correctly to show the actual architecture.

Summary

The P&ID is the primary engineering reference for a process system from detailed design through to decommissioning. Reading one fluently requires familiarity with three things: the equipment symbols (ISO 10628-2), the instrument bubble notation and tag structure (ISA 5.1), and the project-specific conventions that supplement the standard. The line designation links every pipe on the drawing to its specification. The instrument tag decodes the function of every device. The control loop connects measurement to controller to final element. The revision history tells you whether the drawing in your hand is the one everyone else is working from. Master these four and any P&ID becomes readable.