PFD and P&ID standards: content, symbols, instrument identification and review

Process diagrams are the controlling documents of a process plant. The process design is expressed on them, the safety study is conducted on them, and the plant is modelled, built and operated from them. This note sets out what each diagram type must contain, the standards that govern symbols, identification and sheets, and a structured method for reviewing a P&ID.

Technical note, RB Solutions Ltd, 22 October 2026

Three levels of process diagram

BS EN ISO 10628-1 classifies diagrams for the chemical and petrochemical industry into block diagrams, process flow diagrams and piping and instrumentation diagrams, and specifies the minimum information each must carry [1]. The levels form a hierarchy. Each refines the one above without contradicting it.

Table 1. Content of the three diagram levels, after BS EN ISO 10628-1 [1]
Diagram	Purpose	Required content	Excluded content
Block flow diagram (BFD)	Process concept and overall balance	Process stages as rectangles, main streams, principal inputs and outputs with flows	Individual equipment, control
Process flow diagram (PFD)	Process design basis	All process equipment with tags, main lines and flow direction, stream numbers with stream table, operating conditions, principal control loops, duties of major equipment	Pipe sizes and classes, minor valves, full instrumentation
Piping and instrumentation diagram (P&ID)	Detailed design, safety study, construction and operation	All equipment, every line with number, size and specification, all valves and fittings, all instruments and control functions, safety devices, specification breaks, vents, drains, utility connections, interlock references	Physical layout and line routing to scale

The PFD is the drawing on which the mass and energy balance is presented. Every stream number on it corresponds to a column of a stream table generated from the balance calculation. The P&ID adds everything needed to specify, build, operate and safeguard the plant. It is not a scale drawing. Layout and routing belong to plan, section and isometric drawings and to the 3D model.

Graphical symbols: BS EN ISO 10628-2

BS EN ISO 10628-2 defines the graphical symbols for equipment, piping, valves and fittings on these diagrams [2]. Symbols are built from basic shapes with distinguishing features, such as vessel outlines, pump and compressor symbols, heat exchanger types, and valve bodies with actuator symbols. Consistency of symbol size within a drawing set is a practical requirement. Like items are drawn at the same size, and lines meet symbols at defined connection points, so that the diagram reads unambiguously and can be checked automatically. Line types distinguish main process lines, secondary and utility lines, and instrument signal lines. The signal line types follow the instrumentation standard adopted for the project.

Instrument identification: ISA-5.1 and BS ISO 3511

Instrument and control functions are identified by a tag of letters and a loop number. The structure is defined in ANSI/ISA-5.1-2024 [3], published in October 2024 under the title Instrumentation and Control Symbols and Identification. BS ISO 3511 Parts 1 to 4 define closely related functional representation for process measurement control [4]. Whichever standard the project adopts is declared on the legend sheet and applied consistently.

First letter and succeeding letters

The first letter denotes the measured or initiating variable. Succeeding letters denote readout or passive functions, then output functions, then modifiers [3]. A tag such as FIC-101 therefore reads as flow (F), indicating (I) controller (C) in loop 101. The associated transmitter is FT-101 and the final element is FV-101. All instruments in one loop share the loop number.

Table 2. Common identification letters, after ANSI/ISA-5.1 [3]
Letter	As first letter (measured variable)	As succeeding letter (function)
A	Analysis (pH, conductivity, turbidity, dissolved oxygen)	Alarm
C	User's choice	Control
E	Voltage	Sensor (primary element)
F	Flow	Ratio (modifier)
H	Hand (manual)	High (modifier)
I	Current	Indicate
L	Level	Light; Low (modifier)
P	Pressure	Point (test connection)
S	Speed or frequency	Switch; Safety (modifier)
T	Temperature	Transmit
V	Vibration	Valve, damper or louvre
Z	Position	Driver, actuator or unclassified final element

Balloon types and location

The tag is written in a symbol, the balloon, whose shape and line convention indicate the type of device and where its function is accessible to the operator [3]:

Circle, no line: discrete instrument, field mounted.
Circle with a solid horizontal line: discrete instrument in a primary location normally accessible to the operator, such as the central control room panel.
Circle with a dashed horizontal line: discrete instrument in a location not normally accessible, such as behind a panel.
Circle inscribed in a square: shared display or shared control, typically a function in a distributed control system (DCS) presented on an operator screen.
Diamond inscribed in a square: programmable logic controller (PLC) function.
Hexagon: computer function.
Diamond: interlock or binary logic, with the logic itself defined on a cause-and-effect chart or logic diagram referenced from the P&ID.

The balloon shows the architecture of the control system as well as the function. A reviewer can see from it which functions are operator-adjustable, which are executed in the PLC, and which reside in an independent safety system.

Line numbering conventions

Every line on a P&ID carries a unique line number. The project line-numbering procedure defines its format, which typically comprises nominal size (DN), fluid or service code, a sequential number, piping class (material and pressure rating specification), and insulation or tracing code, for example DN100-WW-1012-A1-N. The line number changes at each piping class change and at defined process boundaries. The line list generated from the P&ID carries design pressure, design temperature, test pressure and the hydraulic design flow for each line. These link the diagram to the hydraulic and mechanical calculations.

Valve fail positions and specification breaks

Every actuated valve shows its position on loss of motive power or signal, by abbreviation or arrow. FC denotes fail closed, FO fail open, FL fail locked in last position, and FI fail indeterminate [3]. The fail position is a safety decision, made by considering the consequence of each position, and recorded for HAZOP. A chemical dosing valve normally fails closed. A cooling water valve on an exothermic duty normally fails open.

A specification break is shown wherever the piping class changes. Typical locations are the downstream side of a pressure-reducing valve, the boundary between a corrosive and a non-corrosive service, and the interface between suppliers' scopes. The break symbol and the change of line number together ensure that the upstream class rating covers every condition that can reach the break, including closed-valve and failure cases.

Relief and safeguarding representation

Pressure relief valves, bursting discs, vacuum breakers, flame arresters and breather valves are shown with their set pressures, sizes and discharge destinations. Safety-instrumented functions are identified distinctly from basic process control. Their trip initiators and final elements are tagged so that the safety requirements specification can be traced to the drawing. Locked-open and locked-closed valves, for example isolations under a relief valve, are marked "LO" or "LC". Overpressure scenarios that the drawing protects against are listed in the relief study, not on the drawing, but each relief device on the drawing must correspond to a scenario in that study.

Reference designation and drawing sheets

Equipment tags may follow a project-specific scheme or the reference designation system of BS EN IEC 81346 [5]. The latter structures designations by function (prefix =), product (prefix −) and location (prefix +), with letter codes for object classes defined in Part 2. On multi-discipline projects, an 81346-based scheme allows one designation to be shared across process, mechanical, electrical and control documents.

Drawing sheets follow BS EN ISO 5457, which defines the A-series sizes, the frame, the borders and the grid reference system [6]. Title blocks follow BS EN ISO 7200, which defines which data fields are mandatory and which optional, together with their field lengths [7]; the identification number, title, date of issue and sheet number are among the mandatory fields. BS EN ISO 5457 places the title block in the bottom right-hand corner of the drawing area [6].

Worked example: sheet and drawing-area geometry

Inputs. A P&ID is to be drawn on an A2 sheet to BS EN ISO 5457. The A-series sizes derive from A0 of area 1 m² with side ratio √2, each subsequent size being half the area of the preceding one [8]. BS EN ISO 5457 specifies a border of 20 mm on the left-hand (filing) edge and 10 mm on the other edges [6].

A_{n} = 2^{- n} m^{2}, \frac{L_{n}}{W_{n}} = \sqrt{2} \Rightarrow W_{n} = 2^{- (2 n + 1) / 4} m, L_{n} = 2^{(1 - 2 n) / 4} m

where A_n is the area of size An (m²), and W_n and L_n are the short and long sides (m) before rounding to whole millimetres in the standard.

Route 1, from the formula. For n = 2, W = 2^−5/4 = 0.4204 m and L = 2^−3/4 = 0.5946 m, which round to the standard 420 mm × 594 mm [8]. The drawing area inside the frame is (594 − 20 − 10) × (420 − 10 − 10) = 564 mm × 400 mm, which is 0.2256 m² or 90 % of the sheet.

Route 2, by halving. A0 is 841 mm × 1,189 mm [8]. Halving the long side twice gives A1 at 594 × 841 mm and A2 at 420 × 594 mm. The area is 0.420 × 0.594 = 0.2495 m², against the theoretical 0.25 m², the difference arising from rounding to whole millimetres. The aspect ratio 594/420 = 1.414 equals √2 to four significant figures.

The calculation fixes the space for symbols, the title block and the legend before drafting starts. If the content of a P&ID does not fit the drawing area at the symbol size used throughout the set, it is split across additional sheets with off-sheet connectors carrying the full drawing number of the continuation. Symbols are not shrunk to fit.

A structured P&ID review method

A P&ID is reviewed systematically, line by line and loop by loop, against four tests:

Continuity. Every line has a source and a destination, every off-sheet connector matches its counterpart, and every stream in the PFD appears.
Controllability. Each degree of freedom of the process (flows, levels, pressures, temperatures, compositions) is controlled or deliberately left free. No two controllers fight over the same variable, and every control valve has a manipulated variable it can actually influence.
Isolation and maintenance. Every item that requires maintenance can be isolated, vented, drained and flushed. Positive isolation is provided where the hazard requires it. Instruments have root valves.
Safety. Every credible overpressure, underpressure, overfill, reverse-flow and over-temperature case is covered by a relief device or safety function. Fail positions are consistent with the safe state. Interlocks are referenced.

Findings are recorded against the line or tag, closed by revision, and the revision clouded and listed in the revision table of the title block.

The P&ID as the basis for HAZOP

A hazard and operability study under BS EN 61882 systematically applies guide words such as NO, MORE, LESS, REVERSE and OTHER THAN to process parameters at defined nodes, in order to identify deviations, their causes, consequences and safeguards [9]. The P&ID is the primary design representation examined. Nodes are marked on it, safeguards are read from it, and actions arising change it. The quality of the HAZOP is bounded by the completeness of the P&ID. A study conducted on a drawing without fail positions, set pressures or interlock references cannot evaluate the safeguards it is meant to test.

From P&ID to plan views and 3D plant model

The P&ID defines the topology of the plant: what connects to what, in which line class, through which valves and instruments. It does not define geometry. Plant layout drawings, plan and section general arrangement views and the 3D model add geometry. Their connectivity must match the P&ID exactly. In modern practice, equipment, lines and inline items are placed in the 3D model with the P&ID tags and line numbers as attributes, and a consistency check between model and P&ID is run before isometrics are issued. Plan views and general arrangements are then extracted from the model, so that layout, access, maintenance envelopes and pipe routing are developed from a single verified source.

References

BS EN ISO 10628-1:2015 Diagrams for the chemical and petrochemical industry. Part 1: Specification of diagrams. London: BSI.
BS EN ISO 10628-2:2012 Diagrams for the chemical and petrochemical industry. Part 2: Graphical symbols. London: BSI.
ANSI/ISA-5.1-2024 Instrumentation and Control Symbols and Identification. Research Triangle Park: International Society of Automation. isa.org
BS ISO 3511 Process measurement control functions and instrumentation. Symbolic representation. Parts 1 to 4. London: BSI.
BS EN IEC 81346-1:2022 Industrial systems, installations and equipment and industrial products. Structuring principles and reference designations. Part 1: Basic rules; and BS EN IEC 81346-2:2019 Part 2: Classification of objects and codes for classes. London: BSI.
BS EN ISO 5457:1999+A1:2010 Technical product documentation. Sizes and layout of drawing sheets. London: BSI.
BS EN ISO 7200:2004 Technical product documentation. Data fields in title blocks and document headers. London: BSI.
BS EN ISO 216:2007 Writing paper and certain classes of printed matter. Trimmed sizes. A and B series. London: BSI.
BS EN 61882:2016 Hazard and operability studies (HAZOP studies). Application guide. London: BSI.

Related technical notes

RB Solutions Ltd provides PFD creation, P&ID creation and review to BS EN ISO 10628 and ISA-5.1, and develops plan views, general arrangements and 3D plant design from a client P&ID.

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