What standards define P&ID symbols?

Equipment, piping and valve symbols for the process industries are set out in ISO 10628 Part 2, and instrument symbols and tags in ISA 5.1, updated in 2024. Many companies and projects also use their own legend sheets, so always check the legend on the drawing set.

How do I read an instrument tag on a P&ID?

The first letter shows what is measured, for example F for flow, P for pressure, T for temperature and L for level. The following letters show what the instrument does, such as I for indicate, T for transmit, C for control and V for valve, and the number links it to its loop.

What does a circle with a line through it mean on a P&ID?

In the common ISA convention, a plain circle is an instrument mounted in the field, and a solid horizontal line through it means the function is available to the operator in a central control room. A square around the circle usually shows a shared display, such as a distributed control system.

P&ID Symbols: Valves, Pumps, Vessels and Instruments Explained

A plain reference to common P&ID symbols for lines, valves, pumps, vessels, heat exchangers and instruments, and what each tells a buyer about what must be quoted.

P&ID symbols are the standard shapes used on piping and instrumentation diagrams to show process lines, valves, pumps, vessels, heat exchangers and instruments. Equipment, piping and valve symbols for the process industries are set out in ISO 10628 Part 2 [1], and instrument symbols and tags in ISA 5.1, which was updated in 2024 [2]. Each project also issues a legend sheet that shows the exact symbols it uses, so the legend always wins over any general guide, including this one.

For a buyer, every symbol is a line on a purchase order. This reference groups the common symbols by family, describes them in plain words, and shows what each family means for the equipment list, the valve list, the instrument index and the RFQ.

Why this matters now

Two developments make symbols matter more, not less. ISA 5.1, the standard for instrument and control symbols and identification, was reorganised in 2024, with new symbols, a new title that adds "Control", and its guidance annexes moved into separate technical reports [2]. And P&IDs are increasingly exchanged as data: DEXPI, the open exchange format, builds on ISO 10628 for equipment and on IEC 62424 for instrumentation [3], and tools such as Autodesk's DEXPI bridge for AutoCAD Plant 3D include an ISO 10628 configuration [4]. A symbol drawn consistently today can become structured data that software reads tomorrow.

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How to read any symbol

Every item on a P&ID has three parts:

The shape, which shows what kind of item it is.
The tag, a short code that identifies the item uniquely, such as P101A for a pump or FT 101 for a flow transmitter.
The connections, which show what it is joined to and by what kind of line.

Read the legend first, then the title block for the drawing number and revision, and then follow the process from left to right. ISO 10628 Part 1 sets out how flow diagrams in the chemical and petrochemical industry are classified, what they contain and how they are drawn [5]. The Process Industry Practices standard for P&IDs, PIC001, sets out what information belongs on the drawing for nine classes of equipment; for a pump, for example, that includes the equipment number, service and capacity [6]. Our guide on how to read a P&ID walks through a full sheet.

Lines

Line	Usual appearance	What it means	What it leads to
Main process line	Solid, heavier line with a line number	Pipe carrying process fluid	Line list, piping take off
Minor or utility line	Solid, lighter line	Utility or smaller process connection	Line list
Electrical signal	Dashed line	Signal between instruments	Instrument cabling scope
Pneumatic signal	Line with short diagonal strokes at intervals	Instrument air signal	Instrument tubing scope
Data link	Line with small circles at intervals	Digital communication	Control system scope
Traced or jacketed line	Process line with an extra parallel or dashed line	Heat tracing or jacket	Tracing and insulation scope

Line numbers usually encode size, fluid, sequence and piping class. They are the starting point for the line list, valve list and instrument index.

Valves

Most valve symbols start from the same basic shape: two triangles pointing at each other, tip to tip, like a bow tie, drawn across the line. What sits in the middle, or on top, tells you the type.

Valve	Usual appearance	Typical use	What the supplier needs
Gate	Plain bow tie	On and off isolation	Size, rating, material, end connections
Globe	Bow tie with a filled dot in the middle	Throttling	As above, plus flow and pressure drop
Ball	Bow tie with a small circle in the middle	Quick isolation	As above, plus full or reduced bore
Butterfly	Bow tie with a short line across the middle, or a circle with a disc	Larger lines, low pressure drop	Size, rating, seat and disc materials
Check	Bow tie with one side filled, or an arrow	Stops reverse flow	Size, rating, type, orientation
Diaphragm	Bow tie with a curved line on top	Hygienic or corrosive service	Body lining, diaphragm material
Control valve	Valve body with an actuator drawn on top, often a dome	Automatic control	Datasheet with flow cases for sizing
Pressure relief valve	Angle body with a spring drawn as a zigzag	Overpressure protection	Set pressure, relieving case, size

Manual valves feed the valve list and bulk valve packages. Control valves and relief valves usually get their own datasheets and packages.

Pumps and compressors

Item	Usual appearance	What the supplier needs
Centrifugal pump	Circle with the outlet leaving at the top or side, often with a small triangle or line inside	Pump datasheet: duty, liquid, NPSH available
Positive displacement pump	Circle or rectangle with an internal mark for the type, such as gears or a diaphragm	Datasheet with flow, pressure, viscosity
Compressor	Trapezoid, wide at the inlet and narrow at the outlet	Datasheet with gas, flows, pressures
Fan or blower	Circle with blades, or a simple fan outline	Flow, static pressure, temperature
Vacuum pump	Pump symbol marked for vacuum, often with a separate package outline	Suction pressure, gas load, seal liquid

A pair of pumps tagged A and B usually means one running and one standby, and both go on the equipment list.

Vessels, tanks and columns

Item	Usual appearance	What the supplier needs
Vertical pressure vessel	Tall rectangle with rounded top and bottom	Mechanical datasheet, nozzle schedule
Horizontal drum	Long rectangle with rounded ends, often on saddles	As above
Atmospheric storage tank	Rectangle with a flat or slightly peaked roof	Tank datasheet, design code
Column	Tall vessel with dashed lines for trays or a hatched zone for packing	Internals specification, nozzle schedule
Agitated reactor	Vessel with a shaft and impeller drawn inside and a motor on top	Vessel and agitator datasheets
Jacketed vessel	Vessel with a second outline around the lower part	Jacket design conditions

Each vessel tag goes onto the equipment list with its design pressure, temperature and materials.

Heat exchangers

Item	Usual appearance	What the supplier needs
Shell and tube exchanger	Circle with a zigzag line through it, or a long outline showing shell and channel	TEMA style datasheet
Plate exchanger	Rectangle with several parallel vertical lines	Duty, fluids, temperatures, pressure drops
Air cooler	Rectangle with a fan drawn above or below	Duty, ambient design temperature
Reboiler or condenser	Exchanger symbol connected to a column	Datasheet, including phase change

Other common items

Small items are easy to miss and often end up unpriced, because they sit on the line rather than standing out as equipment.

Item	Usual appearance	Why it matters to the buyer
Strainer	Line with a small Y shape or basket outline	Often bought with valves or as bulk items
Filter	Vessel outline with a dashed internal element	Element type and spares
Sight glass	Small circle or rectangle on a line or vessel	Pressure rating and gasket materials
Steam trap	Small square or circle marked T on a steam line	Bought in bulk by type and size
Spectacle blind	Figure of eight shape across the line	Size and rating; often supplied with piping
Reducer	Short tapered section between two line sizes	Part of the piping take off
Flexible hose	Wavy line section	Pressure, temperature and fluid compatibility
Package boundary	Dashed or dash dot outline around a group of items	Shows a supplier's scope: everything inside is in the package

The package boundary deserves special attention. Everything inside it should be in the package supplier's scope, and every line crossing it is a tie in point that someone else must connect. Mark these points on the RFQ drawings so bidders price the same scope.

Instruments and tags

ISA 5.1 provides a uniform way to show and identify instrumentation and control functions [2]. The common conventions are:

Element	Usual appearance	Meaning
Field instrument	Plain circle	Mounted in the field
Control room instrument	Circle with a solid horizontal line through it	Available to the operator in the main control room
Behind panel	Circle with a dashed horizontal line	Not normally accessible to the operator
Shared display	Circle inside a square	Function in a shared control system, such as a distributed control system
Computer function	Hexagon	Function in a separate computer
Programmable logic	Diamond inside a square	Function in a programmable logic controller

Inside the bubble sits the tag. The first letter is the measured variable, for example F flow, P pressure, T temperature, L level or A analysis. The following letters are the functions, for example I indicate, T transmit, C control, V valve, A alarm, S switch or E primary element. The number is the loop. So FT 101 is a flow transmitter in loop 101, and FV 101 is the flow control valve in the same loop.

Four panels describing P&ID symbol families in words: lines, valves, rotating and static equipment, and instruments, each with what it leads to in procurement

From symbol to purchase package

Symbol family	Goes onto	Bought as
Process lines	Line list	Bulk piping by piping class
Manual valves	Valve list	Valve packages by type and rating
Control and relief valves	Instrument index or own lists	Control valve and relief valve packages
Pumps, compressors, fans	Equipment list	Individual RFQs with datasheets
Vessels, tanks, columns	Equipment list	Fabricated equipment packages
Heat exchangers	Equipment list	Exchanger packages with TEMA sheets
Instruments	Instrument index	Instrument packages by type

Practical example

In this illustrative example, a buyer reads a small loop on a P&ID for a solvent feed. A plain circle on the line from pump P101A is tagged FT 101: a field mounted flow transmitter. A circle inside a square in the control room area is tagged FIC 101: a flow indicating controller in the distributed control system. A valve with an actuator dome downstream of the pump is tagged FV 101: the flow control valve. Dashed lines join the three.

That one loop produces three purchases: a flowmeter with its instrument datasheet, a control valve with its sizing data, and configuration work in the control system. A buyer who can read the loop can check that all three are in scope before the RFQ goes out.

Where ChemCapital fits

ChemCapital's P&ID to RFQ turns symbols and tags into procurement items. Its DEXPI import reads equipment, instruments, valves, nozzles and pipe segments where the file follows supported structures, and more than 300 tag patterns are recognised as standard, with tag rules that can be set per project. PDFs and scans can also be uploaded for extraction with confidence scores and human review, and accepted items are grouped into packages that can become RFQs. For the data format behind it, see DEXPI explained.

Ready to turn your P&ID symbols into purchase packages? Start free with ChemCapital.

Informational only, not procurement or legal advice. Cited names, frameworks, and statistics belong to their respective owners.

P&ID Symbols: Valves, Pumps, Vessels and Instruments