Piping and Instrumentation Diagram

What is a Piping and Instrumentation Diagram (P&ID)?

A piping and instrumentation diagram, or P&ID, is a detailed engineering drawing of a process plant that shows all piping, equipment, valves, instruments and control loops, together with their interconnections and unique tag numbers. The P&ID is the central reference for design, construction, commissioning, maintenance and safety studies. For OT teams it is also the most complete map of everything in the field that measures, controls and switches.

🎯 What is a P&ID used for?

A P&ID is not a poster for the wall but a working document that follows a plant through its entire lifecycle:

• Design — process, mechanical and instrumentation engineers use it to define which sensors, control valves and protective devices are needed
• Safety studies — a HAZOP works through the P&ID node by node, applying guide words such as more, less and no flow
• Commissioning — loop checks and function tests are signed off tag by tag
• Maintenance and modifications — every change goes through change management and ends with an updated as-built P&ID
• Security reviews — the P&ID reveals which instruments and controllers exist and how they depend on each other

🔄 How does a P&ID differ from a PFD or loop diagram?

A P&ID is often confused with other process documents. Each drawing has its own level of detail and audience:

The P&ID therefore tells you what exists and how it is functionally connected, but not where a pipe physically runs; that belongs in isometrics and layout drawings.

📐 Which standards define P&ID symbols?

There is no single global P&ID standard. In practice you will mostly meet these:

• ANSI/ISA-5.1 — the American standard for instrument symbols and identification letters, first published in 1949. The 2024 edition, renamed Instrumentation and Control Symbols and Identification, is supported by two technical reports: ISA-TR5.1.02 (identification) and ISA-TR5.1.03 (graphic symbols).
• ISO 10628 — part 1 (2014) specifies block diagrams, PFDs and P&IDs; part 2 (2012) contains the symbols for equipment, piping and valves.
• IEC 62424 — defines how process control engineering requests (PCE requests) are drawn in a P&ID and how they are exchanged with engineering tools using the CAEX format. Edition 2.0 was published in 2016. In Germany, DIN EN 62424 replaced the older DIN 19227-1 from 2010, since when a PCE request there is drawn as an oval rather than a circle.
• ISO 14617 — a general library of graphical symbols for technical diagrams, including a part on measurement and control functions.

German and other European chemical companies in particular combine ISO 10628 for equipment with IEC 62424 for instrumentation, while oil and gas projects usually follow ISA-5.1. Many operators also apply their own company standard on top. Always start with the project’s legend sheet.

🔤 How are instrument tags built up?

An instrument tag such as FIC-101 consists of identification letters and a loop number. The first letter gives the measured variable, the succeeding letters the function. All instruments in one control loop share the loop number; a suffix such as A or B distinguishes redundant instruments.

Examples: FT is a flow transmitter, TIC a temperature indicating controller, PDT a differential pressure transmitter, LAH a level alarm high and PSV a pressure safety valve. Instruments wired to the control system carry the same number there as a tag, which makes the P&ID the key that links the field to the system.

🧩 Which lines and symbols appear on a P&ID?

• Process line — solid line, often heavier, labelled with line number, nominal size and piping specification
• Pneumatic signal — line with double diagonal hash marks
• Electrical signal — dashed line, for example a 4-20 mA signal
• Data link — line with small circles for a fieldbus or system bus
• Circle — discrete instrument; a horizontal bar means it is operated from the control room, no bar means it is mounted in the field
• Circle inside a square — function in a shared system such as a DCS or SCADA
• Hexagon — computer function; diamond inside a square — PLC logic, often within an SIS

🔧 How do you read a P&ID step by step?

Take the flow control of a feed pump as a worked example:

1. Start with the legend — check which standard, line types and abbreviations the project uses.
2. Follow the process stream — from tank via pump to reactor, in the direction of the flow arrows.
3. Find the measurement — in the line sits a measuring element tagged FT-101: a flow transmitter that measures the flow rate.
4. Follow the signal — a dashed line runs to FIC-101, a circle in a square with a bar: a flow controller in the DCS, visible to the operator.
5. Find the final element — from FIC-101 a signal goes to FV-101, the control valve, often via an I/P converter (FY-101) when the valve is pneumatically actuated.
6. Understand the control — the PID controller compares the measured flow with the setpoint and adjusts the valve. Together the three tags form one control loop with loop number 101.
7. Look for safeguards — is there an FAL-101 (low flow alarm) or a separate SIS function with its own tag number?

🔐 Why does a P&ID matter for OT security?

For an OT Security Engineer the P&ID is an underrated source. It shows which instruments communicate via HART, a fieldbus or a data link, and which systems execute the control. That makes it:

• Input for the asset inventory — every instrument and controller has a tag, while network scans often miss passive or analogue devices
• A basis for zones and conduits — in a risk assessment according to IEC 62443-3-2 the P&ID helps you group process functions in the zones and conduits model, for instance placing the SIS in its own zone
• A tool for impact analysis — which valve could an attacker open, and which mechanical safeguard, such as a PSV, would still catch the consequences?

Because a P&ID contains so much process knowledge, it is sensitive information in its own right. Restrict who can access it and do not share it with suppliers in an uncontrolled way.

💾 What is a digital P&ID and what does DEXPI do?

More and more P&IDs are no longer flat drawings but intelligent models in tools such as AVEVA Diagrams, Hexagon Smart P&ID or AutoCAD Plant 3D, where every symbol is an object with properties. The DEXPI initiative, which began as a ProcessNet working group (DECHEMA and VDI) and is now governed by the association DEXPI e.V., develops an open exchange format so P&IDs can move between tools from different vendors. Version 1.3 appeared in 2021; DEXPI 2.0, released in October 2025, merges the P&ID and process specifications and introduces its own DEXPI XML format replacing the earlier Proteus schema. A digital P&ID like this is a building block for a digital twin and for automatically generated DCS configurations.

⚠️ What are common P&ID mistakes?

• Outdated drawings — the P&ID was not updated after a modification, so it is no longer as-built
• Mixed standards — ISA letters combined with IEC 62424 notation without a legend
• Duplicate or inconsistent tags — the same loop number used for two loops, or mismatches between the P&ID and the DCS database
• Too much detail — cable numbers and terminals belong in the loop diagram, not on the P&ID
• Missing fail-safe positions — control valves should show whether they open (FO) or close (FC) on loss of air

❓ Frequently asked questions

What does P&ID stand for?

P&ID stands for piping and instrumentation diagram. A P&ID shows all piping, equipment, valves and instruments of a process plant together with their tag numbers. The term process and instrumentation diagram is also used and means the same thing.

What is the difference between a PFD and a P&ID?

A PFD shows only the main streams, major equipment and operating conditions of a process. A P&ID goes much further and includes every pipe, valve, instrument and control loop. The PFD comes first in the design, and the P&ID is developed from it.

What does FIC mean on a P&ID?

FIC on a P&ID stands for flow indicating controller: a controller that displays and controls the flow rate. The F is the measured variable (flow), the I stands for indicate and the C for control. The number after it is the loop number of the control loop.

Which standard should you use for P&ID symbols?

For instrument symbols on a P&ID, ANSI/ISA-5.1 (2024 edition) is the most widely used standard worldwide. In Europe, ISO 10628 for equipment and piping and IEC 62424 for process control requests are also common. The standard that applies to your project is stated on the legend sheet.

Which software is used to draw a P&ID?

A P&ID is usually drawn in specialised CAE software such as AVEVA Diagrams, Hexagon Smart P&ID or AutoCAD Plant 3D. These tools store every symbol as an object with properties, so instrument lists and tags can be generated automatically. The DEXPI standard allows P&IDs to be exchanged between such tools.

Is a P&ID confidential information?

Yes, a P&ID contains detailed knowledge of the process, control loops and safeguards of a plant. An attacker could use a P&ID to deliberately create a hazardous situation. Treat P&IDs as confidential OT documentation with restricted access.

📌 In summary

A piping and instrumentation diagram (P&ID) is the detailed map of a process plant: every pipe, valve, instrument and control loop is shown with a unique tag number. If you can read the identification letters and symbols of ISA-5.1, ISO 10628 or IEC 62424, you understand how the process is measured and controlled, and you have a strong basis for asset inventory and zoning according to IEC 62443.