How to Read a P&ID: Symbols, Instrument Tags & Signal Lines

In Part 1, we learned what a Piping and Instrumentation Diagram (P&ID) is, why it is important, and how a P&ID differs from a Process Flow Diagram (PFD).

Now let's move to one of the most important skills for an instrumentation engineer: understanding the symbols, instrument tags, valves, piping and signal lines used on a P&ID.

Once you understand these symbols, a complicated P&ID becomes much easier to read.

Note: P&ID symbols and line conventions can vary between companies and projects. The examples below are simplified educational representations. Always refer to the approved project legend and applicable engineering standards when working on an actual project.

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1. Equipment Symbols on a P&ID

Process equipment represents the major physical equipment used in an industrial plant. Equipment is normally identified by an equipment tag.

Common equipment includes:

• Pumps
• Storage tanks
• Pressure vessels
• Heat exchangers
• Compressors
• Reactors
• Filters

Pump

Pump — Typical Tag: P-101

A pump transfers liquid from one location to another by adding energy to the fluid.

On a P&ID, a pump may be shown together with suction and discharge piping, isolation valves, check valves, instruments and other associated equipment.

Tank / Vessel

Vessel — Typical Tag: V-101

Vessels and tanks can be used for storage, separation, mixing, reaction or other process functions.

Heat Exchanger

Heat Exchanger — Typical Tag: E-101 / HX-101

A heat exchanger transfers heat between two fluids. The exact symbol depends on the type of exchanger and project drafting standard.

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2. Piping Symbols

Piping connects process equipment and allows liquids, gases, steam and other fluids to move through the plant.

When reading a pipe on a P&ID, look for:

• Line number
• Pipe size
• Service
• Flow direction
• Pipe specification
• Valves
• Drains
• Vents
• Instrument connections

Basic Process Line

Process line with flow direction

The arrow indicates the direction of process flow in this simplified example.

Line Number

A process pipeline normally receives a unique line number.

For example:

6"-P-101-150-CS

The meaning of each part depends on the project line-numbering philosophy. It may contain information such as pipe size, service, sequence number, pressure class or material specification.

Never assume the format is universal. Always check the project's line list and P&ID legend.

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3. Valve Symbols

Valves are used to isolate, regulate, divert or control process flow.

Some of the most common valves encountered on P&IDs are:

• Gate Valve
• Globe Valve
• Ball Valve
• Butterfly Valve
• Check Valve
• Control Valve
• On-Off Valve
• Pressure Relief Valve

Gate Valve

Gate Valve

Gate valves are commonly used for isolation and are generally not selected for continuous throttling service.

Globe Valve

Globe Valve

Globe valves are often used where flow regulation or throttling is required, depending on the service.

Ball Valve

Ball Valve

Ball valves are widely used for quick isolation and shut-off applications.

Butterfly Valve

Butterfly Valve

Butterfly valves are commonly used on larger-diameter pipelines because of their compact construction.

Check Valve

Check Valve

A check valve is designed to permit flow primarily in one direction and help prevent reverse flow.

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4. Control Valve Symbol

A control valve is a final control element. It receives a command from the control system and changes the process flow to maintain the desired operating condition.

Control Valve — Example Tag: FCV-101

A typical flow control loop can be represented as:

FT → FIC → FCV

• FT = Flow Transmitter
• FIC = Flow Indicating Controller
• FCV = Flow Control Valve

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5. Instrument Symbols

Instrumentation symbols identify devices that measure, indicate, transmit, or control process variables.

The most common process variables are:

• Pressure
• Flow
• Level
• Temperature

Pressure Transmitter — PT

PT — Pressure Transmitter

Flow Transmitter — FT

FT — Flow Transmitter

Level Transmitter — LT

LT — Level Transmitter

Temperature Transmitter — TT

TT — Temperature Transmitter

Important: The exact shape of an instrument bubble and the line connecting it to the process or control system can depend on the applicable instrumentation standard and project drafting convention.

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6. Common Instrument Tags

Tag	Typical Meaning	Variable
PT	Pressure Transmitter	Pressure
PI	Pressure Indicator	Pressure
FT	Flow Transmitter	Flow
FI	Flow Indicator	Flow
LT	Level Transmitter	Level
LI	Level Indicator	Level
TT	Temperature Transmitter	Temperature
TI	Temperature Indicator	Temperature

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7. How to Read an Instrument Tag

Instrument tags are extremely important when reading a P&ID.

Consider the example:

FT-101

In a typical ISA-style identification:

• F identifies Flow as the measured variable.
• T identifies Transmitter as the function.
• 101 is the identification or loop number assigned by the project.

Therefore, FT-101 generally means Flow Transmitter, identification 101.

For another example:

PT-201

P = Pressure and T = Transmitter.

So PT-201 generally identifies a Pressure Transmitter.

Always verify the exact tag interpretation using the project's instrument index and tagging philosophy.

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8. Controller Tags

Controllers perform control functions based on the measured process variable and the desired set point.

Tag	Typical Meaning
PIC	Pressure Indicating Controller
FIC	Flow Indicating Controller
LIC	Level Indicating Controller
TIC	Temperature Indicating Controller

For example:

LIC-101 generally represents a Level Indicating Controller.

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9. Understanding a Complete Control Loop

Now let's connect the individual symbols together.

Suppose we have a tank where the liquid level must be maintained at a particular set point.

The basic control sequence is:

LT → LIC → Control Valve

Step 1 — Measurement

The LT measures the liquid level inside the tank.

Step 2 — Signal

The measured value is transmitted to the control system.

Step 3 — Controller

The LIC compares the measured level with the desired set point.

Step 4 — Control Action

The controller calculates the required corrective action.

Step 5 — Final Control Element

The control valve changes position to regulate the process flow.

This creates a closed-loop control system.

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10. Signal Lines on a P&ID

Signal lines show the connection between field instruments, controllers and other control-system elements.

Common signal types include:

• Electrical
• Pneumatic
• Hydraulic
• Digital communication
• Capillary connections where applicable

4–20 mA Signal

A very common analog instrumentation signal is 4–20 mA.

For example:

FT → DCS

The flow transmitter may represent its measured range using a 4–20 mA signal that is received by an analog input channel in the control system.

For example, a transmitter configured for:

0–100 m³/h

could be scaled by the control system so that the 4–20 mA signal corresponds to the configured flow range.

The exact scaling and signal architecture depend on the transmitter, I/O system and project design.

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11. P&ID Symbol Cheat Sheet

Symbol / Tag	Typical Function
P	Pump
V	Vessel
E	Heat Exchanger
PT	Pressure Transmitter
FT	Flow Transmitter
LT	Level Transmitter
TT	Temperature Transmitter
PIC	Pressure Indicating Controller
FIC	Flow Indicating Controller
LIC	Level Indicating Controller
TIC	Temperature Indicating Controller
FCV	Flow Control Valve

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12. How an EPC Instrumentation Engineer Should Read a P&ID

An EPC design engineer should not read a P&ID only to identify instruments.

You should read it from a design perspective.

For every instrument, ask:

• What process variable is being measured?
• Where is the measurement taken?
• What instrument technology is required?
• What is the process connection?
• Is an impulse line required?
• Is a root valve required?
• Is a manifold required?
• Where does the signal terminate?
• Is the signal analog or digital?
• Is the instrument connected to PLC, DCS or SIS?
• Is an alarm required?
• Is an interlock involved?
• Is the instrument part of a safety function?

This is the difference between simply recognizing a P&ID and actually using one for instrumentation design.

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13. Documents Used Alongside a P&ID

During an EPC project, the P&ID is normally used together with several other engineering documents.

• Instrument Index
• Instrument Datasheets
• Instrument Hook-up Drawings
• Loop Diagrams
• Cable Schedule
• Junction Box Schedule
• I/O List
• Control Valve Datasheets
• Cause & Effect Matrix
• Instrument Location Plan

Understanding how these documents relate to the P&ID is an important skill for an instrumentation design engineer.

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14. Common Beginner Mistakes

1. Memorizing Symbols Without Understanding the Process

Knowing that PT means Pressure Transmitter is not enough. Understand why the transmitter is installed and what happens with its measurement.

2. Ignoring the P&ID Legend

Different projects can use different graphical conventions. Always check the approved project legend.

3. Ignoring Signal Direction

Trace the signal from the field instrument toward the controller and then to the final control element.

4. Treating a P&ID as a Physical Layout

A P&ID represents process functionality and connections. It is not normally an exact physical installation drawing.

5. Assuming Every Tagging System Is Identical

ISA-based identification provides a framework, but individual projects can have additional conventions. Always verify the project instrument index and tagging philosophy.

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15. Quick Method to Read Any P&ID

1. Identify the major process equipment.
2. Find the main process flow direction.
3. Follow the main piping.
4. Identify isolation and control valves.
5. Find pressure, flow, level and temperature instruments.
6. Decode the instrument tags.
7. Trace the signal lines.
8. Identify controllers and final control elements.
9. Check alarms and interlocks.
10. Refer to the P&ID legend whenever a symbol is unfamiliar.

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16. P&ID Interview Questions

What is a P&ID?

A P&ID is a detailed engineering drawing that represents process equipment, piping, valves, instrumentation and control-related connections.

What does PT mean?

PT generally means Pressure Transmitter.

What does FT mean?

FT generally means Flow Transmitter.

What does LT mean?

LT generally means Level Transmitter.

What does FIC mean?

FIC generally means Flow Indicating Controller.

What is a control valve?

A control valve is a final control element used to regulate process flow or another process variable based on a control signal.

Why should you check the P&ID legend?

Because graphical symbols and line conventions can vary between projects. The approved legend defines the conventions used on that particular project.

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Key Takeaways

• Learn the meaning behind P&ID symbols rather than simply memorizing them.
• Follow the process flow from equipment to equipment.
• Learn how to decode instrument tags.
• Trace the complete control loop from measurement to controller to final control element.
• Understand the difference between field instruments and control-system functions.
• Always check the approved project P&ID legend.
• For EPC design work, connect P&ID information with datasheets, loop diagrams, hook-ups, cable schedules, I/O lists and cause & effect documents.

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Continue to Part 3

In Part 3, we will go beyond individual symbols and learn how to read a complete P&ID from beginning to end.

We will cover:

• How to trace the process through a P&ID
• How to identify complete control loops
• How to read alarms
• How to identify interlocks
• How to understand control valve information
• How to identify instrument connections
• How P&ID information is converted into EPC design documents
• A practical P&ID example

Remember: Don't just memorize the symbols. Understand the process, trace the signal, and ask why every instrument and valve is present.