🔹 Introduction
If you’ve ever opened a P&ID (Piping and Instrumentation Diagram) and felt lost among the circles, lines, and cryptic tags like PT-101 or FV-203A, you’re not alone.
P&IDs are the blueprints of process control systems. They combine mechanical equipment, piping, and instrumentation into a single engineering drawing that tells how the process works and how it’s controlled.
This blog explains — in simple, practical terms — how to read, interpret, and understand P&ID symbols and instrument tags. Whether you’re a student, maintenance technician, or a design engineer, mastering this skill is essential in the instrumentation world.
🔹 What Is a P&ID?
A P&ID is a diagram showing the functional relationship between piping, process equipment, and control instrumentation.
It includes:
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Process lines (pipes)
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Valves and equipment (pumps, vessels, exchangers)
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Measurement instruments and control loops
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Electrical and pneumatic signal connections
In short:
💡 “A P&ID tells you how every part of a plant talks to each other — mechanically and electronically.”
🔹 Difference Between PFD and P&ID
| Feature | PFD (Process Flow Diagram) | P&ID (Piping and Instrumentation Diagram) |
|---|---|---|
| Purpose | Shows overall process flow | Shows detailed control & instrumentation |
| Focus | Major equipment & streams | Valves, lines, control loops, interlocks |
| Level of Detail | Simplified | Detailed |
| Used By | Process engineers | Instrumentation, control, and operations teams |
🔹 Understanding Instrument Symbols
Every instrument in a P&ID is represented by a circle (or balloon) with letters and numbers inside.
Example:
Here’s how to read it:
| Part | Meaning |
|---|---|
| P | First letter = Measured variable → Pressure |
| T | Second letter = Function → Transmitter |
| 55 | Tag number = Unique identifier for that loop |
So PT-55 means: Pressure Transmitter number 55.
🔹 Instrument Tag Letter Codes (ISA 5.1 Standard)
| Variable (1st Letter) | Meaning |
|---|---|
| F | Flow |
| P | Pressure |
| T | Temperature |
| L | Level |
| A | Analysis |
| H | Hand / Manual |
| S | Speed / Position |
| Function (2nd Letter) | Meaning |
|---|---|
| I | Indicator |
| T | Transmitter |
| C | Controller |
| V | Valve |
| R | Recorder |
| S | Switch |
| E | Element / Sensor |
| Z | Final Control Element (Actuator) |
🔹 Examples of Common Instrument Tags
| Tag | Description | Typical Use |
|---|---|---|
| FT-101 | Flow Transmitter | Measures flow rate |
| FIC-101 | Flow Indicating Controller | Maintains flow setpoint |
| FV-101 | Flow Control Valve | Regulates flow |
| LT-202 | Level Transmitter | Measures tank level |
| LIC-202 | Level Controller | Controls tank level |
| LV-202 | Level Control Valve | Adjusts inlet/outlet flow |
| TT-301 | Temperature Transmitter | Measures process temperature |
| TIC-301 | Temperature Controller | Controls process temperature |
| TV-301 | Temperature Control Valve | Modulates steam or cooling water |
| PSH-401 | Pressure Switch High | Triggers alarm at high pressure |
| LSL-201 | Level Switch Low | Indicates low tank level |
🔹 P&ID Instrument Connections and Symbols
The type of signal between instruments is shown using different line types.
🔹 Typical P&ID Control Loop Example
Let’s look at a flow control loop example:
Explanation:
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FT-101 measures process flow and sends a 4–20 mA signal to FIC-101.
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FIC-101 compares actual flow with setpoint and outputs a control signal.
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FV-101 (control valve) adjusts the flow accordingly.
This loop forms a closed control loop.
🔹 Bubble (Balloon) Symbols and Location
🔹 Instrument Index and Tagging Rules
Every project maintains an Instrument Index — a master list of all instruments in the plant.
The tag must include:
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Loop number
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Device type
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Unit/area code (optional)
Example Tag Convention:
Example:
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U01 → Unit 1 (e.g., Crude Distillation)
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PT → Pressure Transmitter
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101 → Loop number
🔹 Reading Interlocks and Alarms on P&ID
Interlocks and alarms are often shown with logic symbols or functional diagrams beside the control loop.
Example:
Meaning:
If high pressure switch PSH-401 is activated, it will trip the pump to prevent overpressure.
🔹 Common P&ID Symbols (Simplified Overview)
🔹 Practical Tips for Reading P&IDs
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Start with the process flow – follow the main lines and direction of flow.
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Identify key control loops – look for transmitter → controller → valve combinations.
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Note the tag numbers – they usually follow consistent logic per plant or unit.
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Check signal types – pneumatic or electronic?
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Locate interlocks and alarms – safety loops are always marked distinctly.
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Refer to the Instrument Index – to cross-check device details.
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Remember: P&ID is not just for design — it’s your best friend during commissioning, troubleshooting, and maintenance.
🔹 Real-World Example: Level Control in a Storage Tank
Working:
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LT-202 measures tank level.
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LIC-202 compares measured level to setpoint.
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LV-202 adjusts outlet flow to maintain the desired level.
If LSL-202 (low level switch) activates → pump stops automatically to protect it from dry running.
🔹 Keywords
P&ID symbols, instrument tag meaning, how to read P&ID, ISA 5.1 instrumentation symbols, P&ID control loops, instrument index, instrument tag numbering, piping and instrumentation diagram tutorial.
🔹 Conclusion
Understanding P&ID symbols and instrument tags is one of the most fundamental skills in instrumentation engineering.
A P&ID is not just a drawing — it’s a map of your entire control system.
Once you learn how to interpret it:
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You can troubleshoot faster.
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You can verify installations.
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You can design control loops confidently.
📘 “If you can read the P&ID, you can understand the plant.”
Master this skill — and you’ll stand out in commissioning, design, and maintenance roles across any industry.
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