P&ID diagrams (Piping and Instrumentation Diagrams) are full of strange-looking circles, letters, and lines. These are called instrument symbols, and they show sensors, controllers, and indicators used in a plant.
For someone new to engineering or chemical plants, these symbols can be confusing. But don’t worry! Once you understand the basic logic, it all becomes much easier.
In this article, we’ll explain the most common P&ID instrument symbols in simple English, so you can start reading diagrams with more confidence.
1. What Are Instrument Symbols?
Instrument symbols are visual codes used in P&ID drawings to show things like:
- Temperature sensors
- Pressure gauges
- Flow controllers
- Level indicators
They help engineers understand what is being measured, where, and how it is controlled.
2. The Basic Format of Symbols
Most symbols follow a structure like:
FT
Where:
- F stands for the measured variable (e.g., Flow)
- T stands for the function (e.g., Transmitter)
Here are some common letters:
- T = Temperature
- P = Pressure
- F = Flow
- L = Level
- I = Indicator
- C = Controller
- V = Valve
So, for example:
- PI = Pressure Indicator
- TC = Temperature Controller
- FV = Flow Valve
3. How to Read the Symbols
Look at the letters and think:
- What is being measured?
- Is it just showing the value (Indicator)?
- Is it also controlling something (Controller)?
- Does it send data somewhere (Transmitter)?
Symbols often appear inside circles. Lines connected to them show where the signal goes—solid lines are physical connections, and dashed lines are electric or pneumatic signals.
4. Common Examples
| Symbol | Meaning |
|---|---|
| TI | Temperature Indicator |
| PT | Pressure Transmitter |
| LIC | Level Indicator Controller |
| FCV | Flow Control Valve |
Summary
P&ID instrument symbols may look like code at first, but they’re built from simple rules:
- First letter = what you measure (T, P, F, L)
- Next letters = what it does (I, T, C, V)
- Use the shape and line type to understand how it’s connected
Start with the basics and build your knowledge slowly. With time, you’ll be reading P&IDs like a pro!