# Reading a P&ID: the piping and instrumentation diagram

> The P&ID is the reference drawing of a process plant: equipment, piping, instruments and control loops. Reading it is a core skill in automation as much as in operations.

- Source : https://industryhub.site/en/learn/process/lire-un-pid
- Track : Process
- Level : Practitioner
- Reading time : 6 min
- Language : English

## The pivot document of engineering

The P&ID — *Piping and Instrumentation Diagram* — is the reference drawing of a process plant. Where the process engineer thinks in equipment, the instrument engineer in sensors and the control engineer in loops, the P&ID is the common document that brings them together. It is what the plant is built, operated, troubleshot and modified from, throughout its life.

## What you find on it

A P&ID shows, with no scale or proportion but with rigour:

- every **piece of equipment**: vessels, pumps, exchangers, columns, each with a unique tag;
- every **pipe**: diameter, material, fluid, line number;
- every **valve**: control, isolation, safety;
- every **instrument**: sensors, transmitters, controllers, and the loops that link them.

## Instruments: the language of bubbles

Each instrument is drawn as a circle, a "bubble". A plain circle means a local instrument, close to the process; a circle crossed by a horizontal line means an instrument accessible in the control room, typically in the [DCS](/en/learn/automation/plc-dcs-scada). The connections read too: a solid line for piping, dashes for an electrical signal, other conventions for pneumatic or digital bus. Reading these circles and lines means reading the control scheme without opening the controller.

## Tags: letters that speak

The letter code, standardised by the ISA (standard ISA-5.1), follows a simple logic. **The first letter denotes the measured variable**: F for flow, L for level, P for pressure, T for temperature, A for analysis. **The following letters denote the function**: I indicator, C controller, T transmitter, V valve. So "FIC" is a flow indicating controller, "LT" a level transmitter, "PV" a pressure valve. Once this code is in your head, a P&ID reads like a sentence.

| Position | Common letters |
|---|---|
| 1st letter (variable) | **F** flow · **L** level · **P** pressure · **T** temperature · **A** analysis |
| Following letters (function) | **I** indicator · **C** controller · **T** transmitter · **V** valve · **S** switch/safety · **A** alarm |

## From the P&ID to the signal

Behind each bubble sits a real signal to wire and scale — most often a 4-20 mA loop whose endpoints must be converted into a physical value, which is what a [4-20 mA scaling calculation](/en/tools/instrumentation/4-20ma-scaling) does. The P&ID is therefore the bridge between drawing and controller: each drawn loop becomes an input, an output and a control block in the control system.

## Don't confuse PFD and P&ID

Upstream of the P&ID sits the **PFD** (*Process Flow Diagram*), simpler: it gives the overview, the major equipment and the main streams, without the detail of valves or instruments. The PFD answers "what does the process do?"; the P&ID answers "how is it built and controlled?". You move from one to the other as engineering progresses, from concept to detail. The [ISA-88](/en/standards/isa-88) and [ISA-95](/en/standards/isa-95) standards then structure, respectively, batch recipes and the link to enterprise management.

## Key takeaway

The P&ID shows every piece of equipment, line and instrument of a plant with standardised symbols. Reading it means turning a drawing into an understanding of the process and its control — without opening the controller.