How to Read a P&ID: Symbols, Tags, and a Worked Example

A Piping and Instrumentation Diagram — universally shortened to P&ID — is the master reference document for any process plant. It shows every piece of equipment, every pipe, every valve, and every instrument on a single drawing, together with the control loops that hold the process at the right temperature, pressure, level, or flow.

If you have ever stared at one and felt like you were reading hieroglyphics, you are not alone. P&IDs pack an enormous amount of information into a compact space using a standardised symbol language defined by ISA-5.1. Once you crack the code, you can extract in thirty seconds what would otherwise take thirty minutes of verbal explanation: what the process does, where the measurements are taken, how control is achieved, and which field devices talk to which PLC I/O channel.

This guide walks you through the complete symbol library, explains instrument bubble notation and ISA tag letter codes, covers line types, and then follows a real level-control loop from the transmitter in the field all the way to the ladder logic in the PLC.

What Is a P&ID (and How It Differs from a PFD and BFD)

Three drawing types describe a process at different levels of detail. Engineers and technicians routinely confuse them because the names sound similar.

A Block Flow Diagram (BFD) is the highest-level view. Each block represents an entire section of the plant — Reactor, Separator, Distillation Column — with arrows showing how streams flow between sections. There are no instruments, no valves, and no pipe specifications.

A Process Flow Diagram (PFD) adds one level of detail: major equipment is drawn to recognisable symbols (vessels, pumps, heat exchangers), key process streams carry flow rates, temperatures, and pressures, and the major control loops appear in simplified form. PFDs are used during design and front-end engineering to establish mass and energy balances.

A P&ID is the fully detailed engineering drawing. It shows:

• Every pipe, with its size and specification
• Every valve, including manual isolation valves and control valves
• Every instrument and its tag number
• Every control loop, including the signal lines between field devices and the control system
• Utility connections (air, steam, nitrogen)
• Safety devices (relief valves, rupture discs)

The key practical difference: a P&ID is the document you work from during commissioning, operations, and maintenance. A HAZOP study is conducted against the P&ID because it captures every potential deviation from design intent.

The P&ID Symbol Library

ISA-5.1 defines a standardised graphic vocabulary. Most engineering organisations publish their own symbol legend sheet at the front of every P&ID set, but the core shapes are consistent across industries worldwide.

Equipment Symbols

Symbol type	Typical shape	Common examples
Vertical vessel	Upright capsule / cylinder	Storage tanks, reactors, absorbers
Horizontal vessel	Horizontal capsule	Flash drums, knockout pots
Centrifugal pump	Circle with inlet arrow	Process pumps, transfer pumps
Compressor / blower	Triangle pointing into flow	Air compressors, gas blowers
Heat exchanger	Rectangle with internal dashed line	Shell-and-tube, plate-frame
Fired heater	Rectangle with flame symbol	Furnaces, heaters
Column / tower	Tall rectangle with internal trays	Distillation, absorption

Valve Symbols

Valves on a P&ID use a "butterfly" shape — two opposing triangles pointing inward — with modifications to indicate type:

• Gate valve: two open triangles, tips touching at the pipe centreline
• Globe valve: two triangles with a circle at the pinch point
• Ball valve: two triangles with a filled circle
• Butterfly valve: horizontal line through the pipe symbol with a cross-bar
• Control valve: two triangles plus an actuator symbol on top (spring, diaphragm, or motor)
• Pressure safety valve (PSV): one triangle plus a vertical bar indicating the spring

Control valves carry a fail position annotation — FC (fail closed), FO (fail open), or FL (fail last) — which is critical for safety and process design.

Instrument Symbols

Every field device is drawn as a circle (sometimes called a "bubble" or "balloon"). The line style of the circle tells you where the instrument is located and how it is accessed:

• Solid-line circle: locally mounted in the field (visible and accessible to an operator walking the plant)
• Single horizontal line through the circle: mounted on a panel or in the control room
• Double horizontal line through the circle: mounted in the control system (DCS or PLC), not physically accessible in a conventional sense
• Dashed-line circle: in a remote or inaccessible location

Inside the circle, the ISA tag code identifies the instrument's function — covered in detail in the next section.

Instrument Bubbles and Location Classification

The instrument bubble is the core information unit of a P&ID. Understanding its anatomy lets you answer three questions at a glance: what does the instrument measure, what does it do with that measurement, and where does it live?

The horizontal dividing line inside the circle splits the tag into two rows. Above the line: letter codes. Below the line: the loop number. The combination is globally unique within a plant — no two instruments share the same tag.

ISA Tag Letter Codes

The ISA-5.1 standard assigns a letter alphabet to measured variables and instrument functions. You only need to memorise the most common dozen or so; the rest you can look up on the legend sheet.

First Letter — Measured Variable

The first letter identifies what is being measured or initiated:

Letter	Measured variable
A	Analysis (composition, quality)
F	Flow
L	Level
P	Pressure
T	Temperature
S	Speed / frequency
W	Weight / force
Z	Position / dimension
U	Multivariable

Subsequent Letters — Function

The letters that follow describe what the instrument does with the measurement:

Letter	Function
I	Indicator (local or remote display)
T	Transmitter (sends a 4–20 mA or digital signal)
C	Controller (computes a control output)
R	Recorder
A	Alarm
H	High
L	Low
S	Switch
V	Control valve (final element)
E	Primary element (e.g. orifice plate)
Y	Relay or compute function

Reading a Loop Tag in Full

Take the tag TT-201:

• T (first letter) = Temperature — this instrument measures temperature
• T (second letter) = Transmitter — it converts the measurement to a signal
• 201 = Loop number 201 — it belongs to temperature control loop 201

Its companion instrument is TIC-201:

• T = Temperature
• I = Indicator — there is a display
• C = Controller — the control algorithm lives here
• 201 = Same loop 201

The tag suffix convention TT-201 can also be written as TT201 or 201-TT, depending on site convention. All three refer to the same device.

For transmitters that output a 4-20mA current loop signal, the tag string tells you exactly what physical signal to expect at the PLC analog input terminal.

Other common tag strings you will encounter frequently:

Tag	Meaning
FE-301	Flow element (orifice plate, venturi) — loop 301
FT-301	Flow transmitter — same loop, sends the signal
FIC-301	Flow indicating controller — where the PID runs
FCV-301	Flow control valve — the final element
FI-301	Flow indicator only (no control)
LSH-401	Level switch, high — loop 401
PSL-502	Pressure switch, low — loop 502
TRC-601	Temperature recording controller

P&ID Line Types

Lines on a P&ID are not all the same. The line style tells you what medium is being carried or what type of signal is being transmitted.

Process lines (main pipes carrying product) are drawn as solid bold lines. Pipe size and specification (e.g. 4"-P-A1A-150#-CS) may annotate the line.

Utility lines (cooling water, steam, compressed air, nitrogen) are typically solid lines of lighter weight, or may use a standard colour coding on coloured drawings.

Signal lines define the communication medium between instruments:

Line style	Signal type
Solid thin line	Electrical signal (4-20mA, discrete, Ethernet)
Dashed line - - -	Pneumatic signal (3-15 psi instrument air)
Dotted line ·····	Capillary / hydraulic signal
Line with slashes —//—	Software / data link (bus, fieldbus, DCS highway)

Knowing the signal line type is critical when you start tracing loops: a pneumatic signal means you need an I/P (current-to-pressure) converter somewhere in the chain, while an electrical signal feeds directly to a PLC analog input card. Visit our instrumentation hub for deeper coverage of signal types and field wiring.

Reading a Real Loop Step by Step

The best way to build P&ID fluency is to walk a complete control loop from measurement to final element. We will use a tank level control loop — one of the most common loops in any process plant.

Step 1 — Identify the Process Stream

Start at the pipe that feeds the tank. The bold horizontal line on the left is the feed stream. It carries product from upstream. Read the pipe annotation for size and specification.

Step 2 — Find the Final Element

Scan along the feed pipe for a control valve symbol — the two opposing triangles with an actuator on top. This is LCV-201 (Level Control Valve, loop 201). The actuator circle carries the letters FC, meaning the valve fails closed on loss of signal or instrument air. A closed valve stops feed, which is the safe failure mode for an overfill scenario.

Step 3 — Locate the Sensing Element

On the tank wall you find LT-201 — a transmitter circle with no line through it, meaning it is field-mounted (locally accessible). The tag tells you it is a Level Transmitter on loop 201. It will output a 4-20 mA signal proportional to level: 4 mA at the low-level span point, 20 mA at the high-level span point. See our dedicated pressure transmitter guide for a close look at differential-pressure level transmitters, which are common in this application.

Step 4 — Trace the Signal to the Controller

A thin solid line (electrical signal) runs from LT-201 to a double-line circle labelled LIC-201. The double line tells you the controller is inside the DCS or PLC, not a standalone instrument panel. LIC = Level Indicating Controller — it displays the current level and runs the PID algorithm. For a deep dive on tuning this algorithm, see the PLC PID tuning guide.

Step 5 — Follow the Output Back to the Valve

A second signal line leaves LIC-201 and routes back to LCV-201. This is the controller output — a 4-20 mA signal that drives the valve actuator. In practice there is usually an I/P (current-to-pneumatic) converter between the PLC output card and the valve, converting the electrical signal into 3-15 psi instrument air to stroke the diaphragm actuator. The I/P may or may not appear explicitly on the P&ID depending on the detail level.

Step 6 — Check Alarms and Interlocks

Above and below the normal operating level you will often see additional instrument bubbles: LSH-201 (Level Switch High) and LSL-201 (Level Switch Low). These discrete devices trigger alarms and safety shutdowns independently of the continuous control loop. A HAZOP study will have established at what level these switches activate and what protective action they initiate.

Summary: Reading the Loop as a Sentence

"The inlet feed rate to TK-101 is controlled by LCV-201 (fail-closed). Tank level is measured by LT-201 (field-mounted, 4-20 mA) and transmitted to LIC-201 inside the PLC, which calculates a PID output and adjusts LCV-201 to maintain the level setpoint."

One sentence. Every word is visible on the P&ID if you know the language.

From P&ID to PLC — Mapping a Loop to I/O and Logic

This is where P&ID reading becomes a practical automation engineering skill. Once you can read a loop on paper, the next step is mapping it to the PLC hardware and software that implement it.

The I/O List Comes from the P&ID

Every instrument bubble with an electrical signal line going to a double-line (DCS/PLC) bubble becomes a row in the I/O list — the document that maps field devices to PLC terminals. For loop LIC-201:

I/O point	PLC channel	Signal type	P&ID tag	Eng. units
LT-201 input	AI0	4-20 mA analog input	LT-201	0-5 m
LCV-201 output	AO0	4-20 mA analog output	LCV-201	0-100% open
LSH-201 input	DI14	24 VDC discrete input	LSH-201	HIGH alarm
LSL-201 input	DI15	24 VDC discrete input	LSL-201	LOW alarm

Scaling the Analog Input

The 4-20 mA signal from LT-201 arrives at the PLC analog input card across a 250-ohm burden resistor (1-5 V at the card terminals). The PLC converts this to a raw integer count (typically 0-27648 in Siemens, 0-32767 in Allen-Bradley). You must scale this to engineering units for the PID block:

Level (m) = (RawCount / MaxCount) × Span + LowLimit

For a 0-5 m level transmitter with a 16-bit card:

Level (m) = (RawCount / 27648) × 5.0

Our 4-20mA current loop explained article covers the exact scaling formulas and PLC register math in detail, including how to handle square-root extraction for differential-pressure flow transmitters.

The PID Function Block

Once the scaled PV (process variable) reaches the CPU, it feeds a PID function block — PID_Compact in TIA Portal, PIDE in Studio 5000, or a generic PID_FBD block in IEC 61131-3. The block's inputs are:

• PV: the scaled level from AI0
• SP: the level setpoint (e.g. 2.5 m), written by the operator from HMI
• Mode: Auto / Manual / Cascade

The block output (0-100%) connects to AO0. See the PLC PID tuning complete guide for how to choose Kp, Ki, and Kd values that keep the level stable without excessive valve hunting.

The Analog Output and I/P Converter

AO0 sends a 4-20 mA signal to an I/P transducer (current-to-pneumatic converter) mounted on or near LCV-201. The I/P converts 4 mA to 0.2 bar (3 psi) — valve fully closed — and 20 mA to 1.0 bar (15 psi) — valve fully open. This pneumatic signal strokes the diaphragm actuator of LCV-201. See the companion article on control valve sizing and actuator types for more detail.

Why ISA Tags Make PLC Programming Easier

When you name PLC variables to match the P&ID tag — LT_201_PV, LIC_201_SP, LCV_201_OUT — the program is self-documenting. Any engineer familiar with the P&ID can read the ladder or FBD without a separate variable list. This convention also makes it trivial to trace a fault: the alarm tag in the HMI, the variable name in the PLC program, and the device on the P&ID all share the same three-character code.

For a deeper look at sensor types that feed these loops, our types of industrial sensors overview covers the measurement technologies behind each first letter in the ISA alphabet (F, L, P, T, and more). The pressure transmitter explained article then goes deep on the most common sensor type for both pressure and differential-pressure level measurement.

Practical Tips for Reading Any P&ID

1. Always read the legend sheet first. Site-specific symbols, colour codes, and tag conventions are defined there. Do not assume an unfamiliar symbol follows the ISA default.

2. Follow streams, not equipment. Start at a feed point and trace the pipe through the system. Control loops will become visible as you cross each measurement point.

3. Count the control loops. Each unique loop number (e.g. 101, 201, 301) represents one closed-loop control function. Count them to estimate PLC I/O requirements before you start designing the cabinet.

4. Note every alarm and interlock. Switches (S in the second letter group) and high/low annotations (H, L) identify safety layer devices. These drive discrete inputs and become rungs in the safety PLC or SIL-rated logic.

5. Check fail positions. Every control valve should declare FC, FO, or FL. If it does not, the P&ID is incomplete — raise a comment in the design review.

6. Match revision numbers. P&IDs are living documents. Always verify you are reading the issued-for-construction (IFC) revision, not a superseded draft.

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Frequently Asked Questions

How do you read a P&ID?

Start with the legend sheet to understand site-specific symbols. Then trace a process stream from source to destination, identifying each piece of equipment by its symbol shape and tag number. For every control loop, find the sensing element (tag ending in T for transmitter or E for primary element), follow the signal line to the controller bubble (tag ending in C), and then follow the output signal to the final element (tag ending in V for valve). The ISA tag letters tell you what is measured and what function is performed; the loop number ties all elements of the same loop together.

What are P&ID symbols?

P&ID symbols are a standardised graphical vocabulary defined by ISA-5.1. Equipment (vessels, pumps, heat exchangers) has distinct shapes. Valves use a double-triangle "butterfly" shape modified to show valve type and actuator. Instruments are drawn as circles (bubbles) whose interior letter codes identify the measured variable and function, and whose line style identifies physical location. Signal lines use solid, dashed, or dotted lines to distinguish electrical, pneumatic, and software communication.

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

A Process Flow Diagram (PFD) shows the major equipment and key process streams with their operating conditions (temperature, pressure, flow), but only includes major control loops in simplified form. It is used for design and mass-balance calculations. A P&ID shows every pipe, every valve, and every instrument in full detail, including signal lines, control loop logic, safety devices, and utility connections. The P&ID is the working document for construction, commissioning, operations, and maintenance.

What do the letters on a P&ID mean?

The letters inside an instrument bubble follow the ISA-5.1 tag code standard. The first letter identifies the measured or initiating variable: F = Flow, L = Level, P = Pressure, T = Temperature, A = Analysis, S = Speed. The subsequent letters describe the instrument's function: I = Indicator, T = Transmitter, C = Controller, R = Recorder, A = Alarm, H = High, L = Low, S = Switch, V = Control valve. For example, FIC = Flow Indicating Controller; LSH = Level Switch High; TT = Temperature Transmitter.