How to read PFD & P&ID diagrams

Engineering drawings serve as a common language for design, operation, and troubleshooting in process sectors such as oil and gas, chemical, petrochemical, and power production. The most important papers are Process Flow Diagrams (PFDs) and Piping & Instrumentation Diagrams (P&IDs). While both describe the movement of materials through a system, they perform separate functions. Understanding how to understand these diagrams is an essential ability for any engineer, operator, or technician engaged in plant design and maintenance.

This blog gives an organized and practical method to reading PFDs and P&IDs, allowing novices to gain confidence and pros to improve their accuracy.

What is a Process Flow Diagram?

• A Process Flow Diagram (PFD) depicts the entire process flow of a facility. It focuses on significant equipment, material flows, and key process variables to provide a high-level overview of the system.

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A PFD often includes:

• Major equipment such as pumps, compressors, heat exchangers, reactors, and vessels.

• Basic process lines depicting the movement of materials.

• Flow direction arrows.

• Operating conditions: pressure, temperature, and flow rate.

• Utility needs include steam, cooling water, and fuel.

• Mass and Energy Balances

• Minimal instrumentation.

• Equipment capacity and design data

What is a Piping and Instrumentation Diagram?

• A Piping and Instrumentation Diagram (P&ID) provides a more thorough picture of the process. It consists of equipment, pipelines, valves, instruments, control loops, safety systems, and interlocks, which serve as a blueprint for operation and maintenance.

A P&ID often includes:

• All pipes, including size, material, and insulation

• Valves: manual, motorized, control, and safety valves

• Instrumentation symbols include transmitters, indicators, and controllers.

• Control loops and linkages.

• Relief systems and flare lines.

• Equipment tags and numbers.

• Line numbers and specs.

• Ancillary items: strainers, vents, drains, filters.

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How to Read a PFD: Step-by-Step:

1. Begin with the Process Overview:

Begin by determining the main blocks or components. PFDs usually show:

• Feed streams

• Process units

• Product Streams

Understanding the overall picture allows you to navigate the particular flows later.

2. Identify Equipment using Symbols and Tags:

Each piece of equipment is assigned a unique number and is represented symbolically.

For example:

• P-101 may signify a pump.

• E-201, a heat exchanger

• R-301 is a reactor

The actual numbering follows plant-specific criteria, although the forms are typically constant.

3. Trace the material flow:

• Follow the flow arrows to understand

• How feed enters

• How processes change materials.

• How Products Leave

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Pay close attention to split streams, recycling loops, and bypass lines, which are often crucial for evaluating process performance.

4. Take note of the key operating conditions:

PFDs often provide:

• Temperature (degree Celsius)

• Pressure (bar, kPa)

• Flow rate (kg/h, m³/h)

• Composition

These characteristics assist engineers in calculating mass and energy balance.

5. Review the mass and energy balance tables:

Most PFDs contain a summary table. This helps to validate:

• Input versus output fluxes

• Heat responsibilities.

• Pump/compressor power needs.

P&ID Reading: Step-by-Step:

P&IDs need additional information. To understand them well:

1. Learn Standard Symbols:

Standard P&ID symbols are ISA, ISO, or plant-specific.

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Learn the symbols for:

• TI, PI, LIC, FI instruments

• Gate, globe, ball, check, control valves

• Pumps, tanks, heat exchangers

• Process, utility, pneumatic, electrical lines

A legend is usually included with the drawing.

2. Know Tag Numbers:

The structure of instrument and equipment tags is standard. As an example:

PIT-101:

• P=Pressure

• I = Signifying

• T = The transmitter

CV-205:

• CV=Control Valve

These marks indicate device type and function.

3. Follow Piping and Line Numbers:

P&ID piping lines are more elaborate than PFDs. Line numbers often look like:

6”-P-101-CS-150#

Which means:

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• 6-inch pipe

• Discharge pump or process code

• Material: Carbon Steel

• 150# class

Understanding line numbers aids construction and maintenance planning.

4. Examine Valve Setups:

Valves regulate fluid flow.

Common arrangements:

• Pump suction/discharge isolation valves

• Process loop control valves

• Valve checks prevent backflow

• Safety relief valves for flare or vent systems

• Check for bypass lines, double block-and-bleed, and specific valve sequences.

5. Read Control Loops Carefully:

Plant stability depends on control loops.

Loops often include:

• Sensor/Transmitter (PT)

• PC controller

• Final Element (CV)

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Lines between these pieces indicate pneumatic, electrical, or digital signals.

6. Identify safety and protection:

Critical safety systems include:

• Relief valves

• Disks rupture

• Emergency shutoff valves

• High-high-low alarms

These protection devices are prominently shown on P&IDs.

7. Pay Attention to Extras:

Also, P&IDs show:

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• Drains, vents

• Sloped gravity pipelines

• Filters, strainers, flanges

How to read PFD & P&ID diagrams