P&ID Design Flow: BFD to PFD to P&ID

The P&ID Development Sequence

Every P&ID starts simple and grows in detail through three stages:

1. BFD (Block Flow Diagram) - High-level process overview with blocks and arrows
2. PFD (Process Flow Diagram) - Major equipment, flow paths, and operating conditions
3. P&ID (Piping & Instrumentation Diagram) - Complete detail: every pipe, valve, instrument, and control loop

Process to Create a P&ID

Key Steps: From BFD to P&ID

The path from concept to construction-ready P&ID follows a logical progression. You don’t jump straight into pipe sizes and instrument tags-you build up to that level of detail.

Block Flow Diagram (BFD) - Start here. Blocks represent major unit operations (reactors, separators, columns), and arrows show material flow direction. No equipment details, no instrumentation. The goal is alignment on the overall process concept.

Process Flow Diagram (PFD) - Now add substance. Show the actual equipment (heat exchangers, pumps, vessels), indicate operating conditions (temperature, pressure, flow rates), and establish the basic material balance. Still no individual valve or instrument details.

Piping and Instrumentation Diagram (P&ID) - The complete picture. Every pipe with its line number, size, material class. Every valve, instrument, and control loop. The P&ID becomes the master document for construction, commissioning, and operations.

Flow & Instrumentation Diagram (F&ID) - A less common term, sometimes used for simplified P&IDs focused on control system visualization (DCS video pages). Essentially a stripped-down P&ID showing process lines and control loops.

Block Flow Diagrams (“BFD”)

A BFD strips the process down to its essence: boxes for unit operations, arrows for flow direction. Nothing more. This simplicity is the point-BFDs force you to think about the process logic before getting lost in equipment details.

Block Flow Diagram (BFD)

Process engineers typically create BFDs in the early project phases, often with input from the end-user’s process consultants. The diagram becomes a communication tool-everyone from project managers to clients can understand the overall flow without needing to interpret complex symbology.

When reviewing a BFD, you should be able to trace the material path from feed to product in seconds. If you can’t, the diagram needs work.

Pro Tip

Get client sign-off on the BFD before investing significant effort in the PFD. I’ve seen projects where engineering teams developed detailed PFDs only to have the client fundamentally change the process configuration. A frozen BFD protects your schedule and budget.

Process Flow Diagrams (“PFD”)

The PFD is where the process becomes tangible. You’ll see actual equipment-reactors, heat exchangers, columns, separators-with operating conditions annotated: temperatures, pressures, flow rates. Stream tables typically accompany the PFD, providing material and energy balances.

A well-prepared PFD answers the fundamental engineering questions: What equipment do we need? What are the design conditions? What’s the material balance? It doesn’t answer how we’ll pipe and control everything-that’s what the P&ID is for.

Process Flow Diagram (PFD)

Notice what’s missing from a typical PFD: relief systems, pump minimum flow lines, compressor anti-surge systems, isolation valves. These details come later.

Common Mistake

Skipping the heat and material balance (H&MB) validation before finalizing the PFD. If your stream tables don’t close, your equipment will be mis-sized. Always verify the H&MB independently - don’t assume the process simulation is error-free.

Process and Instrumentation Diagram (“P&ID” or P&I)

P&ID Definition

The P&ID is the master document of any process plant. It shows every pipe (with line number, size, and pipe class), every valve, every instrument, and every control loop. Construction crews build from it. Operators run from it. Maintenance teams troubleshoot from it.

Unlike the conceptual BFD or the equipment-focused PFD, the P&ID leaves nothing to imagination. If it’s not on the P&ID, it doesn’t get built.

Who Makes P&ID Diagrams?

P&IDs emerge from collaboration between process engineers (who define the mechanical and process requirements) and instrumentation engineers (who design the control systems). Using standardized symbology like ISA S5.1, each discipline contributes:

Discipline	Contribution to P&ID
Process	Equipment data, process lines, operating conditions
Piping	Line numbers, pipe classes, valve locations
Instrumentation	Field instruments, control loops, safety systems
Electrical	Power supply requirements, signal types

Tools to Design P&ID

The industry has largely moved past hand-drawn P&IDs. Today’s tools range from general-purpose (Microsoft Visio, AutoCAD) to specialized P&ID software with intelligent databases:

Tool	Best For
Microsoft Visio	Small projects, quick drafts
AutoCAD	Basic CAD functionality
SmartPlant P&ID (Intergraph)	Large EPC projects, database-driven design
AutoPLANT (Bentley)	Integration with 3D modeling
AVEVA Diagrams	Enterprise-level process plants

The advanced tools maintain a live database behind the diagram-change a valve size in the P&ID, and the material takeoff updates automatically. For major EPC projects, this integration is essential.

Learn more about the symbols used in P&ID diagrams.

Typical Content of P&ID Diagrams

A complete P&ID captures three categories of information:

Piping Details

• All process lines with line numbers, sizes, fluid service, pipe class, and insulation
• Startup, production, shutdown, emergency, and maintenance lines
• Configuration notes (free draining, low points, high points)
• Connection types at equipment nozzles

Instrumentation and Control

• Field instruments (transmitters, switches, gauges)
• Control valves with failure position and sizing data
• Safety and relief valves with set pressure and capacity
• Signal types (pneumatic, 4-20mA, digital/fieldbus)
• Control loops and interlocks

Project Boundaries

• Battery limits between EPC contractor and owner
• Packaged equipment boundaries
• Line breaks showing transitions between pipe classes or disciplines

Field Note

During construction, always work from the latest approved P&ID revision - never from a superseded version. I’ve seen flanges welded in the wrong orientation, valves installed backwards, and entire instrument runs deleted because someone used an outdated drawing. Establish a rigorous revision control system and enforce it on-site.

P&ID Example

For a simple and complete P&ID see the image below: it shows a simple Wessel level control consisting of an electrical control loop (with level transmitter LT 1, level controller LIC 1, I/P converter LY 1, and level control valve LCV1), and an electrical safety loop (with a level switch of high-level LSHH 2 with level alarm LAHH 2 and safety valve LSV 2 auctioned by the solenoid S); in the diagram, the necessary different supplies, like air (AS), power (ES) and water (WS) are also shown.

P&ID Diagram or P&I Example of a measured and controlled Wessel with discrete electronic instrumentation

Flow and Instrumentation Diagram (“F&ID” or FID)

The F&ID (sometimes called a “Scheme of March”) is derived from the P&ID but focuses on control system visualization. Think of it as a P&ID stripped down for DCS operator screens-showing process lines, main instrumentation, and control loops without all the piping detail.

The example below shows a 3-phase separator handling crude oil from production wells. Three control loops manage the separated phases:

Phase	Location	Controller
Gas	Top right	PC (Pressure Controller)
Oil	Bottom right	LC (Level Controller)
Water	Bottom left	LC (Level Controller)

Overpressure protection comes from two devices: a Pressure Safety Valve (PSV) and a Rupture Disk (PSE).

F&ID Example 3-phase separator unit A simplified example of a 3-phase separator: Gas, Oil, and Water

This simplified diagram demonstrates the four fundamental control concepts: measurement (monitoring process variables), regulation (controlling setpoints), actuation (manipulating valves), and safety (protective actions on limit exceedance).

Differences BFD, PFD, P&ID

Aspect	BFD	PFD	P&ID
Purpose	Conceptual overview	Process design	Construction & operation
Detail Level	Minimal	Moderate	Complete
Shows Equipment	As blocks	With basic specs	With full details
Shows Piping	As arrows	Major lines only	Every line with class
Shows Instruments	No	No	Yes, all of them
Shows Control Loops	No	No	Yes, complete
Typical Users	Management, clients	Process engineers	All disciplines

BFD vs. P&ID

The gap between a BFD and P&ID is enormous. A BFD might fit on a single page and be understood by anyone. A P&ID for the same process could span dozens of sheets and require specialized training to read. The BFD asks “what are we building?” The P&ID answers “exactly how are we building it?”

PFD vs. P&ID

The PFD-to-P&ID transition is where engineering really begins. The PFD establishes the equipment and operating conditions; the P&ID adds everything needed to actually build and operate the plant-relief systems, startup/shutdown lines, minimum flow bypasses, anti-surge controls, isolation valves, instrument details.

If you’re comparing documents, remember: a PFD is for understanding the process; a P&ID is for building and running it.

Related Articles

Engineering Documentation

• Pipe Class vs. Piping Specification - Understanding the differences between pipe classes and piping specifications in project engineering
• Engineering Glossary - Definitions of common piping and engineering terms
• P&ID Symbols List - Complete reference for P&ID symbols and instrumentation conventions

Testing and Inspection

• Pipe Inspections - Visual and dimensional inspection methods for piping materials
• Non-Destructive Testing (NDT) Types - Overview of NDT methods for piping inspection