BFD vs PFD vs P&ID: The Complete Design Flow (BFD to PFD to P&ID to FID)

The P&ID Development Sequence

Every P&ID starts simple and grows in detail through four 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, covering every pipe, valve, instrument, and control loop
4. FID (Fabrication Isometric Drawing): Dimensional fabrication document derived from the P&ID

Engineering document progression: BFD to PFD to P&ID to FID

The Design Flow: BFD to PFD to P&ID

Why There Is a Progression

You never jump straight from a blank page to a construction-ready P&ID. The design flow exists because each stage answers a different question and involves a different audience. The BFD asks “what are we building?” The PFD asks “what equipment and conditions are needed?” The P&ID answers “exactly how do we build, control, and operate it?”

This progression also manages risk. Freezing one level of design before investing effort in the next prevents expensive rework. A fundamental process change caught at the BFD stage costs hours; the same change caught at the P&ID stage costs weeks and potentially millions in re-engineering.

Block Flow Diagram (BFD)

What a BFD Shows

A BFD strips the process down to its essence: rectangles 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.

Each block typically represents a major unit operation: a reactor, a distillation column, a separation system, a compression train. The arrows indicate material flow from feed to product, with major recycle streams shown where applicable.

Block Flow Diagram (BFD)

Who Creates the BFD

Process engineers typically create BFDs in the early project phases, often during feasibility studies or conceptual design, with input from the end-user’s process consultants and technology licensors. The diagram becomes a communication tool that everyone from project managers to clients can understand without needing to interpret complex symbology.

When in the Project Lifecycle

BFDs are developed during the conceptual design and pre-FEED phases. They form the basis for preliminary cost estimates (Class 5 or Class 4 per AACE standards) and are used in project screening and option selection. When multiple process configurations are under consideration, each option will have its own BFD for comparison.

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

Pro Tip

Get client sign-off on the BFD before investing significant effort in the PFD. I have 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 Diagram (PFD)

What a PFD Shows

The PFD is where the process becomes tangible. You will see actual equipment (reactors, heat exchangers, columns, separators, pumps, compressors) with operating conditions annotated: temperatures, pressures, flow rates, and compositions. Stream tables typically accompany the PFD, providing complete material and energy balances.

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

Process Flow Diagram (PFD)

Content of a Typical PFD

Element	Details
Major equipment	Vessels, columns, heat exchangers, pumps, compressors (with tag numbers and basic capacity)
Process streams	Flow direction, composition, temperature, pressure, flow rate
Stream tables	Mass and energy balance data for each numbered stream
Utility streams	Cooling water, steam, instrument air, nitrogen, shown as connections
Equipment list	Summary of all major equipment with design parameters
Operating conditions	Normal and design temperatures/pressures for each equipment item

Notice what is missing from a typical PFD: relief systems, pump minimum flow lines, compressor anti-surge systems, isolation valves, drain and vent connections, startup and shutdown lines, and instrumentation detail. These come later in the P&ID.

Mass and Energy Balance

The PFD is inseparable from the heat and material balance (H&MB). Process simulation software (Aspen HYSYS, Aspen Plus, PRO/II, or UniSim) generates the stream data that appears on the PFD. Each numbered stream has a corresponding row in the stream table, documenting:

• Mass flow rate (kg/h or lb/h)
• Volumetric flow rate (at operating and standard conditions)
• Temperature and pressure
• Composition (molar or mass fractions)
• Vapor fraction
• Enthalpy or heat duty

Common Mistake

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

When in the Project Lifecycle

PFDs are developed during FEED (Front-End Engineering Design), typically as the first major process engineering deliverable. They undergo review by the client, process licensor (if applicable), and the EPC contractor’s internal review team. Approval of the PFD is a prerequisite for starting P&ID development.

Piping and Instrumentation Diagram (P&ID)

P&ID Definition

The P&ID is the master document of any process plant. Unlike the conceptual BFD or the equipment-focused PFD, the P&ID leaves nothing to imagination. If it is not on the P&ID, it does not get built.

The P&ID 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.

Who Creates 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, relief valve sizing
Piping	Line numbers, pipe classes, valve locations, spec breaks
Instrumentation	Field instruments, control loops, safety systems (SIS/SIL)
Electrical	Power supply requirements, signal types, motor-operated valve data
Mechanical	Equipment details, nozzle sizes, pressure ratings

Full Content of P&ID Diagrams

A complete P&ID captures several categories of information:

Equipment

• All process equipment with tag numbers, design conditions, and nozzle sizes
• Packaged equipment boundaries (skid limits)
• Rotating equipment (pumps, compressors) with drivers
• Vessels, columns, heat exchangers, tanks, filters

Piping

• All process lines with line designations: size, fluid service, sequence number, pipe class, and insulation
• Startup, production, shutdown, emergency, and maintenance lines
• Drain and vent connections
• Configuration notes (free draining, low points, high points, slope requirements)
• Spec breaks and connection types at equipment nozzles
• Special piping items: strainers, steam traps, orifice plates, spectacle blinds, expansion joints

Instrumentation and Control

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

Project Boundaries

• Battery limits between EPC contractor and owner
• Packaged equipment boundaries
• Line breaks showing transitions between pipe classes or disciplines
• Tie-in points for future connections or other project phases

Line Numbering Conventions

Every pipe on a P&ID carries a line designation (also called a line number) that encodes essential information in a single string. A typical format:

[Size]-[Service Code]-[Sequence Number]-[Pipe Class]-[Insulation Code]

Example: 6”-HC-1001-A1A-H = 6-inch diameter, hydrocarbon service, sequence 1001, pipe class A1A, hot insulation.

Field	Purpose	Examples
Size	Nominal pipe size	2”, 6”, 24”
Service code	Fluid type identifier	HC (hydrocarbon), CW (cooling water), S (steam), IA (instrument air)
Sequence number	Unique line identifier within the unit	1001, 2045, 3200
Pipe class	Material and pressure rating spec	A1A, B2B, D3C
Insulation code	Insulation requirement	H (hot), C (cold), PP (personnel protection)

A new line number is assigned whenever the pipe size, service, or pipe class changes. The line list is the master register of all line designations on the project.

Instrument Tag Numbering (ISA S5.1)

Instrument tags on P&IDs follow the ANSI/ISA-5.1 standard. Each tag consists of letter codes and a loop number:

[First Letter][Subsequent Letters]-[Loop Number]

The first letter identifies the measured variable. Subsequent letters identify the function:

First Letter	Measured Variable	Example Tags
F	Flow	FT-1001 (transmitter), FIC-1001 (indicating controller), FCV-1001 (control valve)
P	Pressure	PT-2005 (transmitter), PIC-2005 (controller), PSV-2005 (safety valve)
T	Temperature	TE-3010 (element), TT-3010 (transmitter), TIC-3010 (controller)
L	Level	LT-4001 (transmitter), LIC-4001 (controller), LSHH-4001 (high-high switch)
A	Analysis	AT-5002 (analyzer transmitter), AIC-5002 (analyzer controller)

Subsequent Letter	Function
T	Transmitter
I	Indicator
C	Controller
V	Valve
E	Primary element (sensor)
S	Switch
A	Alarm
H	High
L	Low
Y	Relay / compute

A tag like LSHH-4001 reads as: Level (L), Switch (S), High-High (HH), loop 4001. This tag identifies a high-high level safety switch that triggers an emergency action.

Pro Tip

Instrument tag numbering should follow the loop numbering philosophy defined in the project’s instrumentation standards document. Typically, the first digit(s) of the loop number correspond to the P&ID sheet or plant area number. For example, instruments on P&ID sheet 100 use loop numbers 1001-1099. This convention makes it easy to trace any instrument back to its P&ID.

P&ID Example

For a simple and complete P&ID see the image below: it shows a simple vessel 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 actuated 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 vessel with discrete electronic instrumentation

How P&IDs Drive Other Deliverables

The P&ID is not just a drawing; it is the source document for multiple downstream engineering deliverables:

Deliverable	Information Extracted from P&ID
Line list	Every line designation, from/to equipment, pipe class, insulation
Instrument index	Every instrument tag, type, range, location, control loop
Equipment list	All equipment tags, types, design conditions
Valve list	Every valve with tag, type, size, rating, material, actuator
Material take-off (MTO)	Pipe quantities, fittings, valves, special items by line
Cause & effect diagrams	Control and safety logic derived from P&ID interlock annotations
Piping isometrics	Physical routing and fabrication detail for each line

In database-driven P&ID software, many of these deliverables are generated automatically from the P&ID database, which reduces manual effort and transcription errors.

Field Note

During construction, always work from the latest approved P&ID revision, never from a superseded version. I have 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.

Flow and Instrumentation Diagram (F&ID / 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.

In some project contexts, FID also refers to Fabrication Isometric Drawings, the dimensional fabrication documents that take each pipeline from the P&ID and produce a spool-by-spool construction drawing. For detailed coverage of piping isometrics, see the piping isometrics guide.

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).

Design Progression Through EPC Project Phases

The BFD-PFD-P&ID sequence maps directly onto the standard EPC project phases:

Project Phase	Diagram Stage	Purpose	Typical Accuracy
Conceptual / Pre-FEED	BFD	Process option screening, feasibility	Class 5 estimate (+/-50%)
FEED	PFD + Preliminary P&IDs	Process definition, equipment sizing, CAPEX estimate	Class 3 estimate (+/-15%)
Detailed Engineering (30%)	P&IDs under development	Process design freeze, start piping modeling	Design basis frozen
Detailed Engineering (60%)	P&IDs for review	Interdisciplinary review, HAZOP input	Procurement initiated
Detailed Engineering (90%)	P&IDs near-final	Final review, instrument/valve data sheets complete	Construction preparation
IFC (Issued for Construction)	P&IDs approved	Construction-ready, all markups incorporated	Basis for construction
As-Built	P&IDs updated	Field changes incorporated, operations handover	Plant as constructed

HAZOP and the P&ID

The HAZOP (Hazard and Operability Study) is conducted using the P&ID as the primary reference document. A multidisciplinary team systematically examines each P&ID node (a section of the process between major equipment items) using guide words (No, More, Less, Reverse, Other Than) to identify hazards and operability problems.

HAZOP findings often result in P&ID modifications: additional safety instrumentation, relief devices, isolation valves, or changes to control philosophy. This is why the HAZOP is conducted while P&IDs are still in development (typically at the 60% review stage), not after they are issued for construction.

Each diagram stage must be frozen before investing effort in the next: BFD establishes the process concept, PFD adds equipment and operating conditions, and the P&ID captures every pipe, valve, instrument, and control loop needed for construction and operation. The P&ID then feeds all downstream deliverables (line lists, instrument indexes, MTOs, and piping isometrics).

Comparison Table: BFD vs PFD vs P&ID

Aspect	BFD	PFD	P&ID
Purpose	Conceptual overview	Process design	Construction and operation
Detail level	Minimal	Moderate	Complete
Shows equipment	As blocks	With basic specs and tag numbers	With full details and nozzle connections
Shows piping	As arrows	Major lines only	Every line with number, size, and class
Shows instruments	No	No	Yes, all with ISA tags
Shows control loops	No	No	Yes, complete with signal types
Shows safety systems	No	No	Yes (relief valves, SIS, interlocks)
Stream data	No	Yes (stream tables, H&MB)	No (refers to PFD for conditions)
Typical users	Management, clients, process licensors	Process engineers, equipment vendors	All disciplines, construction, operations
Project phase	Pre-FEED / Conceptual	FEED	Detailed engineering through operations
Governed by	No specific standard	ISO 10628	ISA S5.1, PIP PIC001, ISO 10628

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, drain and vent connections, instrument details, and safety systems.

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

Software Used for P&ID Development

Database-Driven P&ID Tools

Modern EPC projects use intelligent, database-driven P&ID software that maintains a live database behind each diagram. When you change a valve size or add an instrument, the database updates automatically, keeping all deliverables consistent.

Software	Vendor	Key Features
SmartPlant P&ID	Hexagon (formerly Intergraph)	Industry standard for major EPC projects; integrates with Smart 3D and SmartPlant Materials
AVEVA Diagrams	AVEVA (Schneider Electric)	Enterprise-level; integrates with AVEVA E3D and AVEVA Engineering
AutoPLANT P&ID	Bentley Systems	Integration with OpenPlant 3D modeling
AutoCAD Plant 3D	Autodesk	Mid-range projects; familiar AutoCAD interface with P&ID database

General-Purpose Tools

Tool	Best For
Microsoft Visio	Small projects, quick conceptual drafts
AutoCAD (plain)	Basic 2D drafting without database functionality
Lucidchart / draw.io	Collaborative online diagramming for early-stage work

Important

For any project with more than 10-15 P&ID sheets, use a database-driven tool. Manual CAD-drawn P&IDs without a backing database are error-prone: line lists, instrument indexes, and valve lists must be maintained manually, and inconsistencies between the P&ID and these documents are almost guaranteed over time.

Standards and References

The following standards govern the creation and interpretation of process diagrams:

Standard	Title	Scope
ANSI/ISA-5.1	Instrumentation Symbols and Identification	Instrument letter codes, symbol shapes, tag numbering
ISO 10628	Diagrams for the Chemical and Petrochemical Industry	Rules for creating PFDs and P&IDs
PIP PIC001	Piping and Instrumentation Diagram Documentation Criteria	Content and format requirements for P&IDs
ISO 14617	Graphical Symbols for Diagrams	General-purpose technical diagram symbols
PIP PNE00003	Line Designation Numbering	Line numbering conventions
ISA-5.4	Instrument Loop Diagrams	Detailed wiring and loop documentation

Common Mistakes in P&ID Development

Avoiding these frequent errors saves significant rework during construction:

1. Missing isolation valves. Forgetting block valves around instruments, filters, or equipment that requires maintenance access. Every item that needs periodic removal or calibration must have proper isolation.

2. Incomplete relief system design. Failing to identify all overpressure scenarios (blocked outlet, fire case, thermal relief, control valve failure). Every pressure source must have a relief path.

3. Inconsistent line designations. Line numbers that do not match between the P&ID, the line list, and the piping isometrics. This causes procurement errors and construction rework.

4. Missing drain and vent connections. Process lines need low-point drains and high-point vents for hydrotest, commissioning, maintenance, and startup operations.

5. Undefined control valve failure position. Every control valve must specify its failure mode (fail-open, fail-closed, or fail-last). An undefined failure position is a safety risk.

6. No spec breaks shown. When piping material class changes between two pipe segments, the P&ID must show the spec break point. Missing spec breaks lead to wrong materials being installed.

7. Copying P&IDs from previous projects without review. Reusing P&IDs from similar plants without verifying applicability to the current project’s design conditions, codes, and client standards.

8. Skipping the HAZOP before IFC. Issuing P&IDs for construction before completing the HAZOP study. Post-IFC safety findings require costly field modifications.

Critical

Never issue a P&ID for construction (IFC) without completing the HAZOP and incorporating all safety-related findings. Post-IFC changes due to missed HAZOP actions are among the most expensive modifications on any EPC project, requiring re-engineering, re-procurement, and field rework.

Frequently Asked Questions

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

A PFD (Process Flow Diagram) shows major equipment, flow paths, and operating conditions (temperature, pressure, flow rates) with material and energy balances. A P&ID (Piping and Instrumentation Diagram) adds every pipe with line numbers, sizes, and pipe classes, plus all valves, instruments, control loops, and safety systems. The PFD is for understanding the process; the P&ID is for building and operating the plant. PFDs typically do not show isolation valves, drain/vent connections, relief systems, or instrument detail.

What does BFD stand for in engineering?

BFD stands for Block Flow Diagram. It is the simplest process diagram, using blocks (rectangles) to represent unit operations and arrows to show material flow direction. BFDs are created during early project phases (conceptual design and pre-FEED) to establish the overall process concept. They do not include equipment details, piping, or instrumentation. Their purpose is to communicate the high-level process logic to all stakeholders, including non-technical audiences.

What standard governs P&ID symbols and instrument identification?

The primary standard is ANSI/ISA-5.1 (Instrumentation Symbols and Identification), which defines instrument letter codes, symbol shapes, and tag numbering conventions. Other relevant standards include ISO 10628 (flow diagrams for process plants), PIP PIC001 (P&ID documentation criteria), and ISO 14617 (graphical symbols for diagrams). Most EPC projects include a project-specific legend sheet that defines the exact symbols used on that project's P&IDs.

What software is used to create P&IDs?

Major EPC projects use database-driven P&ID software such as Hexagon SmartPlant P&ID (formerly Intergraph), AVEVA Diagrams, or Bentley AutoPLANT. These tools maintain an intelligent database behind each diagram, enabling automatic generation of line lists, instrument indexes, and equipment lists. Mid-range projects may use AutoCAD Plant 3D. For quick conceptual work or small projects, Microsoft Visio or browser-based tools like Lucidchart are sometimes used, though they lack database functionality.

At what project phase are P&IDs developed?

P&ID development begins during FEED (Front-End Engineering Design) with preliminary P&IDs based on the approved PFD. During detailed engineering, P&IDs are progressively developed through review stages (typically 30%, 60%, 90%) until they reach IFC (Issued for Construction) status. The HAZOP study is conducted using the P&IDs at approximately the 60% stage. P&IDs continue to be updated during construction (as-built markups) and are maintained throughout the plant's operating life as the definitive record of the installed process.