Process Flow Diagram (PFD) Guide
A Process Flow Diagram (PFD) is a simplified schematic showing the main equipment, piping, and process flow in a chemical plant.
PFD vs P&ID
| Aspect | PFD | P&ID |
|---|---|---|
| Purpose | Overall process understanding | Construction & operation details |
| Detail Level | Low - major equipment only | High - all instruments & valves |
| Process Data | T, P, flow rates, compositions | Limited process data |
| Stage | Conceptual & basic design | Detailed design & construction |
Standard Equipment Symbols
Reactors
- • CSTR - Stirred tank with agitator
- • PFR - Elongated tube/cylinder
- • Batch - Similar to CSTR with batch notation
- • Fixed Bed - Cylinder with internal packing
Separation
- • Distillation Column - Tall vessel with trays
- • Flash Drum - Horizontal/vertical vessel
- • Absorber - Column with packing
- • Filter - Circle with internal lines
Heat Transfer
- • Shell & Tube HEX - Circles with lines
- • Heater - Circle with flame/coil
- • Cooler - Circle with water symbol
- • Condenser - HEX with liquid output
Fluid Handling
- • Pump - Circle with discharge arrow
- • Compressor - Similar with larger size
- • Valve - Bowtie shape on line
- • Tank - Cylindrical vessel
Equipment Numbering Convention
Standard prefixes for equipment tagging:
| Prefix | Equipment Type | Example |
|---|---|---|
| R- | Reactor | R-101, R-102 |
| T- | Tower/Column | T-201, T-202 |
| E- | Heat Exchanger | E-301, E-302 |
| P- | Pump | P-101A/B (spare) |
| C- | Compressor | C-401 |
| V- | Vessel/Tank | V-501, V-502 |
| D- | Drum | D-101 |
| F- | Furnace/Fired Heater | F-601 |
Stream Information
PFD streams typically include a stream table with:
- Stream Number: Unique identifier (1, 2, 3...)
- Temperature: °C or °F
- Pressure: bar, psi, or kPa
- Mass Flow: kg/h or lb/h
- Molar Flow: kmol/h or lbmol/h
- Composition: Mole or mass fractions
- Phase: Liquid, vapor, or two-phase
- Density: kg/m³ or lb/ft³
PFD Standards
Common standards for process diagrams:
- ISA 5.1: Instrumentation Symbols and Identification
- ISO 10628: Diagrams for the Chemical and Petrochemical Industry
- ANSI/ISA-5.1: US instrumentation standard
- BS 5070: British Standard for engineering diagrams
- DIN 28004: German standard for flow diagrams
Design Guidelines
- Flow Direction: Generally left-to-right or top-to-bottom
- Equipment Spacing: Allow room for stream labels and data
- Line Crossings: Minimize; use bridges where necessary
- Consistency: Use uniform symbol sizes and styles
- Clarity: Avoid clutter; use separate sheets if needed
- Recycle Streams: Clearly indicate with dashed lines or labels
- Utilities: Show major utility streams (steam, CW, etc.)
Common Process Configurations
Simple Reaction System
Feed → Heater → Reactor → Cooler → Separator → Product
Distillation Train
Feed → Pre-heater → Column → Condenser/Reboiler → Products
Recycle Loop
Fresh Feed + Recycle → Reactor → Separator → Product + Recycle
References
- ISA-5.1-2022: Instrumentation Symbols and Identification
- ISO 10628-1:2014: Diagrams for the chemical and petrochemical industry
- Turton, R. et al. (2018). Analysis, Synthesis, and Design of Chemical Processes. Pearson.
- Towler, G. and Sinnott, R. (2021). Chemical Engineering Design. Butterworth-Heinemann.