What is a process flow diagram in chemical engineering? A chemical engineering process flow diagram (PFD) is a drawing that shows the major equipment, the main process streams, and the flow of material and energy through a process. It captures the big picture of how a plant works: what units exist, how they connect, and the key operating conditions, without the instrumentation and piping detail found on a P&ID.
Why the PFD matters and why it is not just a schematic
The PFD is the reference document that everyone from process designers to equipment suppliers uses to understand plant intent. It sits between the simple block flow diagram and the detailed P&ID, and it is the level at which mass balances, energy balances, and equipment duties are communicated. In European chemical and petrochemical projects, PFDs are frequently drawn to ISO 10628, the standard for diagrams in the chemical and petrochemical industry, which defines how equipment and flows are represented.
For a supplier like GreyTec, the PFD is where equipment selection begins. Line contents, temperatures, pressures, and flow rates on the PFD drive decisions about materials of construction, corrosion resistance, and whether a stream needs lined piping, duplex stainless steel, or thermoplastic such as PVDF.
What does a PFD include?
Answer: A PFD includes major equipment (reactors, columns, vessels, pumps, compressors, heat exchangers), the main process flow lines connecting them, flow direction, key operating conditions (temperature, pressure, flow), and a stream table that lists the composition and conditions of each numbered stream.
Typical PFD contents:
- Major equipment with tag numbers and descriptive names.
- Primary process streams with directional arrows and stream numbers.
- Key utilities where they interact with the process (steam, cooling water, refrigerant).
- Operating conditions at important points: temperature, pressure, and flow rate.
- A stream table (also called a material balance table) keyed to the stream numbers.
- Control philosophy at a high level, if included, though detailed control belongs on the P&ID.
What is a stream table and why is it the heart of a PFD?
Answer: A stream table is a table keyed to the numbered streams on the PFD that lists each stream’s composition, mass or molar flow, temperature, pressure, and phase. It turns the diagram into a quantified mass and energy balance rather than a picture.
Without a stream table, a diagram is closer to a block flow diagram than a true PFD. The stream table is what lets an engineer verify that mass is conserved, size equipment, and check that the materials of construction suit the actual fluid at the actual conditions. For corrosive services, the stream composition (acid concentration, chloride content, temperature) is exactly the data that determines whether standard stainless steel is adequate or whether duplex, super duplex, or a fluoropolymer lining is required.
How is a PFD different from a BFD and a P&ID?
Answer: A block flow diagram (BFD) shows the process as simple labelled blocks. A PFD adds the major equipment, main streams, and operating conditions with a stream table. A P&ID adds every pipe, valve, instrument, and control loop needed to build and operate the plant. Each stage adds detail.
| Feature | Block Flow Diagram (BFD) | Process Flow Diagram (PFD) | Piping and Instrumentation Diagram (P&ID) |
|---|---|---|---|
| Level of detail | Lowest | Medium | Highest |
| Shows major equipment | As blocks | Yes, with tags | Yes, with full detail |
| Shows all piping | No | Main streams only | Every line, size, and spec |
| Shows instruments and valves | No | Minimal | All instruments, valves, loops |
| Includes stream / mass balance | No | Yes (stream table) | No |
| Primary use | Concept and communication | Design intent and balances | Construction, operation, safety |
Numbered guide: how to read a PFD
- Orient left to right. PFDs usually flow from feed on the left to product on the right.
- Identify the major equipment. Read the tags and names to map the unit operations.
- Trace the main streams. Follow the numbered lines and arrows through the process.
- Check operating conditions. Note temperatures and pressures at key points.
- Read the stream table. Match stream numbers to composition, flow, and phase.
- Note utilities. See where steam, cooling water, and refrigerant enter and leave.
- Flag material-critical points. Wherever a stream is hot, corrosive, or abrasive, mark it for materials review.
What are the common PFD symbols?
Answer: PFD symbols are simplified equipment icons: a tall cylinder for a distillation or absorption column, a horizontal or vertical cylinder for a vessel or drum, a circle with a wedge for a pump, a stylised shell for a heat exchanger, and a circle for a compressor or blower. Streams are lines with arrows and numbers.
Because symbol libraries vary between companies and standards, every PFD carries a legend. ISO 10628 provides a consistent EU basis, while many multinational projects use company standards derived from it. As with any process drawing, read the legend before interpreting the icons.
Checklist: what a complete PFD should contain
- ☐ All major equipment shown with unique tag numbers.
- ☐ Main process streams numbered and directional.
- ☐ Operating temperature and pressure at key points.
- ☐ A stream table keyed to the stream numbers.
- ☐ Major utility interactions shown.
- ☐ A legend defining all symbols used.
- ☐ Revision block and drawing number for control.
- ☐ Enough data to begin equipment materials selection.
Is a PFD enough to build a plant?
Is a PFD sufficient for construction? No. A PFD communicates design intent and balances, but it deliberately omits the piping, valve, instrument, and control detail needed to build and operate a plant. That detail lives on the P&ID. Treating a PFD as a construction document is a common and costly mistake, because line sizes, valve types, relief devices, and control loops are simply not on it.
The PFD and P&ID work together. The PFD tells you what the process does and at what conditions. The P&ID tells you exactly how it is piped, controlled, and protected. Both are needed, and both feed equipment and materials decisions.
More questions (FAQ)
Who produces the PFD? Process engineers develop the PFD during process design, usually after the block flow diagram and before the detailed P&ID.
Does a PFD show pipe sizes? Generally no. Line sizing and specification appear on the P&ID and line list, not the PFD.
What software is used to draw PFDs? Common tools include AutoCAD, Microsoft Visio, and specialist process tools, but the standard governing content in EU projects is often ISO 10628, independent of the software.
How does a PFD help with materials selection? The stream table gives fluid composition, temperature, and pressure, which are the primary inputs for choosing corrosion-resistant materials such as duplex stainless steel or PVDF.
What is the difference between a PFD and a PID? PID is shorthand for P&ID (piping and instrumentation diagram). The PFD is higher level and shows main streams and balances, while the P&ID shows full piping, valves, and instrumentation.
Where does the PFD sit in the project lifecycle?
Answer: The PFD is developed during conceptual and front-end engineering design (FEED), after the block flow diagram and before the detailed P&ID. It is the document that fixes the process configuration and the mass and energy balances that the rest of the project is built on, feeding equipment datasheets, procurement, and eventually the P&ID.
Because the PFD carries the stream conditions, it becomes the primary input to equipment specification. A heat exchanger duty, a pump flow and head, a column diameter, and a vessel volume all trace back to the streams and balances on the PFD. When those conditions include aggressive chemistry or high temperature, the PFD is also where materials-of-construction thinking begins, well before the P&ID adds line numbers and specifications. Getting the PFD right therefore prevents expensive rework later, because errors in the balance or configuration propagate into every downstream document.
How does the PFD support procurement and equipment selection?
Answer: The PFD gives suppliers the operating conditions they need to size and quote equipment: flow rates, temperatures, pressures, and fluid composition from the stream table. For corrosion-critical duties, the stream composition tells a supplier whether standard stainless steel, duplex, a fluoropolymer lining, or another corrosion-resistant material is required.
This is precisely where a supplier like GreyTec engages. A PFD stream that shows a hot, chloride-bearing, or acidic fluid is a clear signal that the equipment on that line needs corrosion-resistant materials selected against the actual conditions, not a generic default. Reading the PFD correctly at the procurement stage avoids specifying equipment that will corrode in service.
More expert questions
Does a PFD show control valves? Generally no. High-level control philosophy may appear, but individual control valves, instruments, and interlocks are detailed on the P&ID, not the PFD.
What is the difference between a PFD and a UFD? A utility flow diagram (UFD) focuses on utility systems such as steam, cooling water, and instrument air, while the PFD focuses on the main process. Larger projects use both.
How detailed is the stream table? A full stream table lists each stream’s flow, composition by component, temperature, pressure, and phase, and sometimes enthalpy and density. It is the quantitative backbone of the PFD.
Can a PFD change during a project? Yes. PFDs are revision-controlled and evolve as the design matures, which is why the revision block and drawing number matter for using the correct version.
What standards govern PFD symbols and layout?
Answer: In the EU, PFDs are commonly drawn to ISO 10628, which defines diagrams for the chemical and petrochemical industry, while instrumentation symbols on the related P&ID often follow ISA-5.1 or IEC conventions. Company standards derived from these are widespread, so every PFD carries a legend to define the exact symbols used on that drawing.
Because symbol libraries vary between organisations, the legend is not optional. Two plants may represent the same pump or exchanger slightly differently, and a reader who assumes rather than checks the legend can misread the diagram. Reading the legend first is a discipline that prevents costly misunderstandings, especially on projects that mix contractor and client drawing standards.
Even more questions
Do PFDs include line numbers? Detailed line numbering and pipe specifications belong on the P&ID and line list. The PFD shows main streams with stream numbers keyed to the stream table, not full line specs.
Can I estimate equipment materials from a PFD alone? You can begin materials thinking from the stream conditions and composition, but final materials selection needs the full duty, including trace contaminants and operating extremes confirmed on later documents.
Key facts at a glance
- A PFD shows major equipment, main streams, operating conditions, and a stream table.
- It sits between the block flow diagram and the P&ID in detail.
- The stream table (mass and energy balance) is what makes it a true PFD.
- EU chemical and petrochemical PFDs are frequently drawn to ISO 10628 (Source: ISO 10628, Diagrams for the chemical and petrochemical industry).
- The PFD is the starting point for equipment and materials-of-construction decisions.
From PFD to the right equipment for corrosive service
A PFD tells you which streams are hot, aggressive, or abrasive. Turning that into reliable, long-life equipment is what GreyTec does. We provide engineering solutions and corrosion-resistant equipment for petrochemical and chemical plants across the Netherlands and the EU, matching materials to the stream conditions your PFD defines.
See our engineering solutions for petrochemical plants, or call to discuss materials selection from your PFD.
Related reading: How the butterfly valve symbol appears on P&IDs and duplex stainless steel corrosion resistance explained.