PFD vs P&ID vs Block Flow Diagram: Which Drawing a Quote Needs

A block flow diagram shows a process as a few blocks for the main steps, joined by the main streams. A process flow diagram (PFD) shows the major equipment, the main streams with their flows, temperatures and pressures from the heat and material balance, and the main control loops. A P&ID (piping and instrumentation diagram) shows every pipe line, valve, instrument and connection, with line sizes, specifications and tags. For pricing, a supplier can give an order of magnitude figure from a block diagram, a budgetary price from a PFD plus a datasheet, and a firm price only when it sees the P&ID detail for its scope.

All three are covered by ISO 10628 Part 1, which sets out the classification, content and representation of flow diagrams for the chemical and petrochemical industry [1].

Why this matters now

The three drawings are converging into one data model. DEXPI 2.0, the open standard for exchanging process engineering data, added block flow diagrams and PFDs to the P&ID model in late 2025 [2], with the aim of handing information from simulation to PFD and from PFD to P&ID without retyping [3]. In the US, Process Industry Practices also published a PFD guideline, PIE001, alongside its long standing P&ID practice [4]. Buyers who know which drawing carries which information will be better placed to use the data as it arrives.

What each drawing shows, and what it leaves out

PIP's PFD guideline requires a heat and material balance, on the drawing or in a separate document, and expects PFDs to give an overview of the control philosophy, so only major instruments appear [4]. The P&ID practice, by contrast, sets out detailed information for nine classes of equipment [4]. Our guide on how to read a P&ID explains its symbols.

Which drawing a quote needs, by estimate stage

AACE International's classification links each class of estimate to how much of the project is defined and to typical methods, from cost and capacity curves at Class 5 to detailed unit costs at Class 1 [5]. Engineering courses note that a PFD with its equipment summary is enough for a preliminary investment estimate [6]. In practice, the drawings line up with the classes like this:

The mapping is a practical guide, not an AACE rule; what matters is that the supplier sees everything that drives its cost. For more on the classes themselves, see what AACE project costing is.

What to send, whichever drawing you have

• Datasheets for the equipment being priced, with the buyer's side complete.
• The equipment list, so the supplier sees where its item sits.
• Battery limits: where the supplier's scope starts and stops, especially for skids.
• The P&ID of the supplier's scope, even at an early revision, for any skid or package. Valves and instruments inside a skid can be a large part of its price.
• Specifications for materials, electrical work and documents.

Two mistakes are common. The first is sending a PFD and expecting a firm price, then treating the later increase as a supplier problem. The second is the reverse: sending a full P&ID set of 40 sheets when the supplier needs two, which slows the bid and hides the scope. Send the sheets that show the supplier's scope, mark its battery limits on them, and say which revision they are.

Practical example

In this illustrative example, a skid supplier is asked for a budget price for a solvent dosing skid from a PFD and a short datasheet. It quotes £180,000.

Three months later the P&IDs arrive. Inside the skid boundary they show 14 instruments, 22 manual valves and 2 control valves that the PFD did not. The firm quote adds:

The firm price is £232,000, 29% above the budget. Nothing went wrong with the supplier's pricing; the budget was set from a drawing that could not show the detail. Sending even a preliminary P&ID of the skid scope with the budget request would have narrowed the gap.

Where ChemCapital fits

ChemCapital's P&ID to RFQ works from the most detailed drawing you have. It imports DEXPI files, reading equipment, instruments, valves, nozzles and pipe segments where the file follows supported structures, and also takes P&ID PDFs and scans for extraction, with confidence scores and human review. Extracted items are grouped into packages and turned into RFQs, so suppliers see the detail that drives their price. For the data format behind it, see DEXPI explained.

Ready to send suppliers the drawing detail their price depends on? Start free with ChemCapital.