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PFD

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A Process Flow Diagram — the higher-level schematic showing a facility's major equipment and main process streams, with the key operating conditions and the heat and material balance data.

The PFD is the process design's summary statement: what the plant does, through which major equipment, at what flows, temperatures and pressures. It precedes the P&IDs in the design sequence — the PFD fixes the process scheme, the P&IDs then elaborate it into every line and instrument — and it stays useful long after, as the document by which anyone new understands the plant in an afternoon rather than a month.

The PFD carries numbers as well as topology: stream data drawn from the simulation and the heat and material balance, which makes it the bridge between process modelling and engineering documents. Early estimating leans on it heavily — factored and parametric methods work from PFD-level definition.

Its failure is drift. As detail design develops and the P&IDs absorb change after change, the PFD is the document least likely to be updated, and a facility can reach operation with a PFD describing the plant as first conceived rather than as built — harmless until someone bases a study on it.

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