01 / Introductory
First principles
A process flow diagram (PFD) communicates major equipment, streams and process balances.
A piping and instrumentation diagram (P&ID) adds operating detail: valves, piping identifiers, instruments and connections.
Neither is a scaled routing drawing.
Read the project legend first: line styles, location marks, tag conventions and revision notes govern interpretation.
Equipment tags such as P-101 or V-101 are project conventions.
Instrument letters identify measured variables and functions; FT is flow transmitter and FIC is flow indicating controller.
A shared loop number connects functions, not necessarily physical devices.
02 / Applied
Worked example
Follow the simplified example from the feed through P-101 and FV-101 to the vessel.
FT-101 measures flow; a signal reaches FIC-101, which compares it with the setpoint and adjusts FV-101.
Solid lines in our teaching SVG represent process connections; dashed lines represent signal connections under its own legend.
A flow of 10 m³/h versus a setpoint of 12 m³/h gives an error of +2 m³/h.
That error alone does not determine controller action: valve actuator direction, process response and controller configuration must agree.
This illustrative loop omits power supply, isolation, drains and safety devices; it is not a construction P&ID.
PV → transmitter → controller (SP − PV) → control valve → process FT-101 → FIC-101 → FV-101
| Tag | Variable | Function |
|---|---|---|
| FT-101 | F | T |
| FIC-101 | F | IC |
| FV-101 | F | V |
03 / Advanced
Engineering decisions
Separate normal control from protection.
A high-high alarm, shutdown trip and mechanical relief are different functions.
Determine loss-of-air and loss-of-power behavior from specified fail action, not from a generic valve icon.
Check isolation and maintenance access, instrument ranges, line specification, interlocks and references to cause-and-effect documents.
A drawing review should trace both the process and the signal path, then reconcile tags with equipment and instrument lists.
ANSI/ISA-5.1 standardizes identification and symbols; a project legend and controlled revision remain necessary.
Do not infer operating limits or safety integrity from this teaching example.
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