Plant Diagrams, P&IDs and Instrument Tags

Plant Schematics & Engineering Drawings

Industrial drawings represent complex physical installations at varying levels of technical detail. The three primary drawings used by power engineers are PFDs, P&IDs, and Electrical Single-Line Diagrams.

1. Process Flow Diagrams (PFD)

A Process Flow Diagram (PFD) is a simplified, high-level schematic illustrating the fundamental thermodynamic and chemical flow paths of a plant process. It presents the big-picture operational relationships without cluttering the drawing with mechanical details:

• Contents: Major process equipment (boilers, steam turbines, condensers, deaerators, major pumps, heat exchangers), primary process fluid flow lines, and basic control loops that govern main plant outputs.
• Stream Data Tables: PFDs feature a standardized data table at the bottom of the drawing keyed to numbered process stream flags. This table details design operating parameters: temperature ($^\circ\text{C}$), pressure ($\text{kPa}$), mass flow rate ($\text{kg/hr}$), fluid phase (liquid, saturated steam, superheated steam), density, and enthalpy balances.
• Exclusions: PFDs intentionally omit minor piping, line sizes, pipe material classes, isolation block valves, bypass lines, manual vents, drains, and secondary instrumentation.

2. Piping & Instrumentation Diagrams (P&ID)

A Piping and Instrumentation Diagram (P&ID)—also known as an Engineering Flow Diagram (EFD) or Mechanical Flow Diagram (MFD)—is the definitive, comprehensive mechanical blueprint of the process plant. It represents the process piping, equipment and control relationships at the required drawing detail; it is not a complete physical fabrication drawing:

• Piping Details: Every pipeline is labeled with an alphanumeric line designation tag defining: Nominal Pipe Size - Fluid Service - Line Number - Pipe Material Class - Insulation Specification (e.g., 4"-STM-1021-A1A-H indicates a 4-inch high-pressure steam line, service number 1021, a material class whose meaning must be found in the project specification, with hot thermal conservation insulation). P&IDs show flow direction arrows, pipe reductions, slope requirements, and expansion loops.
• Mechanical Equipment: Equipment symbols, tags and relevant connections; detailed fabrication dimensions and internals are found in the appropriate equipment drawings.
• Valves & Mountings: Every manual valve (gate, globe, ball, butterfly, check, needle), pressure relief valve (including certified set pressure and orifice size), drain, vent, steam trap, strainer, and spectacle blind.
• Instrumentation & Control Loops: All primary sensors, transmitters, local indicators, controllers, converters, automated control valves, safety switches, and interlocks, depicted using standard ISA symbology.
• Fail-Safe & Interlock Actions: Control valves display their fail-safe position (FO or FC), while instrument lines depict pneumatic, electrical, or software communication media.

3. Electrical Single-Line Diagrams (SLD) & Logic Schematics

• Electrical Single-Line Diagrams (SLD): A simplified schematic where a single solid line represents all three phases of an electrical power distribution system. SLDs show incoming high-voltage utility supplies, main step-down transformers, switchgear lineups, circuit breakers, protective relays, Motor Control Centers (MCCs), uninterruptible power supply (UPS) battery banks, and emergency diesel generators.
• Logic & Functional Diagrams: Depict binary control logic using Boolean logic gates (AND, OR, NOT, TIME DELAY) or cause-and-effect matrices. Widely used to document boiler Burner Management Systems (BMS), emergency shutdown (ESD) sequences, and turbine trip interlocks (e.g., establishing that fuel gas safety shutoff valves will open ONLY IF: Purge Complete = TRUE, Drum Water Level Normal = TRUE, Forced Draft Fan Running = TRUE, with flame proof evaluated at the correct stage of the ignition/run sequence).

ISA-5.1 Instrument Identification & Symbology

The International Society of Automation standard ANSI/ISA-5.1 (Instrumentation Symbols and Identification) provides the universal graphical language used on P&IDs and control schematics worldwide.

1. Instrument Tag Balloons (Circles)

Instruments are represented graphically by circular "balloons" containing an alphanumeric identification code. The internal geometry and dividing lines indicate the physical location and operator accessibility of the device:

ISA-5.1 Instrument Location Symbology
├── Plain Circle: Field-Mounted Device (mounted on local pipe, vessel, or field skid)
├── Circle with Single Solid Line: Control Room Panel / Primary DCS Console (operator accessible)
├── Circle with Dashed Line: Behind-the-Panel / Auxiliary Rack (inaccessible to operator)
├── Circle with Double Solid Line: Auxiliary Local Control Console (e.g., local water treatment skid)
└── Square Enclosing Circle: Shared DCS Display / Software Algorithm Function

2. Alphanumeric Identification Letter Codes

An instrument tag consists of two parts: a Letter Code (indicating variable and function) and a Loop Number (identifying the specific process loop). For example, in tag FIC-104, F is the measured variable, IC is the function, and 104 is the loop number.

3. Instrument Signal Line Symbols

The line connecting an instrument balloon to a process pipe or another instrument indicates the transmission medium:

• Solid Heavy Line: Main process fluid piping.
• Solid Thin Line: Instrument impulse tubing or capillary line.
• Dashed Line (--- --- ---): Standard electrical signal ($4-20\text{ mA}$ DC or digital discrete voltage).
• Line with Double Slashes (--//--//--): Pneumatic signal line ($20-100\text{ kPa}$ / $3-15\text{ psi}$ instrument air).
• Line with Open Circles (--o--o--): Internal software data link, DCS digital bus, or Fieldbus network communication.
• Line with Sine Waves (--~--~--): Guided electromagnetic, radio, or sonic signal.

Source notes

Official scope and technical references checked 2026-10-17: Reference 1; Reference 2.