liminfoP&ID Reference
Free reference guide: P&ID Reference
About P&ID Reference
The P&ID Reference is a comprehensive, searchable quick-reference for Piping and Instrumentation Diagrams, the essential engineering drawings used across the process industry. It covers the ISA 5.1 standard for instrument identification, including first-letter codes for measured variables (F for flow, T for temperature, P for pressure, L for level, and 18 others) and succeeding-letter codes for instrument functions (I for indicating, C for controller, T for transmitter, V for valve, S for switch, A for alarm), along with tag number formatting rules and loop numbering conventions.
The reference provides detailed equipment and valve symbol guides following ISA and PIP standards. Equipment symbols cover centrifugal, reciprocating, gear, and diaphragm pumps with driver notation (M/T/E), shell-and-tube, double-pipe, plate, and air-cooled heat exchangers with TEMA type codes, storage tanks (cone roof, floating roof), pressure vessels, distillation columns, reactors, drums, and compressors/blowers/ejectors. Valve symbols include gate, globe, ball, check, butterfly, and safety valves (PSV/PRV) with fail action notation (FC/FO/FL), set pressure specification, and rupture disk details.
Beyond symbols, the reference covers practical P&ID reading and creation skills: pipe line number formatting with fluid codes, size, spec class, and insulation codes; line type conventions for process piping, instrument signals (electrical, pneumatic, hydraulic, wireless), and special markings (battery limits, spec breaks); instrument bubble notation for field-mounted, control room, DCS, PLC, and SIS functions; and complete control loop configurations for temperature, flow, pressure, and level with all associated elements (sensors, transmitters, controllers, control valves, alarms, switches, and interlocks).
Key Features
- Complete ISA 5.1 first-letter (measured variable) and succeeding-letter (function) code tables with tag examples like TIC, FT, PSV, LSHH
- Equipment symbols for pumps, heat exchangers (with TEMA types), tanks, vessels, columns, reactors, compressors, and blowers
- Valve symbols covering gate, globe, ball, check, and safety valves with fail action (FC/FO/FL) and set pressure notation
- Pipe line number format breakdown: fluid code, size, line number, material class, and insulation code with common fluid codes
- Line type conventions for process piping, electrical/pneumatic/hydraulic instrument signals, and boundary markings
- Instrument bubble notation for field, control room, DCS, PLC, and SIS (safety instrumented system) functions
- Complete control loop configurations for temperature, flow, pressure, and level with all loop components and interlocks
- P&ID drawing standards reference including ISA 5.1-5.4, ASME Y32.11, IEC 62424/61511, and industry standards (PIP, Shell DEP)
Frequently Asked Questions
What do the letters in ISA instrument tags mean?
The first letter identifies the measured variable: F (flow), T (temperature), P (pressure), L (level), A (analysis), and others. Succeeding letters identify functions: I (indicating), C (controller), T (transmitter), V (valve), S (switch), A (alarm), H (high), L (low). For example, TIC-101 means Temperature Indicating Controller loop 101, and LSHH-301 means Level Switch High-High loop 301.
How do I read P&ID equipment tag numbers?
Equipment tags follow the format Letter-Number with an optional suffix: P-101A is Pump 101 unit A, E-201 is Heat Exchanger 201, V-301 is Vessel 301, T-401 is Tank 401, C-501 is Compressor 501. The number series typically indicates the plant area (100s for Area 1, 200s for Area 2, etc.) and letter suffixes distinguish parallel units.
What valve symbols are covered?
The reference covers gate valve (two triangles touching, for isolation), globe valve (triangles with vertical center line, for throttling), ball valve (triangles with center circle, quarter-turn), check valve (triangle with vertical line, backflow prevention), and safety/relief valves (PSV for gas/vapor with pop action, PRV for liquid with proportional opening). Control valve fail actions (FC, FO, FL) and rupture disk symbols are also included.
How do I interpret pipe line numbers on a P&ID?
Line numbers follow the format: fluid code, size, serial number, material class, insulation code. For example, PW-4"-101-A1-H means Process Water, 4-inch, line 101, piping class A1, hot insulation. Common fluid codes include CW (cooling water), ST (steam), CA (compressed air), N2 (nitrogen), and FG (fuel gas). Insulation codes include H (hot), C (cold), P (personnel protection), and T (tracing).
What do the different instrument bubble shapes mean?
Circle indicates field-mounted instruments. Circle with horizontal line means main control room (above line = front panel, below = back panel). Diamond represents DCS/computer functions. Square represents PLC functions. Hexagon indicates SIS (Safety Instrumented System) functions. For example, a diamond with TIC-101 means a DCS-based temperature indicating controller.
How is a temperature control loop configured on a P&ID?
A typical temperature loop includes: TE-101 (temperature element/sensor, field), TT-101 (temperature transmitter converting to 4-20mA, field), TIC-101 (indicating controller in DCS performing PID control, control room), TCV-101 (control valve with pneumatic actuator, field, with FC or FO fail action), plus TAH/TAL alarms and TR recorder as optional elements.
What P&ID line types represent different signal types?
Solid lines represent process piping, dashed lines indicate electrical signals, dash-dot lines represent pneumatic signals, double lines indicate hydraulic signals, wavy lines show capillary tubes, and thin dash-dot-dot patterns represent wireless/software signals. Special markings include arrows for flow direction, X marks for battery limits, and double bars for spec breaks.
Which standards govern P&ID creation?
The primary standard is ISA 5.1 for instrument symbols and identification. Related standards include ISA 5.2 (binary logic), ISA 5.3 (distributed control symbols), ISA 5.4 (loop diagrams), ASME Y32.11 (process flow diagrams), IEC 62424 (P&ID representation), and IEC 61511 (SIS functional safety). Industry standards like PIP, Shell DEP, and Saudi Aramco SAES provide additional requirements.