Blueprint Reading, P&IDs & Pipe Math
Piping and instrumentation diagrams, isometric drawings, process flow diagrams, takeoffs, and pipe calculations.
- Read P&IDs including symbols, line designations, and instrument tags
- Interpret isometric drawings and pipe routing conventions
- Perform pipe math calculations for takeoffs and circumference
- Describe boiler chemistry and caustic embrittlement
Lesson 1
P&IDs - Symbols, Lines & Instrument Tags
What is a P&ID?
A P&ID (Piping and Instrumentation Diagram) is the most important drawing for a pipefitter. It shows the complete piping system schematically, including all equipment, valves, instruments, and pipe connections. A P&ID does not show physical layout or routing - it shows the logical connections between components.
Line Designations
Every pipe line on a P&ID has a line designation that encodes critical information. A typical designation reads: 6"-SS-150-HC-1001
| Part | Example | Meaning |
|---|---|---|
| Size | 6" | Pipe diameter (NPS) |
| Material | SS | Stainless steel |
| Rating | 150 | Pressure class (ANSI) |
| Service | HC | Hydrocarbon |
| Number | 1001 | Unique line number |
Instrument Tags
Instruments on P&IDs use a tag number that identifies function and loop. For example, TIC-101:
- T = Temperature (measured variable)
- I = Indicating (readout function)
- C = Controller (control function)
- 101 = Loop number
Common first letters: T = Temperature, P = Pressure, F = Flow, L = Level
Exam Tip - Instrument Letters
The first letter of an instrument tag is the measured variable: T = Temperature, P = Pressure, F = Flow, L = Level. The remaining letters describe the function: I = Indicating, C = Controlling, T = Transmitting, A = Alarm.
A P&ID shows logical piping connections, not physical routing. Line designations encode size, material, rating, service, and line number. Instrument tags use first letter = measured variable (T, P, F, L).