Learn P&ID symbols and flow diagrams step by step. Beginner-friendly guide to piping design, line numbering system, piping class, ASME B31, and oil and gas piping.
Table of Contents
Introduction
A P&ID (Piping and Instrumentation Diagram) is one of the most important technical documents used in Piping Design and Piping Engineering. It represents how a process system is arranged in a plant, showing equipment, piping, valves, instruments, and control systems in a clear schematic form.
In industries such as oil and gas piping, chemical plants, refineries, and power plants, engineers rely on P&IDs daily. They are used from early design stages through construction, commissioning, operation, and maintenance.
Unlike 3D models or piping isometrics, a P&ID does not show exact dimensions or physical routing. Instead, it focuses on functional relationships and engineering intent—how the system works, how it is controlled, and how it is kept safe.
This article explains P&ID symbols and flow diagrams step by step, using simple language for beginners while maintaining correct technical terms such as line number, fluid code, line designation, line numbering system, piping class, material specification, design pressure, design temperature, insulation, tracing, and relevant standards like ASME B31.
What a P&ID Shows and Why It Is Important
What a P&ID Shows
A P&ID typically shows:
- Process flow between equipment
- Major equipment such as pumps, compressors, vessels, heat exchangers
- Piping lines with line numbers and flow direction
- Line designation and line numbering system
- Piping class and material specification
- Valves (gate, globe, ball, check, control valves)
- Instrumentation and control loops
- Design pressure and design temperature
- Insulation and tracing requirements
- Safety devices such as relief valves and shutdown valves
Why a P&ID Matters
P&IDs are used by different engineering and operational disciplines:
- Piping engineers use them to define line numbers, piping class, insulation, and tracing.
- Stress engineers use design pressure and design temperature from P&IDs for pipe stress analysis as per ASME B31.
- Instrumentation engineers use them to design control loops and signal logic.
- Construction teams use them to understand system configuration and isolation points.
- Operations teams use them during plant startup, normal operation, and troubleshooting.
A mistake or missing information in a P&ID can lead to wrong material selection, unsafe operation, or costly rework.
Standard Symbols and Conventions (Quick Reference)
| Category | Typical Symbols | Purpose |
|---|---|---|
| Lines | Solid; dashed; double | Process, instrument, utility lines |
| Valves | Gate; globe; ball; check | Flow control and isolation |
| Equipment | Pump; heat exchanger; vessel | Major process units |
| Instruments | PI, FT, TI, LIC | Measurement and control |
| Connections | Flanged; welded; threaded | Joint types and maintenance access |
Note: Use ISA S5.1 or company standard symbol legend as the authoritative reference. Always include a legend on the drawing.
Line Numbering System and Line Designation
A robust line numbering system is essential for traceability. A typical line number encodes:
- Service or fluid code (e.g., FW = feedwater, LSH = lube oil, H2S = sour gas)
- Piping class or specification group
- Unique sequence number
Example: FW-101-A
FW= Feedwater (fluid code)101= Line sequence numberA= Loop or area identifier
Practical tip: Link each line number to a piping class entry in the Piping Class Schedule that lists material specification, insulation, tracing, design pressure, and design temperature. This allows procurement and construction to know exactly what material and insulation to use.
Reading a P&ID — Step by Step
- Start with the title block and legend. Confirm the drawing revision, project name, and symbol legend.
- Identify major equipment. Locate pumps, heat exchangers, vessels, and compressors. Note their tag numbers (e.g., P‑101, E‑201).
- Trace the process flow. Follow the main process line from source to sink. Note direction arrows and flow paths.
- Read line numbers and piping class. For each line, record the line number, fluid code, piping class, and reference the Piping Class Schedule.
- Check design conditions. Confirm design pressure and design temperature on the line or in the equipment data. These feed into pipe thickness and flange rating calculations.
- Identify instrumentation and control loops. Note transmitters (FT, PT), controllers (LIC, PIC), and final control elements (control valves). Understand whether the loop is local or DCS‑based.
- Verify safety and isolation. Locate relief valves, block valves, and emergency shutdown devices. Confirm their setpoints and discharge paths.
- Note insulation and tracing. Lines carrying hot or viscous fluids will have insulation and possibly steam tracing; these are shown in the P&ID and must match the piping class.
Example: On a pump suction line LSH‑201, the P&ID shows a strainer, isolation valve, flow transmitter (FT‑201), and suction pressure transmitter (PT‑201). The line number links to Piping Class 3 which specifies CS A106 Gr B, insulation 25 mm mineral wool, and design pressure 10 bar @ 150°C.
Practical Plant Examples
1. Boiler Feedwater System (Power Plant)
- Line designation:
FW‑xxxfor feedwater. - Piping class: Includes low‑alloy steel for high temperature, insulation, and steam tracing.
- P&ID elements: Feedwater pump (P‑101), check valve, control valve (CV‑101), flow transmitter (FT‑101), and level control on the deaerator.
- Design considerations: Ensure design temperature and design pressure for the pump suction and discharge lines are consistent with the boiler operating conditions. Use the P&ID to coordinate pipe stress supports and expansion loops.
2. Crude Oil Transfer (Refinery)
- Line numbering:
CRUDE‑prefix with unique sequence. - Material specification: Often carbon steel with corrosion allowance; some lines may require stainless steel for sour service.
- P&ID focus: Heating coils, heat exchangers, and tracing to maintain viscosity. The P&ID must show tracing and insulation details to prevent wax deposition.
3. Chemical Plant Reactor Feed
- Instrumentation: Multiple control loops (flow, temperature, pressure).
- Piping class: May require special alloys (e.g., Hastelloy) per material specification due to corrosive fluids.
- P&ID role: Defines the piping class and isolation strategy for safe maintenance and emergency shutdown.
Instrumentation and Control Loops
Control loops are shown with standard tags: FT (flow transmitter), FIC (flow indicating controller), CV (control valve). Understand whether the loop is split range, cascade, or override. For example, a temperature control loop on a heat exchanger may use a cascade from a flow controller to a control valve to maintain outlet temperature.
Common Mistakes to Avoid
- Missing piping class references. Every line must reference a piping class; otherwise material and insulation decisions are ambiguous.
- Unclear line numbers. Avoid duplicate or non‑unique line numbers.
- Omitting insulation/tracing. For hot or cold services, failing to show insulation or tracing leads to operational problems.
- Ignoring design conditions. Always cross‑check design pressure and design temperature with equipment datasheets.
Interlinking and Further Reading
- For line numbering best practices, see Learn Piping > Line Designation.
- For calculations related to pipe thickness and flange rating, refer to Calculations > Pipe Thickness and Calculations > Flange Rating.
- For stress analysis and support design, consult Pipe Stress & Supports in the site tree.
Frequently Asked Questions (FAQ)
1. Is a P&ID the same as a PFD?
No. A PFD (Process Flow Diagram) shows overall process flow and major equipment only. A P&ID is much more detailed and includes piping, valves, instrumentation, and control logic.
2. Where do design pressure and design temperature come from?
They come from:
- Process design calculations
- Equipment datasheets
- Applicable codes such as ASME B31
They are confirmed and documented on the P&ID and line list.
3. Why is piping class so important?
The piping class ensures correct material selection, code compliance, safety, and consistency across the plant. Without it, construction and procurement cannot proceed correctly.
4. Can a P&ID be used for fabrication?
No. A P&ID is not a fabrication drawing. It is a schematic document. Fabrication uses piping isometrics derived from the P&ID.
Conclusion
Mastering P&ID symbols and flow diagrams is a foundational skill in piping engineering and piping design. A clear P&ID ties together line designation, piping class, material specification, and instrumentation, enabling accurate design, safe construction, and reliable operation. Practice reading real plant P&IDs, cross‑reference with the Piping Class Schedule, and always verify design pressure and design temperature before proceeding to detailed design or procurement.