What Is a Pipe Class?

A pipe class is a project-specific engineering document that groups all compatible piping components—pipes, fittings, flanges, valves, gaskets, and bolting—for a defined set of service conditions. Every oil and gas facility, refinery, or petrochemical plant relies on pipe classes to ensure that every element in a piping system can safely operate together under the same pressure, temperature, and corrosion environment.

Element	What the Pipe Class Defines
Designation	Alphanumeric code (e.g., A1A, B2C, 1A1) unique to the project
Service	Fluid type and corrosivity (e.g., hydrocarbon, sour gas, utility water)
Design pressure	Maximum allowable working pressure at design temperature
Design temperature	Maximum and minimum operating temperatures
Material spec	ASTM/API grade for pipes (e.g., ASTM A106 Gr. B)
Fittings	Material (e.g., ASTM A234 WPB), type (BW, SW), and dimensional standard
Flanges	Type (WN, SO, blind), rating (150-2500), face finish (RF, RTJ)
Valves	Type (gate, globe, check, ball), class, end connections
Gaskets	Type (spiral wound, ring joint), material, standard
Bolting	Stud bolt and nut grades (e.g., ASTM A193 B7 / A194 2H)
Corrosion allowance	Extra wall thickness in mm (typically 1.5-3.0 mm for carbon steel)
Design code	ASME B31.3 (process), B31.1 (power), B31.4/B31.8 (pipeline)

How Pipe Classes Are Structured

Each pipe class covers a specific combination of material, pressure rating, and service type. A typical oil and gas project may have 30 to 80 pipe classes. The naming convention varies by company. Shell uses alphanumeric codes like “AAA1”; Saudi Aramco uses codes such as “1A1” or “2B2.”

The piping engineer develops pipe classes based on process data sheets, P&IDs, and the project design basis. Each line on a P&ID is assigned a pipe class that governs every component installed on that line.

Who Creates Pipe Classes

The lead piping/materials engineer owns the pipe class document. Input comes from multiple disciplines:

Discipline	Contribution
Process engineering	Operating conditions, fluid properties, corrosion data
Piping engineering	Material selection, component compatibility, sizing
Materials/corrosion	Alloy selection for sour service, high temperature, or corrosive fluids
Mechanical engineering	Equipment nozzle ratings, interface requirements
Procurement	Market availability, lead times, cost feedback

Key Standards Referenced

Pipe classes reference manufacturing and dimensional standards rather than redefining them. Common references include ASTM A106 and A333 for carbon steel pipes, ASME B16.5 and B16.47 for flanges, ASME B16.9 for butt-weld fittings, and API 600/602/6D for valves. The pipe class ties these individual standards into a single, coherent system.

Pro Tip

Always cross-check the pipe class P-T rating against the line list. Mid-project design changes sometimes update one document but not the other, leading to procurement errors. A mismatch between the line list and pipe class is one of the most common causes of material re-orders.

Why Pipe Classes Matter

Without pipe classes, each engineer would independently select materials—risking incompatible pressure ratings, mismatched end connections, or galvanic corrosion between dissimilar alloys. The pipe class eliminates this by pre-engineering every combination. Procurement teams use pipe classes to generate MTOs and RFQs, ensuring the right materials are ordered from the start.

Pipe classes appear on every P&ID symbol as a line designation, linking the engineering drawing directly to the material specification.

Read the full guide to pipe classes and specifications