ASME B31.3 (Process Piping)
ASME B31.3 is the American Society of Mechanical Engineers code governing pressure piping in petroleum refineries, chemical plants, pharmaceutical plants, paper mills, textile mills, semiconductor plants, and cryogenic plants. The code defines the materials acceptable for piping service, the design rules for pressure and temperature, the welding and fabrication requirements, the inspection and testing requirements, and the documentation deliverables (line list, isometric drawings, weld map, hydrotest records). It is one of seven sections in the ASME B31 Code for Pressure Piping; B31.1 covers power piping, B31.4 covers liquid-pipeline transportation, B31.8 covers gas transmission and distribution.
What are the B31.3 fluid service categories?.
Category D covers fluid services where the gauge pressure does not exceed 150 psig, the temperature is between -29°C and 186°C, and the service is not lethal or highly toxic; the design and inspection rules are relaxed. Category M covers fluids that are lethal upon escape (highly toxic gases, certain hazardous liquids); the rules are tightened, more inspection is required, and material restrictions apply. Category K covers high-pressure piping above the normal B31.3 limits; the code provides a separate chapter (Chapter IX). Severe Cyclic Service applies to piping subject to thousands of significant pressure or temperature cycles; fatigue analysis is required. The default category for chemical and petrochemical service is Category O (Ordinary) unless one of the other categories applies.
What does the piping spec (pipe class) carry?.
A piping spec (pipe class) is a document maintained by the engineering contractor or operating company that translates B31.3 requirements into specific pipe components for a given service. The piping spec specifies the pipe material and schedule per nominal size, the flange rating and material, the gasket type, the valve construction (body, trim, packing), and acceptable fittings. A piping spec is identified by a short code (A1A, B2C, D3E, etc.) that appears on the line number. The combination of pipe spec, service, and process conditions determines whether the line meets B31.3 design requirements.
What does the line list carry per B31.3?.
The line list is the project register of every named pipe spec on the P&ID set. Each row carries the line number, nominal size, schedule, pipe spec, material, service, design pressure, design temperature, insulation, tracing, and the connected-equipment references at each end. The line list is referenced by the piping detailer for isometric drawings, by the stress engineer for flexibility and support design, by the inspector for pressure-test scoping, by the fabricator for material takeoff, and by operations for the lifetime piping integrity management plan.
Frequently asked.
What is the difference between B31.1 and B31.3?
B31.1 governs power piping (steam, boiler, and turbine service) in power plants. B31.3 governs process piping in chemical, petrochemical, refining, pharmaceutical, and related industries. The two codes have similar design approaches but different scope. A power plant's main steam line is B31.1; the refinery's hydrocarbon piping is B31.3.
Does B31.3 require post-weld heat treatment (PWHT)?
PWHT requirements depend on the pipe material, wall thickness, and the service. The code provides tables specifying minimum PWHT temperatures and times per material P-number and wall thickness. Carbon steel pipe in non-severe service typically does not require PWHT below specific wall thickness thresholds; higher-alloy steels (chromium-molybdenum), stainless steels for stress relief, and severe-service applications typically require PWHT.
What is the difference between B31.3 and API 570?
B31.3 is the design and construction code. API 570 is the in-service inspection code for the same piping after it goes into operation. API 570 references B31.3 for the design baseline and adds requirements for periodic inspection intervals, condition assessment, remaining-life calculations, and repair / replacement criteria. Together they cover the full lifecycle of process piping.
Does the pipe class designation come from B31.3?
B31.3 provides the design framework; the specific pipe class designations (A1A, B2C, etc.) are project-specific. Engineering contractors and operating companies maintain pipe-class libraries (often hundreds of classes covering different service categories, materials, and pressure ratings) that conform to B31.3 design rules and the company's specific service experience.