What Is ASME B31.1? Power Piping Code

Quick Answer: ASME B31.1 is the Power Piping Code. It governs the design, fabrication, installation, inspection, and testing of piping systems in electric power generating stations, industrial plants, and institutional facilities—specifically the steam, water, and auxiliary piping connected to boilers and power generation equipment.

Scope of ASME B31.1

ASME B31.1 applies to piping systems associated with:

System	Examples
Boiler external piping	Main steam, hot reheat, cold reheat, feedwater, blowdown
Power generation piping	Turbine extraction, condenser, heater drain, desuperheater
High-pressure/high-temperature steam	Steam distribution headers, pressure-reducing stations
Geothermal piping	Steam and brine from geothermal wells to power plant
Nuclear auxiliary (non-nuclear)	Balance-of-plant piping in nuclear facilities

The code boundary typically starts at the boiler external piping connection and extends to the first isolation valve at each downstream system. Process piping beyond this boundary falls under ASME B31.3.

ASME B31.1 vs. ASME B31.3

Parameter	ASME B31.1 (Power Piping)	ASME B31.3 (Process Piping)
Primary application	Power plants, boiler piping	Refineries, chemical plants, process facilities
Design factor	More conservative (lower allowable stress)	Less conservative (higher allowable stress)
Weld joint efficiency (E)	Uses quality factor from ASME B31.1 tables	Uses weld joint efficiency from B31.3 tables
Examination	Typically more NDE required for given conditions	Examination based on fluid category
Hydrostatic test pressure	1.5 x design pressure	1.5 x design pressure
Pneumatic test	1.2 x design pressure	1.1 x design pressure
Creep range design	Detailed rules for high-temp steam service	Covered but less emphasis
Fluid service categories	Not categorized by toxicity	Normal, Category M, High Pressure

Key Design Requirements

Wall Thickness

The minimum wall thickness formula under B31.1 is:

t_m = (PD) / (2(SE + Py)) + A

Where A = additional thickness for corrosion, erosion, and threading. The allowable stress S is taken from ASME B31.1 Table A-1, which is generally more conservative than B31.3 values for the same material and temperature.

Material Requirements

B31.1 lists approved materials in its mandatory appendices. Common piping materials include ASTM A106 (carbon steel), ASTM A335 (alloy steel for high-temperature service), and ASTM A312 (stainless steel). Material selection must account for sustained high-temperature operation, thermal fatigue, and creep.

Examination and Testing

Requirement	B31.1 Rule
Radiographic examination	100% for high-pressure/high-temperature boiler piping; percentage reduces for lower categories
Welding qualification	ASME Section IX
PWHT	Required for carbon steel over specific thickness limits and for Cr-Mo steels
Hydrostatic test	1.5 x design pressure, maintained for at least 10 minutes
Hardness after PWHT	Mandatory per code requirements

Pro Tip

When a power plant includes both steam generation piping (B31.1) and process piping for associated chemical treatment or fuel handling (B31.3), clearly define code boundaries on the P&ID. Different codes mean different examination, testing, and documentation requirements. Mixed-code projects require careful coordination between disciplines.

In-Service Inspection

ASME B31.1 Chapter VII provides rules for operation, maintenance, and modification of existing piping. However, many power plant operators supplement B31.1 with API 570 inspection practices, especially for high-temperature piping where creep, graphitization, and thermal fatigue are concerns.

Boiler external piping is also subject to jurisdictional requirements—many states and provinces mandate periodic inspections by authorized inspectors per the National Board Inspection Code (NBIC).

Read the full guide to LEL/UEL and flammable gas safety