ASME B31.1 vs B31.3 vs B31.4
ASME B31.1 (Power Piping), ASME B31.3 (Process Piping), and ASME B31.4 (Pipeline Transportation Systems for Liquids and Slurries) are three distinct sections of the ASME B31 Code for Pressure Piping. Each governs a specific category of piping system with different scope, design philosophy, safety factors, and examination requirements.
When Each Code Is Used
ASME B31.1 applies to piping in power-generating stations: steam, water, oil, gas, and air services associated with boilers, turbines, and auxiliary equipment. It governs piping from the boiler external headers to the turbine and associated systems.
ASME B31.3 applies to piping within petroleum refineries, chemical plants, pharmaceutical facilities, gas processing plants, and other process industries. It covers piping from equipment nozzles to equipment nozzles within the plant boundary.
ASME B31.4 applies to pipeline systems transporting crude oil, condensate, natural gasoline, NGL, LPG, liquid CO2, and liquid alcohol between production facilities, terminals, refineries, and pump stations. It governs cross-country and in-plant pipelines carrying liquid hydrocarbons.
Specifications and Comparison
| Parameter | B31.1 (Power) | B31.3 (Process) | B31.4 (Liquid Pipeline) |
|---|---|---|---|
| Scope | Power plant piping | Process plant piping | Liquid hydrocarbon pipelines |
| Typical facilities | Power stations, cogeneration | Refineries, chemical plants | Cross-country pipelines, terminals |
| Allowable stress basis | 1/4 UTS or 2/3 Sy | 1/3 UTS or 2/3 Sy | 72% SMYS (design factor 0.72) |
| Wall thickness formula | t = PD/2(SE + Py) + A | t = PD/2(SE + PY) + A | t = PD/2(F x E x SMYS) |
| Hydrostatic test | 1.5 x design pressure | 1.5 x design (stress-corrected) | 1.25 x MOP (4 hours minimum) |
| Weld examination | Per Table 136.4 | Per examination category | Per Table 434.8.5 |
| Radiography | 100% for high-energy; sampling for others | 5-100% per fluid category | 10-100% depending on location class |
| PWHT requirements | Per Table 132 | Per Table 331.1.1 | Per Section 434.8.3 |
| Corrosion allowance | Specified by designer | Specified by designer | Typically 1/16” to 1/8” |
| Design life | Not specified (typically 30-40 years) | Not specified | Pipeline integrity management program |
| Piping class approach | Pipe class specifications | Pipe class specifications | Pipeline spec per route |
Comparison: Design Approach
The fundamental difference lies in the safety factor applied to allowable stress:
- B31.1 uses 1/4 UTS, resulting in the most conservative (thickest) wall design. This reflects the higher consequence of failure in power plant piping near boilers and turbines.
- B31.3 uses 1/3 UTS, allowing approximately 33% higher stress than B31.1 for the same material and temperature. Process piping relies on operator training and process controls to manage risk.
- B31.4 uses a percentage of SMYS (design factor F = 0.72 for most locations), which is specific to pipeline engineering and accounts for pipeline location class, population density, and surveillance requirements.
Other B31 Sections
| Code | Scope |
|---|---|
| B31.5 | Refrigeration piping |
| B31.8 | Gas transmission and distribution piping |
| B31.9 | Building services piping |
| B31.12 | Hydrogen piping and pipelines |
For carbon steel and API 5L line pipe material selection under each code, refer to the applicable code’s material tables and the pipe class specification.
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