ASME B31.1 vs B31.3
ASME B31.1 (Power Piping) and ASME B31.3 (Process Piping) are the two most widely used piping design codes in the oil and gas, petrochemical, and power generation industries. Both codes govern the design, materials, fabrication, examination, and testing of pressure piping systems, but they differ in scope, safety philosophy, and specific technical requirements.
Scope of Each Code
ASME B31.1 covers piping found in power plants: steam, water, oil, gas, and air services for electric power generation, heating systems, and geothermal installations. ASME B31.3 covers piping within petroleum refineries, chemical plants, pharmaceutical facilities, and similar process industries.
The boundary between the two codes is typically defined at the power plant battery limit or at the first block valve from a boiler or turbine.
Key Differences: B31.1 vs B31.3
| Parameter | ASME B31.1 (Power Piping) | ASME B31.3 (Process Piping) |
|---|---|---|
| Primary scope | Power plants, boilers, turbines | Refineries, chemical plants, gas processing |
| Basic allowable stress (S) | S = min(1/4 UTS, 2/3 Sy) at temperature | S = min(1/3 UTS, 2/3 Sy) at temperature |
| Weld joint efficiency (E_j) | Applied as a multiplier to basic allowable stress | Applied to allowable stress; quality factor E_j = 0.60-1.00 |
| Sustained stress equation | Eq. 11: S_L = PD/4t + 0.75i*M_A/Z <= S_h | Eq. 16: S_L = PD/4t + 0.75i*M_A/Z <= S_h |
| Displacement stress range | Eq. 1C: S_E = sqrt(S_b^2 + 4S_t^2) <= S_A | Eq. 17: S_E = sqrt(S_b^2 + 4S_t^2) <= S_A |
| Allowable stress range (S_A) | S_A = f(1.25 S_c + 0.25 S_h) | S_A = f(1.25 S_c + 0.25 S_h) + f*S_unused |
| Occasional loads (wind/seismic) | Eq. 12-13: S_L + S_occ <= k*S_h (k = 1.15 to 1.20) | Eq. 18-20: sustained + occasional <= k*S_h |
| Liberal stress provision | Not available | Allows unused sustained stress to be added to displacement stress range |
| Minimum wall thickness (t_min) | t = PD/2(SE + Py) + A | t = PD/2(SE + PY) + A (similar formula) |
| NDE requirements | More prescriptive; 100% RT for high-energy lines | Based on examination categories (normal, severe cyclic, Category M) |
| Hydrostatic test pressure | 1.5 x design pressure (minimum) | 1.5 x design pressure x (S at test temp / S at design temp) |
| Pneumatic test | Permitted with restrictions | Permitted at 1.1 x design pressure (with stress ratio correction) |
| Material restrictions | Stricter for boiler external piping | More flexible; allows broader range of materials |
Stress Allowable Comparison
| Material | Temperature | B31.1 Allowable (S_h) | B31.3 Allowable (S_h) | Difference |
|---|---|---|---|---|
| A106 Gr. B (CS) | Ambient (38 deg C) | 15,000 psi | 20,000 psi | B31.3 allows 33% more |
| A106 Gr. B (CS) | 400 deg F (204 deg C) | 15,000 psi | 20,000 psi | B31.3 allows 33% more |
| A312 TP304 (SS) | Ambient | 15,000 psi | 20,000 psi | B31.3 allows 33% more |
| A335 P11 (alloy) | 900 deg F (482 deg C) | 11,000 psi | 14,600 psi | B31.3 allows ~33% more |
The higher allowable in B31.3 reflects the 1/3 UTS basis versus the 1/4 UTS basis in B31.1. This means B31.1 generally requires thicker pipe walls for the same design conditions.
Examination and Testing Comparison
| Requirement | B31.1 | B31.3 |
|---|---|---|
| Visual examination | 100% of all welds | 100% of all welds |
| Radiographic testing (RT) | 100% for boiler external piping; sampling for others | Per examination category: 5% (normal), 20% (severe cyclic), 100% (Category M fluid) |
| Hardness testing | Required for P-Nos. 3, 4, 5, 6, 10 after PWHT | Required for similar materials after PWHT |
| Hydrostatic test hold time | 10 minutes minimum | 10 minutes minimum (30 minutes per some project specs) |
| PWHT requirements | Per Table 132 | Per Table 331.1.1 |
The applicable piping code—B31.1 or B31.3—determines the design rules and material requirements referenced in the pipe class specification.
Leave a Comment
Have a question or feedback? Send us a message.