An Overview of ASME BPV Code Section VIII, Division 1

Scope

ASME Section VIII Division 1 applies to the design and construction of pressure vessels operating at internal or external pressures exceeding 15 psig (103 kPa). It covers:

• Unfired pressure vessels
• Vessels with compressed gases, liquids, vapors, or a combination
• Temperatures typically above -50°F (-45°C), but low-temperature rules exist

Exclusions

• Piping (ASME B31.3), Boilers (ASME I) and Small vessels (<6″ diameter, 5 cu.ft volume).

Key Terms:

• MAWP: Maximum Allowable Working Pressure (design basis).
• MDMT: Minimum Design Metal Temperature (for impact testing).

Structure of the Code

The Code is divided into parts and appendices:

Part UG: General Requirements

• Definitions, materials, design rules, and mandatory requirements
• Rules for welding, testing, marking, stamping, and inspection

Design Requirements – Wall Thickness Calculations

Cylindrical Shells (UG-27)

t = (P × R) / (S × E – 0.6P)

P = Design pressure (psi)
R = Inner radius (in)
S = Allowable stress (ASME II-D)
E = Joint efficiency (Table UW-12)

Heads (UG-32)

Ellipsoidal (2:1): t = (P × D) / (2SE - 0.2P)
Torispherical: Crown radius ≤ vessel diameter.

Nozzle Reinforcement (UG-37 to UG-45)

Area Replacement Method: Compensate for metal removed by nozzle cutout.
Minimum Nozzle Thickness: tₙ = (P × Rₙ) / (Sₙ × E – 0.6P) + corrosion allowance 

Hydrostatic Test (UG-99)

• Pressure: 1.3 × MAWP × (S_test / S_design).
• Duration: 30 minutes minimum (no leaks).

NDE Requirements

Method	Extent	Acceptance Criteria
RT	100% for Category A	No cracks, lack of fusion
UT	10% for thickness >1″	Flaws < 1/4″ depth
PT/MT	All welds	No linear indications >1/8″

Part UW: Welding

• Acceptable welding processes and techniques
• Requirements for joint design and inspection (e.g., RT/UT, PWHT)
• Joint efficiency tables (critical for calculating vessel thickness)
• WPS/PQR: Qualify per ASME IX.
• Joint Categories:
  Type 1: Full penetration butt welds (vital for pressure parts).
  Type 2: Lap welds (limited to non-critical applications).

B. Post-Weld Heat Treatment (PWHT)

Part UF: Flat Heads and Covers

• Design of flat heads under pressure (e.g., blind flanges, bolted covers)

Part UB: Pressure Vessels Built with Forgings

• Construction rules for forged pressure-containing components

Part UC: Cylindrical Shells and Heads

• Design formulas for common shapes (cylinders, hemispheres, ellipsoidal and torispherical heads)

Part UHA / UHT / UCI / UCL / etc.: Material-Specific Rules

• Special rules for materials like carbon steel (UCS), austenitic stainless steel (UHA), high alloy steels, cast iron, etc.

Impact Testing (UG-84)

• Required if: MDMT < -55°F (carbon steel) or thickness >1″ at MDMT < -20°F.
• Exemptions: Austenitic SS, normalized materials.

Part UHX: Heat Exchangers

• Tube-to-tubesheet joints, design, and testing of heat exchanger components

Part U-1 to U-2: Scope and Limitations

Limitations of Division 1

• Not suited for high-pressure applications (above 3000 psi): Use Division 2
• Not suitable for lethal service without additional precautions
• Not optimized for very thin or very thick sections: Use special design consideration