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Section 9 overview Pressure Vessel Welding ** - Coggle Diagram
Section 9 overview Pressure Vessel Welding **
Here is a mind map outlining the key aspects of pressure vessel welding based on the provided sources:
Why Critical?
Operate under high pressure and temperature
Welding mistakes can lead to catastrophic failures (e.g., boiler explosion)
High-stakes process
Demands meticulous attention
Ensures safety and operational reliability
Governing Standards & Codes
ASME Boiler and Pressure Vessel Code (BPVC)
Primary standard worldwide
Governs design, fabrication, and maintenance
Sections:
Section VIII:
Rules for Construction of Pressure Vessels
Three divisions for different pressure requirements
Section IX:
Welding and Brazing Qualifications
Framework for ensuring weld quality
Applies to vessels operating at > 15 psig
Covers welding, heat treatment, and hardness testing
Complements Sections VIII and IX
Specifies requirements for materials, design, fabrication, nondestructive examination
Approved Welding Processes
Shielded Metal Arc Welding (SMAW)
Versatile, various materials and positions
Gas Tungsten Arc Welding (GTAW)
Required for all passes of small piping butt welds and nozzles (< 25.4 mm NPS)
Requires high-frequency start and post-purge for most materials
Filler metal always required
Required for root passes of single-sided groove welds without backing (stainless steel/nickel)
Submerged Arc Welding (SAW)
Preferred for thick materials
Deep penetration capabilities
Gas Metal Arc Welding (GMAW) & Flux Cored Arc Welding (FCAW)
Commonly used
Restrictions on short-circuiting mode (except structural attachments)
Welding Preparation
Fundamental for high-quality welds
Critical Steps:
Material Selection:
Based on operating conditions and pressure
Surface Preparation:
Cleaning, prevent rust and cross-contamination
Joint Design:
Proper design for application and process
Assembly:
Careful with fixtures and positioning
Preheating:
When required by material or procedure
ASME code includes specs for materials, assembly, safety
Inspection & Testing (Quality Control)
Verifies weld integrity
Nondestructive Examination (NDE)
X-ray Inspection:
Subsurface cracking/inclusions (high cost, critical joints)
Ultrasonic Testing:
Surface and subsurface defects
Visual Examination:
Surface defects/discontinuities (initial assessment)
Magnetic Particle Examination:
Surface/near-surface discontinuities
Penetrant Examination:
Surface-breaking defects
Leak Testing:
Verifies pressure containment
Supervised by qualified personnel (Level III, CWI)
Hydrostatic and Pneumatic Testing
Hydrostatic Testing:
Filled with water, pressurized (leaks, integrity)
Pneumatic Testing:
Certain applications where hydrostatic is impractical
Weld failure during testing leads to significant costs (removal, rework)
Common Welding Defects & Failure Prevention
Types of Weld Defects:
Surface Defects:
Mismatch, geometry, undercuts, overlap, cracks, porosity, arc strikes, spatter
Subsurface Defects:
Underbead cracks, internal porosity, slag inclusions, incomplete fusion/penetration, solidification/liquation cracks
Slag Inclusions:
Non-metallic, weakens joint, inhibits inspection
Even small defects can cause catastrophic failure
Service Failure Mechanisms:
Brittle Fracture:
Rapid, unstable crack growth (negligible deformation)
Ductile Failure:
Preceded by plastic deformation
Fatigue:
Progressive crack growth from cyclic loading
Corrosion-Related Failures:
General, fatigue, stress-corrosion cracking
Hydrogen-Induced Cracking:
High-strength materials
Historic examples of failures due to weld discontinuities
Certification & Qualification Requirements
Ensures safe, compliant fabrication
Welder and Procedure Qualifications (ASME Section IX)
Employ welders with documented qualifications
Detailed procedures essential
Certification involves:
Welding Procedure Specifications (WPS):
Directions for production welds
Procedure Qualification Records (PQR):
Proof of sound welds with required properties
Welder Performance Qualification (WPQ):
Demonstrates welder skill
Organizational Certifications:
ASME "U" Stamp:
Fabricate Section VIII Division 1 vessels
ASME "R" Stamp:
Repairs and alterations (National Board)
Certified Welding Inspectors (CWI):
Crucial for compliance and safety
Repair & Maintenance
Special considerations for material degradation
Repair Welding Procedures:
Guidelines for flaws detected during inspection
Consider material degradation factors (strength loss, ductility loss, embrittlement, corrosion, SCC, fatigue/creep)
Repair welds require same inspection/testing as original
Lifecycle Documentation
Essential for demonstrating regulatory compliance
Includes design, material certifications, WPS, welder qualifications, inspection/test results, repair records
Periodic inspection and maintenance are vital
Regular assessment of welds is critical
Conclusion
Strict adherence to standards is crucial
ASME BPVC is the foundation for safe practices
Requires meticulous preparation, skilled execution, rigorous inspection, ongoing monitoring
Proper certification ensures compliance
Following rules prevents devastating consequences