Tube Failures In High Pressure Boilers Dr V T Sathyanathan M.E., Ph.
Dr V T Sathyanathan, M.E., Ph.D.
Defects and Failures
Defects and their significance with respect to failures is a complex subject. Brittle Failures Crack propagation without appreciable plastic deformation Ductile Failures Plastic deformation with gradual propagation of cracks
Dr V T Sathyanathan, M.E., Ph.D.
Classification of Service Failures
Design (structural, design notches, joint location or welding configuration) related Materials (selection and handling) related Base Metal Defects (introduced during raw material manufacture and shaping) Fabrication Defects In - service Defects
Dr V T Sathyanathan, M.E., Ph.D.
Design Considerations
Flexibility for Thermal Fatigue Cracking Resistance Avoid sharp corners and notches for Fatigue Resistance and gradual taper (4 : 1) Proper Stress Concentration factors Proper Weld Joint design
Dr V T Sathyanathan, M.E., Ph.D.
Material Selection & Handling Considerations
Graphitisation and Embrittlement
Proper choice of steels especially stainless steels Proper choice of welding consumable Minimise abrupt material transitions (DMW) Choice against temper embrittlement
J-Factor = (Si + Mn)x(P + Sn)x 104 (to be less than 160, elements in weight %) X-Factor = ( 10P + 5 Sb + 4 Sn + As) (to be less than 15, elements in ppm) Step Cooling Cycle,
Impact test at sub-zero temperatures
Combination Heat Treatment Feasibility Material Handling and surface protection in storage Hydrogen Induced Cracking and Sulphide Stress Cracking Resistance for sour service applications
Dr V T Sathyanathan, M.E., Ph.D.
Base Metal Defects
In Wrought and forged Products
Mechanical Notches :
Laminations - Severe inclusions aligned parallel to surface Laps - Surface defect in rolling, parallel to length, at an angle Scabs - Scale rolled into surface Slivers - Metal surface ruptures, rolled into the surface Bark - Intergranular penetration of oxides and scale Seams - Surface defect, parallel to rolling, linear fissures
Metallurgical Notches :
Hot Shortness Surface Carburisation / Decarburisation
Dr V T Sathyanathan, M.E., Ph.D.
Casting Defects
Mechanical Notches
Hot Tears and Cracking Gas and Blow Holes Unfused chaplets Inclusions Internal Shrinkage
Metallurgical Notches
Hot Shortness
Dr V T Sathyanathan, M.E., Ph.D.
Fabrication Defects
Cold Bending (Excessive Thinning, Ovality) Hot Bending (Reduced Hot Ductility problem) Weld Defects Improper Heat Treatment Surface Cleanliness (for SS, for welds before welding etc.)
Dr V T Sathyanathan, M.E., Ph.D.
Weld Defects
Improper Fit up Root Oxidation in SS welds Burn Through & Porosity Hot Cracks & Cold Cracks Concavity Slag Inclusions Crater Cracking Undercut Lack of Fusion Stray Arcing
Dr V T Sathyanathan, M.E., Ph.D.
In-service Defects
Boiler Tube Failures Will be covered as Failure Analysis
Dr V T Sathyanathan, M.E., Ph.D.
Boiler Tube Failures
Boiler Tube Failures - main cause of forced outages in electric utility steam generating boilers Single tube Failure in a 500 MW Rs. 5 to 6 Crores (replacement power charges for 3-4 days to repair) besides affecting Plant Morale.
Dr V T Sathyanathan, M.E., Ph.D.
Boiler Tube Failures (22 Primary Mechanisms)
Stress Rupture
Short Term Overheating High Temperature Creep Dissimilar Metal Welds
Fatigue
Vibration Thermal Corrosion
Water-side Corrosion
Caustic Corrosion Hydrogen Damage Pitting Stress Corrosion Cracking Low Temperature Waterwall Coal Ash Oil Ash
Erosion
Fly Ash Falling Slag Soot Blower Coal Particle
Fire-side Corrosion
Lack of Quality Control
cleaning Maintenance damage Chemical Excursion damage Material Defects Welding Defects
- indicates that such problems have not been reported in India
Dr V T Sathyanathan, M.E., Ph.D.
Short Term Overheating
Steam / Water cooled tubes Plugged by debris, scale etc. High Heat Transfer / Improper firing Low water/steam flow due to poor circulation / upstream leak
Corrective Action
Prevent Blockage Maintain Drum level Assure Coolant flow Reduce over firing Redesign tubing to promote flow Relocation of horiz. / inclined tubes to avoid film boiling
Dr V T Sathyanathan, M.E., Ph.D.
Dr V T Sathyanathan, M.E., Ph.D.
Dr V T Sathyanathan, M.E., Ph.D.
SHORT TERM OVERHEATING
Dr V T Sathyanathan, M.E., Ph.D.
SHORT TERM OVERHEATING TRANSFORMED MARTENSITE SHORT TERM OVERHEATING ORIGINAL STRUCTURE
Dr V T Sathyanathan, M.E., Ph.D.
High Temperature Creep
Typical Locations Steam cooled Tubes
Partially choked Radiant Heat Zone Gas Blockage Incorrect Material Material Transition Higher stress due to weld attachment
Corrective Action RLA Fluid flushing Material up-grades Tube shielding
Dr V T Sathyanathan, M.E., Ph.D.
Dr V T Sathyanathan, M.E., Ph.D.
LONG TERM OVERHEATING RANDOM GRAPHITISATION LONG TERM OVERHEATING OXIDE NOTCHES LONG TERM OVERHEATING EYEBROW GRAPHITISATION LONG TERM OVERHEATING CREEP MICROCRACKS
Dr V T Sathyanathan, M.E., Ph.D.
LONG TERM OVERHEATING OVERHEATING, CREEP INCORRECT MATERIAL OVERHEATING BULGING, SATELLITE SCALE CRACKING OVERHEATING WATERSIDE DEPOSITS
Dr V T Sathyanathan, M.E., Ph.D.
LONG TERM OVERHEATING WATERSIDE DEPOSTS & DAMAGE DUE TO TUBE INSIDE TUBE
Dr V T Sathyanathan, M.E., Ph.D.
Dr V T Sathyanathan, M.E., Ph.D.
Dr V T Sathyanathan, M.E., Ph.D.
Dr V T Sathyanathan, M.E., Ph.D.
Dissimilar Metal Welds
Typical Locations
At SH / RH dissimilar weld joints : Temperature / Stress excursions
Corrective Action
Repair/Replacement Relocating the weld Use of Ni-base filler Frequent inspection
Mechanism : 1. The formation of carbon depleted zone on the ferritic side of the transition from the ferritic to austenitic structure is the initial step and any treatment which enhances the formation of this zone will enhance the failure probability. 2. The carbon depleted soft feerritic zone is constrained by the sorrounding harder and stronger material and is subjected to strains induced by thermal expansion mismatch, bending, vibration and pressure. 3. The strain accumulation in the carbon-depleted zone is relieved by creep at elevated temperature. 4. Creep damage in the form of cavitation, grain boundary sliding and tearing results in cracking in the carbon depleted zone along and adjacent to the weld interface Dr V T Sathyanathan, M.E., Ph.D.
Dr V T Sathyanathan, M.E., Ph.D.
DISSIMILAR METAL WELD FAILURE
Dr V T Sathyanathan, M.E., Ph.D.
Cont to Tube Fail2
Dr V T Sathyanathan, M.E., Ph.D.
