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ROOT CAUSE ANALYSIS AND CORRECTIVE MEASURES OF DISSIMILAR WELD JOINT FAILURE IN A HIGH PRESSURE STEAM PIPE

PROBLEM DEFINITION

  • During the operation of a petrochemical plant (located in Middle East), recurring appearance of cracks in dissimilar metal welding joint of a critic, high pressure steam pipe has been detected (2010-2014).
  • Cracks appear periodically over the same area, which is the butt weld joining two pieces made of different materials A-335-P91 and SS347H. Weld material is INCONEL625.
  • 24” diameter and great thickness due to severe operating conditions: tª=530ºc, p=110 bar
  • None of the reparations conducted so far has been effective to stop recurring breakage after 6-12 months.
root cause analysis and corrective measures of dissimilar weld joint failure in a high pressure steam pipe
Root cause analysis of dissimilar weld joint failure

Fig. 1: 24″ High pressure steam pipe actual disposition and welding features

Dissimilar weld joint failures in high pressure steam pipes
Fig. 2: Failures sequence

Methodology

Methodology for root cause analysis of dissimilar weld joint failure in a high pressure steam pipe

Design and Fabrication Data:

  • Welding detail
  • Welding process
  • Pre and Post welding thermal treatments
  • Materials (properties, storage…)
  • Inspections and NDT

Operation Data

  • Previous observations and Evidences
  • Failures record and details (critical area pictures, crack pictures…)
  • Macrographs, metallography, hardness tests, traction tests, chemical composition.

Regular Operation

  • Pipe and welding on operation conditions
  • Welding process
  • Residual stresses

Best Mechanical and Process Engineering Practices

  • Simulation: FEA
  • International Codes (ASME, FFS)
  • Physical Modelling (understanding how it works)
  • Tests and Hypothesis Modelling
  • Corrective Measures Modelling

2.1: DETAIL LEVEL 1

  • Piping and welding analysis under operation conditions

2.2: DETAIL LEVEL 2

  • Welding process simulation

2.3. DETAIL LEVEL 3

  • Tests to obtaining residual stresses

Observations and Evidence

  • Laboratory Tests

Root Cause Analysis and Determination of Failure Cause

  • Specific proposal of corrective measures
  • Modification of welding detail + Pipe spool
  • PWH
  • Modification of welding process and pass sequences
  • Modification of welding filler material

1. Problem Awareness

1.1 Failure identification

⇒  FAILING EVERY 6-12 MONTHS

identificación
Dissimilar Weld Joint Failures

1.2 Location and Size Craks

  • CIRCUMF. between 100º-270º
  • LENGTH. 250mm-400mm
  • Crack on melting area
  • buttering Inconel 625 – P91
Localización de las grietas y tamaño de las grietas:
Inspecciones y ensayos NDT

1.3 Inspections and NDT

✔  Full cracks VT and superficial cracks (PT-NDE)
✔ Internal part of the pipe in crack location ⇒ inadequate filling

FIG9

1.4 Fractography, Steroscopic Examination and Chemical Composition

✔ Sreroscopy ⇒ oxide deposits ⇒ CAPING PASS
✔ Fractography ⇒oxide fragments on internal and external surfaces
✔ Chemical composition ⇒acording to materials specifications requirements

Fractografía, examinación, esteroscópica y composición química de tubería crítica
Steroscopic examination in high pressure steam pipe

1.5 Metalography

✔ Tempered martensite structure in P91, HAZ
✔ Dendritic austenitic structure in INCONEL and Buttering
✔ Tempered martensitic structure with ferrite in crack location area
✔ Presence of nucleation of voids and spheroidal graphite

Metalography fig15

1.6 Hardness test

✔ Identification of high hardness points in HAZ areas of P91 and INCONEL, coincident with the location of cracks

Ensayos de dureza en soldaduras disimilares
Hardness tests in high pressure steam pipe fig 17

1.7 Welding process and heating treatments

✔ Warm-up 200ºC
✔ Welding GTAW
✔ Welding 5g Weld / 2G buttering
✔ PWHT in buttering ⇒ no PWHT in welding

procedimiento de soldadura y tratamientos termicos
FIG19

1.8 Operation conditions And support systems

✔ Support Systems and stress Calculation with no anomalies detected
✔ Insulation temperatures are higher in leakages area

condiciones de operacion y sistemas de apoyo
Operating contitions and support systems in dissimilar weld joint failure
CADE-evaluation-of-plausible-failure-causes

2. Physical characterization

2.1 Detail Level 1- Pipe & Weld Analysis under Operation Conditions

Detail Level 1- Pipe & Weld Analysis under Operation Conditions
stationary, linear static and non-linear static pipe and weld analysis
Analysis against plastic collapse, fatigue and creep failure

2.2 Detail Level 2- Welding Process Simulation

CADE welding process simulation
CADE consideration of pre-welding thermal treatment and welding pass thermal simulation
Determination of stress states and residual stresses
Transient thermal analysis in a high pressure steam pipe
FIG31
Structural analysis in a high pressure steam pipe
Structural analysis in a high pressure steam pipe

2.3 High Pressure Steam Pipe Failure Causes

Failure causes in a high pressure steam pipe

Solution: Corrective Measures

Installation of Spool Pipe section between P91 and SS347H

Installation of Spool Pipe section between P91 and SS347H ​

Definition of new PWHT

✔    Optimal PWHT treatment on weld area is defined
         ⇒    Decreasing residual stresses, decreasing material hardness, improving toughness
✔   Spacial care with reached temperatures (do not exceed 760ºC)
         ⇒    Fragilization issues
✔ Verification of residual stresses by means of FEA, and physical tests

Definición de nuevo PWHT

Modification of Welding Pass Sequence

CADE modification of welding pass sequence

ABOUT THE PROJECT

  • Solution proposed has solved a critical and recurring problem which involved a high cost in terms of plant availability.
  • Root cause analysis and project execution was subjected to hard constraints of time, and had to be solved over a 2-months period (remaining time until Turnaround).
  • Solid and consistent methodology thanks to simulation.

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