Problem Definition
- During operation of a petrochemical plant (Middle East), recurring appearance of cracks in dissimilar metal welding joint of a critic, high-pressure steam pipe have 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 operation conditions: tª=530ºc, p=110 bar
- None of the reparations conducted so far has been effective à recurring breakage after 6-12 months.
Fig. 1: 24″ High pressures steam line actual disposition and welidng features
Methodology
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
Analysis and determination of Failure Cause
- Specific proposal of corrective measures:
- Modification of welding detail + Spool Pipe
- PWHT
- Modification of welding process and pass sequences
- Modification of welding filler material
1. Problem Awareness
2. Physical characterization
2.1 Detail Level 1- Pipe & Weld Analysis under Operation Conditions
2.2 Detail Level 2- Welding process simulation
2.3 Faiure Causes
3. Solution. Corrective Measures
3.1 Installation of Spool Pipe section between P91 and SS347H
3.2 Definition of new PWHT
3.3 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.
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.