Introduction
A small leak from top tubesheet-to-tube welds prompted further inspection of the 1¼Cr- ½Mo Ammonia Converter Boiler Feed Water (BFW) Exchanger during a planned shutdown. Further cracks were identified in the top channel to tubesheet butt weld that operated at 700 °F. After grinding halfway through thickness with these cracks still being present, excavations of the 2 1/8” thick shell were stopped. As well, attempts to repair the top tubesheet-to-tube welds resulted in further cracks developing. The BFW exchanger was scrapped after being in service for 7 years. In contrast, previous exchangers in this service had operated without known cracks for more than 15 years. The findings as to the causes and potential measures to obtain a better service life are presented below.
Vessel Background Information
The Ammonia Converter BFW Exchanger:
- Has a process pressure of 2075 psig. [Design pressure 2200 psig] [Design temperature 800oF]
- Carries boiler feed water on the shell side. [Design pressure 1750 psig] [Design Temperature 625oF]
- Is welded with a double submerged arc welding (DSAW) procedure.
- The DSAW Weld metal is ASME Section II Part C SFA 5.23, Grade EB2.
- Top channel has a wall thickness of 2 1/8” and an ID of 35 ½ inches.
- Has channels made of ASME SA387 Grade 11 Class 2 plate.
- Has its butt welds Post Weld Heat Treated (PWHT) at the time of manufacture as per ASME Section VIII, Division 1 requirements.
- Is installed in the vertical position. The Top Tubesheet is hotter than the Bottom one.
- Has a H2 content of approximately 55-60%.
- The hydrogen partial pressure is approximately 1150 – 1254 psia.
- Operates below the Nelson curve for 1¼Cr- ½Mo in API 941.
Lessons Learned
- Rigorous control of the post weld heat treatment is fundamental.
- Verification of the hardness of the process side heat affected zones is important. Meticulous surface preparation is required to obtain meaningful field hardness measurements.
Part A: Top Channel to Tube Sheet Weld
Visual and Magnetic Particle Inspections (MPI)
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