Abnormal Cracks Led to Premature Decommissioning of Boiler Feed Water Exchanger – What Happened and Why?

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 800^oF]
• Carries boiler feed water on the shell side. [Design pressure 1750 psig] [Design Temperature 625^oF]
• 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.