Inspectioneering
Inspectioneering Journal

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

By Ana Benz at IRISNDT, Dr. Donald H. Timbres at D. & E. Consulting, Inc. , and Matt Stroh. This article appears in the September/October 2014 issue of Inspectioneering Journal

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 Hcontent 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)

 
Figure 1(a) and (b). Vessel ImagesFigure 1(a) and (b). Vessel Images
 
The type of vessel is illustrated in Figures 1(a) and 1(b). The photographs show the vessel’s Inside Diameter (ID) surfaces after attempting to excavate the cracks on the top channel to tubesheet weld. Magnetic particle inspections showed cracks mainly transverse to the welding direction. The cracks are mainly confined to the deposited weld metal (DWM). See Figures 2-4.

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