Cracking of AISI T22 Reheater Pendant Assembly Tubing of Steam Boiler
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CASE-HISTORY—PEER-REVIEWED
Cracking of AISI T22 Reheater Pendant Assembly Tubing of Steam Boiler B. Erenburg . A. Zilberberg . E. Iskevitch
Submitted: 14 March 2019 ASM International 2019
Abstract The Power Station Unit steam boiler was supplied in 1995. In the period of 2008–2013, a number of forced outages were documented at the reheat pendant assemblies. Based on laboratory analysis, it has been concluded that all investigated tubes failed due to propagation of fatigue cracks along the high crown seal welds, caused by sway movement fluctuations of assemblies while combustion gas flows. The main factor in the formation of fatigue cracks was the poor quality of the crown seal welds, and the main reason for the development of fatigue cracks in the tube wall was the rigid and non-flexible fixing of the tubes in the crown seal boxes. It has been assumed that fatigue cracks currently exist in many reheater tubes. In an attempt to locate leaks in the reheater tubes, leak tests were conducted with acoustic equipment, and validation tests were conducted using soap bubbles and a borescope. Using this method, a high number of leaks were revealed during the long planned outage in April–December 2016. The most surprising fact was that in every one of approximately ten tests there were revealed a few new additional leaks. It was necessary to carry out brief failure analysis due to the outage, to understand the causes of failure, to avoid similar failures and to return the unit to operation. Keywords Crack growth Failure analysis Heat-affected zone Microstructure
B. Erenburg (&) A. Zilberberg E. Iskevitch Israel Electric Corp, Haifa, Israel e-mail: [email protected]
Introduction The reheater rear assembly is supplied as part of the boiler in 1995 and consists of 141 reheat rear pendant assemblies (sections); each section has five loops ‘‘U’’ form, all together ten tubes in each assembly, as shown in Fig. 1. Tube outer diameter is 2.5 inches. Since 2008, a number of forced outages due to leaking tubes occurred and documented. All leaks occurred at the reheater’s exit bundle. (Five tubes are marked in Fig. 1.) Based on laboratory failure analysis results, the following is concluded and agreed upon: 1.
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All investigated tubes failed due to propagation of fatigue cracks along the high crown seal welds, caused by sway movement fluctuations of assemblies while conducting combustion gas flow. The main factor in the formation of fatigue cracks is the poor quality of the sealing welds made by the boiler manufacturer. The main reason for the development of fatigue cracks in the tube wall is due to the rigid fixing/welding of the tubes in the seal boxes. Their non-optimal design led to low flexibility. Fatigue cracks currently exist in many reheater tubes (roughly about 60–70%). Therefore, in order to prevent a scenario of multiple failures, it is necessary to employ nondestructive methods for defect detection.
The ultrasonic testing method (UT) has found to be ineffective due to tube inaccessibility.
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