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Engineering Failure Analysis 11 (2004) 873–893 www.elsevier.com/locate/engfailanal

Creep failures of overheated boiler, superheater and reformer tubes
D.R.H. Jones
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Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK Received 1 February 2004; accepted 8 March 2004 Available online 21 July 2004

Abstract Internally pressurised tubes are criticalcomponents in heat-exchanger applications, such as boiler water tubes, steam superheater elements and chemical plant reformer tubes. Tubes in such applications are vulnerable to temperature excursions: as a consequence the material may enter the creep regime, and creep deformation (bulging) and even fracture (longitudinal rupture) may subsequently occur, with serious consequences. It is estimatedthat 10% of all power-plant breakdowns are caused by creep fractures of boiler tubes. In general, 30% of all tube failures in boilers and reformers are caused by creep. This paper gives details of four case studies in which internally pressurised tubes failed by creep bulging and rupture (two boilers, one superheater and one reformer). The conditions of temperature and time under which the failuresoccurred are deduced from the morphology of fracture and the changes in microstructure, and are correlated with the deformation-mechanism and fracture-mechanism maps for the tube materials. Ó 2004 Elsevier Ltd. All rights reserved.
Keywords: Boiler failures; Boiler tube; Chemical-plant failures; Creep; Creep rupture

1. Introduction Internally pressurised tubes are critical components inheat-exchanger applications, such as boiler water tubes, steam superheater elements and chemical plant reformer tubes. Tubes in such applications are vulnerable to temperature excursions: as a consequence the material may enter the creep regime, and creep deformation (bulging) and even fracture (longitudinal rupture) may subsequently occur, with serious consequences. It is estimated that 10% of allpower-plant breakdowns are caused by creep fractures of boiler tubes. In general, 30% of all tube failures in boilers and reformers are caused by creep. This paper gives details of four case studies in which internally pressurised tubes failed by creep bulging and rupture (two boilers, one superheater and one reformer). The conditions of temperature and time under which the

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Tel.:+44-1223-332600; fax: +44-1223-332662. E-mail address: drj1000@eng.cam.ac.uk (D.R.H. Jones).

1350-6307/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2004.03.001

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D.R.H. Jones / Engineering Failure Analysis 11 (2004) 873–893

failures occurred are deduced from the morphology of fracture and the changes in microstructure, and are correlated with thedeformation-mechanism and fracture-mechanism maps for the tube materials.

2. Case study 1: creep rupture in a water-tube boiler 2.1. Description of boiler The first case study describes the investigation of the creep rupture of a water tube in a large water-tube boiler, which resulted in considerable consequential damage and down-time. As part of the investigation, it was considered necessary toestablish the temperature at which the failure occurred, to estimate the time it took for the tube to burst, and to correlate these findings with the operating records of the boiler. Fig. 1 is a schematic of the boiler. For simplicity, the figure shows only two banks of hot tubes (risers) and two banks of cool tubes (downcomers). In the actual boiler, there were 12 banks of risers and 12 banks ofdowncomers. Each bank consisted of a parallel array of 90 tubes spaced regularly along the length of the drums. The total number of tubes in the boiler was 2160. The drums were 17-m long and 1.7 m in diameter. The average length of the water tubes was 10 m. Technical data are as follows: 2.1.1. Boiler operating parameters Maximum evaporation rate: 500 tonnes hÀ1 , Maximum heat flux through riser...