Origins of tensile stress-induced circumferential cracking of waterwall tubes in boilers C.H. Hsueh* and I.G. Wright Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA *E-mail: hsuehc@ornl.gov ABSTRACT The leakage and rupture of boiler tubes in power plants is a serious problem that can lead to unscheduled and costly outages. The predominant failure location of current concern is circumfer- ential cracking on the fireside of waterwall tubes in the furnace waterwall section of a boiler. Although there is basic agreement that cracking results from a combination of thermal fatigue and corrosion, a complete explanation of the basic phenomena needed to establish the root causes of this problem is lacking. The purpose of the present study was to analyse the sources of the tensile stress responsible for initiating circumferential cracking and to identify the key parameters controlling this tensile stress. The results of analytical modelling suggested that a combination of increasing tube wall temperature with increasing thickness of internal oxide layers, and temperature spiking due to deslagging events eventually may result in tensile stresses sufficient to crack the fireside oxide and initiate the development of circumferential cracks. This scenario also led to suggestions for reducing tensile stresses in waterwall tubes which, in turn, would be expected to delayyavoid circumferential cracking and improve the reliability of waterwall tubes. Keywords: waterwall tubes, boiler, cracking, creep, stresses 1. INTRODUCTION Waterwall tubes constitute a major part of the structure of the furnace of a power station boiler which serves to absorb thermal energy from the products of combustion. These tubes run in a harsh environment, and contact of the gas, flame, and (where coal or oil is burned) slag with the waterwall tubes can cause corrosion damage, leakage, and rupture of the tubes which can have serious implications for the safety and cost-effectiveness of the thermal power plant. According to statistics from the Electric Power Research Institute (EPRI), the equivalent availability loss due to tube failures in US steam boilers is between 2.5 and 3% and, of these, corrosion fatigue in waterwalls is ranked as the second most important cause [1]. The typical construction of such boilers in the US employs waterwalls made from vertically-oriented ferritic steel tubes of the order of 31 mm ID66 mm wall, which are joined by uncooled spacers or webs (approximately 10 mm wide and 6 mm thick) of the same material attached at the mid-point of the tubes (across a diameter), as suggested schematically in Figure 1(a). These tubes are assembled into panels several metres wide and long, which are welded together to form the furnace enclosure or waterwall. The construction usually MATERIALS AT HIGH TEMPERATURES 25(4) 287–294 287 0960-3409/4/2008 # 2008 Science Reviews 2000 Ltd doi: 10.3184/096034008X394428 Figure 1 (a) Schematic transverse section of tubes in a waterwall panel. (b) Schematic drawing of a longitudinal cross section of a waterwall tube showing the scales on both the fireside and the waterside, the slag on the fireside, and the deposit on the waterside. The tube is constrained from movement in the axial direction because of the welding attachment. (a) (b)