The power generating industry has become proficient at predicting how long a component will last under a given set of operating conditions. This chapter explains how such predictions are made in the case of boiler tubes. It identifies critical damage mechanisms, progressive failure pathways, and relevant test and measurement procedures. It describes life assessment methods based on hardness, wall thickness, scale formation, microstructure, and creep. It also includes a case study on the determination of the residual life of a secondary superheater tube.

Black liquor recovery boilers are an integral part of the kraft pulping process used in paper mills. Besides recovering chemicals for reuse, they also produce process steam. High operating temperatures and exposure to process chemicals and combustion byproducts make recovery boilers susceptible to corrosion. This chapter describes some of the problems that the pulp and paper industry has solved as well as ongoing issues and concerns. It includes an in-depth review of 304L cladding failures involving coextruded composite tubes used as floor tunes in the lower furnace, as superheater tubes, and for other purposes such as smelt openings. It provides detailed images showing cracks on the outer surfaces of the tubes and explains where the tubes were used and how they were operated.

Managing corrosion continues to be a challenge for operators of modern boilers worldwide. This chapter addresses the corrosion-related problems that can occur in boilers burning municipal solid waste (MSW). It describes corrosion mechanisms associated with different environments and alloys. It also discusses corrosion protection methods for furnace waterwalls and superheater tubes in waste-to-energy boilers.

This chapter describes some of the most effective tools for investigating boiler tube failures, including scanning electron microscopy, optical emission spectroscopy, atomic absorption spectroscopy, x-ray fluorescence spectroscopy, x-ray diffraction, and x-ray photoelectron spectroscopy. It explains how the tools work and what they reveal. It also covers the topic of image analysis and its application in the measurement of grain size, phase/volume fraction, delta ferrite and retained austenite, inclusion rating, depth of carburization/decarburization, scale thickness, pearlite banding, microhardness, and hardness profiles. The chapter concludes with a brief discussion on the effect of scaling and deposition and how to measure it.

Boilers are often classified based on the maximum operating temperature and pressure for which they are designed. Classifications, in ascending order, are subcritical, supercritical, ultra-supercritical, and to advanced ultra-supercritical. At each higher operating point comes greater efficiency, as well as greater demand on construction materials. This chapter discusses the primary requirements for boiler tube materials, including oxidation and corrosion resistance, fatigue strength, thermal conductivity, and the ability to resist creep and rupture. It also provides information on various steels and alloys, covering cost, engineering specifications, and ease of use.

This chapter discusses material-related problems associated with coal-fired burners. It explains how high temperatures affect heat-absorbing surfaces in furnace combustion areas and in the convection pass of superheaters and reheaters. It describes how low-NOx combustion technology, intended to reduce NOx emissions, accelerates tube wall wastage. It also covers circumferential cracking in furnace waterwalls, thermal fatigue cracking induced by waterlances and water cannons, superheater-reheater corrosion, and erosion in fluidized-bed boilers.

Fireside corrosion can be a serious problem in oil-fired boilers and in refinery furnaces fired with low-grade fuels. This chapter provides an overview of fireside or oil-ash corrosion and the problems it can cause in utility power boilers and petrochemical refinery furnaces. It explains how oil-ash corrosion affects waterwalls, superheaters, and reheaters as well as metal tube supports and hangers.

Boiler tubes subjected to cyclic or fluctuating loads over extended periods of time are prone to fatigue failure. Fatigue can occur at relatively low stresses and is implicated in almost 80% of the tube failures in firetube boilers. This chapter covers the most common forms of boiler tube fatigue, including mechanical or vibrational fatigue, corrosion fatigue, thermal fatigue, and creep-fatigue interaction. It discusses the causes, characteristics, and impacts of each type and provides several case studies.

Boiler tubes operating at high temperatures under significant pressure are vulnerable to stress rupture failures. This chapter examines the cause, effect, and appearance of such failures. It discusses the conditions and mechanisms that either lead to or are associated with stress rupture, including overheating, high-temperature creep, graphitization, and dissimilar metal welds. It explains how to determine which mechanisms are in play by interpreting fracture patterns and microstructural details. It also describes the investigation of several carbon and low-alloy steel tubes that failed due to stress rupture.

Many metallic components, such as retorts in heat treat furnaces, furnace heater tubes and coils in chemical and petrochemical plants, waterwalls and reheater tubes in boilers, and combustors and transition ducts in gas turbines, are subject to oxidation. This chapter explains how oxidation affects a wide range of engineering alloys from carbon and Cr-Mo steels to superalloys. It discusses the kinetics and thermodynamics involved in the formation of oxides and the effect of surface and bulk chemistry. It provides oxidation data for numerous alloys and intermetallics in terms of weight gain, metal loss, depth of attack, and oxidation rate. It also discusses the effect of metallurgical and environmental factors such as oxygen concentration, high-velocity combustion gas streams, chromium depletion and breakaway, component thickness, and water vapor.

Combustion byproducts such as soot, ash, and abrasive particulates can inflict significant damage to boiler tubes through the cumulative effect of erosion. This chapter examines the types of erosion that occur on the fire side of boiler components and the associated causes. It discusses the erosive effect of blowing soot, steam, and fly ash as well as coal particle impingement and falling slag. It also includes several case studies.

This chapter examines boiler tube failures attributed to operation-related causes. It discusses failures due to rapid start-ups, excessive load swing, excessive heat inputs, poor water chemistry control, and water-treatment methods.

Fossil fuels produce many byproducts that, if not fully combusted, put boiler tubes at risk. Fuel ash, chemical residues, and process heat pose the greatest threat and are the primary contributors to fireside corrosion. This chapter covers various types of fireside corrosion such as waterwall, fuel ash, and hot corrosion, acid dew-point or cold-end corrosion, and polythionic acid corrosion. It also addresses stress corrosion cracking and includes relevant case studies.

Boiler tube failures associated with material defects are often the result of poor quality control, whether in primary production, on-site fabrication, storage and handling, or installation. This chapter examines quality-related failures stemming from compositional and structural defects, forming and welding defects, design defects, improper cleaning methods, and ineffective maintenance. It also includes case studies and illustrations.

This chapter discusses the effects of corrosion on boiler tube surfaces exposed to water and steam. It describes the process of corrosion, the formation of scale, and the oxides of iron from which it forms. It addresses the primary types of corrosion found in boiler environments, including general corrosion, under-deposit corrosion, microbially induced corrosion, flow-accelerated corrosion, stress-assisted corrosion, erosion-corrosion, cavitation, oxygen pitting, stress-corrosion cracking, and caustic embrittlement. The discussion is supported by several illustrations and relevant case studies.

This chapter describes the metallurgy, composition, and properties of steels and other alloys. It provides information on the atomic structure of metals, the nature of alloy phases, and the mechanisms involved in phase transformations, including time-temperature effects and the role of diffusion, nucleation, and growth. It also discusses alloying, heat treating, and defect formation and briefly covers condenser tube materials.

Coal-based thermal power plants play a major role in the welfare of many nations and the overall global economy. This chapter describes the basic equipment requirements and operating principles of thermal power plants, particularly subcritical, supercritical, and ultra-supercritical types.

This chapter explains how materials selection becomes much more difficult when designing for high-temperature corrosion environments. It also discusses the use of cladding and weld overlays to compensate for material deficiencies and prolong service life.

Boilers are engineered systems designed to convert the chemical energy in fuel into heat to generate hot water or steam. This chapter describes boiler applications and types, including firetube boilers, watertube boilers, electric boilers, packaged boilers, fluidized bed combustion boilers, oil- and gas-fired boilers, waste heat boilers, and black liquor recovery boilers. It also describes the operation and working principle of utility or power plant boilers, covering conventional subcritical and advanced supercritical types.

Elevated-temperature failures are the most complex type of failure because all of the modes of failures can occur at elevated temperatures (with the obvious exception of low-temperature brittle fracture). Elevated-temperature problems are real concerns in industrial applications. The principal types of elevated-temperature failure mechanisms discussed in this chapter are creep, stress rupture, overheating failure, elevated-temperature fatigue, thermal fatigue, metallurgical instabilities, and environmentally induced failure. The causes, features, and effects of these failures are discussed. The cooling techniques for preventing elevated-temperature failures are also covered.