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.

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.

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.

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.

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.

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.

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 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.

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 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.

This chapter provides an outline of the failure modes and mechanisms associated with most boiler tube failures in coal-fired power plants. Primary categories include stress rupture failures, water-side corrosion, fire-side corrosion, fire-side erosion, fatigue, operation failures, and insufficient quality control.

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.

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 lists all of the references cited in the chapters on damage mechanisms.

Water chemistry is a factor in nearly all boiler tube failures. It contributes to the formation of scale, biofilms, and sludge, determines deposition rates, and drives the corrosion process. This chapter explains how water chemistry is managed in boilers and describes the effect of impurities and feedwater parameters on high-pressure boiler components. It discusses deposition and scaling, types of corrosion, and carryover, a condition that occurs when steam becomes contaminated with droplets of boiler water. The chapter also covers water treatment procedures, including filtration, chlorination, ion exchange, demineralization, reverse osmosis, caustic and chelant treatment, oxygen scavenging, and colloidal, carbonate, phosphate, and sodium aluminate conditioning.

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.

Depending on the operating environment and the nature of the applied loading, a structure can fail by a number of different modes, including brittle fracture, ductile fracture, plastic collapse, fatigue, creep, corrosion, and buckling. These failure modes can be broken down into the categories of fracture, fatigue, environmental cracking, and high-temperature creep. This article discusses each of these categories, as well as the benefits of a fitness-for-service approach.

The toughness of a material is its ability to absorb energy in the form of plastic deformation without fracturing. It is thus a measure of both strength and ductility. This chapter describes the fracture and toughness characteristics of metals and their effect on component lifetime and failure. It begins with a review of the ductile-to-brittle transition behavior of steel and the different ways to measure transition temperature. It then explains how to predict fracture loads using linear-elastic fracture mechanics and how toughness is affected by temperature and strain rate as well as grain size, inclusion content, and impurities. It also presents the theory and use of elastic-plastic fracture mechanics and discusses the causes, effects, and control of temper embrittlement in various types of steel.

This chapter provides a detailed overview of the creep behavior of metals and how to account for it when determining the remaining service life of components. It begins with a review of creep curves, explaining how they are plotted and what they reveal about the operating history, damage mechanisms, and structural integrity of the test sample. In the sections that follow, it discusses the effects of stress and temperature on creep rate, the difference between diffusional and dislocation creep, and the use of time-temperature-stress parameters for data extrapolation. It explains how to deal with time dependent deformation in design, how to estimate cumulative damage under changing conditions, and how to assess the effect of multiaxial stress based on uniaxial test data. It also includes information on rupture ductility, creep fracture, and creep-crack growth and their effect on component life and performance.