Power Boilers: A Guide to the Section I of the ASME Boiler and Pressure Vessel Code, Second Edition
14 Creep and Fatigue Damage during Boiler Life
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The Foreword to Section I states that the objective of the Section I rules is “to afford reasonable protection of life and property and to provide a margin for deterioration in service so as to give a reasonably long safe period of usefulness.” Sooner or later, almost all equipment wears out, and boilers are no exception. The factors that cause the gradual deterioration of a boiler are many. Among them are erosion and corrosion, which cause a gradual wastage and thinning of pressure parts. Two other significant factors that can limit the life of a boiler are creep and fatigue, processes that can cause progressive deterioration of material properties without wastage and thinning. This deterioration is characterized as creep damage or fatigue damage. For practical purposes when discussing boilers, it can be said that creep damage occurs only at elevated temperature, about 700°F (370°C) and above for carbon and low alloy steels and 800°F (425°C) and above for the high alloy steels typically used in superheater construction. Fatigue damage can occur over the entire range of boiler operating temperature, but as will be explained later, components in elevated-temperature service are more susceptible to fatigue damage. These two damage mechanisms are discussed separately, but their effects are additive.