1 Basic Concepts

Many vessels and equipment components encounter elevated temperatures during their operation. Such exposure to elevated temperature could result in a slow continuous deformation, creep, of the equipment material under sustained loads. Examples of such equipment include hydrocrackers at refineries, power boiler components at electric generating plants, turbine blades in engines, and components in nuclear plants. The temperature at which creep becomes significant is a function of material composition and load magnitude and duration.

Components under loading are usually stressed in tension, compression, bending, torsion, or a combination of such modes. Most design codes provide allowable stress values at room temperature or at temperatures well below the creep range, for example, the codes for civil structures such as the American Institute of Steel Construction and Uniform Building Code. Pressure vessel codes such as the American Society of Mechanical Engineers Boiler and Pressure Vessel (ASME B&PV) Code, British, and the European Standard BS EN 13445 contain sections that cover temperatures from the cryogenic range to much high temperatures where effects of creep are the dominate failure mode. For temperatures and loading conditions in the creep regime, the designer must rely on either in-house criteria or use a pressure vessel code that covers the temperature range of interest. Table 1.1 gives a general perspective on when creep becomes a design consideration for various materials. It is broadly based on the temperature at which creep properties begin to govern allowable stress values in the ASME B &PV Code. There may be other specific considerations for a particular design situation, e.g., a short duration load at a temperature above the threshold values shown in Table 1.1. These considerations will be discussed later in this chapter in more detail.