4 Analysis of Asme Pressure Vessel Components: Load-Controlled Limits

The ability of pressure vessel shells to perform properly in the creep range depends on many factors such as their stress level, material properties, temperature range, as well as operating temperature and pressure cycles. American Society of Mechanical Engineers' (ASME) Sections I and VIII-1, which are based on design-by-rules rather than design by analysis, allow components to be designed elastically at elevated temperature service as long as the allowable stress is based on creep-rupture data obtained from Section II-D. In many cases, however, additional analyses are required to ascertain the adequacy of such components at elevated temperatures. The effect of creep on long-time exposure of various components in power boilers to high temperatures is of interest. The effect of continuous startup and shutdown of heat recovery steam generators (HRSG) on the life expectancy at elevated temperatures is one of the design issues. Another safety concern is the effect of long-time exposure of equipment used in the petrochemical industry to high temperatures.

These additional analyses include evaluation of bending and thermal stresses as well as creep and fatigue. The sophistication of the analyses could range from a simple elastic solution to a more complicated plastic or a very complex creep analysis. The preference of most engineers is to perform an elastic analysis because of its simplicity, expediency, and cost effectiveness. The approximate elastic analysis is not as accurate as plastic or creep analysis, which reflects more accurately the stress-strain interaction of the material. However, elastic analysis is sufficiently accurate for most design applications. The ASME's elastic procedure compensates for this approximation by separating the stresses obtained from the elastic analysis into various stress categories in order to simulate, as close as practically possible, the more accurate plastic or creep analysis.