Stress Analysis and Engineering Design of Reducer Bend under Internal Pressure

Abstract

Standard reducer bends has structure function of both bend and reducer pipe at the same time, but there's only few engineering application and relevant report on it. Although lacking in relevant strength design standard, it still has obvious advantage in some individual case. By comparison of analytic formulas of circumferential or longitudinal thin membrane with that of similar structure pipe specified in ASME B31.1-2012 Power Piping and ASME B31.3-2011 Process Piping separately under internal stress, we know that these two have almost the same structure form and affected factors. In order to carry out internal pressure strength design of reducer bends, by comparison and analysis of analytic formulas of circumferential and longitudinal thin membrane under internal pressure and test stress measurement under internal pressure, the result shows that analytic formulas value is apparently larger so that it is conservative to be applied in engineering. This error arises from the difference between ideal model and actual pipe. Actually, reducer bend is belonging to neither the thin wall structure nor the rotated shell of axial symmetry, but a kind of hyperboloidal shell with 2 main curvatures changed gradually complicated. Therefore, it is not appropriate to analyze such kind of reducer bends with the thin membrane theory in thin wall and axial symmetry structure. The reducer bends strength formula derived directly from circumferential stress formula of reducer bends under internal pressure is also conservative. The reducer bends fabricated with the new technology combined with half seamless and half seam has sound quality, and the internal pressure strength is sufficient to meet requirements of fittings used in engineering.