Fatigue design in penstocks – comparison of the nominal stress and structural stress method for common details

Abstract

AbstractPenstocks in hydro power plants have a very long working life. They can have high pressure variations during service life, especially in pumped storage power plants caused by the dynamic pressure. Therefore, the fatigue design can be a dominating factor in the design of penstocks. In this paper, two common used construction details in penstocks are investigated with the nominal stress and the structural hot‐spot stress method for fatigue verification. The investigated construction details are: i) a wall thickness transition of the penstock with a weld along the circumference and ii) a ring stiffener welded to the penstock. The results of the different methods for fatigue verification for these two construction details are compared with each other, with variation of different geometric parameters. Also an analytical solution for the stresses in the hot‐spot for the two construction details, based on the semi‐membrane theory of cylindrical shells, was developed. Based on that stresses, stress concentration factors kHS are calculated and are available for practical applications in form of presented Tables in this paper. With the calculated stresses by the semi‐membrane theory in the hot‐spot it can be shown, that they are nearly equal to the hot‐spot stresses of a Finite‐Element‐Analysis, based on the IIW recommendation. Finally, a recommendation for fatigue verification of the two common details in penstocks is given.