Fatigue behaviour and design of quarter turn valve actuators

Actuators are machines used to move valves in order to ensure the correct control of process fluid flows, in industrial and Oil&Gas application. To operate efficiently, valve actuators must be powerful but also reliable in order to guarantee the structural integrity of all its components under the service number of cycles required by the end-users, always more often greater than the minimum prescribed by the main standards of the sector. The fatigue assessment method currently adopted for the design of valve actuators produced by FLOWSERVE-Limitorque company, industrial partner of the research activities carried out in the present thesis is based on the nominal stress approach. Therefore, the current company design rules for the fatigue assessment of the components usually do not take into consideration local stress effects. An oversimplified design approach might be a particularly delicate issue when a structural failure could have safety and environmental related consequences. The present thesis focuses on the fatigue life assessment of the scotch yoke, which is the core component of a quarter turn valve actuator typically used in Oil&Gas sector. The thesis is arranged in five chapters. Chapter 1 introduces the valve actuation sector, explaining the different applications and describing the type of actuators. Furthermore, the main standards of the sector are described. Chapter 2 is devoted to a review of the technical literature on fatigue assessment and design of industrial components with particular attention to the fatigue strength evaluation of notched and welded components. Chapter 3 describes the experimental fatigue tests on scotch yokes. After a description of the testing set-up, the results of two different geometries of scotch yoke are presented. Chapter 4 describes the finite element analyses and the post-processing methods to predict fatigue life of the scotch yoke. The two geometries of the experimental campaign are considered in the simulations. Fatigue life assessment is performed according to some critical plane criteria based on local strain components. A dedicated post-processing software is used to obtain full-field maps of estimated fatigue life. In addition, fatigue life estimation is carried out by considering a strain energy method along with a so-called peak stress method. Finally, a comparison between the results of the different analyses carried out on the different geometries is described in Chapter 5, together with a comparison between the numerical and experimental results.