Computation of dynamic stresses in piston rods caused by unsteady hydraulic loads

The stresses in piston rods, a very important part of the blade-control system in Kaplan turbines, were analyzed to predict failure conditions. The turbine blades suffer from pressure oscillations whose forces are transferred to the piston rods. The 3D unsteady flow through the Kaplan turbine was simulated to predict the forces on the piston rod. The unsteady Reynolds-averaged Navier–Stokes (RANS) equations with the SST κ–ω turbulence model were solved to model the flow within the entire flow path of the Kaplan turbine. The unsteady hydraulic forces on the blades were then used as the boundary condition for dynamic analysis of piston rods either fixed with a nut or fixed with a retainer ring for various operating conditions. The results show that the mean stress and the dynamic stress for the nut structure were less than for the retainer ring structure because the nut structure had a pretightening force. The dynamic stress with the retainer ring structure reached 23.7 MPa at the high head low output condition with a very low safety coefficient, which caused the final breakage of the rod. The predicted stress concentration position in the rod agreed well with the fracture position.