A Method for Assessing the Turbine Generator Set Shaft-Line Behavior in Accidental Situations

Ensuring the operating safety of electrical power plants is a major issue for operators. Regarding turbine generator sets, a risk was identified due to the potential loss of one or more low pressure last stage blades of the turbine which could lead to a large unbalance, and then to an accident. This kind of accident is generally avoided using several means among which robust design, condition monitoring and periodic non destructive inspections. Nevertheless, safety studies must be undertaken. This led researchers to develop realistic numerical methods to predict the effects of such an accident. In this framework, EDF R&D has developed its own method to describe the most accurately the dynamic behavior of a shaft-line in an accidental situation caused by a blade loss. The objective is to evaluate the loads on the bearings in these conditions and compare them to the maximum design loads provided by the manufacturers. This methodology is composed of two phases. The first one focuses on the study of the shaft-line behavior before the blade loss considering the linear behavior of the bearings oil film: a preliminary static computation is made under gravity loads considering the altimetry of the bearings. Loads on the bearings resulting of this calculation are used by a bearing code in order to compute the dynamic coefficients associated with the oil film. Then, the harmonic response to unbalance is calculated for different unbalance positions. The objective is to find the critical positions, critical rotational speeds and the most loaded bearings, for which a nonlinear modeling of the oil film is to be considered. The second phase starts at the instant which follows the blade loss and simulates the shutdown of the turbine. The simulation of the transient response under unbalance is performed taking into account the nonlinear behavior of the most loaded bearings oil films. The aim is to estimate the maximum loads that the bearings have to support, especially when the rotor to stator contact occurs. In this paper, the steps described above are detailed and applied on an industrial study. Results and performances are presented as well.