Detailed life assessments cover both the pressure retaining components (e. g. casing, case cover, closure bolting, and stuffing box gland) and also the rotating element of the pump. The shaft and other components of the rotating element of HTS pumps are critical sub-components that must meet all the operational conditions for design life. Fatigue is the degradation mechanism of concern, particularly given that it is very difficult to get any degradation indicator prior to shaft failure.

To perform fatigue life assessment of operating pump shafts, a number of techniques have been used, specifically tailored to reflect actual plant conditions (as distinct from the more conservative design-basis assumptions). The methodology provides a more accurate estimate of shaft fatigue life for the actual plant-operating environment.

The fatigue life of the shaft is very dependent upon the radius provided at each of the shaft notches or grooves, which are the primary sources of stress concentration. The notch radii are not always specified on the supplier’s drawing or often not verified after manufacturing, even if specified. Hence it is important that the actual notch radii be known for accurate determination of the fatigue life of the pump shaft. To do this, replicas are taken of actual groove profiles on a plant pump shaft (usually a spare one). Then the actual shaft notch radii are determined from the replicas in a metrology laboratory. The stress concentration factors based on these actual groove radii are then used in subsequent fatigue evaluation.

Fatigue life assessment for operating mechanical loadings further involves a number of other special techniques to accurately represent actual plant conditions. Bending stresses in the shaft are a particularly important loading. Specialized analysis techniques have been developed to represent the pump and motor bearing stiffness in the structural model and to enable use of actual operational vibration data on the pump/motor set in the shaft bending loading assessment.

Thermal fatigue life assessment of pump internals is a relatively complicated and expensive undertaking. However, a detailed thermal and stress analysis approach has also been developed to assess thermal conditions associated with cold injection flows during normal and abnormal operation.

While HTS pump shafts and other important internals are non-pressure boundary components, the above techniques have been used in fatigue assessment of these components for the original plant life. These methodologies have proven to reflect the excellent plant performance to date of HWR HTS pumps and are useful approaches to assess fatigue life extension capability of these important sub-components.