Case study

High thermal-stress components - Fatigue life prediction

The challenge

The feedwater nozzles inside most conventional and nuclear power station boilers exist within an environment of continuous high-pressure, and extreme temperature variations that can significantly reduce the fatigue life of even the most robust components (see Fig. 1).

Throughout their operational life, they endure a range of transient loads, often passing beyond those encountered in normal continuous operation, whilst thermal stresses placed on the nozzle (caused by passing cool feedwater into a hot environment) gradually reduce the life of critical welds through fatigue.

A previous study had predicted that the life of most feedwater nozzles was significantly less than the rest of the plant. Frazer-Nash was therefore asked to conduct a more indepth fatigue life prediction study for one such set of nozzles.

Our solution

Our first challenge was to model the thermally driven flow of hot water within the feedwater pipe sleeve, which was difficult as the flows within this narrow slot were entirely governed by natural convection and no data existed. However, using advanced Computational Fluid Dynamics, we were able to build a computer generated model of the feedwater temperature distribution, and validate the results against published German experiments on a similar geometry.

After introducing over 20 different thermal transients to the fluid-thermal-structural modelling, in order to predict the stresses generated as the ambient conditions changed, we found that the model had given us a complete set of low cycle data which was not previously available. And by applying further high cycle requirements to this data, we were able to predict the precise fatigue life and critical defect sizes using conventional R5 and R6 analyses (J-integral analysis was not required for this study).

Benefits

The critical success factor in this instance was that the analysis carried out by FrazerNash was able to generate data which was not previously available, and use this data to predict a greatly extended operating life. As a result, there was no need for our client to undertake expensive routine replacement of the nozzles, and the thorough documentation of the project led to swift regulatory approval.

Screenshot 2020 05 03 At 21.31.17
Figure 1: simulated temperature distribution inside a feedwater nozzle

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