This paper presents a concept design methodology to establish robust designs against thermal fatigue of 2″ and 3″ thermal tee branch sizes on 14″ pipework, which are subjected to relatively hot and cold fluid turbulent mixing, for use in Pressurised Water Reactor Plant.

Thermal tees can be subjected to extremely demanding thermal fatigue conditions, e.g. high temperature fluctuations causing high stress ranges where hot and cold fluid mix at the tee position from different branches of the system, these conditions ultimately limiting design life.

Prior to conducting a full design justification to the ASME Section III code [1], which for these components Rolls-Royce has justified by a section NB3200 approach using finite element analysis and computational fluid dynamics, analysis/iteration time can be saved, and the likelihood of a robust design being found increased, by understanding the effect and significance of geometric features of the tees. Fundamentally, establishing which features have a greater influence on the thermal fatigue performance of the tee and setting maximum and minimum values for these features. This paper presents an approach that can be used in the concept design phase to understand the influence of variables such as: branch throat internal diameter, run versus branch reinforcement, inclusion of integral orifices and branch fluid flow rate, and also of how they interact with each other in relation to providing a code compliant design. The approach is also used to size such features so that they are away from ‘cliff edges’ in performance, i.e. away from values that are likely to produce high stress levels and reduce design life.

The paper covers: the variables chosen to be investigated, the methodology including the associated stress models to understand the effect of variable change and positioning in the ‘design landscape’, and identifies which geometric features should be maximised or minimised in size to maximise thermal fatigue life.

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