Analysis: The objective of the fatigue failure analysis is to determine the allowable life cycles & cumulative damage occurred at the circumferential weld located between the two forged tees of the connecting branch lines  based on the the fatigue design curves and the stress cycles generated during ONE operational cycle. The one complete operational cycle corresponded to 24hrs and involves the coking cycle, of each of the three Coker sets of  successively. The operational load combination was applied in a time based manner, such that the operational cycle from start to end (24hrs) is covered. The thermal gradient that gives rise to cyclic stresses is transient in reality. This evaluation requires applying an accurate transient thermal gradient to arrive at the true nature of the stress cycles generated thereof. However, this can be possible by completing a CFD (or knowing the temperature profile accurately enough i.e. ⁰F/12inches), and then applying the temperature gradient to the solid model as a function of time, resulting in stress cycles. Hence for the purpose of simplification, the following method of loading was adopted to generate the Thermal cyclical loadings As the type of temperature loading witnessed by the BlowDown header is Extremely Complex for this purpose the complete thermal loading of the BD header has been broken down into simpler but realistic scenarios, which individually but independently generate their respective Type and Number of stress cycles. The main objective is to identify all those operational instants, which could lead to Stress extremes that follow every Stress reversal For a temperature loading, it is the thermal swing at which the stress reversal occurs immediately following a stress extreme.