The mixing of process streams with large differences in temperature typically on the order of 300°F or greater can result in cracking due to thermal fatigue. Over the years failures of these “thermal mix points” have been reported but little in the way of estimating life or probability of cracking has been published outside the nuclear industry where this form of cracking has been a major issue. This paper studies the application of a thermal fatigue model originally applied to thermal mixing problems in nuclear piping for use in petrochemical applications. The dependence of predicted life on variables such as temperature cycling amplitude temperature cycling frequency and distance from mix point is explored. Cracking and failure data from field experience are then compared to the predictions of the strain based model. Finally results from a stress based finite element analysis which incorporates computational fluid dynamics modeling are presented.