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Strain Based Modeling of Thermal Fatigue at Mix Points

Product Number: 51312-01134-SG
ISBN: 01134 2012 CP
Author: Vianett B Munoz
Publication Date: 2012
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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.
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.
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Thermal fatigue is a specific type of fatigue failure mechanism that is induced by cyclic stresses generated due to repetitive fluctuations in the temperature. These temperature change in the material induces thermal expansion (or contraction) and if surrounding material or external constraints hinder this expansion thermal stresses arise. The degree of damage is affected by the magnitude and frequency of the temperature swings and typically appears in the form of one or more surface cracks on the component.This paper presents two case histories of thermal fatigue failure along with detailed metallurgical analysis of stainless steel pressure components in a downstream Aromatics plant.The first case study is on failure of a UNS31803 duplex stainless steel heat exchanger tubing that occurred after four and half years in service. The damage was in the form of multiple parallel circumferential surface cracks with few cracks propagating through the wall thickness of tube. The equipment was subjected to a temperature differential of about 100 deg C every 4 hours of operation during alternate heating & cooling cycle.The second failure discussed is on 6” diameter UNS32100 process piping after around ten years of service. This is primarily a thermal mix point wherein demineralized water is injected in a hot gas stream resulting in a temperature gradient of more than 100 Deg C. The failure mode was also characterized by many circumferential cracks at location of injection as well as about 2-3 diameters downstream injection point.Keywords: Thermal Fatigue Stainless Steel Cyclic stresses Temperature differential

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