Creep-Fatigue Interaction in Coke Drums: An Approach Based on API 579-1/ASME FFS-1 2007

Abstract

Coke Drums are critical equipment in refineries due to variable temperature and pressure. The temperature is also very high and coke drums work in the creep range for some duration of one full cycle. The phenomenon of creep along with fatigue plays a very crucial role in failure of coke drums. In the present study, a coke drum is subjected to pressure–temperature reversal with each cycle of 48 hours duration. Temperature and pressure varies from 65 to 495 °C and 1.72 to 4.62 bar, respectively. The material of construction is 1.25Cr-0.5Mo. This material is in the creep range beyond 454 °C as defined by ASME Section II, Part D. This coke drum is in creep range for 23 hours of one full cycle. The skirt to shell and cone junction is a critical portion of coke drum because it is highly susceptible to creep and fatigue simultaneously. In addition to this, the skirt is provided with slots at specific pitch all around circumference to induce flexibility. This has great influence on creep and fatigue due to localized stress. API 579-1/ASME FFS-1 2007 is used for the creep fatigue interaction in the present case. Non-linear transient thermal analysis is coupled with the elastic-plastic structural analysis for calculation of stresses and strains. These stresses are further used in deducing a creep damage factor and strains are used for deducing permissible cycles for fatigue as per the approach based on API 579-1. A macro is developed in the commercially available Finite Element Code ANSYS 11 for calculation of the creep damage factor and permissible cycles for fatigue. The creep damage factor works out to be 0.01 and this has been used for deducing a fatigue damage factor from creep-fatigue interaction curve. The fatigue damage factor is found to be 0.9. The macro results are validated with theoretical calculations.