Non-Linear Creep-Buckling Analysis: An Approach Based on WRC-443 for Development of Allowable Compressive Stresses in Coke Drums

Coke Drums are critical equipment in refineries due to variable temperature and pressure. The temperature is also very high and coke drums works in the creep range for some duration of one full cycle. 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. Design temperature of 510 °C and total Operating Weight of coke drum is 2500 tons. The skirt is to be check against the operating weight, operating pressure & wind load/earthquake load at high temperature which causes the compressive stresses in skirt. The phenomenon of creep along with buckling plays a very crucial role in failure of skirt of coke drums. In addition to this, the skirt is provided with slots at specific pitch all around circumference to induce flexibility for fatigue which weakens the skirt for compressive loading. The material of construction is 1.25Cr-0.5Mo. The temperature limit of 1.25Cr-0.5Mo is 482 °C as per external pressure chart & Appendix-3 of ASME Section II, Part D. Design temperature of coke drum is 510 °C & as design temperature is exceeding the temperature limit, allowable compressive stress from ASME Section II, Part D, Subpart 3 can not be used for design. Thus, an allowable compressive stress for 1 Hr and 100,000 Hr has been developed using Non-linear creep-buckling analysis with WRC-443 to check the skirt against induced compressive stresses. The isochronous curve including accumulated creep strain has been developed for 1 Hr & 100,000 Hr using API 579-1/ASME FFS-1 2007. Non-linear creep buckling analysis at 1 Hr & 100,000 Hr has been carried out in ANSYS using isochronous stress-strain curve as material properties. An induced stress in skirt obtained from analysis has been used in WRC-443 for calculation allowable compressive stress in skirt. An allowable compressive stress works out to be 227.8 MPa & 86.8 MPa at 1 Hr and 100,000 Hr, respectively.