Analysis: Linear Static Structural-Thermal FEA, by utilizing Elastic stress analysis to establish the primary + secondary stresses in the tubesheet + welds by simulating plastic collapse.

Equipment:Sulfur Recovery Unit first Sulfur Condenser generating HP Steam on the shell side with ASME Sec-I designed Tubesheet.

Conclusion: Reducing the thermal gradient across tube-sheet to 675F-295F when combined with the reduced internal pressure to 75psig helps significantly in lowering the primary + secondary stresses in the tubesheet thickness and ligaments to within an acceptable range. The presence of a Bonded condition of the annulus space between the tube OD and the tubesheet-hole ID surfaces is required to lower the alternating stress range enough to increase in the fatigue design life of the tube-to-tubesheet welds significantly above the minimum cycles required for a 30 year reliable service life.

Results:Overall, the existing design of the first sulfur condenser under Max-OPE load: 670psig + 725F-295F thermal gradient was found to promote failure by ratcheting of the tubesheet thickness ligaments and knuckle region thickness due to presence of high primary + secondary stresses. Additionally, the Crevice-type joint configuration resulting from the presence of an annulus space between the tubesheet hole and the outside of the tube area is mainly responsible for the failure of the tube-to-tubesheet welds in the peripheral region. This failure would occur within 32 cycles due fatigue caused by thermo-mechanical loading considering all the start-up, shutdown cycles experienced by the sulfur condenser during its operating history.

Benefits:This FEA assessment lent credibility and provides appropriate recommendations to implement the correct design change to fix the issues with sulfur condensers. The conclusions of this analysis also provides the technical basis to support the failure mechanism. A project on improving company-wide sulfur condenser issues was implemented enabling the Sulfur Recovery Unit to safely and reliably continue to operate without any disruption throughout the SOR to EOR 5-year scheduled period, resulting in maximizing SRU throughput.

Problem: The tubesheet suffered numerous tube leaks failure considered to be a case of repeated failure of the tube-to-tubesheet welds. This caused multiple shut-downs in the SRU operations resulting in loss of availability of SRU to support refinery operations and production $$$.