Equipment:Sulfur Recovery Unit Reaction Furnace Waste-Heat-Boiler generating LP Steam on the shell side with ASME Sec-I designed Tubesheet.

Problem:Currently the Sulphur Reaction Furnace WHB was designed and constructed following the design rules of ASME BPVC Sec-I, which considers them to be steam generators or boilers. Plant inspections have experienced difficulties testing and inspecting the components of a SRU reaction furnace because of the mixture of ASME BPVC design rules of Sec-VIII & Sec-I, which specify different inspection intervals. These difficulties include limited accessibility for inspecting the shell side of the waste heat boiler (WHB), with an inspection interval of 24 months, without removing the tubes that are designed to the ASME BPVC VIII rules, with an inspection interval of 36 months. The reclassification of the equipment to ASME VIII was thus requested to be completed to allow revision of the T&I interval to 36 months similar to  all other equipments interval in the Plant. This caused unnecessary shut-downs in the SRU operations resulting in loss of availability of SRU to support refinery operations and production $$$.

Analysis: As a Firetube boiler, the tubesheet is designed in accordance to the requirements of PART PFT, Requirements For Firetube Boilers. As ASME Sec-VIII Div-1 requires that the welded flathead (tubesheet) is to be treated as unstayed surface, this does not permit to take credit for strength welded tubes (process gas service). The only way to verify this excessive tubesheet calculated thickness would be by running additional calculations using design by analysis ASME Sec-VIII Div-2 rules. The analytical results of ASME Sec-VIII Div-2 rules will have to be then compared and confirmed with a FEA solution. Linear Static Structural-Thermal FEA was performed , by utilizing Elastic stress analysis to establish the primary + secondary stresses in the tubesheet + welds by simulating plastic collapse.

Results Conclusion:Within a detailed stress evaluation based on categorization and linearization of stresses, it was shown in the report that all stresses are lower than their limits. Therefore, the specified design pressures are acceptable. Therefore, Sulfur Reaction Furnace WHB that was designed and constructed following the design rules of ASME BPVC I, was established to also comply with the design requirements of ASME BPVC Sec-VIII. On basis of this, an inspection interval of 36 months for the entire equipment was acceptable.Reducing the thermal gradient across tube-sheet to 675⁰F-295⁰F 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.

Benefits:This FEA assessment lent credibility and provided appropriate recommendations to implement the correct design change to fix the issues with code reclassification. This enabled 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.