Interim Boxup of Crack weld Joint of Ammonia Synthesis Converter

This study is based on self-motivation and exclusively for learning purpose by comparing the real-life vs the software simulation results. It is based on the design evaluation (using modern software) of the low alloy plate used for interim repair box-up carried out on cracked shell weld joint of Ammonia Converter  back in 2013.

Plate welding was carried out using temper bead welding technique. The interpass temperature were maintained above 250C. The root pass was carried out using GTAW technique. After DPT, SMAW technique was adopted. After final DPT, the plate was heated above 400C and put under insulation for cooling.

The purpose of this evaluation was to check the results from the modern FEA software and compare them to the well-evaluated and proven short-term boxup. The results from this study will help us using modern software as an another great tool for validation and selection of box-up material thickness and its dimensions and help us making bold decisions for such type of critical boxup activity.

The CAD model of the plate was prepared as per actual plate dimensions (110mm x50mmx20mm) used for box up at that time. The fillet weld was integrated in the plate CAD model.  

 A very fine solid Jacobian mesh was generated with Total elements= 107737Ea to get accurate results.

 The fillet weld was constrained and a pressure of 150Kgf/cm2 was applied on the bottom of the plate in the software. 

Considering the Yield Strength of the material , the maximum Von mises stresses observed toward the tip of the longest fillet weld found to be 1457 kgf/cm2. This stress value is at the root pass interface between the plate and the shell of the converter.

 The minimum Factor of safety (FOS) that is (Yield strength/Max von misses stress) was found 4.34 (Normally FOS above 1 is consider to be a safe design).

The maximum displacement of 0.009mm was observed at the centre of the plate.

CONCLUSION:

The software result validates the selection of plate material and its dimensions used in  box-up.
FEA results also shows that the fillet weld is the weaker part of this boxup. The fillet weld of this dimension plate can withstand pressure upto 500 kgf/cm2 before failure.

Falling of Zblock in Primary Reformer Convection Section and FEA interpretations

Primary Reformer  is a critical equipment that reform process gas at elevated  temperature.  Total 364 burners installed on the side wall of the furnace heating 168 catalyst tubes in the radiation chamber. The hot flue gases pass through many tube bundles inside the convection section and transfer heat to process fluid before emitting into the atmosphere.

The smoke hood roof of the furnace is covered with the Z-block installed on 6mm thick carbon steel plates. More than 1500 zblocks of dimension 305mm x 305mm x 305mm installed on the smoke hood roof. Recently we had an experience of falling of z-blocks resulted in huge distortion in the carbon steel plates due to intense heat (~1150C) at that portion. Due to fall of one zblock, CS plate crossed its yield point at that particular zblock location and a small portion of it fell down.

The following FEA study is based on the non-linear analysis carried out to see the effect of high temperature on CS plate. The study is based on 12sec exposure of 1100C at the center (305mm square area) of the CS plate of dimension 2000mm x 1000mm x 6mm. Temperature of 1100C was applied considering falling of 305mm zblock  at the center of the CS plate. The solution took 6minutes to complete on 64bit system core i3 computer.

Gravity, the overall skin temperature of the plate (~170C) and the weight of Z-block at the remaining portion of the plate was considered in the study. Minor vacuum pressure was also applied to meet the real world requirement.

Following is the picture of the carbon steel plate after fall of Zblock. The bulge portion can be seen clearly. The plate crossed the elastic limit and the deformation you see is permanent deformation (Plastic region).

 At the center portion of the following mesh 1100C temperature was applied for 12sec. Also the plate was constrained all around the edges.

The following picture shows the Von mises stress plot at 12sec of the study.The stresses clearly shows the weaker portions of the plate at the fallen zblock location.

The displacement plot shows 51mm deformation at the fallen Zblock location which matches with the actual plate installed.One can also see in the following picture  how much deformation took place at each 1sec interval.

The

Below picture shows the weakest portion (Red one) that fell down. The shape of the created holes almost resembles comparing FEA and the actual.

Conclusion at your end.