EQUIPMENT: Reactor Effluent / Debutanizer Feed HP Exchanger, TEMA type DEU in 91MM BTU.Hr duty has Hydrocarbon on the shell-side and Hydrocarbon + Hydrogen on the tube-side. The design pressure / temperature = 1570psig / 620°F (Shell-side) = 2350psig / 620°F (tube-side). This two phase stream contains H2 (partial pressure 1600 psi), H2S, NH3 and H2O(v). 740-800°F reactor effluent vapors pass through a series of exchangers to give up heat to various process streams. The materials are SA336 F22 (2¼ Cr.1 Mo.) on shell-side & SA213 TP321 (18 Cr. 10 Ni-Ti) on tube-side.

ANALYSIS: Elastic-Plastic Structural Stress Analysis was performed based on mechanical loads and transient thermal loads.

RESULTS-CONCLUSION: Due to the propriety design of this HP channel box enclosure, the following design features could have resulted in the failure observed. The inherent design requires that 3 independent fillet welds terminate at the location above (Inside of CB). The multiple start-stop of welding at this corner may have aided the cracked weld. A dynamic force loading from the tube- inlet flow during operation, impinging on the horizontal channel box plate, caused excessive stress and fatigue-type cracking in the connection weld. All materials involved in it are metallurgical similar, hence thermal related stresses do not seem a possibility. At this time it is anticipated that the crack-like flaw was only present in the fillet welds connecting the channel-box and the channel-barrel.

PROBLEM: This is a fillet weld, all around size = 8mm, SS 347 filler on SS 321 base material (Channel box), the parent metal and weld metal are of the same metallurgy. Also, there is supposed to be a 2mm GAP originally designed between the butting edge of the channel-box plate and the ID of the channel-barrel. This gap may have turned into a very tight gap after this unit has experience operating service. DPT when performed at this location, with the existing weld grinded off, could give a false warning of the presence of a crack. Inspection shall make a note of this during DPT. The bottom north and south lower corners of the channel box pass partition plate fillet welds revealed cracks. It has also been understood from inspection-repair history that these crack have appeared before similarly during the last operating cycle