Corrosion fatigue cracking of tube coils in an actifier column catalytic cracker

摘要

Tube coils made of 25Cr-20Ni austenitic stainless steel were horizontally installed in the fluidized bed of an actifier column of a catalytic cracker installation in an oil refinery unit. Catalyst particles and flue gases were moved in the fluidized bed. When the catalyst lost activity, carbon in the catalyst was burned out in the fluidized bed to recover the activity of the catalyst. Meanwhile, a steam-water mixture was formed with a pressure of 4 MPa and saturation temperature of 250 deg C in the tube coils by the heating of the flue gases. Thus, the heat in the fluidized bed was utilized to generate steam. However, after the installation had been in service for about 40 days, leakage occurred in the tube coils. In general the positions of leaks were in the upper part of the tubes within about 6 m of the inlet. Microscopic analyses indicated that cracks initiated at local corrosion pits where chloride ions present in the feedwater enriched and accumulated. The crack propagated in an intergranular or transgranular manner. Obvious striations were found on the crack surfaces at some positions. Based on the failure analysis and heat transfer calculation. failure of the tube coils was mainly caused by the effects of corrosion fatigue. The lifetime of the tube coils can be prolonged by changing the steam-water flow conditions.