Fracture Analysis On Girth Weld Of Oil-gas Separator In Catalytic Cracking Unit

In a petrochemical company, a R1341 oil-gas separator of catalytic cracking unit is a horizontal pressure vessel, which was designed in December 1974 and put into operation in October 1984. In December 2008, two transverse cracks appear in the lower positon of the girth weld of the R1341 oil-gas separator, which was deal with iron clay. One month later, six transverse cracks appear in the girth weld again, which was treated with the same method. The catalytic cracking unit shutdown in February 2009 and the R1341 need to be updated.Cut one part of 450 mm spacing from the girth weld as test samples and respectively take corresponding samples in the weld and parent parts for macroscopic inspection, chemical composition analysis, metallographic observation, hardness test, tensile and impact test, fracture morphology observation and micro EDS analysis. Chemical composition analysis shows that the steel plate and weld metal both has high content of S and P. Energy spectrum analysis also confirmed that the existence of MnS inclusion. Accordingly, Hydrogen induced cracking in R1341 is inevitable in wet hydrogen sulfide environment.Recent years, the petrochemical company processed large amount of high Sulphur crude, leading to the increasing of H2 S concentration. Hydrolysis have dramatically increased the number of hydrogen ions and hydrogen ions adsorb to the steel surface, forming a hydrogen atom and spreading in steel. The hydrogen atom gathers in girth weld, especially MnS inclusions, and combine into hydrogen molecules, causing the hydrogen induced cracking.The petrochemical production unit was designed for low sulfur crude originally. Operating from 1984 to 2005, it has been processing low Sulphur crude. Since 2006, with the rising of sulfur content of crude oil, the equipment corrosion are apparent. Oil-gas separator R1341 was made of 16 MNR, not for heat treatment, cannot meet the requirement of long-term operation in wet hydrogen sulfide environment.According to the above analysis, the following proposals are put forward for technical improvements. Firstly, in wet H2 S environment the hydrogen sulfide corrosion resistant material is selected for equipment. Secondly, adopting reasonable manufacturing process to make sure that the S and P content in the girth weld do not exceed the standard. Thirdly, inner anti-corrosion methods are took to improve the corrosion resistance of equipment. Finally, great importance is attached to the process corrosion protection work, such as strengthening the management of water injection and agent injection.