Stress Corrosion Cracking Assessment And Safety Analysis Of Stainless Steel Evaporators In Glycol Production

Stainless steel has been paid attention due to its excellent performance, including goodcorrosion resistance and mechanical properties. Just so, it has been extensively used in differentfields, such as chemical industry, nuclear energy engineering industry etc. However, for lots ofstainless steel equipment,various kinds of premature failures are occurred during the serviceperiod because of the combined action of stress and medium, which results in nonplannedshutdown and enormous economic losses. Accordingly, stress corrosion cracking is a typicalfailure. During the operating process,lots of meshed cracks and longitudinal cracks normal to thewelded joints were found on several glycol evaporators in YangZi Petrochemical Company. Themetallographic replica and failure analysis results showed that the cracking was caused byintergranular stress corrosion crack (IGSCC). For this cracking phenomenon, the research contents were as follows: 1. The IGSCC behaviors of three kinds of stainless steel 1Cr18Ni9Ti, SAF2205 and 316Lwere studied by means of standard Strauss experiment. The loaded specimens used inintergranular corrosion test were self-designed. Experimental results showed that the sequenceof IGSCC resistance listed as follow: SAF2205, 316L, 1Cr18Ni9Ti.and SAF2205 is the mostsevere. 2. According to the real operating environment of glycol evaporator,the corrosionphenomena of the three kinds of stainless steel welded joints under sensitized and weldedconditions was investigated by the same loading method in the ambient media containingformate,acetic acid and Cl-. Experimental results show that,the susceptibility of the threekinds of stainless steel to IGSCC is low. Slight stress corrosion cracking phenomenon occurredaccompanying with a little dot corrosion. On the other hand, sensitised treatment will decreasethe IGSCC resistance of stainless steel. 3. During the assembly process of glycol evaporator, forced assembly between the headand cylinder may be produced. The relationship between the misalignment magnitude and stresswas simulated by the finite element analysis software ABAQUS. The simulation results showthat the stress is less than the yield strength when the magnitude of misalignment is less than6mm. 4. The safety analysis of IGSCC of glycol evaporator has been made by use of the fault III摘 要tree analysis method. The analysis results show that the environmental media is the main factorthat results in the IGSCC of glycol evaporator, such as Cl-,SO4 . On the other hand, 2-magnitude of stress, welding parameters and undetected cracks are also the importantinfluencing factors and the action can be ranged as: magnitude of stress, welding parameters,undetected cracks. and magnitude of stress is the most important. From the above investigations,several conclusions can be drawn as the follow: in order toassure the safety operation of the whole equipment and prevent the occurrence of corrosioncracking, the media in glycol evaporator should be controlled and corrosive media, such as Cl-and SO4 , should be avoided, During the design and manufacture processing of stainless steel 2-evaporator, the materials having resistance of IGSCC should be selected; Residual stresses dueto misalignment and welding process should be reduced; and the welding parameters should beoptimized in order to decrease the sensitized degree in HAZ of welded joints.