Failure analysis on outer tube breaks of a double-sleeve rapid cooling heat exchanger

Abstract

Failure analysis of a double-sleeve rapid cooling heat exchanger used for ethylene cracking process is presented in this study. The outer tubes of the heat exchanger break four times in succession. The inner tube and the outer tube materials of the heat exchanger are 13CrMo4-5 and SA106Gr.B, respectively. Based on the analysis of fracture appearance, chemical composition, mechanical properties, oxidation products and metallographic microstructure, it is found that short-time local overheating resulted in the four outer tube breaks. The observed pearlite spheroidization of SA106Gr.B indicate the temperature at the break positions exceed the phase transition critical temperature AC1（735℃）. Based on the analysis of the heat transfer process and boiler water circulation, it is found that the heat transfer deterioration of in a single set of double-sleeve, i.e., water is completely converted into superheated saturated steam, is the root cause of short-time local overheating. In view of the above failure mechanism, several countermeasures were proposed including the verification of the circulation ratio to ensure that the heat absorption per unit volume of cooling water is within the threshold value should be conducted in the design process, considering the uneven distribution of cooling water and cracking gas at the inlet and the influence of shock vibration caused by water and cracked gas, an appropriate safety factor should be considered for the design of gap / water volume during operation, as well as promoting the monitoring of vital parameters (e.g., the temperature during operation).