When using heat exchangers, it is necessary to avoid excessive flow rates to prevent corrosion fatigue
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This post was last edited by Wang Wei2 on 2024-1-28 at 10:31. When using heat exchangers, it is necessary to avoid excessive flow rates in order to prevent corrosion fatigue. Example: In the thermal cracking unit of a certain factory, the stainless steel tube bundles in four heat exchangers suffered severe damage after 2 to 4 years of use; numerous macroscopic cracks appeared on the surface of the tubes, and some of them even broke off. Upon inspection, the pipe cracked from the outside in. Tube side fluid: gasoline, containing 20–60 mg/L of H2S and 0–77 mg/L of Cl-, with a pH of 4.7 and a temperature of 60–110°C. Shell side fluid: silicone oil, sulfides, water. Analysis: The cause of the damage was corrosion fatigue resulting from the combined effect of vibration during operation and the medium in the shell side. Vibration, on the other hand, is caused by an excessive flow rate. Vibrations caused by fluid disturbances in the shell side are the main reason for the alternating loads on the heat exchanger tube bundle. Calculations of the natural frequency of this heat exchanger tube bundle and various vibration patterns caused by fluid disturbances show that when the flow rate of the fluid in the shell side is greater than 485 T/day, the vibration frequencies generated by the Karman vortices formed by the fluid around the tube bundle coincide with the natural frequency of the tube bundle, causing it to vibrate. The tube bundle – especially the first span at the feed end – is subjected to a high-frequency alternating load. Of the four heat exchangers, two exceeded the critical flow rate for Karman vortex vibration over 1,124 operating days, with 207 such days, while the other two also had 130 such days. Furthermore, the heat exchanger tubes are straightened before delivery, resulting in indentations on their surfaces; corrosion fatigue cracks form and propagate preferentially within these indentations.
When using the heat exchanger, it is necessary to control the flow rate to prevent excessive flow from causing increased vibration of the tube bundle and leading to corrosion fatigue. In the case in question, the stainless steel heat exchanger experienced Karman vortex vibration at a frequency matching the natural frequency of the tube bundle due to an excessive flow rate of the fluid in the shell side. Combined with surface indentations, this led to the formation and propagation of corrosion fatigue cracks, ultimately resulting in severe damage to the tube bundle. Controlling the flow rate of the heat exchanger to avoid excessive flow rates and surface defects is an important measure to prevent corrosion fatigue. .