Failure Analysis of the Tube Bundle on Air Cooler of Alkylation Equipment

In order to find out the failure mechanism of the tube on the air cooler and avoid its failure again. The macroscopic morphology of the tube was visually inspected, the metallographic morphology of the tube was analyzed with a metallographic microscope, the microscopic morphology of the tube was observed with a scanning electron microscope, and the chemical composition of the tube matrix and the corrosion products on the tube were analyzed with an energy spectrometer. Visual inspection showed that the thickness of the tube wall was significantly reduced and reddish-brown products were produced on the tube, but the surface of the tube away from the failure location was intact. The metallographic morphology of the tube matrix is consistent with that of standard 20# steel, and the products on the tube are loosely porous, with a chemical composition consisting mainly of Fe, Zn, O and S. The chemical composition of the tube matrix lacks three elements, Cr, Ni and Cu, and the material used for tube bundle manufacture is unsuitable, which reduces the corrosion resistance of the tube material and possesses the material factor for corrosion to occur. The tube bundle at the junction with the tube plate has both internal stress and local breakage of the surface galvanized layer caused by manufacturing factors, the latter leading to the formation of small anode and large cathode electric coupling pairs; under the combined effect of internal stress and electric coupling effect, the overall corrosion of the tube substrate is accelerated and eventually leaks due to thinning and perforation. The outer wall of the tube is in contact with oxygen-rich air or cooling water medium, O2 becomes the depolarizing agent of the electrochemical cathodic process; while the inner wall of the tube is a hydrocarbon medium containing sulfur components, H+ ions become the depolarizing agent of the electrochemical cathodic process. In the oxygen-rich medium, the surface of the outer wall of the tube is very easy to occur oxygen depolarization electrochemical corrosion, while in the sulfur-rich medium, the surface of the inner wall of the tube is very easy to occur hydrogen depolarization electrochemical corrosion. Therefore, when manufacturing the air cooler, ensure that the material of the air cooler tube bundle is consistent with the design material to avoid large deviations. Avoid local damage to the galvanized layer as much as possible, and carefully check the galvanized layer on the outer surface of the tube bundle after manufacture, and repair any local damage as soon as possible; in addition, eliminate the internal stress at the connection as much as possible. Regularly check the connection between the air cooler bundle and the tube plate to ensure that there are no gaps in the connection and that the galvanized layer is not damaged; regularly remove the dust from the air cooler bundle and drain the water and dirt in the dead ends and low places to prevent the generation of corrosive media. © 2022, Chongqing Wujiu Periodicals Press. All rights reserved.