Case studies of heat exchanger corrosion damage – Causes of corrosion in heavy oil catalytic water coolers and solutions

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This post was last edited by Wang Wei2 on 2020-5-23 09:55. Causes of corrosion in heavy oil catalytic water coolers and solutions 1. Overview: A heavy oil catalytic light diesel cooler with the model number FB1000-240-40-6; the tube bundle is made of 10# material, and there are 564 heat exchange tubes. Its operating conditions are shown in the table below: Operating Conditions, Medium Temperature (°C), Pressure (MPa): Tube side, Shell side, Tube side, Shell side, Tube side, Shell side. Light diesel, Circulating water: 120, 30, 20.3. 2. Usage status: The tube bundle of this cooler has been in use for about 10 months, and 12 heat exchange tubes have leaked. It is not common for corrosion to occur to this extent in the use of water coolers. From the appearance of the tube bundle, there is slight corrosion on the metal surface of the tube sheet (oil side); there are no obvious pitting defects, and the amount of rust deposits on the surface is minimal. The outer wall of the tube bundle (on the circulating water side) is covered with a thick layer of scale; beneath this scale layer lies a thick layer of rust. On the metal surface under the rust layer, there are numerous pitting defects, and in some areas the original metal surface is no longer visible – the metal surface is quite rough. 3. Analysis of the causes of tube bundle corrosion: Since this tube bundle has been in use for about 10 months, pitting corrosion has occurred on a large number of tubes at a rapid rate, which falls outside the normal range of acceptable usage. The reasons for scale formation and metal corrosion of circulating water under these operating conditions are as follows: 3.1 Analysis of the causes of scale formation. In most coolers, water flows through the tube side, and this water contains calcium and magnesium ions as well as bicarbonates. When the cooling water flows over the metal surfaces involved in heat transfer, the following reactions occur: Mg2+ + HCO3- + H2O = MgCO3↓ + Mg(OH)2; 3MgCO3 + CO2. Ca2+ + 2HCO3- = H2O + CO2 + CaCO3↓. When polyphosphates are added to the water as corrosion inhibitors, the following reaction takes place: 3Ca2+ + 2PO43- = Ca3(PO4)2↓. In addition, oxygen dissolved in the cooling water can also cause metal corrosion and the formation of rust, according to the reaction: 2Fe + 2H2O + O2 = 2Fe(OH)2↓. As a result of these reactions, scale gradually forms on the heat transfer surfaces, along with the formation of rust. When the cooler is in operation, the heat exchange efficiency is severely reduced due to the effect of scale buildup. In some individual tubes, the heat exchange tubes became clogged within less than a year of use. 3.2 Analysis of corrosion causes We know that the corrosion of metal surfaces by water is primarily electrochemical corrosion; in the corrosion cell, the cathodic reaction is mainly the reduction of oxygen, while the anodic reaction is the dissolution of iron. Corrosion of metals beneath scale is accelerated due to the autocatalytic effect of their inherent electrochemical corrosion. 3.3 Influence of other factors: Since circulating water is used as the medium in the shell layer of this tube bundle, it exacerbates the corrosion of the outer wall of the tube bundle. The reason is that, when the pressure and flow rate of the circulating water remain constant, the shell section is equipped with multiple baffle plates; as a result, the circulating water flows through this shell section in a direction parallel to those baffle plates, which increases the flow resistance compared to when the water flows inside tubes. Due to the right angles in the baffle plates, dead zones exist in the flow of circulating water, and under heating conditions, scale tends to form more rapidly in these dead zones. Furthermore, there are gaps between each tube in the tube bundle and the baffle plates; if these gaps meet the geometric dimensions required for gap corrosion to occur, electrochemical gap corrosion is likely to take place. Furthermore, a decrease in the circulating water volume exacerbates the rate of scale formation. Therefore, judging from the corrosion condition of the outer wall at the site, it is much more severe than the corrosion that occurs in the tubes where circulating water flows. 4. The effect of tube bundle scale and rust layers on heat transfer: As we know, the thermal conductivity of scale and metal corrosion products is much lower than that of metals, typically ranging from 0.058 to 5.8 W/(m•℃). The thermal conductivity of metal pipes is 6 to 10 times greater than that of scale; in other words, for the same heat transfer area, the thermal resistance of scale is 6 to 10 times higher than that of metal. Therefore, when the circulating water exchanges heat with the heat medium through the shell layer, it indeed performs better in the early stages of operation compared to when the circulating water flows through the tube layer. However, after being used for a period of time, its heat exchange advantage changes due to the relatively fast scaling rate of the circulating water in the shell. Therefore, once 1–2 millimeters of rust accumulates on the outer wall of the tube, its thermal resistance becomes very high. Therefore, when such a tube bundle is in use for a period of time of this kind, its heat transfer efficiency is still not as good as that of a tube with circulating water inside it. Therefore, the heat exchange efficiency is low. 5. Conclusion Through the above analysis, firstly, the circulation of water in the coolant shell accelerates the corrosion of the metal surface on the outer wall of the tube bundle, thereby reducing its service life. Secondly, compared to circulation water flowing through the tube layer, circulation water flowing through the shell layer tends to form scale more easily, which increases the thermal resistance; moreover, the rate at which heat is transferred is faster in the shell layer, resulting in a lower average thermal efficiency per cycle compared to that in the tube layer. Based on the comprehensive analysis above, the circulating water in the process conditions of this water cooler should be changed to the tube side, which helps to extend the service life of the tube bundle and maintain the heat exchange efficiency of the water cooler.