Case studies on heat exchanger corrosion and protection – Corrosion of the tube bundle in the interstage cooler (E-312) of a heavy oil catalytic compressor and solutions

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Analysis of the tube bundle corrosion issue in the compressor inter-stage cooler (E-312) 1. Process conditions: The rich gas, with a temperature of 40–60°C and a pressure of 0.22 Mpa, flows at a rate of 700 Nm3/min. It enters the first compression stage via the pneumatic control valve at the compressor inlet, where it is compressed to a pressure of 0.64–0.68 Mpa; at this point, the temperature of the fluid is around 95°C. Entering the interstage cooler, to prevent the formation of ammonium salt crystals in the cooler and to remove harmful substances such as H2S, purified water is injected at a flow rate of 8000 kg/h before the compressed rich gas enters the cooler. After being cooled to 40°C by the gas cooler, it enters the inter-stage liquid separation tank for gas-liquid separation. 2. Operation status of the compressor inter-stage cooler: The compressor inter-stage cooler is an auxiliary device included in the compressor unit; it was designed and manufactured by Shenyang Blower Factory. Its specifications are DN1200×9397, with a heat exchange area of 626 m2. It is a custom-designed heat exchanger, where the working medium in the tube side is circulating water, while the working medium in the shell side is the compressed gas from the first stage of the compressor. Its operating conditions are quite stringent: the operating medium is rich gas containing around 1000 ppm of H2S, and during operation the rich gas is pressurized by a compressor; as a result, some of the heavier components change phase from gas to liquid, resulting in phase transitions. This means that the tube bundles of the inter-stage coolers operate in conditions highly susceptible to H2S corrosion. During the first shutdown for maintenance, it was found that the tube bundles of the inter-stage coolers were severely corroded after one year of operation, with a blockage rate of 55% in some tubes, rendering them unusable. The issue was resolved by reusing the tube sheets and installing new tubes, with a three-layer composite coating applied to protect the tube bundles against corrosion. Pressure tests conducted during shutdowns in 2003 and 2005 showed no leaks. During this maintenance inspection, no significant corrosion was found on the surface of the heat exchange tubes. The original anti-corrosion coating in a 5 mm area adjacent to the floating tube sheet had basically completely peeled off; the anti-corrosion coating on the main body of the heat exchange tubes was largely intact, while the anti-corrosion coating on the inside of the tubes was gone. No significant corrosion was observed on the surface of the tube sheet. When testing the floating head using maintenance circulating water, it was found that the leakage occurred at the junction between the heat exchange tubes and the floating head tube sheet, indicating that the main cause of the leakage was corrosion at the areas where the anti-corrosion coating of the original tube bundle had been damaged over the five years of operation. The interruption in the circulating water of the unit that occurred on the 10th of this month was an indirect cause. 3. Analysis of corrosion causes: Carbon steel suffers severe corrosion when used in an environment rich in gas. The medium contains H2S, NH3, as well as various other substances such as CO2, CN–, and oil; the operating temperature is around 95°C. When carbon steel operates under such conditions, the form of corrosion it experiences is electrochemical corrosion. 3.1 Damage to the coating surface: If the cooling coil tubes are made of carbon steel, their service life is only 1 year. Using a three-layer composite coating provides corrosion protection for the inner and outer walls of the tube bundle for nearly 6 years. The three-layer composite coating anti-corrosion layer features a Ni-P alloy coating of 20–30 microns thick near the metal surface. The intermediate layer and the surface layer are non-metallic polymer coatings; the difference is that the intermediate layer serves as a thickening layer, while the outer wall functions as a sealing layer. The function of each layer is as follows: ① Ni-P alloy coating – First, a layer of Ni-P alloy coating is applied to the metal surface; this coating provides strong adhesion to the metal. It is a coating with a high porosity, which increases the area of the surface layer. An enlarged metal coating and non-metallic materials can achieve better adhesion. ②The intermediate layer: this coating is a thick-coating type with fillers added, and its main function is to cover the irregularities in the metal coating. ③The sealant layer is a coating without fillers, primarily intended to reduce the porosity of the coating and enhance its corrosion resistance. The main component of this non-metallic coating is a high-temperature material made of silicone-modified epoxy resin. Based on its 6 years of use, this coating performs well and has met the expected goals. Since this coating is a new material when in use, there is no defined lifespan for it. This protection method has been discontinued now. Because in environments containing corrosive agents, smaller molecular gases and these agents can easily penetrate into organic coatings, disrupting their original molecular structure; as a result, the glass transition temperature decreases, which reduces the coating’s heat resistance. The surface coating softens, bubbles form, the coating hardens, and it breaks down, losing its function. 3.2 The gas-rich side of the shell [1]: Many sulfides in crude oil are decomposed into H2S during catalytic cracking; simultaneously, nitrogen compounds in the crude oil are also present in the cracking products in certain proportions. Among these, 1–2% of the nitrogen compounds exist in the form of HCN, thereby creating a corrosive environment composed of HCN, H2S, and H2O. The presence of HCN accelerates the corrosion of H2S-H2O. Cyanide ions have two effects in alkaline H2S-H2O solutions: they dissolve the FeS protective film formed by hydrogen sulfide, thereby accelerating the corrosion caused by H2S. As the presence and concentration of CN increase, the corrosive effect on the equipment also increases. Secondly, it can remove corrosion inhibitors from certain solutions, further exacerbating corrosion. As the concentration of cyanide ions increases, the sensitivity to uniform corrosion, stress corrosion, and localized corrosion all increases. FeS formed as a result of the reaction between H2S and iron can cover the surface of steel when the pH value is above 6, providing good protection; the corrosion rate decreases over time. However, if CN－ is present in the medium, it causes FeS to dissolve to form the complex ion Fe(CN)4-6, accelerating corrosion. The interruption of the circulating water in the installed unit accelerated the corrosion of the tube bundle. It can be seen that carbon steel is highly susceptible to corrosion in HCN–H2S–H2O. 3.3 Corrosion of the inner wall of tubes【2】: In most coolers where water flows through the tube side, this corrosion occurs due to the presence of calcium and magnesium ions as well as bicarbonates in the cooling water. As a result of use, scaling gradually forms on the heat transfer surface, accompanied by the formation of rust. When the cooler is in operation, the heat exchange efficiency is significantly 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. Furthermore, the presence of scale can lead to under-scale corrosion on the inner wall of the tubes, reducing the service life of the tube bundle. 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. Therefore, corrosion of metals beneath scale is accelerated due to the autocatalytic effect of their inherent electrochemical corrosion. 4. Remedial Measures 4.1 Selection Criteria for Stainless Steel: Based on the resistance of austenitic stainless steel materials to pitting corrosion, intergranular corrosion, and stress corrosion, the order is 1Cr18Ni9Ti → 0Cr18Ni9 (304) → 0Cr18Ni11Ti (321) → 00Cr19Ni10 (304L). By considering the overall benefits, the use of 0Cr18Ni10Ti (321) can effectively solve this problem. Because 0Cr18Ni10Ti low-carbon stainless steel has excellent resistance to intergranular corrosion, it can prevent such corrosion. Cr–Ni forms a very thin and dense oxide film on the surface of the steel, which prevents further oxidation or corrosion. The use of chromium-nickel austenitic stainless steels with low carbon content, such as 0Cr18Ni10Ti, can reduce the hazards of grain boundary sensitization. After implementing the aforementioned anti-corrosion measures, the corrosion problem of carbon steel can be effectively resolved. It ensures the long-term safe operation of the equipment. Hydrogen sulfide corrosion occurs in alkaline solutions, where chloride ions do not cause corrosion. Chloride ions cause corrosion in acidic solutions, whereas hydrogen sulfide does not cause corrosion in acidic solutions. HIC (hydrogen-induced cracking) mostly occurs under acidic conditions. Under alkaline conditions, chloride ions do not cause corrosion of stainless steel【3】. 4.2 Performance under identical conditions: When the tube bundle of the air cooler in the heavy oil catalysis Workshop No. 1 was put into use in October 1992, it was made of 10# material; by May 1994, 45 tubes had become clogged. During the period from 1994 to 1995, internal leaks occurred, causing the air compression unit to stop operating and forcing waste gas to be vented through a flare. In 1995, the tube bundle was replaced with one made of 1Cr18Ni9Ti material. Since then, up to the year 2007, that is, for more than 12 years, no further clogging has occurred, and the performance has been good. Operating conditions: pressure of 0.5 Mpa, temperature of 70/30°C; the medium involved is rich gas and circulating water ； Specification: DN1200. Furthermore, in most domestic heavy oil catalytic units of the same type, the material used for the tube bundles is stainless steel 5. Based on the above analysis and actual operating conditions, it can be seen that the main cause of corrosion of the tube bundles is corrosion caused by rich gas in the shell; however, corrosion of carbon steel surfaces due to circulating water, in the absence of any protective measures, is also severe. 5.1 Based on the above analysis, the use of 0Cr18Ni10Ti can resolve the rich-gas corrosion in the intercooler tube bundle. 5.2 The inlet and outlet valves of the air compressor are repaired or replaced during equipment maintenance; it is recommended to replace the original pneumatic inlet butterfly valve (DN700) and outlet pneumatic motor gate valve (DN350) with ordinary gate valves, in order to ensure safe maintenance of the air compressor unit and its auxiliary equipment as well as system isolation. It is also necessary to commission the design department to install valves and bypass lines at the outlet of the inter-stage cooler, to facilitate on-line maintenance. Tests were conducted prior to this shutdown, and it was confirmed that the compressor can operate safely with inter-stage coolers removed under low load conditions. 5.3 Strengthen the management of the unit to ensure the safe and stable operation of this tube bundle until the unit is shut down.