Corrosion protection of tube sheet in shell-and-tube heat exchangers (methyl chlorides)

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I. Tubular Heat Exchangers Tubular heat exchangers are the most widely used type of heat exchanger in the production processes of the chemical industry and other sectors. It is mainly composed of a shell, tube sheet, heat exchange tubes, tube box, baffle plates, etc. The required materials can be made of ordinary carbon steel, red copper, stainless steel, or special materials. During heat exchange, one fluid enters through the connection pipe at the end cap and flows inside the tube, exiting through the outlet pipe at the other end of the end cap; this is referred to as the tube side ； Another fluid enters through a tube connected to the shell, and exits through another tube on the shell; this is referred to as the shell side. II. Problems existing in chlorohydrocarbon manufacturing enterprises: The tubular cooler for methyl chlorides is a very important piece of equipment in chlorohydrocarbon manufacturing plants, accounting for a large proportion in the production of process equipment. It uses cooling by a substance of the same type; the medium in the shell side is monochloromethane, while the medium in the tube side is a mixture of low-temperature dichloromethane and trichloromethane. In production, corrosion and leakage often occur at the tube sheet surface and the tube side ports. As a result, first, trace amounts of dichloro- and trichloromethane mixtures end up in the chloromethane stream, which directly leads to the product being unqualified ； Second, chloromethane is mixed into the dichloro- and trichloromethane mixture in the pipeline, resulting in waste of material. At the same time, the frequent occurrence of such problems directly affects the safe and continuous operation of the equipment, increasing the company’s maintenance time and labor costs. For a long time, the issue of leakage caused by tube sheet corrosion has not been properly resolved, posing a challenge for the industry. 1. Equipment parameters: Dimensional parameters: Φ300mm; Inlet temperature: 120°C; Outlet temperature: 80°C; Cooling medium temperature: 20°C; Material of the tube side: brass; Medium in the shell side: chloromethane; Medium in the tube side: mixture of dichlorine and trichlorine; Pressure on the shell side: 1.4 Mpa; Pressure on the tube side: 0.8 Mpa. 2. Problem analysis: A. Chemical corrosion. Chlorides are strong solvents that can cause severe corrosion of metals ； B. Stress corrosion. Since the tubes and tube sheets are connected by welding, welding defects can occur, and stress can cause cracking, pitting, or leakage at the welded areas. C. Enterprises regularly use high-pressure water cleaning to clean the tubes; since methane chlorides are rich in chloride ions, acidic substances are generated in the production environment, leading to tube damage and leakage of the medium. D. Effect of temperature. Corrosion is a chemical reaction; for every 10°C increase in temperature, the corrosion rate increases by 3 to 5 times. When the operating temperature of the medium reaches 120°C, it accelerates the corrosion of the tube sheet, thereby leading to leaks and fluid leakage. 3. Traditional approaches: For these reasons, when a mixture of dichloroform and trichloroform is detected in the crude product produced by the enterprise, it should immediately shut down the tube sheet and locate the source of the leak ； Furthermore, enterprises regularly open the tube box every 30 days or so for cleaning and inspection, venting the chloromethane present in the shell side and filling it with nitrogen; leaks are detected in the tube side and tube sheets by spraying soapy water. After each inspection, welding repairs are carried out on the leaking areas, and new equipment is installed once a certain percentage of the pipe system has been sealed. III. Law on the Protection of Polymer Materials: Using polymer materials to apply organic surface coatings for corrosion protection is one of the effective anti-corrosion measures currently in use. Surface adhesive protection can be widely used for the repair of areas subjected to abrasion, cavitation, and corrosion, as well as as a pre-protective coating. It possesses good chemical resistance, as well as excellent mechanical and adhesive properties. Compared with traditional welding repairs, anti-corrosion coating protection technology features simple installation, low cost, high safety, and good repair results. A well-established material for this application is Fushi Lan EE-101. The EE-10 material is based on a phenolic epoxy system; it contains no volatile substances, allowing it to be used safely in sealed environments without shrinking, and it provides a long-lasting protective coating for components. IV. Application process: 1. Perform a pressure test on the tube sheet to check for any leaks at the joints where the tubes meet the tube sheet (leaks can be repaired using Fosjoas 2211F metal repair material; this material can also be used to fill holes in the surface of the tube sheet) ； 2. Dry the components that require protection again to ensure their surfaces are dry and free of any water traces ； 3. First, chamfer the inner pipe end, then use a sandblaster to sand the surface of the tube sheet to be protected (the area 2 cm away from the pipe end is also sandblasted), achieving the Sa2.5 standard ； 4. Use compressed air (dry and free of impurities) to blow away the dust, and then immediately clean the surface that has been sandblasted with acetone or anhydrous ethanol, ensuring thorough cleaning ； 5. The treated surface should be sprayed with a pretreatment agent and allowed to dry to prevent the formation of an oxide layer again ； 6. Apply the evenly mixed Fosei Blue EE-101 material uniformly over the surface of the tube sheet, ensuring a thickness of no less than 0.5 mm. The inside of the tube ends should also be coated with Fosei Blue EE-101 material, with a depth of no less than 2 cm ； 7. Material curing. Cure for 24 hours at 24°C; for every 11°C increase in temperature, the curing time is reduced by half ； 8. After the material has cured, use an electric spark detector to inspect the coating for defects, and repair the defective areas ； 9. The material can be installed and used once it meets the curing requirements. During installation, the repaired area should be protected from impacts and knocks. IV. Application Images V. Application Feedback: After applying the Fushilan EE-101 material for anti-corrosion treatment, based on feedback from one year of ongoing use, corrosion and leakage at the tube sheet connections have been virtually eliminated. This has **improved the efficiency and service life of the tube sheets, reduced the frequency of replacements as well as the workload for workers, lowered the costs associated with equipment replacement, enhanced product quality, and ensured the safe and continuous operation of the enterprise.