Corrosion and protection case -----Fracture analysis and improvement measures of floating head bolts of petrochemical acid water stripping heat exchanger

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This post was last edited by Wang Wei 2 on 2020-5-24 10:40 Analysis on fracture of floating head bolts of petrochemical acid water stripping heat exchanger and improvement measures 1. Usage situation A certain petrochemical sulfur 640,000 tons/year acid water stripping unit was only used twice for 4 months in 28 months, and each use was 9 months apart. During the maintenance period, it was found that 20, 5, 12, 5, 4 and 5 floating head bolts of the six heat exchangers of E-401, 404, 406, 408D, 409 and 415 were broken respectively. 2. Fracture analysis 2.1 Fracture shape From the perspective of the broken outer tube of the bolt, the cross section is broken at the root of the thread perpendicular to the center line of the bolt. Judging from the fracture surface, it is a brittle fracture. No reduction in cross-section was found, indicating that no shrinkage deformation occurred during fracture, see Figure 1, Figure 2, and Figure 3. 2.2 Fracture analysis The operating conditions and bolt parameters of the heat exchange equipment are shown in Table 1 and Table 2. Table 1 Operating conditions Serial number Process number Equipment name Specification Model Temperature ℃ Medium pressure Mpa Shell Shell Shell 1E401 Raw water cooler BES700-2.5-120-6/25-4I3065 Circulating water Raw water 0.31.45 2E404 Condensate Cooler BJS600-2.5-90-6/25-2I30116 Circulating Water Condensate 0.350.15 3E406 Purified Water Raw Water Secondary Heat Exchanger BES800-2.5-145-6/25-6I68.1132.1 Raw Water Purified Water 1.10.45 4E408D purified water raw water primary heat exchanger BES800-2.5-160-6/25-4I122146.6 raw water purified water 10.5 5E409 purified water raw water tertiary heat exchanger BES600-2.5-85-6/25-4I40.3112.8 raw water purified water 1.20.4 6E415B purified water cooler BES800-2.5-170-6/25-2I3060 circulating water purified water 0.30.33 Table 2 Bolt parameter serial number process number equipment name specification model material total quantity (set) fracture quantity (set) 1E401 raw water cooler M20×25040Cr4020 2E404 Separated liquid cooler M20×21040Cr245 3E406 Purified water raw water secondary heat exchanger M20×25040Cr 4012 4E408D Purified water raw water primary heat exchanger M20×25040Cr 405 5E409 Purified water raw water tertiary heat exchanger M20×21040Cr 284 6E415B purified water cooler M20×25040Cr 405 Because the raw water of the sulfur stripping unit contains H2S, NH3, CO2, CN-, phenol and oil and other media, the pH value is 10±5, and the temperature of the raw water is 65-70°C. When carbon steel works under such operating conditions, the metal surface of carbon steel will corrode severely. The form of corrosion is electrochemical corrosion, and stress corrosion cracking is prone to occur at welding joints and metal base metals that are subject to bending forces. That is to say, in the same environment, HB with high hardness is more susceptible to corrosion than low hardness. This phenomenon can be explained by the stress corrosion of the tank wall plate weld bead of the sulfur acid water tank but the lack of corrosion of the tank wall base material [1]. The cause of corrosion is: Since H2S is dissolved in water, it reacts with metal in a corrosive reaction: H2S + Fe - FeS + H2 ↑ FeS reacts with HN3 to generate HN3HS, which is deposited on the metal surface and causes corrosion under scale. Sulfide stress corrosion cracking (SCC) can also be caused in areas with stress concentration. H3N＋H2S - NH4HS Ammonia is very easily soluble in water (solubility volume ratio 700: 1), ammonia and water combine to form a relatively stable crystal hydrate NH3 under low temperature conditions. H2O。 But its melting point is lower -78.85℃, and its electrolytic formula is as follows: NH3。 H2O-NH4++OH- produces ions and electrolyte, forming electrochemical corrosion. Coupled with the combined action of hydrogen sulfide and ammonia, corrosion is aggravated. When there is cyanide (CN-), when the pH value is greater than 7.5, cracking increases as the CN- concentration in the medium increases. When HN3HS reacts with H3N: HN4HS+HN3-(HN4)2S Ammonia sulfide (HN4)2S can increase the solubility of H2S in water * * Increase, improve the HS- concentration. Because the bolts are in a locked state whether the equipment is out of use or in use. This shows that there is always tensile stress in these bolts. In addition, the bolt material currently used is 40Cr steel, with a carbon content of about 0.4%. HB hardness is 317, which is a high-hardness steel. Therefore, there is a high tendency of stress corrosion [2]. In addition, if no protective measures are taken for the shutdown device, the corrosion of the equipment will be much more severe than when it is continuously operated. The main reason is that there is a lot of air during the shutdown process, and the substances generated by the oxidation of the air and harmful media in the equipment can strongly corrode the equipment. Therefore, the fracture of the bolt is caused by the tensile stress in the bolt itself. When the PH value in the equipment is alkaline and there is wet hydrogen sulfide in the equipment, there is an external condition for the start-up and shutdown of the device. The result of the compound action of the two. 3. Solution measures According to the above analysis, the following methods can be adopted to solve the problem of stress corrosion fracture of bolts.: ⑴The bolt material used in the original design is 40Cr, and the HB hardness is 317. The hardness is too high and is not suitable. This time, it is recommended to use 30CrMoA as the bolt material in order to reduce the carbon content and enhance toughness. The HB hardness is 229, which is reduced by 88. The processing requires that the HB hardness is not greater than 200. In this way, the hardness of the material is reduced and the toughness is increased. This increases resistance to stress corrosion. ⑵It is recommended that after the device is shut down, the front and rear pipe boxes should be removed and the floating head bolts should be loosened. Doing so removes the tensile stress on the bolt. Stress corrosion of bolts is avoided. In addition, the pipe boxes before and after the heat exchange equipment are removed can achieve ventilation conditions, and at the same time, the corrosion of the pipe bundle can be reduced. After such treatment, stress corrosion fracture of the bolts is avoided.