Thread Content
An equipment protection method for shutting down petrochemical plants is introduced. 1. Corrosion conditions of the shut-down plants: With the continuous development of the petrochemical industry, the advent of a market economy, and adjustments in production processes, production plants are shut down for certain periods of time every year. After being shut down, the equipment protection issue becomes very prominent; if proper protection is not provided, corrosion occurs at a greater rate than in devices that are still in operation, resulting in significant economic losses. Therefore, the equipment protection issue related to shutdown devices is a matter that deserves attention. Initially, there was insufficient awareness of the corrosion condition in the decommissioned equipment; only simple treatment was carried out on the inner walls of the equipment pipes in such devices. That is, nitrogen is used to completely displace the air from the system for protection, with the pressure of the nitrogen maintained at around 0.2 Mpa. Since other production units require nitrogen (for equipment maintenance), the shut-down units often cannot provide the necessary amount of nitrogen. There are certain difficulties in maintaining pressure with nitrogen. Equipment corrosion surveys were conducted on the two units that are usually taken out of service at refineries – the atmospheric and vacuum distillation units as well as the thermal cracking unit – for a period of about 1 year. The situation is as follows: the degree of corrosion on the inner walls of the devices that have been taken out of service is much more severe than that in the production devices. In the gasoline cooler of the atmospheric and vacuum distillation unit, an internal inspection revealed severe corrosion on the outer wall of the tube bundle; a large amount of rust deposits had blocked the gaps between the tubes in the bundle. The inner wall of the tube bundle is also severely corroded (this is related to the incomplete cleaning of the tube bundle surface after shutdown, which leads to an accelerated rate of corrosion). This caused two of the tube bundles to be scrapped. The corrosion of the product water separation tanks at the top of the unit is also quite severe; for example, in the horizontal type of product water separation tank located near the inlet, the thickness of the rust deposits was about 10–15 millimeters, and approximately 500 kilograms of rust were removed from the inner wall. There are highly uneven rust pits on the inner wall of the tank. In tower equipment, the top of the tower is more severely corroded than the bottom. Based on the corrosion condition of the inner surfaces of the units in the thermal cracking plant, it can be seen that the most severe corrosion occurs at the areas where the metal surface comes into contact with hydrogen-rich oil, and this corrosion is more severe than that in areas in contact with heavy oil. Therefore, equipment corrosion and protection require not only proper care for the protection of operating units but also sufficient attention to the corrosion and protection of units that are shut down. 2. Analysis of corrosion causes: Although measures were taken after the unit was taken out of service, such as purging with steam, industrial air, and nitrogen replacement. However, judging from the inspection of equipment corrosion, the effect is not good. There is a large amount of water on the inner walls of the equipment and pipes, and the air humidity inside is very high. There is a considerable amount of moisture on the metal surface, forming a continuous water film. At the same time, a considerable portion of the metal surface is submerged in water. Since the necessary condition for corrosion to occur is the coexistence of oxygen and water, the water adhering to the metal surface serves as an excellent thin layer of electrolyte. Electrochemical corrosion occurs at the defects on the metal surface. 3. Basis for material selection: Based on the corrosion condition of the devices that had been out of use, anti-corrosion treatment was applied to those devices that had been inactive for a long time. Since the necessary conditions for metal corrosion are the simultaneous presence of oxygen and water, preventing corrosion can be achieved by removing either one of them. Gu successively carried out deoxygenation and water treatment on these two sets of equipment. The process was as follows: (1) All equipment and pipelines were first purged with steam to remove oil and other contaminants from the metal surfaces. ⑵ Blow out the steam-treated equipment and pipelines with an industrial air to increase the dryness of the metal surface. ⑶ Check the sealing condition of the equipment and pipelines. ⑷ The air in the system is purged using nitrogen from the treated equipment and pipelines; the nitrogen pressure is set at 0.4–0.5 Mpa, after which it is stabilized around 0.2 Mpa, with the nitrogen pressure being monitored continuously. Although these two sets of equipment have been treated, given the actual conditions of production, it is possible to maintain the nitrogen pressure during winter; however, as spring arrives, the production equipment undergoes maintenance one after another. Maintenance requires a large amount of nitrogen, and the nitrogen available for the protected equipment is not sufficient to meet this demand. The device was protected with nitrogen for over a year; when the equipment was opened for inspection, it was found to be severely corroded, failing to achieve the intended purpose. Given the above circumstances, it is reasonable to adopt a method of \"disabling the boiler protector\". This protective agent has been used for several years to protect shut-down boilers. Practice has shown that the anti-corrosion effect is good. However, the degree of corrosion in boilers is not as severe or complex as that in petrochemical plants; the area to be protected is nearly 4,000 cubic meters, and there is no precedent for using such a protective agent in production facilities. Based on the actual conditions of the refining plant and through investigation and comparison, it is believed that the use of this corrosion inhibitor can provide protection for idle equipment. Because the boiler inhibitor that is discontinued is a type of gas-phase corrosion inhibitor. This corrosion inhibitor is a solid organic substance that can volatilize automatically at room temperature. Its mechanism of action involves the formation, under the influence of air and moisture, of a protective layer on the metal surface through diffusion, dissociation, and adsorption; this layer prevents the formation of corrosion products, thereby achieving anti-corrosion effects. This corrosion inhibitor overcomes the disadvantages of traditional vapor-based corrosion inhibitors, such as poor volatility, insufficient penetration, and low overall anti-corrosion performance. It possesses notable advantages such as high protection efficiency, strong penetration, and a large corrosion inhibition radius. This corrosion inhibitor evaporates from its location, passes through the water film on the metal surface, reaches and adheres to the metal surface, thereby isolating it from water. Thus, it provides protection for the metal surface. Moreover, compared with conventional corrosion inhibitors, this one provides excellent protection for metal surfaces at close distances, at distant distances, and beneath scale, thanks to its high volatility and permeability. Based on the above characteristics of this corrosion inhibitor, it is considered feasible to use it on shut-down equipment to protect the equipment and the inner walls of pipelines. 4. Equipment protection and methods: These two sets of equipment utilize protective agents, with a total construction volume of nearly 3,600 m3. The process of filling the protective agent is as follows: (1) Protection of the tower and containers: Open the inlet of the equipment, and fill the protective agent onto the tray surfaces or at the bottom of the containers in a uniform manner, at a rate of 1.5–2 Kg/m3 per layer. Place the protective agent on a tray or plastic sheet, then seal it after filling. ⑵ Protection of heat exchangers: After the powdered medication is mixed evenly using industrial air in a mixer, it is blown into the shell side and tube side respectively through the inlet or outlet of the heat exchanger; once installation is complete, the vents are closed. To monitor the protection effect, a certain number of carbon steel test plates were installed at appropriate locations on both the tower and the container. The corrosion inhibition effect on the protected metal surface is compared through the inspection of the degree of strip corrosion. 5. Effects of protection: The two sets of devices provided protection for 11 months. By checking the degree of corrosion on the test plates placed inside the protected equipment, no signs of corrosion were found; the metal surfaces remained free of any loose oxide layers, just as they were before protection was applied. 6. Conclusions ⑴ It has the effect of removing the rust layer. For example, at the bottom of the distillation column in a thermal cracking unit, before a protective agent is installed, the rust layer is thick and hard. After nearly 11 months of protection, it can be seen that most of the rust layer originally attached to the tower wall has fallen off, revealing the metal surface. The protective agent forms a uniform protective layer on the metal surface. This protective agent has strong penetration ability. ⑵ The protection effect is excellent; after one year of protection, inspection of the test pieces made of 10# carbon steel in the field showed that there were no signs of corrosion on them, just as they were when first installed. It can be said that the corrosion inhibition rate reached 100%, and it could be seen from the surface of the protected metal that no loose rust deposits were formed. Therefore, this protective agent has comprehensive performance, as it can protect metals both when they are in a dry state and when they are in a humid state. ⑶ Using this protective agent saves time and effort. When loading the equipment, it is not necessary to use a drying device; simply drain the water from inside the equipment, add the chemical in one go, and then seal the equipment. Do not turn on the device during the protection period. Using a \"boiler protector\" to protect idle petrochemical equipment is an effective method.