Avoid the coexistence of oxygen, hydrogen chloride gas, and water vapor when using the equipment

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Case of damage: The chlorine used in the evaporator of a certain factory was sourced from two storage tanks. A large storage tank holds commercial chlorine, which contains no hydrogen chloride but does contain a small amount of dissolved water (about 30 ppm). The chlorine contained in the second large storage tank is produced from hydrogen chloride, with approximately 3.5% (mole percentage) hydrogen chloride, and very little water (less than 5 ppm). Both storage tanks and their outlet pipes are made of carbon steel, as it is corrosion-resistant for this type of dry gas medium. When designing the pipes leading into the evaporator, only one main pipe was installed for convenience. The outlet pipes of both storage tanks are connected to the main pipe. Just a few months after operation began, the main pipe behind the connection point suffered severe corrosion damage, resulting in a large release of chlorine gas. Later, Hastelloy C-276 (a nickel-molybdenum-chromium alloy with excellent corrosion resistance) pipes were used as a replacement, but their corrosion rate was also high. Problem analysis: This is a corrosion issue caused by improper pipeline design. Although chlorine containing trace amounts of water, and dry chlorine containing a small amount of hydrogen chloride, have little corrosive effect on carbon steel, the corrosivity **increases** when oxygen, hydrogen chloride gas, and water vapor are present together. This is because the presence of hydrogen chloride gas reduces the solubility of water in chlorine, raises the dew point, and makes it easier for condensate to form. This corrosion problem is difficult to resolve from a material perspective, whereas changing the design can solve it effectively and cost-effectively. The proper design is to equip each of the two storage tanks with an outlet pipeline leading directly to the evaporator, so that the two types of chlorine do not mix in the pipelines.