The piping design is unreasonable, resulting in severe corrosion of the pipes
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This post was last edited by Wang Wei2 on 2024-12-19 at 11:48. The chlorine gas used in the evaporator of a certain factory is sourced from two storage tanks. A large storage tank contains commercial chlorine, which contains no hydrogen chloride whatsoever, 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 about 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 piping leading to the evaporator, for the sake of convenience, only one main pipe was installed. 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, it was replaced with Hastelloy C-276 (a nickel-molybdenum-chromium alloy with excellent corrosion resistance), but its corrosion rate was also high. 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 very 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 reduces water’s solubility in chlorine gas, 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.
Since the design uses only one main pipeline to connect the two storage tanks, chlorine containing a small amount of hydrogen chloride mixes with water-containing chlorine in the pipeline, creating an environment with greater corrosivity. In this case, the presence of hydrogen chloride and water promotes the formation of more condensate in chlorine gas, exacerbating the corrosion of the pipeline materials. The proper approach is to equip each storage tank with separate pipelines connected directly to the evaporator, in order to avoid mixing of different types of chlorine and reduce the risk of corrosion. .
That month, the main pipe behind the connection point suffered severe corrosion damage, resulting in a large release of chlorine gas. Later, it was replaced with Hastelloy C-276 (a nickel-molybdenum-chromium alloy with excellent corrosion resistance), but its corrosion rate was also high. 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 very 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 reduces water’s solubility in chlorine gas, 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.