Corrosion problems caused by unreasonable pipeline design
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The chlorine used in the evaporator of a certain factory is taken from two storage tanks. A large storage tank holds commercial chlorine, which contains no hydrogen chloride but has 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 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. 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 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 install an outlet pipeline from each of the two storage tanks directly to the evaporator, so that the two types of chlorine do not mix in the pipelines.
In this case, due to a design flaw, chlorine gases of two different properties mixed in the main pipe, resulting in severe corrosion problems. Although the carbon steel pipes used in the original design are corrosion-resistant to single dry gas media, their corrosivity increases significantly when hydrogen chloride, oxygen, and water vapor are present together. In this case, the presence of hydrogen chloride reduces the solubility of water in chlorine gas, making it easier for water to condense within the gas and thereby exacerbating corrosion. To solve this problem, the pipeline system should be redesigned to prevent the mixing of chlorine of two different properties in any part of the pipelines. Specific improvement measures include: 1. **Separate pipeline design**: Design independent outlet pipelines for each storage tank, connecting directly to the evaporator, to ensure that the two types of chlorine do not mix in the pipelines. 2. **Material selection**: Although Hastelloy C-276 offers good corrosion resistance, it is necessary to consider whether there are more suitable materials available during design, or to adjust the alloy composition based on actual operating conditions in order to further improve corrosion resistance. Through these design adjustments, corrosion issues can be effectively reduced, preventing similar safety accidents, while also improving the economic efficiency and safety of the entire system. .
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If the evaporators are far apart and a single main pipe is desired, it is necessary to use lined pipes.