On the selection of chemical process pumps

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Corrosion has always been one of the most troublesome hazards for chemical processing equipment; a slight carelessness can lead to equipment damage, and in severe cases, it can cause accidents or even disasters. According to relevant statistics, about 60% of damage to chemical processing equipment is caused by corrosion; therefore, when selecting chemical pumps, it is essential to pay attention to the scientific choice of materials. There is a common misconception that stainless steel is a \"universal material,\" and that stainless steel pumps are the ideal choice in any medium or environmental condition – this is very dangerous. Below are the key points for selecting materials for chemical pumps when dealing with some commonly used chemical media: 1. Sulfuric acid – As one of the highly corrosive media, sulfuric acid is an important industrial material with a wide range of applications. Sulfuric acid at different concentrations and temperatures causes varying degrees of corrosion to materials. For concentrated sulfuric acid with a concentration of over 80% and a temperature below 80°C, carbon steel and cast iron exhibit good corrosion resistance; however, it is not suitable for sulfuric acid that flows at high speeds, and thus cannot be used as material for pumps and valves ； Common stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) also have limited utility in sulfuric acid media. Therefore, pump valves for transporting sulfuric acid are usually made of high-silicon cast iron (which is difficult to cast and process) or highly alloyed stainless steel (grade 20 alloy). Fluoroplastics exhibit good resistance to sulfuric acid, making the use of fluorinated-lined pumps (F46) a more economical option. 2. Hydrochloric acid: The vast majority of metal materials are not resistant to corrosion by hydrochloric acid (including various stainless steel materials); high-molybdenum high-silicon iron can only be used in hydrochloric acid at temperatures below 50°C and with a concentration of less than 30%. Unlike metal materials, the vast majority of non-metallic materials exhibit good corrosion resistance to hydrochloric acid; therefore, rubber-lined pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choices for transporting hydrochloric acid. 3. Nitric acid: Most metals are rapidly corroded and damaged by nitric acid. Stainless steel is the most widely used material resistant to nitric acid; it exhibits good corrosion resistance to nitric acid at all concentrations at room temperature. It is worth noting that stainless steels containing molybdenum (such as 316 and 316L) do not have better corrosion resistance to nitric acid than ordinary stainless steels (such as 304 and 321), and in some cases their resistance is even lower. For high-temperature nitric acid, titanium and titanium alloy materials are typically used. 4. Acetic acid: It is one of the most corrosive organic acids; ordinary steel suffers severe corrosion in acetic acid at all concentrations and temperatures. Stainless steel is an excellent material resistant to acetic acid, and 316 stainless steel containing molybdenum can also be used in high-temperature environments as well as with dilute acetic acid vapor. For demanding applications such as high-temperature, high-concentration acetic acid or environments containing other corrosive substances, high-alloy stainless steel pumps or fluoroplastic pumps can be used. 5. Alkalis (sodium hydroxide): Steel is widely used in sodium hydroxide solutions at temperatures below 80°C and concentrations up to 30%. Many factories still use ordinary steel at temperatures of 100°C and concentrations below 75%; although corrosion increases, it remains cost-effective. Ordinary stainless steel does not have a significant advantage over cast iron in terms of corrosion resistance to alkaline solutions; it is not recommended to use stainless steel unless a small amount of iron is allowed to be present in the medium. For high-temperature alkaline solutions, titanium and titanium alloys or highly alloyed stainless steels are commonly used. 6. Ammonia (ammonium hydroxide): Most metals and non-metals suffer only mild corrosion in liquid ammonia and ammonium hydroxide solutions; only copper and its alloys are not suitable for use. 7. Saltwater (seawater): Ordinary steel experiences a low rate of corrosion in sodium chloride solutions as well as in seawater and brackish water; coating protection is generally required ； Various types of stainless steel also have very low uniform corrosion rates, but local corrosion may occur due to chloride ions; therefore, 316 stainless steel is generally the better choice. 8. Alcohols, alkanes, esters, and ethers. Common alcohol-based media include methanol, ethanol, ethylene glycol, propanol, etc.; alkanes include propane, butane, etc.; ester-based media comprise various methyl esters, ethyl esters, etc.; ether-based media include dimethyl ether, diethyl ether, butyl ether, etc. These substances generally have no corrosive properties, and most common materials can be used with them. When making a selection, it is necessary to consider the properties of the medium as well as relevant requirements in order to make a suitable choice. It is also worth noting that alcohols, esters, and ethers are soluble in various rubbers, which helps to avoid mistakes when selecting sealing materials.