How to choose chemical pumps (share)

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Corrosion has always been one of the most troublesome hazards of chemical equipment. If you are not careful, it may damage the equipment, or cause accidents or even disasters. According to relevant statistics from gear pump manufacturers, about 60% of the damage to chemical equipment is caused by corrosion. Therefore, when selecting a chemical pump, you must first pay attention to the scientific nature of material selection. There is usually a misunderstanding that stainless steel is a "universal material". It is very dangerous to use stainless steel regardless of the medium and environmental conditions. Let’s talk about the key points of material selection for some commonly used chemical media.: 1. As one of the highly corrosive media, sulfuric acid is an important industrial raw material with a wide range of uses. The corrosion of materials by sulfuric acid at different concentrations and temperatures is quite different. For concentrated sulfuric acid with a concentration above 80% and a temperature less than 80°C, carbon steel and cast iron have good corrosion resistance, but they are not suitable for high-speed flowing sulfuric acid and are not suitable for use as materials for pump valves. ； Ordinary stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) also have limited use 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) and high-alloy stainless steel (No. 20 alloy). Fluorine plastic has good sulfuric acid resistance, and using a fluorine-lined pump (F46) is a more economical choice. 2. Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and molybdenum-containing high-silicon ferrosilicon can only be used in hydrochloric acid below 50°C and 30%. Contrary to metal materials, most non-metallic materials have good corrosion resistance to hydrochloric acid, so rubber-lined pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choices for transporting hydrochloric acid. 3. Nitric acid. Most general metals are rapidly corroded and destroyed in nitric acid. Stainless steel is the most widely used nitric acid-resistant material and has good corrosion resistance to all concentrations of nitric acid at room temperature. It is worth mentioning that the corrosion resistance of molybdenum-containing stainless steel (such as 316, 316L) to nitric acid is not only no better than that of ordinary stainless steel (such as 304, 321), but sometimes even worse. For high-temperature nitric acid, titanium and titanium alloy materials are usually used. 4. Acetic acid is one of the most corrosive substances among organic acids. Ordinary steel will be seriously corroded in acetic acid at all concentrations and temperatures. Stainless steel is an excellent acetic acid-resistant material. 316 stainless steel containing molybdenum is also suitable for high temperatures and dilute acetic acid vapor. For demanding requirements such as high temperature, high concentration acetic acid or other corrosive media, high alloy stainless steel or fluoroplastic pumps can be selected. 5. Alkali (sodium hydroxide) steel is widely used in sodium hydroxide solutions below 80°C and with a concentration of 30%. There are also many factories that still use ordinary steel at temperatures below 100°C and 75%. Although corrosion increases, the economy is good. The corrosion resistance of ordinary stainless steel to alkali liquid has no obvious advantage compared with cast iron. As long as a small amount of iron is allowed to be incorporated into the medium, stainless steel is not recommended. For high-temperature alkaline solutions, titanium and titanium alloys or high-alloy stainless steel are often used. 6. Ammonia (ammonium hydroxide) Most metals and non-metals corrode very slightly in liquid ammonia and ammonia (ammonia hydroxide). Only copper and copper alloys are not suitable for use. 7. Salt water (sea water) The corrosion rate of ordinary steel in sodium chloride solution, sea water, and salt water is not very high, and it generally needs to be protected by paint. ； Various types of stainless steel also have very low uniform corrosion rates, but may cause localized corrosion due to chloride ions. Generally, 316 stainless steel is better. 8. Alcohols, similar media, esters, ethers. Common alcohol media include methanol, ethanol, ethylene glycol, propanol, etc. Similar media include propylene glycol and butylene glycol, etc., ester media include various methyl esters, ethyl esters, etc., and ether media include methyl ether, ethyl ether, butyl ether, etc. They are basically non-corrosive and can be used with commonly used materials. Reasonable selection should be made based on the properties of the medium and related requirements. It is also worth noting that homologs, esters, and ethers are soluble in a variety of rubbers, so avoid mistakes when selecting sealing materials. There are many other media that cannot be introduced one by one here. In short, you must not be casual and blind when selecting materials. You should consult more relevant information or learn from mature experience. Cooling issues: The transportation of high-temperature media puts forward higher requirements on the structure, materials and auxiliary systems of the pump. Let’s talk about the cooling requirements of different temperature changes and the company’s applicable pump types.: 1. For media with a temperature lower than 120°C, a special cooling system is usually not set up, and the own medium is often used for lubrication and cooling. Like DFL(W)H chemical pump, DFL(W)PH shielded chemical pump (when the temperature exceeds 90℃, the protection level of the shielded motor should be H level) ； The DFCZ ordinary type and IH chemical pumps use a suspension structure to allow the upper temperature limit to reach 140°C~160°C.℃ ； The maximum operating temperature of IHF fluorine-lined pump can reach 200℃ ； Only the CQB ordinary magnetic pump's operating temperature does not exceed 100°C. It is worth mentioning that media that is prone to crystallization or contains particles should be equipped with a sealing surface flushing pipeline (interfaces are left in the design). 2. For media above 120°C and within 300°C, a cooling cavity must be provided on the pump cover, and the sealing chamber must also be connected to coolant (must be equipped with a double-end mechanical seal). When the coolant is not allowed to penetrate into the medium, the medium itself should be cooled and then connected (this can be achieved through a simple heat exchanger). 3. For high-temperature media above 300°C, not only the pump head part needs to be cooled, but also the suspension bearing chamber should be equipped with a cooling system. The pump structure is generally in the form of a central support. The mechanical seal is preferably a metal bellows type, but the price is high (the price is more than 10 times that of ordinary mechanical seals). Sealing without leakage is the eternal pursuit of chemical equipment. It is this requirement that has contributed to the increasing application of magnetic pumps and canned pumps. However, there is still a long way to go to truly achieve no leakage, such as the life problems of the magnetic pump isolation sleeve and the canned pump shield sleeve, the pitting corrosion problem of the material, the reliability of the static seal, etc. Now some basic information about sealing is briefly introduced as follows:: 1. Sealing form For static sealing, there are usually only two forms: gasket and sealing ring, and the O-ring is the most widely used sealing ring. ； For dynamic seals, chemical pumps rarely use packing seals, mainly mechanical seals. Mechanical seals are divided into single-end and double-end, balanced and unbalanced types. The balanced type is suitable for sealing high-pressure media (usually refers to pressure greater than 1.0MPa). The double-end mechanical seal is mainly used for high-temperature, easy-to-crystallize, viscous, particle-containing and toxic and volatile media. The double-end mechanical seal should inject isolation liquid into the seal cavity, and its pressure is generally 0.07~0.1MPa higher than the medium pressure. 2. Sealing material: The static seal material of chemical pumps is generally fluorine rubber, and polytetrafluoroethylene material is used in special cases. ； The material configuration of the dynamic and static rings of the mechanical seal is more critical. Carbide is not the best for cemented carbide. On the one hand, the price is high. It is unreasonable to have no hardness difference between the two, so it is best to treat them differently according to the characteristics of the medium. The viscosity of the medium has a great influence on the performance of the pump. When the viscosity increases, the lift curve of the pump decreases, the lift and flow rate of the optimal working conditions decrease, and the power increases, so the efficiency decreases. Generally, the parameters on the sample are the performance when conveying clean water, and should be converted when conveying viscous media (the correction coefficients of different viscosities can be found in the relevant conversion chart). For the transportation of slurries, pastes and viscous liquids with higher viscosity, it is recommended to use screw pumps. http://blog.hc360.com/portal/per ...sp?blogName=myfen *