Corrosion-resistant centrifugal pumps are necessary equipment for lifting and transporting chemical media in the chemical industry, and corrosion resistance is the most important issue for chemical equipment. Careless selection of the design can damage the equipment or cause an accident. 60 to 80% of the damage of chemical equipment is basically caused by corrosion. Therefore, when selecting corrosion-resistant centrifugal pumps, attention must be paid to the scientific nature of material selection.
When considering corrosion resistance factors, it must be noted that no material can be suitable for all liquids handled in the chemical processing industry. Materials with universal corrosion resistance are the most ideal corrosion-resistant materials. Corrosion-resistant materials are materials that can withstand various acids, alkalis, salts and corrosive gases. Corrosion-resistant materials are generally divided into two types: metal corrosion-resistant materials and non-metallic corrosion-resistant materials.
Corrosion-resistant materials are graded according to their corrosion resistance. From the perspective of use, it is generally divided into three levels:
(1) Corrosion resistance level. After the material is corroded, its basic physical and mechanical properties remain unchanged or change very little, which can ensure the use of working conditions.
(2) Still corrosion resistant level. After the material is corroded, its physical and mechanical properties are reduced to varying degrees, but it can still meet the requirements of the working conditions.
(3) Not corrosion resistant level. After the material is corroded, the physical and mechanical properties change greatly, which cannot meet the requirements of working conditions.
The corrosion resistance of corrosion-resistant materials is related to the concentration of acids, alkalis, salts and corrosive gases, temperature, humidity and usage conditions. In use, if there is abrasion, collision and thermal shock, it will accelerate the damage of corrosion-resistant materials. Various corrosion-resistant materials have their limited scope of use, such as cast iron and carbon steel resistant to concentrated sulfuric acid and not resistant to dilute sulfuric acid; stainless steel is resistant to the corrosion of medium-dilute concentration of nitric acid or organic acid, and is not resistant to the corrosion of dilute sulfuric acid: FRP materials only Resistant to dilute acids, not alkalis. Most non-metallic corrosion-resistant materials can only resist corrosion below 60°C, but cannot withstand corrosion above 60°C. Some materials can only resist acid corrosion at medium or low concentrations, but not concentrated Acid corrosion; some can resist non-oxidizing acid corrosion, but not oxidizing acid abrasion. Therefore, when choosing corrosion-resistant materials, we must understand their basic properties and use their strengths according to their characteristics. The following analysis of corrosion-resistant pumps made of commonly used corrosion-resistant materials is mainly based on the corrosion properties of chemical media as follows
sulfuric acid:
Sulfuric acid is a strong corrosive medium. It is an important industrial raw material with a wide range of uses. The corrosion of sulfuric acid with different concentrations and temperatures to materials varies greatly. For concentrated sulfuric acid with a concentration of more than 80% and a temperature of less than 80°C, carbon steel and cast iron are better The corrosion resistance is very suitable for corrosion-resistant pump materials: ordinary stainless steels such as 304 (r18Ni9), 316 (00r18Ni12Mo2Ti) are not resistant to any concentration of sulfuric acid, and the pump valve of high silicon cast iron material has the best effect of transporting sulfuric acid, but the casting and processing are difficult and difficult. The price is expensive, so it is impossible to universally use
hydrochloric acid:
Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and ferrosilicon containing molybdenum can only be used for hydrochloric acid with a concentration of less than 30% at 50°C. Most non-metallic materials (glass reinforced plastics, engineering plastics, fluoroplastic alloys, etc.) have good corrosion resistance to hydrochloric acid. Among them, phenolic glass reinforced plastics is resistant to hydrochloric acid at any concentration of various temperatures and is the best choice for transporting hydrochloric acid.
Nitric acid:
Most metals are rapidly corroded and destroyed in nitric acid. Stainless steel is the most widely used nitric acid resistant material. It has good corrosion resistance to nitric acid below 90% concentration at room temperature. For high temperature and high concentration nitric acid, fluoroplastic alloys are usually used. And 316 stainless steel containing molybdenum, in some cases, cast aluminum alloy and aluminum can also be used.
acetic acid:
It is one of the most corrosive substances in organic acids. Ordinary steel will be severely 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 temperature and dilute acetic acid steam. . For demanding requirements such as high temperature, high concentration of acetic acid or other corrosive media, high alloy stainless steel or fluoroplastic alloy pumps can be used.
Alkali (sodium hydroxide):
Generally, it is not very corrosive. Cast steel, cast iron, and stainless steel pumps can be applied, but glass fiber reinforced plastic pumps are not resistant.
Ammonia (Ammonia Hydroxide):
Most metals and non-metals have slight corrosion in liquid ammonia and ammonia (ammonium hydroxide), only copper and copper alloys are not suitable for use
Salt water (sea water):
Ordinary steel has a low corrosion rate in sodium chloride solution, sea water and salt water, and generally must be protected by paint; all kinds of stainless steel also have a very low uniform corrosion rate, but may cause localized corrosion due to chloride ions, usually 316 stainless steel better,
Alcohols, ketones, esters, ethers:
Common alcohol media include methanol, ethanol, ethylene glycol, propanol, etc., ketone media include acetone, butanone, etc., ester media include various methyl esters, ethyl esters, etc., and ether media include methyl ether, ethyl ether, etc. Butyl ether, etc., they are basically not corrosive, so ordinary stainless steel can be used, and a reasonable choice should be made according to the properties of the medium and related requirements when selecting it. In addition, it is worth noting that ketones, esters, and ethers have solubility to a variety of rubbers, and avoid mistakes when choosing sealing materials.
For pumps that transport chemical media containing solid particles, when the convection parts are required to be made of wear-resistant materials, such as: AH, AHH type wear-resistant slurry pump material is high-strength new engineering plastic UHBWPE, which is a modified ultra-high molecular weight (500 10,000 or more) polyethylene. Among plastics, it has the most excellent wear resistance, which is much higher than stainless steel, and its impact resistance, creep resistance and excellent corrosion resistance are equivalent to F4, and it has unique properties such as non-adhesion. .
Selection of sealing materials for corrosion-resistant centrifugal pumps:
The selection of materials for the mechanical seal of corrosion-resistant chemical centrifugal pumps is also particularly important. The pump is easy to leak, the service life is not long, pollutes the environment, and causes safety hazards. Mechanical seals generally use fluoroelastomers, and PTFE materials are used in special circumstances; the material configuration of the dynamic and static rings is more critical, and the two should have poor hardness. , For example, cemented carbide and cemented carbide for moving and static rings have the highest price, but not the best.
Therefore, it is best to treat differently according to the characteristics of the medium. You can use silicon carbide to silicon carbide alumina ceramics to tetrafluoride, silicon carbide to alumina ceramics, cemented carbide to cemented carbide (such as particle-containing media) for users to choose and configure. When considering the selection of corrosion-resistant materials, the input cost of pump procurement must also be considered. Although the advanced level of non-metallic corrosion-resistant materials (glass reinforced plastics, engineering plastics) and metallurgical technology has produced many new materials, the closest material to achieve the ideal effect of corrosion resistance is ceramics. Ceramics have high hardness and wear resistance and excellent corrosion resistance. Except for hydrofluoric acid, fluorosilicic acid and concentrated alkali, they can almost resist the corrosion of various media. It is very difficult to manufacture corrosion-resistant centrifugal pump products from ceramic materials. It's hard to achieve.
