Since the will, corrosion is one of the most troublesome hazards of chemical equipment. If you are careless, you may damage the equipment, or cause accidents or even disasters. According to relevant statistics, about 60% of the damage of chemical equipment is caused by corrosion. Therefore, when selecting chemical pump, we should first pay attention to the scientificity of material selection. There is usually a misunderstanding that stainless steel pumps are "universal materials". It is very dangerous to hold stainless steel pumps regardless of media and environmental conditions. Here are the key points of material selection for some common chemical media:

1. Sulfuric acid

As one of the strong corrosive media, sulfuric acid is an important industrial raw material with a wide range of applications. Sulfuric acid with different concentration and temperature has great corrosion difference on materials. For concentrated sulfuric acid with concentration above 80% and temperature less than 80 ℃, carbon steel and cast iron have good corrosion resistance, but it is not suitable for high-speed flow sulfuric acid and is not suitable for pump valve materials; Ordinary stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) are also limited to sulfuric acid medium. Therefore, the pump valve for transporting sulfuric acid is usually made of high silicon cast iron pump (casting and processing are difficult) and high alloy stainless steel pump (No. 20 alloy). Fluoroplastics have good resistance to sulfuric acid. It is a more economical choice to use fluorine lined pump (F46).

2. Hydrochloric acid

Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and molybdenum containing high silicon iron can only be used for hydrochloric acid below 50 ℃ and 30%. Contrary to metal materials, most non-metallic materials have good corrosion resistance to hydrochloric acid, so lined rubber 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 damaged 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 and 316L) to nitric acid is not only better than that of ordinary stainless steel (such as 304 and 321), but sometimes even worse. For high temperature nitric acid, titanium and titanium alloy materials are usually used.

4. Acetic acid

It is one of the most corrosive substances in 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 can also be used for high temperature and dilute acetic acid steam. For high temperature and high concentration acetic acid or other corrosive media, high alloy stainless steel or fluoroplastic pump can be selected.

5. Alkali (sodium hydroxide)

Steel is widely used in sodium hydroxide solution below 80 ℃ and within 30% concentration. Many factories still use ordinary steel at 100 ℃ and below 75%. Although the corrosion increases, it has good economy. The corrosion resistance of ordinary stainless steel to alkaline solution has no obvious advantages compared with cast iron. Stainless steel is not recommended as long as a small amount of iron is allowed in the medium. For high temperature lye, titanium and titanium alloy or high alloy stainless steel are mostly used.

6. Ammonia (ammonia hydroxide)

The corrosion of most metals and nonmetals in liquid ammonia and ammonia (ammonia hydroxide) is very slight, and 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, seawater and salt water is not too high, so it must be protected by coating; All kinds of stainless steel also have low uniform corrosion rate, but may cause local corrosion due to chloride ion. 316 stainless steel is usually better.

8. Alcohols, ketones, esters and ethers

Common alcohol media include methanol, ethanol, ethylene glycol and propanol, ketone media include acetone and butanone, ester media include various methyl esters and ethyl esters, and ether media include methyl ether, ethyl ether and butyl ether. They are basically non corrosive, and common materials can be applicable. The specific selection should also be made according to the properties of the media and relevant requirements. In addition, it is worth noting that ketones, esters and ethers are soluble in a variety of rubber, so mistakes should be avoided in the selection of sealing materials. There are many other media that cannot be introduced here one by one. In short, when selecting materials, we must not be arbitrary and blind. We should consult more relevant materials or learn from mature experience.

Influence of cooling problem on selection of chemical pump

The transmission of high-temperature medium puts forward higher requirements for the structure, materials and auxiliary system of the pump. Here are the cooling requirements of different temperature changes and the applicable pump types of the company:

For the medium with temperature lower than 120 ℃, no special cooling system is usually set, and its own medium is mostly used for lubrication and cooling. Such as DFL (W) H chemical pump and DFL (W) pH shielded chemical pump (when the temperature exceeds 90 ℃, the protection grade of shielded motor shall be grade H); The upper temperature limit of dfcz ordinary and IH chemical pumps can reach 140 ℃ ~ 160 ℃ due to the suspension structure; The maximum operating temperature of IHF fluorine lined pump can reach 200 ℃; Only CQB ordinary magnetic pump can operate at a temperature not exceeding 100 ℃. It is worth mentioning that the media easy to crystallize or containing particles shall be equipped with sealing surface flushing pipeline (interfaces shall be reserved in the design).

For the medium above 120 ℃ and below 300 ℃, generally, a cooling chamber must be set on the pump cover, and the sealing chamber should also be connected with coolant (double face mechanical seal must be equipped). When the coolant is not allowed to penetrate into the medium, it should be connected after cooling its own medium (which can be realized through a simple heat exchanger).

For high-temperature media above 300 ℃, not only the pump head 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 central support. The mechanical seal is preferably of metal bellows type, but the price is high (the price is more than 10 times that of ordinary mechanical seal).

Influence of sealing problem on selection of chemical pump

No leakage is the eternal pursuit of chemical equipment. It is this requirement that leads to the increasing application of magnetic pump and shielded pump. However, there is still a long way to go to achieve no leakage, such as the service life of magnetic pump isolation sleeve and shield pump shield sleeve, material pitting corrosion, reliability of static seal and so on. Some basic information about sealing is briefly introduced as follows:

For static seal, there are usually only two forms of seal gasket and seal ring, and O-ring is the most widely used seal ring; For dynamic seal, chemical pumps rarely use packing seal, mainly mechanical seal. Mechanical seal can be divided into single end face and double end face, balanced type and non-equilibrium type. Balanced type is suitable for sealing of high-pressure media (usually pressure greater than 1.0MPa). Double end face mechanical seal is mainly used for high-temperature, crystallizable, viscous, granular and toxic volatile media, Isolation fluid shall be injected into the sealing cavity of double face mechanical seal, and its pressure is generally 0.07 ~ 0.1MPa higher than the medium pressure.

Sealing material the static sealing material of chemical pump generally adopts fluororubber, and Teflon material is used only under special circumstances; The material configuration of the dynamic and static rings of mechanical seals is more critical. Cemented carbide is not the best for cemented carbide. On the one hand, the two have no hardness difference and are unreasonable. Therefore, it is best to treat them differently according to the characteristics of the medium.

The viscosity of the medium has a great impact on the performance of the pump. When the viscosity increases, the head curve of the pump decreases, the head and flow under the best working condition decrease, while the power increases, so the efficiency decreases. Generally, the parameters on the sample are the performance when transporting clean water. When transporting viscous medium, conversion shall be carried out (refer to relevant conversion charts for correction coefficients of different viscosities). Screw pump is recommended for the transportation of slurry, paste and viscous liquid with high viscosity.