The petrochemical industry occupies a very important position in the national economy, and chemical process pumps, as key supporting equipment, are also receiving increasing attention from people. Due to the complex characteristics of chemical media and the constantly increasing requirements of users, as a manufacturer, how to select and which aspects should be paid attention to? It appears particularly important. Here are some selection methods for common problems with chemical pumps: corrosion resistance has always been one of the most troublesome hazards of chemical equipment. Slight carelessness can cause equipment damage, and serious accidents or even disasters. According to relevant statistics, about 60% of the damage to chemical equipment is caused by corrosion. Therefore, when selecting chemical pumps, the first thing to pay attention to is the scientific selection of brass materials. There is usually a misconception that stainless steel is a “universal material”, and it is very dangerous to hold stainless steel out regardless of the medium and environmental conditions.
Here are some key points for selecting materials for commonly used chemical media:
1. As one of the strong corrosive media, sulfuric acid is an important industrial raw material with a wide range of uses. The corrosion of materials varies greatly with different concentrations and temperatures of sulfuric acid. For concentrated sulfuric acid with concentrations above 80% and temperatures below 80 ℃, carbon steel and cast iron have good corrosion resistance, but they are not suitable for high-speed flow of sulfuric acid and are not suitable as materials for pump valves; Ordinary stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) are also limited in their use for sulfuric acid media. Therefore, pump valves for transporting sulfuric acid are usually made of high silicon cast iron (with high difficulty in casting and processing) and high alloy stainless steel (alloy 20). Fluoroplastics have good sulfuric acid resistance, and using a fluorine lined pump (F46) is a more economical choice. Applicable products include: IHF fluorine lined pump, PF (FS) strong corrosion resistant centrifugal pump, CQB-F fluorine plastic magnetic pump, etc.
2. Most metal materials are not corrosion-resistant (including various stainless steel materials), and high silicon iron containing molybdenum can only be used at temperatures below 50 ℃ and 30%. Contrary to metal materials, the vast majority of non-metallic materials have good corrosion resistance, so lined rubber pumps and plastic pumps (such as polyethylene, fluoroplastics, etc.) are the best choices for transportation. Applicable products include: IHF fluorine lined pump, PF (FS) strong corrosion resistant centrifugal pump, CQ polymer magnetic pump (or fluoroplastic magnetic pump), etc.
3. Most metals in nitric acid are rapidly corroded and destroyed. 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 stainless steel containing molybdenum (such as 316 and 316L) not only has lower corrosion resistance to nitric acid than 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. The company’s applicable products include: DFL (W) H chemical pump, DFL (W) PH shielded chemical pump, DFCZ process pump, DFLZP self priming chemical pump, IH chemical pump, CQB magnetic pump, etc., made of 304 material.
4. Acetic acid is one of the most corrosive substances in organic acids. Ordinary steel will severely corrode in all concentrations and temperatures of acetic acid. Stainless steel is an excellent acetic acid resistant material, and 316 stainless steel containing molybdenum can also be suitable for high temperatures and dilute acetic acid vapors. For demanding requirements such as high temperature and high concentration acetic acid or containing other corrosive media, high alloy stainless steel or fluoroplastic pumps can be selected.
5. Alkaline steel is widely used in solutions below 80 ℃ and within 30% concentration. Many factories still use ordinary steel at temperatures below 100 ℃ and 75%. Although corrosion increases, it has good economic efficiency. Compared with cast iron, ordinary stainless steel has no significant advantage in corrosion resistance to alkaline solution. As long as a small amount of iron is allowed to be added to the medium, stainless steel is not recommended. For high-temperature alkaline solution, cnc turning titanium and titanium alloys or high alloy stainless steel are often used. The general cast iron pumps of the company can be used for low concentration alkaline solution at room temperature. When special requirements are met, various types of stainless steel pumps or fluoroplastic pumps can be used.
6. (Hydrogen oxidation) The corrosion of most metals and non-metals in liquid and (hydrogen oxidation) is very mild, and only copper and copper alloys are not suitable for use. Most of the company’s products are suitable for conveying.
7. Salt water (seawater): Ordinary steel has a low corrosion rate in sodium chloride solution, seawater, and salt water, and generally requires coating protection; Various types of stainless steel also have a low uniform corrosion rate, but may cause localized corrosion due to chloride ions. Usually, 316 stainless steel is preferred.
8. Alcohols, ketones, esters, and ethers. Common alcohol media include methanol, ethanol, ethylene glycol, propanol, etc. Ketone media include butanone, etc. Ester media include various methyl esters, ethyl esters, etc. Ether media include,, butyl ether, etc. They are basically non corrosive and can be used as commonly used materials. When selecting them, reasonable choices should also be made based on the properties and relevant requirements of the media. It is also worth noting that ketones, esters, and ethers have solubility in various types of rubber, so errors should be avoided when selecting sealing materials. There are many other media that cannot be introduced here one by one. In short, when selecting materials, one should not be arbitrary or blind, and should consult more relevant materials or learn from mature experience.
The transportation of high-temperature media poses higher requirements for the structure, stainless steel materials, and auxiliary systems of pumps. Below, we will discuss the cooling requirements of different temperature changes and the pump types applicable to the company:
For media with a temperature below 120 ℃, a dedicated cooling system is usually not set up, and the medium itself is often used for lubrication and cooling. For example, DFL (W) H chemical pump and DFL (W) PH shielded chemical pump (when the temperature exceeds 90 ℃, the protection level of the shielded motor should be H level); Due to the use of suspension structure, the DFCZ ordinary and IH chemical pumps can achieve a temperature limit of 140 ℃~160 ℃; The maximum operating temperature of the IHF fluorine lined pump can reach 200 ℃; Only CQB ordinary magnetic pumps can be used at temperatures not exceeding 100 ℃. It is worth mentioning that media that are prone to crystallization or contain particles should be equipped with sealing surface flushing pipelines (with interfaces reserved during design).
2. For media above 120 ℃ and below 300 ℃, a cooling chamber must generally be installed on the pump cover, and the sealing chamber should also be connected to the cooling liquid (equipped with a double end mechanical seal). When the cooling liquid is not allowed to penetrate the medium, it should be cooled and connected (which can be achieved through a simple heat exchanger). At present, the company has DFCZ chemical process pumps, GRG high-temperature pipeline pumps, and HPK hot water circulation pumps (under development) available for selection. In addition, CQB-G high-temperature magnetic pumps can be used for high-temperature media up to 280 ℃
For high-temperature media above 300 ℃, not only the pump head part needs to be cooled, but the suspension bearing chamber should also be equipped with a cooling system. The pump structure is generally in the form of a central support, and the mechanical seal is preferably a metal corrugated pipe type, but the price is high (more than 10 times that of a regular mechanical seal). Currently, DFAY centrifugal oil pumps can operate at temperatures up to 420 ℃ (under development). Sealing without leakage is the eternal pursuit of chemical equipment, and it is this requirement that has led to the increasingly expanding application of magnetic pumps and shielded pumps. However, there is still a long way to go to truly achieve no leakage, such as the lifespan issues of magnetic pump isolation sleeves and shielding pump shielding sleeves, material pitting issues, reliability issues of static seals, and so on.
Here is a brief introduction to some basic information about sealing:
1. Sealing form
For static sealing, there are usually only two forms: sealing gasket and sealing ring, and O-ring is the most widely used sealing ring; For dynamic seals, chemical pumps rarely use packing seals, mainly mechanical seals. Mechanical seals can be divided into single and double face, balanced and non balanced types. Balanced types are suitable for sealing high-pressure media (usually referring to pressure greater than 1.0MPa). Double face mechanical seals are mainly used for high temperature, easily crystallized, viscous, granular, and toxic volatile media. Double face mechanical seals should inject isolation liquid into the sealing chamber, Its pressure is generally 0.07-0.1MPa higher than the medium pressure.
2. Sealing materials
The material for static sealing of chemical pumps is generally fluorine rubber, and in special cases, polytetrafluoroethylene material is used; The material configuration of mechanical sealing dynamic and static rings is crucial, and hard alloy is not the best choice for hard alloy. The high price is one aspect, and there is no difference in hardness between the two, which is not reasonable. Therefore, it is best to treat the viscosity issue differently based on the characteristics of the medium (note: the typical configuration of mechanical seals and pipeline systems in the eighth edition of API 610 is detailed in Appendix D)
The viscosity of the medium has a significant impact on the performance of the pump. As the viscosity increases, the head curve of the pump decreases, and the head and flow rate under optimal operating conditions also decrease, while the power increases, resulting in a decrease in efficiency. The parameters on the general sample are the performance when transporting clean water, and conversion should be carried out when transporting viscous media (the correction coefficients for different viscosities can be found in relevant conversion charts). For the transportation of high viscosity slurries, pastes, and viscous liquids, it is recommended to use a screw pump. DFGG single screw pump is suitable for medium viscosity up to 1000000 cst.
Link to this article：Key Points and Difficult Issues in the Selection of Chemical Process Pumps
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