Introduction to experience in selecting chemical process pumps

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The petroleum and chemical industries hold a very important position in the national economy, and chemical process pumps, as key supporting equipment, are attracting increasing attention. Given the complex nature of chemical media, along with the increasing demands for environmental protection, how should chemical engineering technicians select chemical pumps? What aspects should be paid attention to? This article provides a brief overview of the selection of common chemical pumps, taking into account the characteristics of related products in various chemical companies, in the hope of assisting customers in carrying out their work more effectively in this field. I. Corrosion Issues 　　Corrosion has always been one of the main hazards associated with chemical processing equipment. Even a slight carelessness can lead to equipment damage, and in more severe cases, it can result in 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 corrosion-resistant material, and that it can be used in any medium or under any environmental conditions – this is very dangerous. Below, we will discuss the key points regarding material selection for some commonly used chemical media: 1. Sulfuric acid. As one of the highly corrosive media, sulfuric acid is an important industrial raw material with a wide range of applications. Sulfuric acid at different concentrations and temperatures has significantly different corrosive effects on materials. For concentrated sulfuric acid with a concentration above 80% and a temperature below 80°C, carbon steel and cast iron exhibit good corrosion resistance. However, it is not suitable for sulfuric acid flowing at high speeds; thus, these materials cannot be used as materials for pumps and valves ； Common stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) also have limited applicability 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), but their difficulty in processing and high cost render them less attractive. Fluoroplastic alloys exhibit excellent resistance to sulfuric acid, making the use of fluorine-lined pumps (F46) a more economical option. Applicable products include: IHF fluorine-lined pumps, FSB highly corrosion-resistant centrifugal pumps, IMD fluoroplastic magnetic pumps, etc. 　　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 engineering plastics and fluoroplastics) are suitable choices for transporting hydrochloric acid. Applicable products include: CQB fluoroplastic leak-proof acid-resistant pumps, CQF engineering plastic magnetic pumps (or fluoroplastic magnetic pumps), etc. 　　3. Nitric acid: Most metals are rapidly corroded and damaged by nitric acid. Stainless steel is a widely used material resistant to nitric acid; it exhibits good corrosion resistance against 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, fluoroplastic alloy materials are typically used. Applicable products include: CQB fluoroplastic alloy magnetic pumps, FSB fluoropolymer alloy centrifugal pumps, etc. 　　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 agents, high-alloy stainless steel or fluoroplastic pumps can be used. Such as CQB magnetic drives pumps, CQ stainless steel magnetic drives pumps. 　　5. Bases (sodium hydroxide) generally do not have strong corrosivity, but crystallization occurs in most alkaline solutions; therefore, an FSB-type fluoropolymer alkali pump equipped with a mechanical seal made of siliconized graphite can be used. 　　6. Ammonia (ammonium hydroxide): Most metals and non-metals suffer only mild corrosion in liquid ammonia and ammonia water (ammonium hydroxide); only copper and its alloys are not suitable for use. Most of the company’s products are suitable for transporting ammonia and ammonia solutions. Pumps such as the CQF engineering plastic magnetic pump and the FSB fluoropolymer centrifugal pump are quite good options. 　　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. Such as IH Chemical centrifugal pumps. 　　8. Alcohols, ketones, esters, and ethers. Common alcohol-based media include methanol, ethanol, ethylene glycol, propanol, etc.; ketone-based media include acetone, butanone, etc.; ester-based media include various methyl esters, ethyl esters, etc.; ether-based media include dimethyl ether, diethyl ether, butyl ether, etc. These substances generally have low corrosivity, so ordinary stainless steel can be used for them. However, a proper choice should still be made based on the properties of the medium and relevant requirements. It is also worth noting that alcohols, esters, and ethers are soluble in various rubbers, which helps to avoid mistakes when selecting sealing materials. 　　There are many other media that cannot be listed here one by one. In short, when selecting materials, one must not act casually or blindly; it is advisable to consult relevant information and draw on proven experience. II. Sealing Issues 　　Leaklessness is an eternal goal for chemical processing equipment, and it is precisely this requirement that has led to the increasing use of magnetic pumps. However, there is still a long way to go before true leaklessness can be achieved, such as the lifespan issue of magnetic pump isolation sleeves, the pitting problem of materials, and the leakage issues of static seals, among others. 　　1. Sealing types: For static seals, there are generally only two types – gaskets and seals; among these, O-rings are the most widely used ； For dynamic sealing, chemical pumps rarely use packing seals; mechanical seals are preferred instead. Mechanical seals come in single-face and double-face types, as well as balanced and unbalanced versions. Balanced mechanical seals are suitable for sealing high-pressure media (typically those with pressures greater than 1.0 MPa). Double-face mechanical seals are used mainly for media that are high-temperature, prone to crystallization, viscous, contain particles, or are toxic and volatile. An isolation fluid must be injected into the sealing chamber using double-face mechanical seals, and its pressure is generally 0.07–0.1 MPa higher than the pressure of the medium. 　　2. Sealing materials: Fluororubber is generally used as the material for static sealing in chemical magnetic pumps, with polytetrafluoroethylene being used only in special cases ； The material selection for the stationary and rotating rings of mechanical seals is quite important; it’s not sufficient to use carbide against carbide. While high hardness is an important factor, it’s also unreasonable if there is no difference in hardness between the two components. The choice of materials should be determined based on the characteristics of the medium being handled. 　　(Note: Appendix D of API 610, eighth edition, provides detailed specifications for the typical configurations of mechanical seals and piping systems.) III. Viscosity issues The viscosity of the fluid has a significant impact on the performance of the pump. As viscosity increases, the pump’s head curve declines; both the head and flow rate under operating conditions decrease, while power consumption increases, resulting in reduced efficiency. The parameters for typical samples represent the performance when transporting clean water; adjustments are necessary when transporting viscous media (the correction factors for different viscosities can be found in the relevant conversion charts). For transporting slurries, pastes, and viscous fluids with high viscosity, it is recommended to use mortar pumps; corrosion-resistant and wear-resistant mortar pumps are suitable for media that are viscous or contain particles. IV. Principles for Pump Selection When designing equipment and systems, the design institute must determine the purpose and performance parameters of the pump and select the appropriate type of pump. This choice begins with selecting the type and design of the pump. So, what principles should be used to choose a pump? What is the basis for this? 　　1. The type and performance of the selected pump must meet the requirements of process parameters such as flow rate, head, pressure, temperature, net positive suction head, and suction lift.
2. The pump must also meet the requirements related to the characteristics of the medium being transported.
For pumps used to transport flammable, explosive, toxic, or valuable media, it is required that there be zero leakage from the shaft seal; alternatively, leak-free pumps should be used, such as magnetically driven pumps (which have no shaft seal and utilize indirect magnetic drive).
For pumps handling corrosive media, corrosion-resistant materials must be used for the wetted parts; examples include fluoroplastic corrosion-resistant pumps.
For pumps transporting media containing solid particles, wear-resistant materials must be employed for the wetted parts; when necessary, the shaft seal should be flushed with a clean liquid. 　　3. Mechanically, it is required to have high operational flexibility, low noise, and minimal vibration. 　　4. Calculate the investment cost associated with pump procurement correctly; for more details, see: How to Correctly Calculate the Investment Cost of Purchasing Pumps. 5. When transporting highly corrosive substances such as concentrated sulfuric acid or concentrated nitric acid, or when transporting flammable and explosive materials, and in environments where there must be no contamination at all, magnetic pumps can be used, such as the CQB series of magnetic pumps and the IMD series of magnetic pumps. If self-priming is required, the FZB fluoroplastic self-priming pump can be chosen. 　　6. Centrifugal pumps feature high rotational speed, compact size, light weight, high efficiency, large flow rate, simple structure, pulse-free fluid delivery, stable performance, ease of operation, and convenient maintenance. Therefore, when there are no special requirements regarding operating conditions, centrifugal pumps are a good choice. 　　7. For pumps used to transport chemical media containing solid particles, when wear-resistant materials are required for the flow components: (slag) slurry pumps are a suitable choice. 　　8. When the liquid level of the medium is below the pump’s installation position: a self-priming pump should be selected. 　　9. Select the model based on the pump’s performance curve; when no suitable model is available in the performance parameters table for the required application, refer to the pump’s performance curve to choose an appropriate type of pump.