【Original】Application of Ceramic Valves in Acetylene Calcium Carbide Slurry

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Chlor-alkali chemistry is one of the most fundamental technologies in the chemical industry. It is widely used in various fields such as the chemical industry, light industry, textile industry, metallurgical industry, and public utilities. ​ The chlor-alkali industry is primarily an industrial process that involves the electrolysis of brine to produce caustic soda, chlorine, and hydrogen. It is one of the important basic chemical industries in China at present. PVC is the key downstream product, and the acetylene production stage constitutes the most fundamental part of its three core stages; 70% of domestic production relies on the calcium carbide process. For the shutdown and regulation of calcium carbide slurry media, conventional metal or lined valves can no longer meet the requirements on site; frequent replacements and maintenance cannot ensure the safe and stable operation of the process. Imported ceramic ball valves can effectively solve such problems. Process overview of the acetylene section: The qualified calcium carbide processed by the calcium carbide crushing system is fed into the calcium carbide finished product storage tank, from where it is conveyed to the calcium carbide feeding hopper via an inclined screw conveyor. It is then continuously added to the generator through an electromagnetic vibrator. Inside the generator, the calcium carbide reacts with water; the by-product of this hydrolysis reaction is a slurry containing approximately 10% solid content, at a temperature of around 75 °C. It overflows through sealed pipes into the slurry buffer tank, with a pressure of 1–3 kPa. The crude acetylene gas produced by the hydrolysis reaction escapes from the top of the generator; after passing through a spray pre-cooler and a positive water seal, it enters the spray cooling tower. From there, it goes on to the packed-bed cooling tower before reaching the gas holder. Alternatively, it can be compressed by a compressor and then subjected to gas-liquid separation as part of the acetylene purification process. The process flow is shown in Figure 1: https://mmbiz.qpic.cn/mmbiz_png/nkaH7FkqpP8R5JpvLmkic2E0u5ibSePZKlVIhU9Uz90raxqRRpwQlq7mD58Zbb9891ALRkwtIl7RF4iaDE9hnR23Q/?wx_fmt=png Figure 1: Process flow of the acetylene section. Characteristics of the medium: The main component of calcium carbide slag slurry is calcium hydroxide; the main components are listed in the table. Its solubility in water is low, and as the temperature drops, solid calcium hydroxide particles gradually precipitate from the solution, forming a thin paste with poor fluidity. In addition, solid impurities in calcium carbide that do not participate in the reaction (such as ferrosilicon, sand, coke, etc.) are also present in this slurry, which makes it easy for a compacted solid mixture to form when the slurry stays still; this can lead to blockages in valves or pipes. Moreover, substances such as ferrosilicon, sand, and coke have large particle sizes and high hardness, which can easily cause wear and sticking problems in valves. Therefore, due to the special conditions of the medium, there are many issues regarding the selection of valves. https://mmbiz.qpic.cn/mmbiz_png/nkaH7FkqpP8R5JpvLmkic2E0u5ibSePZKl78g3WMBjlHDWrfQCdwnibqHSqDSS6I6ZQog3AsEe28eUdQxo52BNr6w/?wx_fmt=png Main components of calcium carbide slurry; Current application status of ball valves in systems involving calcium carbide slurry; Technical issue 1: Medium factors – Due to the highly alkaline nature of the slurry water, long-term use of it causes corrosion of the valve cores, leading to malfunctions. Additionally, frequent opening and closing of the valves results in severe corrosion and wear caused by the medium, often resulting in internal leakage or poor sealing, which significantly affects the stable operation of the system. Prolonged corrosion of the valve by calcium carbide slurry water has resulted in inflexible operation of the valve core, preventing it from closing promptly in emergency situations and potentially leading to a risk of evacuation. Furthermore, the calcium carbide slurry contains large amounts of ferrosilicon as well as unreacted pieces of calcium carbide, which can easily cause damage to the ball valve stem. This makes repairs difficult in subsequent maintenance tasks and can lead to internal leaks. 2. Operating frequency factor: The operating frequency of valves at the calcium carbide slurry location in the acetylene section varies significantly. In some locations, it may remain closed or open for a month or even longer without any operations being performed. On the contrary, some locations need to be turned on and off several times per hour. In some flow control valves, the opening and closing position of the valve changes frequently, sometimes even several times per minute. Valves with frequent actuation reduce the likelihood of failures caused by scaling and deposits, but they are prone to wear. 3 Material selection factors: Valves used in this location are usually those with surface coatings or made of special metal materials. However, in the case of control valves used at the calcium carbide slurry location, changes in the flow path during operation cause variations in the fluid inside the valve; the flow velocity of the fluid is higher than that in the pipes connected to it. The valve core operates in an environment subject to severe wear, and coupled with the corrosive nature of the slurry, valves made from such materials often cannot be used for a long time. Economic Issue 1: Maintenance Costs. In the production process of the chlor-alkali industry, the cost associated with valves constitutes a part of the product cost. The funds required to maintain inventory of valves also typically account for a significant portion of a company’s total working capital. Due to the harsh operating conditions in the calcium carbide slurry area of the acetylene unit, valves need to be replaced frequently. This is highly detrimental to the safe operation of the entire equipment as well as to cost control for maintenance activities. Moreover, it poses a significant risk of emergency shutdowns, resulting in substantial losses. Solutions for ceramic ball valves: Ceramic ball valves not only help to make industrial piping systems more efficient and improve their sealing performance, but also contribute to a safer and more stable operation of associated equipment systems. They play a very positive role in environmental protection as well as energy conservation and emission reduction. In the 21st century, characterized by rapid technological innovation, industry professionals both domestically and internationally are increasingly focusing on ceramic valves, which has led to an expansion of their areas of application. Material properties of ceramic valves: With the advancement of science and technology, particularly in the fields of energy and space technology, materials are required to be used in harsh environments, which has led to increasingly stringent demands regarding their resistance to high temperatures, corrosion, and wear. Ceramic materials possess excellent properties such as high strength, high hardness, high wear resistance and corrosion resistance, as well as a low coefficient of expansion and low weight – characteristics that are difficult for metal materials to match. They are widely used in various fields and play an increasingly important role. The table shows a comparison of the basic properties of ceramic and steel materials. Comparison of the basic properties of ceramic materials and steel materials 1. Hardness: Ceramic materials are brittle materials. During hardness testing, pseudo-plastic deformation such as compressive shear occurs in the area where the indenter presses in; as a result, it is difficult to directly relate the hardness of ceramic materials to their strength. However, high hardness and good wear resistance are among the main advantages of ceramic materials. Mohs hardness belongs to the scratch method and is generally applicable to ceramic and mineral materials. It can only indicate the order of hardness from highest to lowest; materials with a higher hardness level can scratch the surface of those with a lower hardness level, but it cannot provide specific hardness values. The earlier standard divided Mohs hardness into 10 grades, while the new standard divides it into 15 grades in sequence, as shown in the table. Mohs hardness scale. Fracture toughness: Ceramics undergo brittle fracture; therefore, they exhibit high crack sensitivity. Given this characteristic of ceramics, their fracture mechanical properties are important indicators for evaluating their mechanical performance. The most commonly used parameter for assessing the fracture mechanics of ceramic materials is fracture toughness (KIC). To fully utilize the advantages of ceramics and expand their practical applications, it is necessary to find ways to significantly enhance and improve their toughness. The table shows the toughness values of ceramic materials. The toughness value of ceramic materials; structural characteristics of ceramic valves. The acetylene section in the chlor-alkali industry requires a large number of valves. By understanding the characteristics of the manufacturing process, selecting the appropriate type of valve based on the features of each system in the installation, addressing issues related to valve design, and choosing suitable materials, it is possible to improve their wear and corrosion resistance, extend the service life of the equipment, and ensure stable operation of the process. For cut-off valves, the flow channel and ball core are usually of O-type design, which is used to control the flow of fluid. Pneumatic ceramic ball valves consist of a limiter, solenoid valve, filter pressure regulator, ceramic ball valve, and air supply circuit. Imported ceramic ball valves feature O-ring ball cores and valve seat sealing surfaces with a roughness of less than 0.1μm, enabling a sealing performance between the ceramic ball core and the valve seat that is superior to that of metal ball valves. In addition, the ceramic ball core and valve seat possess self-grinding properties, low opening and closing torque, and high corrosion resistance. The metal part between the ceramic-lined flow channel and the valve body is completely isolated, resulting in excellent performance in applications involving media with high corrosion resistance and purity requirements. https://mmbiz.qpic.cn/mmbiz_png/nkaH7FkqpP8R5JpvLmkic2E0u5ibSePZKl72icibTiblFza0psicavVlYiaVP1qXnrBA0PBWReCoWI1FJjERpcibbfA0Ow/?wx_fmt=png V-type control valves: For control valves, imported V-type ball valves are often chosen; such valves consist of an actuator and a V-type valve body. The valve internals are made of special ceramics. There is a shearing action between the V-shaped open ball and the valve seat; when the medium contains fibers or solid particles, the ball does not get stuck, thus maintaining good sealing performance and adjustability. The flow characteristic of the V-type ball valve is equal percentage flow characteristic; the flow change is small at low opening degrees, which ensures precise control as well as stable operation of the valve. Special ceramics possess excellent wear resistance, and the flow paths are lined with such ceramics as well, which prevents direct erosion by fluids and extends the valve’s lifespan. Commonly used brands of imported ceramic ball valves at present include: cera system from Samson, Flowserve from the United States, Masonielian from GE in the United States, Metso from Finland, and SSZ from the United States. Corrosion and wear: Imported structural ceramics exhibit excellent chemical stability; they hardly react with strong acids and bases. Additionally, they possess high hardness, enabling them to resist the abrasive effects of media containing hard particulate matter. These excellent chemical and physical properties ensure that the imported ceramic ball valves can not only effectively resist severe corrosion, but also maintain their internal structure under harsh operating conditions, preventing any wear-related damage that could lead to valve malfunction, internal leakage, or external leakage. Short service life: The service life of imported ceramic valves can be 6 to 8 times that of metal valves, which reduces the number of shutdowns and valve replacements on site. This not only improves production efficiency but also reduces the frequency and cost of manual maintenance, thereby effectively extending the service life of the valves. The maintenance costs are high. Imported ceramic valves use imported structural ceramic materials for the valve’s sealing components and wear-prone parts, which enhances the durability, corrosion resistance, and sealing performance of these valves, **thereby extending their service life. The use of imported ceramic valves can **reduce the frequency of repairs and replacements on-site, enhance the safety and stability of the associated equipment and operating systems, and save on maintenance costs.