【Original】Special Applications of Ceramic Valves in Liquid Chlorine Media

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Liquid chlorine is one of the main products of the chlor-alkali industry; it is widely used in various industrial processes and is classified as a hazardous chemical and a highly toxic substance. To facilitate use over long distances, chlorine gas filling has become an essential part of the production process; however, this process poses risks of production accidents such as leaks, explosions, and poisoning. The process of filling liquid chlorine is complex, and automatic shut-off valves and control valves are required to ensure the safety and reliability of the entire process. Due to the properties of the medium, high pipeline pressure, low temperatures, and negative pressure during the vacuuming phase, there are high requirements for the selection of valves. Ceramic ball valves perform well in such operating conditions. Liquid chlorine, whose chemical name is liquid chlorine, is a yellow-green liquid with a boiling point of -34.6 °C and a melting point of -103 °C. It vaporizes into a gas at normal pressure; inhalation can cause severe poisoning. It is highly irritating and corrosive, and can ignite and explode when mixed with other flammable gases in the presence of light. It can also react with most substances. The chlorine gas produced by electrolysis has a high temperature (85°C) and contains a large amount of water; after cooling and drying to remove the water, it is pressurized and cooled to be liquefied, changing from a gaseous state to a liquid state, which reduces its volume and makes it easier to store and transport. The packaging of liquid chlorine is a very important step in the chlorine production process. At present, the level of automation in this process is low, as it relies entirely on manual operation, which makes it difficult to control. Process flow for liquid chlorine filling: In recent years, the process of filling liquid chlorine has gradually evolved toward mechanization and automation. The filling methods that have been used include gas compression, vaporizer compression, submersible pump methods, and shielded pump methods. Gas pressurization method: The process of transporting and filling liquid chlorine using the gas pressurization method is simple and not complicated to operate; however, it places demands on the airtightness of the entire system. High standards are required for the quality of raw materials, and there is a high risk of errors being made by operators. The process requires the use of dry nitrogen or compressed air to pressurize the corresponding liquid chlorine tank to a certain pressure, usually between 0.8 and 1.0 MPa, and high pressure is then used to transfer and fill the liquid chlorine in that tank. The waste gas generated in the tank is evacuated for treatment with sodium hypochlorite. Liquid chlorine storage tanks have numerous accessories, including inlet and outlet pipelines, safety valves, pressure gauges, and related valves. The carburetor pressurization method operates on the principle of a compression ratio as high as 450 between chlorine gas and liquid chlorine; when the liquid chlorine vaporizes, it expands rapidly, resulting in an extremely high vapor pressure. The liquid chlorine in the storage tank is required to be fed into the liquefier at atmospheric pressure either by gravity flow or under pressure from high-pressure nitrogen, with the liquid level kept within specified upper and lower limits ； Hot water at a temperature range of 45–65°C (with a maximum of 80°C) is used for heating, thereby vaporizing the liquid chlorine in the vaporization tank and increasing its pressure; the pressure is kept within the range of 0.8–1.2 MPa. The vaporization pressure of the liquid chlorine is utilized to force the liquid chlorine out of the storage tank and into packaging. The residual pressure is sent to the sodium hypochlorite system for treatment. Diagram of filling by carburetor pressurization method. Submersible pump method: Filling by the submersible pump method involves adding an intermediate tank behind the liquid chlorine storage tank, with multi-stage vertical pumps installed on this intermediate tank. During packaging, liquid chlorine flows from the liquid chlorine storage tank to the intermediate liquid chlorine tank (when the pressure is insufficient, nitrogen or compressed air is used to maintain the pressure). Once the liquid level in the intermediate tank exceeds the specified minimum level and reaches a certain height, the corresponding process valves are opened, and the submersible pump is started to carry out the transfer and filling process. Process diagram for filling by submersible pump method; Shielded pump method. The shielded pump method also belongs to the mechanical power-based filling approach, but it offers a higher level of automation compared to the submersible pump method. It offers higher work efficiency, simpler operation, and easy maintenance. There is no nitrogen trichloride accumulation; the storage tank only needs to have its waste discharged regularly. Taking into account the net positive suction head of the canned pump, it should be installed at the bottom of the liquid chlorine tank, at a depth of at least half a meter greater than its net positive suction head. Since the shielded pump is installed directly below the liquid chlorine tank, during filling, the relevant valves are opened in accordance with the process requirements, the shielded pump is started, and filling and transfer operations are carried out directly. Process diagram for transporting and filling using shielded pump method; Current status of the use of ball valves in chlorine loading; The properties of chlorine require that the design of chlorine valves not only feature a simple structure, small size, light weight, reduced material consumption, and compact installation dimensions, but also require a low driving torque, easy and rapid operation, as well as good sealing capabilities and excellent corrosion resistance. In this way, the liquid chlorine valve can maximize the protection of health and the environment during the production, transportation, and use of chlor-alkali products, thereby ensuring safe production. Medium-related issue: When liquid chlorine is being filled and throttled at the valve, the pressure at the valve outlet is lower than that at the inlet; as a result, part of the liquid chlorine vaporizes due to the decrease in pressure. The vaporization process is endothermic, so the temperature at the valve is lower than that in the pipeline, which leads to frosting. Chlorine itself is not very corrosive, but liquid chlorine (which contains moisture) is highly corrosive. The moisture content control levels for liquid chlorine vary among different factories, resulting in varying degrees of corrosion of the valves used in each factory. Impurities within the pipes have a significant impact, including construction impurities from the initial phase of the project as well as those that are inevitable in the pipe processing during subsequent production. Valve issues: The opening and closing motion of the valve causes wear between the valve stem and the packing, resulting in a continuous decline in the sealing strength. The valve will leak when the sealing force is less than the medium pressure ； The opening and closing motion of the valve causes a change in pressure between the valve body and the valve cover; combined with factors such as shocks and resonance, this leads to loosening of gaskets and bolt assemblies, resulting in leaks. All these factors reduce the service life of ordinary liquid chlorine valves, requiring frequent replacements of these valves and leading to production line shutdowns, which is not conducive to the continuous operation of chemical enterprises. Material issues: Valves used in this application are often those with metal surfaces coated or lined with plastic. However, most metal valves have limited resistance to chlorine corrosion, especially when chlorine is present in aqueous form. This applies to various alloy steel valves as well. In such cases, although valves lined with PTFE represent a good option, it is soon found that as these valves are used for an extended period, the torque required to operate them increases, and the problem of PTFE aging becomes apparent.

Economic issues: In the production process of the chlor-alkali industry, the cost associated with valves constitutes a part of the total product cost. The funds required to maintain a stock of valves also account for a significant portion of a company’s working capital. Due to the harsh conditions in the liquid chlorine filling system, valves need to be replaced frequently, which is highly detrimental to the safe operation of the entire equipment as well as to cost control for maintenance. Moreover, the risk of potential emergency shutdowns is very high, resulting in significant losses. Ceramic ball valves as a solution for liquid chlorine filling: After decades of research, ceramics have seen significant development and progress. Today, there are two main categories of structural ceramics: high-strength structural ceramics such as silicon nitride systems, silicon carbide systems, and zirconia/alumina-toughened systems; as well as ceramic matrix composites. These materials are used in various traditional industries such as the machinery industry, automotive industry, chemical industry, paper industry, and textile industry, in applications such as heat engine components, cutting tools, and wear- and corrosion-resistant parts. They contribute to the upgrading of products, thereby enhancing economic and social benefits. By taking advantage of the excellent properties of ceramic materials and combining them with the structural features of ball valves, imported ceramic ball valves exhibit outstanding advantages in conditions of high corrosion and severe wear. Properties of ceramic materials: With the advancement of science and technology, particularly in the fields of energy and space technology, materials are required to be used in relatively 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 materials and steel materials. Comparison of the basic properties of ceramic materials and steel materials --- Corrosion resistance. Ceramic materials possess excellent heat resistance as well as resistance to acid and alkali corrosion; they rarely undergo chemical reactions in such environments, and their shape changes little. Even under high-temperature conditions, the properties of ceramic materials remain fully intact. The corrosion of ceramic materials, in a broad sense, refers to the corrosion that occurs as a result of chemical reactions under harsh conditions such as acids, alkalis, high-temperature gases, and molten salts. This type of corrosion is characterized by a chemical reaction that begins at the grain boundaries, with little to no change in the appearance or surface dimensions. For a long time, ceramic materials have been evaluated based on their electrical, magnetic, thermal, optical, and mechanical properties, while there has been little research on their corrosion characteristics, as well as the changes or even degradation in their properties caused by chemical reactions under harsh environmental conditions. For example, ceramics such as silicon nitride or silicon carbide possess excellent high-temperature properties: they have high strength and hardness, low creep, a small thermal expansion coefficient, and good thermal stability. They are recognized as good high-temperature structural materials, and can therefore be used in corrosive or high-temperature environments where metal materials cannot withstand such conditions. ---Fracture toughness: Ceramics undergo brittle fracture, so they are highly sensitive to cracks. Based on this property of ceramics, fracture mechanics properties are important indicators for evaluating the mechanical performance of ceramic materials. The most commonly used fracture mechanics parameter for evaluating 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. Structural features of ceramic ball valves: Ball valves are a common type of valve used in liquid chlorine pipelines. When in use, ball valves with conventional designs can retain liquid chlorine when closed, posing safety risks. Therefore, when using ball valves in liquid pipelines, valve balls with special structures and materials should be selected. Therefore, greater care must be taken in the selection of valves and their materials. For cut-off valves, the flow channel of such valves and the ball core are typically equipped with O-rings, which are 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. Ceramic ball valves feature O-ring ball cores and valve seat sealing surfaces with a roughness of less than 0.1μm; the sealing performance between the ceramic ball core and the valve seat is superior to that of metal ball valves. In addition, ceramic ball cores and valve seats have self-grinding properties, resulting in low opening and closing torque, as well as excellent corrosion resistance. The ceramic-lined flow channel can be completely isolated from the metal parts of the valve body, making it suitable for applications involving media with high corrosivity or stringent purity requirements. For control valves, imported V-ball valves are commonly used. Among them, the electric V-ceramic control ball valve consists of an electric actuator and a V-ball valve, with the valve core made of high-quality ceramic. There is a shearing action between the V-shaped open sphere and the valve seat; when the medium contains fibers or solid particles, the sphere does not get stuck, thus maintaining good sealing performance. The valve core is made of high-quality ceramic, offering excellent wear resistance. The valve seat protection ring prevents the fluid from directly scouring the valve seat, thereby extending its lifespan. Commonly used brands of imported ceramic ball valves at present include: cerasystem under Samson, Flowserve from the United States, Masonielian under GE in the United States, Metso from Finland, and SSZ from the United States. Basic characteristics of control valves: 1. Regulation performance – It features equal percentage flow regulation, with a stable and consistent CV value. 2. Ball and orifice design – Different V-shaped balls are used, and various angles for the V-shaped openings allow for different regulation capabilities. 3. Optimized CV performance – Low fluid resistance and a high CV value; the ceramic flow channels prevent sticking during operation. 4. During opening and closing, strong shear forces are generated between the ball and the valve seat, which helps to remove fiber particles and other impurities present in the fluid. 5. The ceramic components enable complete isolation of the entire flow channel, thereby preventing contact between the fluid and the metal valve body, and this effectively protects the ceramic valve from corrosion by corrosive fluids. For the filling of liquid chlorine, ceramic valves offer unique advantages: Liquid chlorine is corrosive to valves, but structural ceramics possess good chemical stability; they hardly react with strong acids or bases. Moreover, they have high hardness, enabling them to resist wear from high-hardness particulate media. These excellent chemical and physical properties ensure that ceramic ball valves can not only effectively resist severe corrosion, but also maintain their internal structure under harsh operating conditions, preventing damage due to wear from causing the valve to fail to function properly and leading to internal or external leaks. Short service life: Ceramic valves can have a service life 6 to 8 times longer than that of metal valves, which reduces the number of downtime incidents and valve replacements. This not only improves production efficiency but also decreases the frequency and cost of manual maintenance, thereby effectively extending the service life of the valves. High maintenance costs: Ceramic valves use structural ceramic materials for their sealing and wear-prone components, which enhances the wear resistance, corrosion resistance, and sealing performance of these valves, **thereby extending their service life. The use of ceramic valves can **reduce the frequency of on-site repairs and replacements, enhance the safety and stability of the associated equipment and systems, and save on maintenance costs.