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Basic Principles and Considerations for Selecting Common Valves
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Basic Principles of Valve Selection
First, Clarify Valve Function and Identify Valve Application: When selecting common valves in chemical engineering design, it is important to fully understand their specific functions and accurately identify their intended use. This foundation requires clarifying the valve's key role, fully understanding basic parameters such as the valve's corresponding control method and operating pressure, and mastering its functional role. By comprehensively considering the specific circumstances, a rational selection of common valves can be made.
Second, Be Familiarize Yourself with Valve Characteristics: Valves vary significantly in type and type, and their functions and roles also differ. Valves of varying types exhibit distinct differences in characteristics, production, and application. Therefore, during chemical engineering design, designers must prioritize valve characteristics and fully understand valve characteristics and types to ensure the selected valve meets site requirements and provides safety assurance for chemical production.
Third, Determine Valve End Connections: Valve end connections include threaded, flanged, and welded connections, with threaded and flanged connections being the most commonly used. Threaded connections have certain standard requirements for the nominal diameter of the valve, requiring it to be DN ≤ 50mm. If the diameter significantly exceeds the standard, installation and sealing of the connection will inevitably become significantly more difficult. Flange connections are relatively simple to install and disassemble, and their application range is relatively wide, allowing them to connect pipes of varying diameters and pressures. Welded connections have more stringent standard requirements and offer greater connection reliability. However, welded connections are more difficult to disassemble and reinstall, which limits their applicability. Welded connections are suitable for applications requiring long-term reliable operation or under stringent conditions or high temperatures.
Fourth, valve material selection: When selecting a valve material, the working medium, particularly its physical and chemical properties, must be given special consideration. Furthermore, the cleanliness level of the medium must be fully understood. Relevant regulations should also be comprehensively referenced. Choosing the right valve material can fully guarantee valve quality, performance, and service life. Regarding valve body material selection, cast iron is preferred, followed by carbon steel and stainless steel in that order. Regarding sealing ring material selection, rubber is preferred, followed by copper, alloy steel, and F4 in that order.
Common Valve Selection
1. Gate Valve Selection
Gate valves are a common valve type in chemical production plants. They do not change the direction of liquid flow. When fully open, gate valves have a relatively low resistance coefficient and are suitable for a wide range of diameters, pressures, and temperatures. Compared to globe valves of the same diameter, they can save installation space, making them a more versatile choice. However, gate valves also have some disadvantages, such as their relatively high height and prolonged opening and closing times; the potential for erosion of the sealing surface during opening and closing; certain standards for media cleanliness; and the difficulty of using non-metallic, corrosion-resistant materials in manufacturing. When the gate valve is open, the media behind the gate disc can generate vortexes, causing erosion and vibration of the gate, and potentially damaging the valve seat sealing surface. Therefore, gate valves are not suitable for flow regulation. They are more suitable for effectively controlling the flow in fully open or closed positions. Gate valves are classified by the thread position of the valve stem into two types: rising and non-rising stem. Rising-stem gate valves are preferred in corrosive environments, while non-rising-stem gate valves can be used in low-pressure and non-corrosive environments. Based on the gate disc structure, gate valves can be divided into wedge-type and parallel-type gate valves. Wedge-type gate valves are further divided into single- and double-disc types. Parallel-type gate valves are primarily used in oil and gas transmission systems and are relatively less commonly used in chemical plants. Regarding gate valve applications, gate valves are suitable for media such as steam, oil and gas, or high-temperature oil products, or for frequent opening and closing. However, they are less suitable for media prone to coking. For high-temperature media prone to coking, single-disc wedge-type gate valves are suitable, while double-disc wedge-type gate valves are suitable for media such as steam and oil products.
2. Ball Valve Selection
Ball valves are suitable for low-temperature, high-temperature, and high-viscosity media. Most ball valves demonstrate good suitability for media containing suspended solids. Depending on the sealing material, ball valves can also be selected for granular and powdery media. Full-channel ball valves are less commonly used for flow regulation and more frequently for rapid opening and closing, enabling rapid emergency shutoff operations. Ball valves are suitable for chemical pipelines with strict sealing and wear requirements, high-pressure shutoff, rapid opening and closing, and constricted passages. Ball valves are also suitable for corrosive media, lightweight structures, and low-pressure shutoff. Ball valves also play an important role in low-temperature and cryogenic media. Cryogenic ball valves, equipped with appropriate bonnets, are required in piping systems and equipment handling cryogenic media. Floating ball valves have specific standard requirements for the valve seat material, which must be able to effectively withstand the loads generated by the ball and the working medium. When using large-diameter ball valves, due to the high operating force required, turbine drive is preferred for diameters ≥ 200mm. For larger diameters and pressure requirements, fixed-ball ball valves are recommended. Furthermore, in pipelines handling flammable or toxic media, ball valves must be equipped with fireproof and anti-static features to ensure safety.
3. Check Valve Selection
Check valves are often used for clean media. Check valves have certain requirements for the cleanliness of the media. For diameters ≤ 40mm, lift check valves are suitable. They must be properly designed and installed in horizontal pipelines. For diameters between 50 and 400mm, swing check valves are suitable. They can be properly designed and installed in both horizontal and vertical pipelines. For vertical pipelines, ensure that the media flows from bottom to top. For diameters ≥ 450mm, cushioning check valves are suitable. For diameters between 100 and 400mm, clamp-type check valves are also an option. Swing check valves can withstand high operating pressures, with a PN standard of up to 42 MPa. Due to the different materials used in their components, they can be used in a wide range of working media and temperatures, including steam, corrosive media, and oils, with temperatures between -196°C and 800°C.
4. Globe Valve Selection
During chemical engineering design, globe valves can be considered when high fluid resistance requirements are not met. Globe valves also have a relatively wide range of applications with high-temperature and high-pressure media. Globe valves can be used for steam-generating pipelines with a DN of ≤200 mm. In practice, globe valves can also be used for small valves such as sampling valves, needle valves, and instrument valves, as well as for low-vacuum systems and exhaust valves. Globe valves function to regulate and control pressure and flow, so there are no strict standards for regulation accuracy. Globe valves are preferred for smaller pipe diameters. For toxic media, bellows globe valves are required to ensure adequate sealing performance. Globe valves are not suitable for media with poor cleanliness, large particle deposits, or medium viscosity. During chemical engineering design, when strict sealing performance standards are required, a globe valve should be prioritized. The presence of a sealing ring ensures excellent sealing performance, simplifies gasket replacement, and requires only careful attention to the globe valve's installation, ensuring that the valve's direction matches the flow direction of the media.
5. Choosing a Throttle Valve
Based on a structural analysis, throttle valves and globe valves share significant similarities, differing in that throttle valves lack a throttling element. Considering the overall valve market, throttle valves, despite their relatively small size, offer significant advantages, such as safety and energy savings. However, they also have certain limitations, such as the need for improved control accuracy. Therefore, during chemical engineering design, throttle valves can be designed for media with low temperatures and high pressures. They can also be used for flow and pressure regulation. However, throttle valves are not suitable for media with poor cleanliness or high viscosity.
6. Butterfly Valve Selection
Butterfly valves offer relatively low fluid resistance, rapid opening and closing, and significant advantages in weight, structure, and size, effectively saving material costs. Butterfly valves are more suitable for shutoff and throttling applications. However, for large flow regulation, their usability is limited due to the influence of sealing materials. Butterfly valves are also suitable for liquid media containing suspended solids and for large-diameter pipelines. Continuous improvements in materials, design, and processes have also led to corresponding changes in their performance, expanding their application range. Due to their economical efficiency and flow regulation performance, butterfly valves are preferred where suitable. Designing butterfly valves is also beneficial for applications with low design pressures, large pipeline diameters, and rapid opening and closing. Butterfly valves are categorized as either soft or hard seal types, and the appropriate type should be selected based on the fluid temperature. It is important to note that soft seals significantly outperform hard seals. Among commonly used butterfly valves, large-diameter valves are the most common. They are primarily used for media such as crude oil, oil products, and water, with temperatures below 300°C and pressures below 1.0 MPa.
7. Plug Valve Selection
Plug valves are among the earliest valves designed and used. They offer excellent sealing performance, bidirectional sealing, effortless opening and closing, and a long service life. They are particularly useful in systems with hazardous materials, but their relatively high opening and closing torques result in a relatively high price. Plug valves prevent liquid accumulation within the cavity and contaminate materials in intermittent devices. In chemical pipeline design, plug valves can be used for shutoff and distribution operations, as well as to change the flow direction of media. Plug valves are also common in multi-channel designs. They can connect multiple flow paths, simplifying pipeline system design and avoiding the need for an excessive number of valves and fittings. Plug valves come in two types: non-lubricated and lubricated. Oil-sealed plug valves, driven by forced lubrication, form an oil film, which not only enhances sealing performance but also reduces effort during opening and closing, minimizing damage to the sealing surface. However, attention should be paid to material contamination and regular maintenance is required. Generally, non-lubricated plug valves are more commonly used. Plug valves are more suitable for operating environments with low media temperatures, high viscosities, and rapid opening and closing. They are generally not suitable for media such as steam or those with higher temperatures.
8. Diaphragm Valve Selection
Diaphragm valves have distinct structural characteristics, effectively controlling fluid movement through a rubber or plastic diaphragm. Diaphragm valves offer relatively low fluid resistance and enable bidirectional sealing, making them particularly suitable for low-pressure media, suspended viscous fluids, and corrosive slurries. The operating mechanism is securely separated from the media channel, and the elastic diaphragm effectively shuts off the fluid. Diaphragm valves are commonly used in the design of production systems for industries such as medicine, healthcare, and food. The temperature resistance of the diaphragm material determines the temperature environment in which the diaphragm valve can operate. Based on their structure, diaphragm valves can be divided into straight-through and weir types. Diaphragm valves are suitable for operating environments primarily involving suspended solids, oils, and acidic media, with temperatures not exceeding 200°C and pressures not exceeding 1.0 MPa. Diaphragm valves are generally not suitable for environments with media such as strong oxidizers and organic solvents.
Valve Selection Considerations
When selecting a valve, consider the following factors:
① Fluid Properties. Valves are designed to effectively control fluids. Fluid types vary, such as steam, slurries, gases, and liquids. Some fluids are less clean and may contain suspended solid particles and dust. Therefore, when selecting a valve, a thorough analysis of the fluid properties is necessary to ensure that the valve meets specific standards.
② Valve Function. When selecting a valve, consider its function and the specific operating conditions, such as pressure regulation, flow control, or shutoff, the need for rapid opening and closing, and unidirectional or bidirectional flow. In short, a thorough understanding of the valve's characteristics, application, and function is essential to provide a foundation for appropriate valve selection.
③ Valve Size. When selecting a valve, the standard valve size should be selected based on the type and size of the process piping.
④ Valve operating pressure and temperature. Different valve types have significantly different operating ranges and conditions. To address these varying pressure and temperature conditions, the correct valve type must be selected. Valve materials and pressure ratings should be clearly defined based on the process conditions to provide a reference for valve selection.
⑤ Valve pressure loss. Localized resistance loss is directly related to the valve. Due to structural differences, different valve types exhibit varying resistance levels. Therefore, the allowable standard resistance range must be carefully considered during valve selection.
⑥ Valve material. When selecting a valve, consider the temperature, pressure rating, and fluid characteristics. The valve body, seat, and disc are made of varying materials. Especially when conveying corrosive materials, it's crucial to make appropriate choices based on the material properties to ensure valve performance.