In the diverse realms of modern industry, from the extraction and transportation of oil and natural gas, to the precise regulation in chemical production, and the accurate dosing in food and pharmaceutical industries, control valves play an indispensable role. At the core of these valves lies the trim, which is the key to precise fluid control. The trim is not only the part of the control valve that comes into direct contact with the fluid, but also the critical component that enables the regulation function, ensures sealing performance, and safeguards production safety. This article will delve into the structure, functionality, design considerations, and various types of valve cores of control valve trim, helping readers gain a comprehensive understanding of this extremely important component in industrial automation and how it plays a crucial role in complex industrial environments.
Control valve trim refers to the detachable internal components that come into direct contact with the fluid. Their primary functions are to alter the flow cross-sectional area and to provide guidance for the throttling element. These components include the valve core, seat, cage, stem, noise reducer, anti-cavitation and erosion components, guide, seal, and fasteners. Together, they ensure that the control valve can precisely regulate the flow rate and pressure of the fluid.
Flow Cross-sectional Area Regulation One of the core functions of control valve trim is to change the flow cross-sectional area in a certain proportional manner. This change directly affects the flow rate and pressure of the fluid, thereby establishing the relationship between the flow capacity and the valve core/stem travel. For instance, in chemical production, by adjusting the trim, the precise amount of fluid required for a chemical reaction can be controlled, ensuring the stability and safety of the production process.
Tight Closure and Sealing Another vital function is to ensure that the control valve can close tightly, meeting the specified leakage rate standards. In many industrial applications, such as in the oil, natural gas, and chemical industries, control valves need to maintain good sealing performance under high pressure and high-temperature conditions to prevent fluid leakage, avoid environmental pollution, and prevent safety accidents.
Valve Core (Plug): The valve core is the most critical component of the trim and the movable part of the control valve. It works in conjunction with the seat to regulate the fluid flow by changing the throttling cross-sectional area, thereby achieving process control.
Seat: The seat is the key component that works with the valve core, responsible for sealing and guiding the fluid flow. The material and design of the seat directly affect the sealing performance and reliability of the control valve.
Cage: The cage is used to guide the movement of the valve core and provides additional throttling functions. It can also reduce noise and cavitation.
Stem: The stem connects the actuator and the valve core, transmitting the force and movement from the actuator to ensure the precise movement of the valve core.
Noise Reducer (Flow Divider): The noise reducer is used to reduce the noise generated when the fluid passes through the control valve, enhancing the operational stability of the control valve.
Anti-Cavitation and Erosion Components (AC-trim): These components are designed to prevent cavitation and erosion of the fluid under high pressure differences, extending the service life of the control valve.
Guide: The guide ensures that the valve core maintains the correct direction during movement, reducing wear and leakage.
Seal: The seal prevents fluid leakage, ensuring the sealing performance of the control valve.
Fastener: The fastener is used to secure the trim within the valve body, ensuring its stability.
The valve core is the most critical component of the control valve trim, and its design and material directly affect the performance and reliability of the control valve.
The shape of the valve core determines the flow characteristics of the control valve. Common flow characteristics include linear, equal percentage, quick-opening, and parabolic characteristics.
Linear Flow Characteristic The linear flow characteristic refers to a linear relationship between the relative flow rate of the control valve and the relative displacement (travel). In other words, the change in flow rate per unit displacement is constant. This characteristic results in a relatively large flow change at small openings, offering high sensitivity but making it difficult to control and potentially causing oscillations. Conversely, at larger openings, the relative flow change is small, leading to slow and less responsive regulation. The linear flow characteristic is suitable for applications requiring precise flow control, such as in laboratories and fine chemical production.
Equal Percentage Flow Characteristic The equal percentage flow characteristic means that the relative change in flow rate caused by a unit relative travel change of the control valve is proportional to the relative flow rate at that point. The curve slope increases with increasing travel, also known as the logarithmic flow characteristic. This characteristic results in small flow changes at small openings, providing smooth regulation, while at larger openings, the flow change is significant, offering sensitive and effective control. The equal percentage flow characteristic is suitable for applications requiring flow regulation over a wide range, such as in oil and natural gas transportation.
Parabolic Flow Characteristic The parabolic flow characteristic indicates that the relative change in flow rate caused by a unit relative travel change is proportional to the square root of the relative flow rate at that point. This characteristic is primarily designed to compensate for the poor regulation effect of the linear flow characteristic at small openings. The parabolic flow characteristic is suitable for applications requiring fine-tuning at small openings, although most manufacturers do not offer it as a standard product.
Quick-Opening Flow Characteristic The quick-opening flow characteristic allows for a significant flow rate at small openings, with the flow rate quickly reaching its maximum as the opening increases. Further increases in opening result in minimal flow changes. This characteristic is commonly used in on-off control valves, which can achieve a large flow rate at small openings. The quick-opening flow characteristic is suitable for applications requiring rapid response, such as emergency shut-off valves.
There are various designs for valve core structures, generally divided into two main categories: linear travel and rotary travel.
Single-Seat Control Valve (Globe Valve): The single-seat control valve is typically a top-guided linear travel control valve. The most commonly used valve cores are the plunger-type, cage-type, and needle or cylindrical slot-type valve cores for low-flow applications. Additionally, there are multi-stage valve cores and specially designed valve cores for anti-cavitation and erosion.
Plunger-Type Valve Core: The plunger-type valve core was the primary form of early control valves, featuring a simple structure and good sealing performance. However, its drawbacks include large unbalanced forces, low flow capacity, and high noise levels.
Cage-Type Valve Core: The cage-type valve core was designed to overcome the shortcomings of the plunger-type valve core. It offers advantages such as smaller unbalanced forces, higher flow capacity, and lower noise levels. However, the cage-type valve core has relatively poor tight shut-off and leakage ratings, and its complex structure with numerous components requires more maintenance and inspection.
Rotary travel valve cores change the flow area between the core and the seat through rotational movement. These valve cores are suitable for applications requiring high flow rates and rapid response, such as butterfly and ball valves.
The design and material selection of control valve trim directly affect its performance and reliability. The design of the valve core, seat, and trim reflects the technical capabilities of the control valve manufacturer.
The choice of materials for the valve core and seat, as well as their processing techniques, determine the application and reliability of the control valve. Common materials include stainless steel, hard alloys, ceramics, etc. Stainless steel offers good corrosion resistance and mechanical properties, making it suitable for a variety of conditions. Hard alloys provide higher hardness and wear resistance, ideal for high-pressure and high-wear applications. Ceramic materials are known for their extremely high hardness and corrosion resistance, suitable for extreme conditions.
Unbalanced Valve Core: The unbalanced valve core is a traditional design characterized by a significant pressure difference across the valve core, resulting in large unbalanced forces. This design is suitable for applications with relatively clean media.
Balanced Valve Core: The balanced valve core features balance holes that direct fluid pressure to a balance chamber on the opposite side of the valve core, thereby balancing most of the pressure difference. This design significantly reduces unbalanced forces and is suitable for applications with complex media or those containing solid particles. However, the balance holes are relatively small and prone to clogging, making them unsuitable for dirty media.
The design of the cage-type valve core aims to overcome the shortcomings of the plunger-type valve core. It offers advantages such as smaller unbalanced forces, higher flow capacity, and lower noise levels. However, the cage-type valve core has relatively poor tight shut-off and leakage ratings, and its complex structure with numerous components requires more maintenance and inspection. For example, Fisher's ED series cage valve with a balanced valve core has five dedicated gaskets, and both the anti-squeeze seals and spring-loaded seal rings are considered wear parts and inspection replacement items. The high replacement frequency and the high cost of dedicated gasket spare parts increase maintenance costs. The quick-disconnect design of the cage valve is a response to the durability and frequent maintenance issues of the cage valve trim.
The drilled cage valve core is specifically designed for high-pressure drop applications, effectively eliminating common issues such as cavitation, noise, corrosion, and vibration. The flow characteristic is achieved through the distribution of the drilled holes, with the minimum controllable Cv determined by the gap between the valve plug and the cage, as well as the distance from the sealing surface to the lowest side hole of the cage.
The maintenance and care of control valve trim are crucial for ensuring its long-term stable operation. Regular inspection and replacement of wear-prone components, such as seals and gaskets, can effectively extend the service life of the control valve. Additionally, correct operation and maintenance methods can reduce the likelihood of failures.
Inspect Seals and Gaskets: Regularly check the wear of seals and gaskets and replace any damaged parts promptly. Seals and gaskets are key components of the control valve, and their wear can lead to leakage and performance degradation.
Inspect Valve Core and Seat Wear: Regularly check the wear of the valve core and seat, and repair or replace them as necessary. Wear on the valve core and seat can affect the sealing performance and flow characteristics of the control valve.
Inspect Cage and Stem Wear: Regularly check the wear of the cage and stem to ensure smooth movement. Wear on the cage and stem can cause the control valve to move sluggishly, affecting its regulatory performance.
Avoid Overloading: Prevent the control valve from operating beyond its designed range, which can cause excessive wear and damage to the components.
Regular Lubrication: Regularly lubricate moving parts such as the stem to reduce wear and prevent sticking.
Prevent Media Contamination: Ensure the cleanliness of the fluid medium to prevent solid particles and impurities from entering the control valve, reducing the risk of wear and blockage.
Through the comprehensive exploration of control valve trim presented above, it is evident that these seemingly small components actually embody complex and ingenious design wisdom. They play an irreplaceable role in industrial production. From precisely regulating fluid flow to ensuring sealing performance and adapting to various complex conditions, the design and material selection of the trim directly determine the performance and reliability of the control valve.
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