Cage-Type Control Valve vs. Double-Seat Control Valve

Control valves, as the core components of fluid control systems, play an indispensable role in precisely controlling parameters such as the flow rate, pressure, and level of fluids, vital for ensuring production efficiency, product quality, and the safe operation of systems. Cage-type control valves and double-seat control valves, as important members of the control valve family, have gained widespread application in many industrial settings due to their unique structural characteristics and performance advantages. In this guide, we will talk about the differences of them in terms of the structure, performance, application scenarios.

Structure and Performance Features of Cage-Type Control Valves

Let's first analyze the structure and performance features of cage-type control valves. This section will tell you how the unique design of cage-type control valves enables them to stand out in specific application scenarios.

1. Streamlined Valve Body Passage

The valve body passage of a cage-type control valve is streamlined, which not only enhances the fluid's flow capacity and reduces resistance losses but also improves the overall operating efficiency of the fluid system. Its internal components can be replaced online. During maintenance, there is no need to disassemble the entire valve; simply remove the upper valve cover to take out and replace the internal components. This greatly simplifies the maintenance process, shortens maintenance time, reduces maintenance costs, and enhances the availability and reliability of the equipment. It is particularly suitable for industrial production lines that require long-term continuous operation.

2. Unique Sealing and Balancing Structure

A flexible metal gasket made of asbestos or polytetrafluoroethylene is placed between the valve seat ring and the valve body of a cage-type control valve, which not only ensures sealing and prevents leakage but also enhances the structural stability and sealing performance. Sometimes the cage and the valve seat ring are designed as an integrated unit, with a cylindrical valve core. The flow characteristic is determined by the shape of the cage window, and the window can be adjusted to meet different process requirements. The valve core can also be designed as a hollow plunger. Fluid pressure passes through the valve core, and a sliding sealing washer seals the outlet. The balancing structure reduces thrust but may increase leakage. The appropriate valve type should be selected according to the process requirements.

3. Diverse Flow Directions and Noise Reduction Design

There are two types of fluid flow directions in cage-type control valves: from the center to the outside and from the outside to the center. Fluid usually flows in from the lower part of the cage and out through the openings in the valve cage. The number of throttling openings symmetrically distributed on the cage is generally 3, 4, or 6, and their shape is closely related to the flow characteristic. Replacing the cage can change the flow characteristic to achieve precise control. In applications where noise reduction is required, small holes are drilled in the cage and the valve core to increase resistance, converting velocity head into kinetic energy, which can reduce noise by more than 10 decibels. In addition, a multi-stage pressure reduction method is used to distribute the total pressure drop across stages, preventing flashing and cavitation, reducing noise, extending valve life, and improving system stability.

4. Cavitation Resistance and Sealing Characteristics

The bottom of the valve core of a cage-type control valve is designed as a flat structure, which has significant advantages in the face of cavitation. When cavitation occurs, the impact generated by the rupture of bubbles does not directly act on the valve core but is absorbed by the medium itself. Therefore, cage-type control valves are less affected by cavitation, have a relatively longer service life, and can better adapt to working conditions that are prone to cavitation, providing more reliable assurance for industrial production. However, since the cage and the valve core are sealed with a graphite piston ring, the wear of the sealing ring during long-term operation may cause the leakage of the cage-type control valve to gradually increase, making its leakage slightly larger than that of a single-seat valve. Therefore, when selecting a cage-type control valve, it is necessary to fully consider the requirements of the process flow for sealing performance, as well as the maintenance and replacement cycle of the valve in actual operation, to ensure the sealing reliability and operational stability of the entire system.

5. Balanced and Unbalanced Valves

Cage-type control valves are divided into balanced and unbalanced valves. The balanced valve has a balance hole on the valve core, which can reduce unbalanced force and act as a damper to reduce vibration and instability. It is suitable for complex working conditions with large pressure differences and low noise. The unbalanced valve has no balance hole. The inner surface of the valve seat ring can cooperate with the disc-type valve core to form an equal percentage flow characteristic. The failure mode can be changed by rotating 180 degrees, adapting to different process requirements. In addition, the unbalanced valve is only guided by the valve stem, which is suitable for slurry media, can reduce wear and clogging, and improve reliability and service life.

Structure and Performance Features of Double-Seat Control Valves

After exploring the structure and performance features of cage-type control valves, we will now turn to double-seat control valves. Double-seat control valves also play an important role in industrial fluid control systems with their unique structural design and performance advantages.

1. Dual-Guide Structure and Multi-Spring Actuator

Double-seat control valves feature a dual-guide structure equipped with a multi-spring actuator, offering advantages such as compact structure, light weight, and sensitive operation. The multi-spring actuator can provide a large output force within a small space, ensuring that the valve can quickly and accurately respond to control signals, achieving precise control of the medium's flow rate, pressure, and level. Therefore, double-seat control valves have significant advantages such as compact structure, light weight, sensitive operation, large valve capacity, precise flow characteristic, and convenient disassembly and assembly. These features have led to their widespread application in many industrial settings, especially in processes that require precise control of parameters such as pressure, flow rate, temperature, and level of gases, liquids, steam, and other media, and keeping these parameters stable around the set values.

2. Dual-Valve Core and Dual-Valve Seat Design

The structural characteristic of double-seat control valves is that they have two valve cores and two valve seats. Fluid flows in from the left side, passes through the two valve cores and valve seats, and then converges and flows out on the right side. This dual-valve core and dual-valve seat design allows the upward thrust on the upper valve core and the downward thrust on the lower valve core to be basically balanced, resulting in a smaller unbalanced force on the entire valve core. This characteristic brings many advantages, such as a larger allowable pressure drop. For example, for a DN100 double-seat control valve, the allowable pressure difference can reach 280 kPa. Compared with other control valves of the same caliber, double-seat control valves can pass a larger amount of fluid, and their flow coefficient is relatively higher. The flow coefficient of a double-seat valve of the same caliber is about 20% to 50% higher than that of a single-seat valve. For example, the flow coefficient of a DN100 double-seat valve can reach 160. Therefore, when the same flow coefficient is required, a double-seat valve can use an actuator with a smaller thrust, thereby reducing the system's cost and complexity and improving the economy and reliability of the entire fluid control system.

3. Flexible Installation and Application Limitations

Double-seat control valves feature a top-bottom dual-guiding structure, which allows for flexible installation. The valve body can be interchanged without altering the actuator type, thus accommodating various process requirements. However, there are limitations to this design. It is challenging to achieve simultaneous and complete closure of both the upper and lower valve cores, resulting in a relatively high leakage rate. Moreover, when the materials have different coefficients of linear expansion, the leakage rate can further increase. Although special structures can achieve lower leakage levels, leakage remains a critical issue. Additionally, the complex internal flow path of the valve makes it susceptible to erosion under high-pressure differential conditions, leading to flashing and cavitation. These phenomena can cause damage and failure, making the double-seat control valve unsuitable for applications involving high-pressure differentials, fibrous media, and highly viscous fluids, as they are prone to clogging or wear.

4. Flow Characteristic and Control Precision

The flow characteristic of a control valve is determined by the shape of the valve core. For double-seat control valves, the flow characteristic curve is typically gentle. This means that changes in the valve opening result in relatively stable and uniform flow variations. This characteristic enables double-seat control valves to achieve high control precision during the regulation process, making them well-suited for processes that require precise control of parameters such as flow rate, pressure, and level. By appropriately selecting the shape and size of the valve core and optimizing the structural design of the valve, double-seat control valves can achieve ideal flow characteristics under various operating conditions, meeting complex industrial control requirements and providing robust support for the automation of industrial production processes.

Comparison of Applications for Cage-Type and Double-Seat Control Valves

Next, we will compare the application scenarios of these two types of control valves. By analyzing their performance in different industrial environments, we can gain a clearer understanding of their respective strengths and limitations, providing a reference for selection in practical applications.

1. Applicable Occasions for Cage-Type Control Valves

Cage-type control valves have significant advantages in industrial applications due to their unique structure. Their online replaceable internal components make them suitable for environments with corrosive or highly abrasive media, such as large chemical plants, reducing downtime and improving efficiency. The balanced structure and noise reduction design enable them to perform well in high-pressure differential and low-noise scenarios, such as high-pressure steam systems in oil refining, reducing noise pollution and improving the working environment. Their cavitation resistance ensures a longer service life in cavitation-prone conditions, such as water treatment or chemical liquid transfer systems, guaranteeing operational and control accuracy while reducing maintenance costs.

2. Applicable Occasions for Double-Seat Control Valves

Double-seat control valves are widely used in applications requiring precise control and high flow capacity due to their compact structure, large flow coefficient, and sensitive operation. For example, in the steam systems of large power plants, they are used to control steam flow and pressure to meet the varying load requirements of power generation units, achieving efficient steam transfer and energy conversion, and improving power generation efficiency. In chemical production, double-seat control valves can quickly respond to control signals and precisely regulate liquid flow and pressure, ensuring stable reaction processes and product quality. However, their higher leakage rate makes them unsuitable for high-pressure differential and complex media applications. A comprehensive assessment of process requirements and operating conditions is necessary to ensure that they meet control and operational stability needs.

Conclusion

Cage-type control valves and double-seat control valves, as important equipment in the field of industrial fluid control, each have unique structural characteristics, performance advantages, and applicable scenarios. Cage-type control valves, with their streamlined valve body passage, online replaceable internal components, balanced structure, noise reduction design, and good cavitation resistance, perform well in high-pressure differential, low-noise, frequently maintained, and cavitation-prone conditions, providing reliable fluid control solutions for industrial production. Double-seat control valves, on the other hand, are widely used in applications requiring precise control, high flow capacity, and high control precision due to their dual-guide structure, multi-spring actuator, dual-valve core and dual-valve seat design, compact structure, large flow coefficient, and sensitive operation. However, double-seat control valves also have limitations, such as higher leakage rates and unsuitability for high-pressure differential and complex media applications.