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Control valves are the terminal actuating elements in industrial process control systems. In automated control systems for continuous industrial production, control valves are used to regulate various process parameters in order to control aspects such as the pressure, temperature, flow rate, and level of fluids. They are often referred to as the “hands and feet” in automated industrial production. The quality of its application is directly reflected in the regulation quality of the system. As a terminal actuator in process control, its importance is now recognized more deeply than before. The effectiveness of control valve application depends not only on product quality and whether the user installs, uses, and maintains it properly, but also on accurate calculation and selection. There are many cases where system instability occurred due to errors in computational selection, with some systems even becoming unusable as a result. Therefore, users and system designers must fully recognize the importance of control valves in the field, and pay sufficient attention to their selection. The general principles for selecting control valves are: on the premise of meeting the required functional requirements, the selected valve should have a simple structure, reliable performance, low cost, a long service life, and be easy to maintain. Below, emphasis is placed on the selection of control valve types and the selection of accessories. 1 Selection of control valve types. There are many ways to classify control valves; a common classification method used both domestically and internationally is based on structure, principle, and function. In total, there are 9 major categories: straight-through single-seat control valves, straight-through double-seat control valves, sleeve control valves, angle control valves, three-way control valves, diaphragm valves, butterfly valves, ball valves, and eccentric rotary valves. These nine types represent the most basic and common valves, and they are often referred to as standard valves. Other valves that are developed by modifying these basic types to suit specific applications are known as special valves. 1.1 Characteristics of standard control valves and their proper selection 1.1.1 Straight-through single-seat control valve: A straight-through single-seat control valve has only one valve core and one valve seat, which makes it possible to achieve a tight seal. It can use a hard seal between metals, or a soft seal using metals in combination with polytetrafluoroethylene or other composite materials. The standard leakage rate is 0.01%C (where C is the rated flow coefficient). It allows for a low pressure difference and has a limited flow capacity; for example, a DN100 single-seat control valve has a permissible pressure difference of only 120 kPa, with a flow capacity of just 100. It features a complex flow path and a simple structure, making it suitable for applications involving clean media with strict leakage requirements and low operating pressure differences. However, smaller-sized control valves (DN1/2, 3/4, 20) can also be used in situations with higher pressure differences; they are among the most widely used control valves. With further design, they can function as shut-off valves as well. The shape of the valve core determines the flow characteristics; it loses its original properties after being eroded, and replacing the valve core can change these flow characteristics. However, the fluid medium exerts a large thrust on the valve core, meaning that the unbalanced force is high; as a result, an actuator with greater thrust is required. Therefore, such control valves are not suitable for applications with high pressure differences and large diameters. When selecting this valve, special attention should be paid to pressure difference verification to prevent it from being pushed open. 1.1.2 Direct-acting two-seat control valve: A direct-acting two-seat control valve has two spools and two seats. Since the upward force acting on the upper spool and the downward force acting on the lower spool are roughly balanced, the unbalanced force acting on the entire spool is small, allowing for a high pressure difference. For example, a DN100 two-seat control valve can tolerate a pressure difference of 280 kPa. It also has a high flow capacity; compared to other control valves of the same diameter, a two-seat control valve can handle more fluid. The flow capacity of a two-seat control valve of the same diameter is approximately 20% to 50% higher than that of a single-seat control valve. For example, the flow capacity of a DN100 two-seat control valve is 160. Therefore, to achieve the same flow capacity, a double-seat control valve can use an actuator with a lower thrust. The two-seat control valve features top and bottom dual guidance, which makes it easy to convert between the normal orientation and the reversed orientation of the valve. Simply by installing the valve core and seat in reverse order, it is possible to change a valve from normal orientation to reversed orientation, or vice versa, without the need to change the type of actuator – whether it is of direct or reverse action. The upper and lower valve cores of a two-seat control valve cannot both be kept closed simultaneously, resulting in a high leakage rate; the standard leakage rate is 0.1%C (where C is the rated flow coefficient) ； It has a complex flow path and is not suitable for applications with high pressure differences, as in such applications the valve is subjected to severe erosion by high-pressure fluids, and flashing and cavitation can occur, further exacerbating the erosion of the valve body. It is also not suitable for controlling media containing fibers or fluids with high viscosity. 1.1.3 Sleeve Control Valve: The sleeve control valve, also known as a cage valve, has as its internal components a valve stem and a valve cage (sleeve). This sleeve can be part of a straight-through single-seat control valve, or it can be used in double-seat control valves or angle control valves, among others. There are two types of configurations: single-seal and double-seal; the former is similar to a single-seat control valve and is suitable for applications that require such a valve design ； The latter is equivalent to a two-seat control valve and is suitable for applications that require two-seat control valves. In addition, it features good stability, easy loading and unloading, convenient maintenance, as well as the ability to reduce noise and the effects of cavitation. However, its price is 50% to 200% higher than that of single- and double-seat control valves, and it requires special wound sealing gaskets. It is widely used, second only to single- and double-seat control valves, but it should not be used for media that are dirty or prone to crystallization, clogging, or scaling. 1.1.4 Angular control valve: The angular control valve is a single-seat control valve with a special valve body structure, suitable for specific piping and fluid applications. It transforms the straight-through valve body into an angular one (equivalent to an elbow), while its throttling and stress mechanisms remain exactly the same as those of a single-seat control valve. It retains the advantages of low leakage and a small allowable pressure difference associated with single-seat control valves. In addition, due to its simple flow path and self-cleaning property, it can be used with dirty media. It can also be further improved into a clog-proof angle valve, suitable for applications involving media containing suspended particles, and is particularly appropriate in situations where installation space is limited. 1.1.5 Three-way control valve: The three-way control valve relies on the guidance provided by its valve core; when changing from air-open to air-close operation, it is necessary to replace the actuator. It should be noted that the meanings of air-open and air-close for this valve differ from those of other control valves, and it is essential to specify whether these modes refer to a horizontal or vertical position. It has three channels and can replace two direct-through single-seat control valves for diverting and combining two streams in applications where the temperature difference is ≤150°C; when DN≤80mm, the combination valve can be used for diversion purposes. 1.1.6 Diaphragm control valve: The diaphragm control valve consists of a corrosion-resistant diaphragm and a valve body lined with a corrosion-resistant material; it has a simple flow path and is suitable for two-position on/off operation in applications involving dirty media or mildly corrosive media. It is one of the earliest control valves. Due to its flow characteristics that are close to those of a quick-opening valve, its control quality is poor. It is also affected by the material of the diaphragm and lining, which prevents its use in high-temperature and high-pressure conditions; generally, the operating pressure is ≤1.6 MPa and the operating temperature is ≤150°C. Additionally, the diaphragm tends to break easily and has a short lifespan, so it is now used in few applications. 1.1.7 Butterfly Valve: A butterfly valve functions by using a section of pipe as its valve body, with a valve disc located in the center to regulate flow; it is the most common type of rotary control valve used for regulation purposes. It is suitable for low-pressure and medium-pressure applications, or in rare cases for high-static-pressure and high-flow applications, but there are limitations on the pressure difference. It is small in size and light in weight, being 4 to 10 times lighter than spherical control valves of the same diameter. It has a favorable diameter-to-price ratio, making it particularly suitable for large-diameter applications; moreover, the greater the diameter of the control valve, the more pronounced this advantage becomes. Generally, when DN>300mm, butterfly valves are usually used for this purpose. 1.1.8 Ball valves: Ball valves are a mature and established type of product; they come in “O”-type and “V”-type variants. They feature the simplest flow path, minimal flow resistance, lowest losses, and self-cleaning capabilities. "The O-ring ball valve is a type of flow-regulating valve with no resistance; compared to valves of the same specification, it has the highest rated flow coefficient, and is commonly used in applications involving large flow rates or dirty media ； "The V-shaped ball valve provides approximately logarithmic flow characteristics and a high control ratio; the shear force is generated as the V-shaped ball core rotates relative to the valve seat, making it particularly suitable for regulating and shutting off high-viscosity, suspended-flow, or dirty, fiber-containing media such as pulp. Ball valves are relatively expensive. 1.1.9 The eccentric rotary valve, also known as the cam-deflection valve, combines the advantages of ball valves and butterfly valves; it features a simple flow path, good \"self-cleaning\" properties, and excellent control capabilities, making it suitable for use with crystalline, viscous, or dirty media ； The valve body has a small size and light weight, and can be assembled flexibly at the installation site without the need to replace any components ； It features a high rated flow coefficient, which is 10% to 30% higher than that of single-seat and double-seat control valves of the same diameter; it also has a large adjustable ratio, reaching up to 100:1 ； The valve seat provides reliable sealing; the flexibility of the valve stem support arm, along with the eccentric rotational movement of the valve stem’s spherical surface, reduces the required operating torque and compensates for certain asymmetries, enabling stable operation under conditions of flow on, flow off, and high pressure differences ； During proportional control, a positioner is required; by adjusting the position of the convex plate in the positioner, it is possible to easily achieve a linear or equal-percentage flow characteristic. 1.2 The selection of special-type control valves is intended for special applications; based on the aforementioned control valves, extending the upper valve cover and adding radiators can be used in low-temperature and high-temperature environments ； An actuator with multiple springs can reduce the size and weight of the entire control valve ； To reduce noise, a low-noise control valve can be designed using a series of noise-reduction measures. In addition, there are also valve-body-separated control valves with a valve-body separated structure to facilitate maintenance and cleaning ； Fast-cutting control valve adopted to meet the rapid requirements of sequential actions ； Low-flow control valve designed for low-flow regulation requirements ； Bellows-sealed control valves and similar devices used to prevent leaks. These special types of control valves are specialized valves designed to meet the requirements of specific manufacturing processes or particular applications, and they are considered non-standard. They are characterized by complex operating conditions, high performance requirements, and small production batches. These control valves are usually developed and improved from standard types of products to meet specific application requirements. Therefore, first, its basic form should be determined based on the non-specific aspects, and then the corresponding modified forms and materials should be identified according to the specific characteristics. The accessories for selected control valves mainly include: valve positioners, valve position switches, pneumatic hold-down valves, pneumatic actuators, solenoid valves, air filters and pressure reducers, handwheel mechanisms, valve position transmitters, and converters, etc. Among them, valve positioners include electrical valve positioners and pneumatic valve positioners; they are primarily used to improve the operating characteristics of control valves, achieve accurate positioning, enhance the linearity of the valve’s position, reduce delays in the transmission of control signals, alter the flow characteristics of the control valves, change the range of response of the valves to signal pressure, and enable step control as well as accurate positioning. It is one of the most important accessories for control valves, and its quality has a direct impact on the performance and quality of the control valve as well as the control system. Below, we focus on several key factors to consider when selecting a valve positioner: 1) Whether the valve positioner can achieve a \"range division\" function, that is, it responds only to certain ranges of the input signal. If the valve positioner can perform this function, then one input signal can be used to control two or more control valves as required ； 2) Is it easy to adjust the zero point and range, is calibration independent, and what is its stability? ； 3) What is the accuracy of the valve positioner? Under ideal operating conditions, for a given input signal, the internal components of the control valve (including the valve spool, valve stem, valve seat, etc.) should always be positioned accurately in the desired position, regardless of the direction of movement or the amount of load exerted on these components ； 4) What is the operating speed of the valve positioner and what are its frequency characteristics? This is because the valve positioner can continuously compare the input signal with the valve position, and adjust its own output based on the difference between them. If the valve actuator responds quickly to such deviations, then the flow rate of the medium per unit time is high; as a result, the control system responds more rapidly to changes in the set point and load. This means that the system error is smaller, and the quality of control is better. Generally speaking, the higher the frequency characteristic, that is, the greater the sensitivity to the frequency response, the better the control performance. It should be noted that the evaluation of frequency characteristics should employ a combination of experimental and theoretical methods, rather than relying solely on theory. Moreover, the experimental methods used in such evaluations must be reliable and scientific, and valve positioners and actuators should be considered together ； 5) How does the positioning resolution change when a valve positioner is combined with a control valve? Positioning resolution has a significant impact on the control quality of the regulation system; higher resolution means that the positioning of the control valve is closer to the ideal value, which allows for effective suppression of fluctuations caused by excessive adjustment of the control valve, thereby ultimately achieving the goal of limiting the periodic variations in the regulated quantity. 6) Whether the maximum rated supply pressure of the valve positioner matches the rated operating pressure of the actuator, whether installation and connection are convenient, what the level of maintenance required is, and so on. Apart from the valve positioner, the other types of accessories are relatively simple, so there is no need to repeat them here. All attachments serve to provide supplementary functions and ensure the proper operation of the control valve. The principle to follow when selecting them is to include only those that are necessary and to discard any unnecessary ones; otherwise, it will only increase the operating costs of the control system and reduce its reliability. 3 Conclusion The proper selection of control valves is the first and most crucial step in their effective use. The quality of this selection directly affects the performance of the control valves, which in turn influences the regulation quality of the system. Of course, the selection process is quite complex; it is also a field of study that requires continuous exploration and summarization through practical application. Therefore, it is necessary to master certain methods and techniques in the selection of control valves, based on a thorough understanding of the relevant professional knowledge; only in this way can they truly fulfill their role as the \"hands and feet\" in the automation control of industrial processes.