The flow characteristics are described in detail, and the method for selecting control valves along with the issues that need attention are provided

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Control valves are commonly used actuators in automatic control systems; they are used to regulate the flow rate of the controlled object. Choosing the control valve correctly is the guarantee of the control quality of the system. A detailed description is provided on the composition and classification of control valves as well as their flow characteristics, and the methods for selecting control valves along with the issues that need attention are outlined. Preface  0 Control valves are commonly used actuators in automatic control systems; they serve as the primary control elements in such systems, directly regulating the flow rate of the medium being controlled. A control valve consists of an actuator and a control mechanism; it receives control signals from a regulator or a computer in order to adjust the flow rate of the medium being controlled, thereby keeping the desired parameter within the specified range and achieving automation of process control. In the field of automatic control, whether the control process is stable depends directly on whether the control valve can operate accurately, thereby enabling the process control to manifest as precise variations in material energy and flow rate. Therefore, different control valves should be selected according to various requirements. Selecting the appropriate control valve is a key issue in pipeline design, and it is also crucial for ensuring the safe and stable operation of the control system. Type selection  1  Control valves generally consist of an actuator and a valve. The control valve is the regulating mechanism of a control valve; it adjusts the flow rate by changing the degree of opening of the valve in accordance with the requirements of the control signal, and it is a throttling element whose local resistance can be varied. A control valve is mainly composed of upper and lower valve covers, a valve body, a valve core, a valve seat, packing, and a pressing plate. In automatic control systems, the main media used for regulating valves are water, steam, etc. In situations with relatively low pressure and single-purpose use, common control valves include straight-through control valves, three-way control valves, and butterfly valves. Actuators can be classified into three types based on the type of energy used: pneumatic, electric, and hydraulic. Pneumatic actuation uses compressed air as a power source, electric actuation uses electricity as a power source, while hydraulic actuation utilizes the pressure of a liquid medium such as oil. Among them, pneumatic actuators have advantages such as simple structure, reliable operation, stable performance, low cost, easy maintenance, and fire/explosion resistance, and they are widely used in many control systems. Although electric actuators are not conducive to fire and explosion prevention, their power supply is readily available, they offer fast signal transmission which facilitates long-distance communication, they are compact in size, operate reliably, are easy to maintain, and are inexpensive. Hydraulic actuators offer the highest thrust, along with high precision in adjustment, fast and smooth operation, but they are large in size and require complex manufacturing processes. 　　(1) Selection of the control valve type The choice of the valve body type for a control valve is the most important aspect in selecting such a valve. Before selecting a valve, it is necessary to conduct a thorough analysis of the medium involved in the control process, as well as the process conditions and parameters. It is important to understand the requirements of the system regarding control valves, and to determine the type of valve to be used based on the data collected. When making a specific choice, the following factors can be taken into consideration: it is primarily based on factors such as the selected flow characteristics and unbalanced forces. ① Valve core shape and structure: When the fluid medium is a suspension containing high concentrations of abrasive particles, the mating surfaces of the valve core and valve seat experience severe friction every time they close. Therefore, the flow path of the valve must be smooth, and the internal materials of the valve need to be hard. ② Wear resistance: Since the medium is corrosive, valves with a simple structure should be chosen as much as possible, provided that their regulatory functions are still met. ③ Corrosion resistance: When the temperature and pressure of the medium are high and subject to significant fluctuations, valves whose valve core and seat materials are less affected by such changes should be used. ④ Temperature and pressure of the medium: Flashing and cavitation occur only in liquid media. In the actual production process, flashing and cavitation not only affect the calculation of the flow coefficient but also cause vibrations and noise, thereby reducing the service life of the valves; therefore, when selecting valves, it is necessary to prevent flashing and cavitation from occurring.  　　⑤ Preventing flashing and cavitation (2) Selection of the control valve actuator Regardless of the type, the output force of an actuator is used to overcome the loads acting on it (primarily referring to forces such as unbalanced forces and unbalanced torques, friction, sealing forces, and gravity). Therefore, for the control valve to function properly, the actuator used must be capable of generating sufficient output force to overcome various resistances, ensuring a tight seal and the proper opening of the valve.  　　① After determining the output force of the actuator, consider that force and select the appropriate actuator based on the requirements of the operating environment. For example, when explosion protection is required at the site, pneumatic actuators should be used, and the junction box must be of explosion-proof type; electric actuators cannot be chosen. If there are no explosion-proof requirements, both pneumatic and electric actuators can be used; however, from an energy-saving perspective, electric actuators should be preferred. For hydraulic actuators, although they are not as widely used as pneumatic and electric actuators, they feature high control precision, fast response speeds, and smooth operation; therefore, in special situations, they are also employed to achieve better control results. Finally, the reliability and cost-effectiveness of the actuator also need to be considered; it is advisable to choose an actuator that operates smoothly, is lightweight, has an attractive appearance, a simple structure, and is easy to maintain.  　　② Analysis of flow characteristics considering the environmental requirements of the actuator 2 2.1 Working principle: In the formula, p1 represents the pressure before the control valve. According to fluid mechanics, a control valve is a throttling element whose local resistance can be varied. For incompressible fluids, the flow rate of the control valve: p2 – pressure behind the control valve; A – throttling cross-sectional area; ξ – resistance coefficient of the control valve; ρ – fluid density. 　　As can be seen from equation (1), when A is constant and Δp = p1 – p2 remains constant as well, the flow rate Q through the valve varies with the resistance coefficient ξ; that is, the greater the resistance coefficient ξ, the smaller the flow rate. The drag coefficient ξ, on the other hand, is related to the structure of the valve and its opening degree. Thus, the output signal of the regulator controls the opening or closing of the valve, allowing the resistance coefficient of the valve to be changed, and consequently the flow rate of the medium being regulated to be altered. 　　2.2 Flow Characteristic Analysis The flow characteristic of a control valve refers to the relationship between the relative flow rate of the medium flowing through the valve and the relative opening degree of the valve. 　　The flow characteristics of control valves include ideal flow characteristics and operating flow characteristics. The ideal flow characteristic refers to the flow behavior when the pressure difference before and after the control valve remains constant. There are four types of such characteristics: linear, equal percentage, parabolic, and quick-open, as shown in Table 1. In actual systems, the pressure drop across the valve is not constant. There are mainly two reasons for this variation: ① Due to the characteristics of the pump, the system pressure generated by the pump increases when the system flow rate decreases; ② When the flow rate decreases, the resistance in the coiled tube also decreases, resulting in a higher pump pressure being applied to the valve. Therefore, the pressure difference before and after the control valve is usually variable. In this case, the relationship between the relative flow rate and the relative opening degree of the control valve is referred to as the operating flow characteristic. Specifically, it can be divided into the operating flow characteristics when pipes are connected in series and those when they are connected in parallel. 4 ideal flow characteristics of control valves Table 1 Flow characteristic properties and features Linear: The relative flow rate of a control valve is directly proportional to its relative opening; in other words, the change in relative flow rate resulting from a unit change in relative stroke is a constant. (1) At low opening degrees, the flow rate changes significantly, while at high opening degrees the flow rate changes less; (2) At low loads, the control performance is too sensitive, resulting in oscillations, whereas at high loads the control is slow and inadequate; (3) It has poor adaptability. 　　In a equal-percentage characteristic, the relative change in flow rate caused by a change in the relative stroke is proportional to the relative flow rate at that point. (1) The percentage change in flow rate caused by a unit change in stroke is the same; (2) The operation remains relatively stable over the entire stroke range, especially at large opening degrees, where the amplification factor is high as well, making the operation more sensitive and effective; (3) It has a wide range of applications and strong adaptability. (1) Its characteristics lie between those of a linear type and a equal percentage type; (2) Its control performance is fairly good, but the valve core is difficult to manufacture.  　　Parabolic characteristics lie between those of straight lines and those of equal percentage points; in practice, equal percentage point characteristics are often used as a substitute. 　　It opens quickly; when the valve stroke is small, the flow rate increases significantly and reaches its maximum value very fast. (1) The flow rate is already high at low opening degrees, and it reaches its maximum value very quickly as the stroke increases; (2) It is generally used for two-position control and program control. 　　(1) Operating flow characteristics when valves are connected in series When control valves are connected in series, changes in the valve opening lead to changes in flow rate. According to fluid mechanics theory, the resistance loss in the pipes is proportional to the square of the flow rate. Once the control valve operates, the flow rate changes, and accordingly the system resistance also changes; thus, the pressure drop across the control valve changes as well. The operating flow rate characteristics when pipes are connected in series are related to the pressure drop distribution ratio. The smaller the pressure drop across the valve, the lower the flow rate at full opening of the control valve, which causes the ideal linear characteristic to be distorted into a fast-opening characteristic, and the ideal equal percentage characteristic to be distorted into a linear characteristic. In practical use, when the control valve is chosen to be too large or when production is not at full capacity, the valve operates at a low opening degree. Sometimes, in order to ensure a certain opening degree of the control valve, the valve is closed further to increase the resistance in the pipeline, thereby reducing the flow rate through the valve. This actually results in a decrease in the pressure drop distribution ratio, distorts the flow characteristics, and deteriorates the quality of regulation. 　　(2) Operating flow characteristics in parallel pipelines Parallel pipelines generally consist of a valve branch and a bypass pipe branch, with the control valve installed on the valve branch pipeline. The control valve is located on the parallel pipeline; when the system resistance remains constant, the ratio of the flow rate when the control valve is fully open to the maximum flow rate in the main pipe decreases as the bypass valve of the parallel pipeline is gradually opened. At this point, although the flow characteristics of the control valve itself remain unchanged, the adjustable range of the system **reduces**, and the range of flow variation that the control valve can manage during operation also **decreases**, to the point where it can no longer perform its regulatory function. To ensure good control performance of the control valve, it is generally considered that the bypass flow should not exceed 20% of the total flow. Selection of flow characteristics  3  Methods for selecting flow characteristics: ① Analytical method based on mathematical calculations; ② Empirical method derived from practical engineering experience. Since the analytical method is both complex and time-consuming, empirical methods are generally used in engineering. Specifically, the following aspects should be considered: (1) Considering the regulation quality of the automatic control system. According to the principle of characteristic compensation in automatic control theory, in order for the system to maintain good regulation quality, it is desirable that the product of the overall open-loop gain and the gains of each individual element remain constant. In this way, by appropriately selecting the characteristics of the valve to use changes in its amplification factor to compensate for changes in the amplification factor of the plant, the overall amplification factor of the system can be kept constant. 　　(2) Selection is based on the changes in pressure drop within the piping system. The pressure drop ratio of a control valve is defined as the ratio of the differential pressure Δp1m across the valve at the maximum flow rate it can control to the system’s overall differential pressure Δp. The flow characteristics of a control valve are closely related to this pressure drop ratio S, as shown in Table 2. Selection of control valve characteristic tables based on pipeline system pressure drop Table 2 Pipeline system pressure drop ratio: 1–0.6, 0.6–0.3, 0.3–0 Actual operating flow characteristic: Linear, equal percentage linear, equal percentage – Inappropriate for regulation Selected flow characteristic: Linear, equal percentage linear, equal percentage (3) Analysis from the perspective of load variations For linear valves, the flow rate changes significantly at low opening degrees, resulting in overly sensitive regulation and prone to oscillations; at high opening degrees, the regulatory effect becomes weak, leading to untimely and insufficient regulation. Therefore, linear valves are not suitable for applications with a low pressure drop ratio S and large load variations. The equal percentage valve operates smoothly and gradually as it approaches closure, while it has a high amplification factor and operates efficiently as it approaches full open position; therefore, it is suitable for applications with large variations in load. The quick-opening characteristic valve exhibits a high flow rate at small stroke lengths; as the stroke length increases, the flow rate quickly reaches its maximum value. It is generally used in applications requiring two-position control and program-controlled operation. (4) Selection is based on the characteristics of the process variable. In general, for process variables that possess self-balancing capabilities, control valves with a proportional flow characteristic can be used; whereas for those without such capabilities, control valves with a linear flow characteristic are appropriate. Selection of control valve diameter  4 The purpose of selecting the diameter of the control valve is to enable a proper combination between the valve and the pipeline, thereby achieving a reasonable linear characteristic that allows for effective control of the system’s regulation. The diameter of the control valve is determined based on the flow capacity required by the process. The flow capacity of the control valve is calculated using the provided process conditions, and then the diameter of the control valve is selected accordingly. 　　To properly select the diameter of a control valve, it is necessary to understand its flow capacity. The flow capacity refers to the flow rate per hour that passes through the control valve when the valve is fully open, the pressure difference across the valve is 9.81 ×104 Pa, and the density of the fluid is 1 g/cm3. The flow capacity of a control valve is an important parameter for selecting the valve and its diameter appropriately. By calculating the flow capacity of the control valve, and by comparing it with the technical parameters provided by the manufacturer, the appropriate diameter for the valve can be determined. For automatic control systems, water is the most common medium that flows through control valves; therefore, water is used as an example here to illustrate the flow capacity of control valves. 　　In practical engineering, valve diameters are classified into different grades, and the C value is usually not a continuous value (the C value calculated through formulas is continuous). Different manufacturers’ products of the same type have different C-value to caliber correspondence tables. After calculating the desired value of C, the manufacturer’s corresponding product data sheet can be consulted to determine the required valve size. The principle for selecting the valve port diameter should be as close to or larger than the calculated value, and should not be smaller than it. 　　5 Precautions
(1) It is unreasonable to select a control valve solely based on the diameter of the connecting pipe, as the regulating performance of the valve has no relation to the flow velocity or diameter of the pipe. Instead, the regulating performance of the valve depends only on the water resistance and flow rate. In other words, once the system equipment is determined, theoretically there is only one ideal size for the valve suitable for that system; there are no multiple options. 　　(2) Regarding the selection of valves, if the valve diameter chosen is too small, on one hand it will increase the resistance in the system; even when the valve is fully open (i.e., at 100% of its opening degree), the system still may not meet the required capacity, leading to serious consequences. On the other hand, the valve will need a large pressure difference supplied by the system in order to maintain an adequate flow rate, which increases the load on the pump and makes the valve more prone to damage, significantly affecting its lifespan. 　　(3) Regarding the selection of valves, choosing a valve with an excessively large diameter not only increases the project costs but also causes the valve to operate frequently at a low percentage range, resulting in reduced regulation accuracy and poorer control performance; it also makes the system susceptible to shocks and oscillations. 　　(4) To ensure the control quality of the system, the best approach is to select a valve diameter that allows for a larger pressure drop within the limits permitted by the system, thereby minimizing the variation in pressure drop during operation. The higher the percentage of the pressure drop at full valve opening relative to the total pump pressure, the smaller the relative change in the valve’s pressure drop, and the more the installation characteristics of the valve approach its inherent properties. 　　(5) In control systems, control valves should operate under constant pressure drop conditions as much as possible, because whether a valve is suitable for use with a coil depends on its inherent properties and flow factor, and these valve parameters are determined by a constant valve pressure drop. 　　In summary, the purpose of designing control valves is to ensure the highest possible quality of system control, which requires a clear understanding of the composition, classification, and characteristics of control valves, as well as the ability to choose the appropriate ones based on this knowledge. Moreover, when sizing control valves for an actual system, it is necessary to conduct a thorough analysis of the entire piping loop, taking various factors into account. Only in this way can the control valve be properly selected to ensure the control quality of the control system.