Brief introduction to actuators

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8-1 What role do actuators play in automatic control systems? Answer: In a process control system, the actuator receives the command signal from the regulator; through its actuating mechanism, this signal is converted into the corresponding angular or linear displacement, which is used to operate the control mechanism and adjust the amount of energy or material flowing in or out of the controlled process, thereby achieving automatic control of the process. In any automatic control system, actuators are an essential component. If sensors are compared to the sensory organs of a control system, then regulators serve as the brain of that control system, while actuators can be likened to the hands that carry out the actual work. http://www.fmdwq.net 8-2 What components make up an actuator? What is the function of each component? Answer: The actuator consists of two parts: the actuating mechanism and the control valve (regulating mechanism). Figure 8-1 shows the external appearance of a pneumatic actuator. The actuator is the driving mechanism of the actuator; it generates a corresponding thrust based on the magnitude of the control signal, thereby driving the control valve to move. The control valve is the regulating component of the actuator; under the thrust exerted by the actuator, it undergoes a certain displacement or rotation, thereby directly regulating the flow rate of the fluid. www.cscyyb.com Figure 8-1: The types and structures of the adjustment mechanisms for various actuators are roughly the same; the main difference lies in the actuators themselves. Therefore, when introducing actuators, they are divided into actuating mechanisms and control valves. It should be noted that in electric actuators, the actuator and the control valve are basically two separate components, whereas in pneumatic actuators, the two are inseparable and form a single unit. 8-3 How are actuator types classified? Answer: Actuators can be classified into three main categories based on the type of energy they use: pneumatic, electric, and hydraulic. (1) Electric actuator: An electric actuator is powered by electricity. Its advantages include easy access to energy, fast operation, rapid signal transmission, the ability to transmit signals over long distances, and ease of use in conjunction with digital devices. Therefore, electric actuators are in a period of development and growth, and they represent a device with promising prospects. Its disadvantages are complex structure, high cost, and low driving force; moreover, electric actuators are generally not suitable for fireproof and explosion-proof applications. However, by using an explosion-proof structure, the requirements for fire and explosion prevention can also be met. (2) Pneumatic actuator: A pneumatic actuator is powered by compressed air, and it boasts advantages such as a simple structure, reliable and stable operation, high output force, easy maintenance, as well as fire and explosion resistance. Therefore, it is widely used in industries such as petroleum, chemicals, metallurgy, and power generation, and is particularly suitable for petroleum and chemical production processes where there is a risk of explosion. Its disadvantage is large latency, making it unsuitable for long-distance transmission (within 150m), and it cannot be connected to digital devices. www.cscyyb.com At present, among the actuators used domestically and internationally, hydraulic ones are quite rare. 8-4 What are the common types of control valves? What are their respective advantages and disadvantages? Answer: There are many types of control valves, but based on the operating mechanism of their valve cores, they can be divided into two main categories: straight-stroke and rotary-stroke types. Control valves in which the valve stem drives the valve core to move in a straight line belong to the linear stroke type, while those in which the valve core moves according to an angle belong to the rotary stroke type. Its common structural types are shown in Figure 8-2 below. Figure 8-2 (I) Straight-stroke type control mechanisms 1. Direct-acting single-seat valve. By \"single-seat\", it is meant that there is only one valve core and one valve seat inside the valve body. As shown in Figures 8-2(b) and (c). It is characterized by a simple structure, low leakage (which can even be completely eliminated), and a low allowable pressure difference. Therefore, it is suitable for applications involving clean media that require low leakage rates and small operating pressure differences. Special attention should be paid to its allowable pressure difference in applications, to prevent the valve from failing to close properly. 2. Direct-through two-seat valve www.cscyyb.com The valve body of a direct-through two-seat control valve contains two valve spools and valve seats. Compared to single-seat valves of the same diameter, its flow capacity is about 20% to 25% higher. Since the forces exerted by the fluid on the upper and lower valve cores can cancel each other out, it is difficult for both valves to close at the same time; as a result, double-seat valves allow for large pressure differences and higher leakage rates. Therefore, it is suitable for applications with a large pressure difference across the valve and low requirements for leakage, in clean media; it is not suitable for high-viscosity or fiber-containing media. 3. Angular valve: The valve body of an angular control valve is right-angled; it has a simple flow path and low pressure loss, making it suitable for controlling the flow of materials with high pressure differences, high viscosity, as well as those containing suspended solids and particles. It is generally used for bottom-inside-out flow, and this type of control valve has good stability. In high-pressure applications, side inlet and bottom outlet can be used to extend the service life of the valve core, but oscillations are likely to occur at low opening degrees. (II) Angular travel type control mechanisms 1. Butterfly valve: The disc of a butterfly valve is used to control the flow rate of fluid through rotation around an axis. It consists of components such as a valve body, baffle plates, baffle shafts, and shaft seals. It features a simple structure, small size, light weight, low cost, and high flow capacity, making it particularly suitable for applications with low pressure differences, large diameters, high flow rates of gases, and fluids containing suspended particles; however, its leakage rate is relatively high. Its flow characteristics are achieved at an angle of 70°. The characteristics before and at equal percentage points are similar; performance becomes unstable beyond 70°, and the characteristics are also poor. Therefore, butterfly valves are typically used within an angle range of 0° to 70°. Butterfly valves are widely used not only in general industries such as petroleum, gas, chemicals, and water treatment, but also in the cooling water systems of thermal power plants. 2. Cam deflection valve: The cam deflection valve, also known as the eccentric rotating valve, is another type of control valve with a novel structure. The center line of its spherical valve core is offset from the center of the rotating shaft; the rotating shaft causes the valve core to rotate eccentrically, allowing it to move forward and downward into the valve seat. www.cscyyb.com Eccentric rotary valves feature advantages such as small size, light weight, reliable operation, easy maintenance, wide applicability, and low fluid resistance. They are suitable for applications with high viscosity, and exhibit good performance when used with fluids such as lime and slurry. 8-5 What is the flow coefficient of a control valve? What factors is it related to? Answer: The flow coefficient of a control valve refers to the volume or mass of fluid that passes through the valve per unit time when the valve is fully open. It indicates the size that the control valve should have according to the process requirements. For incompressible fluids, it is not difficult to derive the expression for the flow rate through a control valve based on the principle of energy conservation for fluids. As can be seen from this expression, the flow rate through a control valve depends on many factors such as the type and properties of the fluid, the operating conditions, as well as the structural dimensions of the valve core and seat. Therefore, in order to define the flow coefficient of a control valve, certain conditions must be specified. The flow coefficient KV can be defined as: the volume of fluid (in m3) that passes through the valve per hour, under conditions where the pressure difference before and after the control valve is 100 kPa and the fluid density is 1000 kg/m3. 8-6 How are the principles for selecting air-open and air-close operation for control valves determined? In a single-parameter control system, how are the positive and negative actions of the regulator determined? Answer: The principle for determining the operating mode of the control valve is to ensure the safety of process equipment and production in the event of a loss of signal pressure. If the valve is in the open position when the signal is interrupted, it is safest to ensure a continuous flow of fluid; in such cases, a air-operated shut-off valve should be used ； If the valve remains in the closed position after the signal pressure is lost, and it is not safest for fluid to pass through, then a normally open valve should be used. In an automatic control system, the regulator, control valve, and process variable should be combined in such a way that they can provide negative feedback within the control system. The general steps are as follows: first, the direction of action on the controlled variable is determined based on the influence of the control variable; then, the air-open or air-close type of the control valve is decided according to the process safety requirements; finally, the positive or negative action of the controller is determined by whether the combination of the process plant, the control valve, and the controller results in a “negative” effect. 【Example 1】 There is a level control system as shown in Figure 8-3. According to the process requirements, a air-operated control valve is selected; what should be the forward and reverse action of the regulator? Solution: First, two rules are established: for www.cscyyb.com, (1) gas-on control valves are assigned the value +A, while gas-off control valves are assigned the value –A ； (2) When the control valve is opened, the regulated parameter increases to +B and decreases to –B. Then A×B=“+” control valve with reversed action ； A×B=“－”: The control valve should be selected for positive action. In the diagram, if the valve is of the air-open +A type and it opens fully, causing the liquid level to drop –B, then: Figure 8-3 shows that (+A) × (–B) = “–”; thus, the regulator operates in a positive mode. 8-7 What is the basic structure of an electric actuator? Answer: An electric actuator receives 0–10mA DC or 4–20mA DC signals from an electric controller, and converts these signals into corresponding angular or linear displacements of its output shaft, thereby controlling the control valve to achieve automatic regulation. The electric actuator consists of two main components: a servo amplifier and an actuation unit, and its structural principle block diagram is shown in Figure 8-3. Figure 8-3 Block diagram of the electric actuator structure www.cscyyb.com. The signal from the regulator drives the motor through a servo amplifier; the motor, in turn, operates the control valve via a reducer. At the same time, a position transmitter feeds back information on the valve stem’s movement to the servo amplifier, thus forming a position-following system. Reliance on position feedback ensures that the input signal is accurately converted into the stroke of the valve stem. 8. What are the main structural types of control valves? What are their respective features? In what occasions is it mainly used? Answer: Type, Characteristics, Main Applications
Straight-through single-seat control valve: Simple structure, low leakage, easy to ensure closure; suitable for small diameters and low pressure differences.
Straight-through double-seat control valve: Low unbalanced force, but higher leakage; the most commonly used type.
Angle control valve: Simple flow path with low resistance; suitable when straight-angle connections are required in the pipeline system, under high pressure differences, with high fluid viscosity, or when the fluid contains a small amount of suspended solids and particulates.
Three-way control valve: Has three inlets and outlets connected to the process pipelines; can be used for splitting or combining flows, as well as for proportional control or bypass control.
Diaphragm control valve: Simple structure, low flow resistance, high flow capacity, and strong corrosion resistance; suitable for handling strong acids, strong bases, highly corrosive fluids, high-viscosity fluids, and those containing suspended particles.
Butterfly valve: Simple structure, light weight, low cost, extremely low flow resistance, but high leakage; suitable for large diameters, high flow rates, low pressure differences, and fluids containing a small amount of fibers or suspended particles.
Ball valve: Both the valve core and the valve body are spherical in shape; suitable for fluids with high viscosity, dirty fluids, and applications requiring two-position control.
Cam flexure valve: Good sealing performance, light weight, small size, and easy installation; suitable for fluids with high viscosity and those containing suspended particles.
Cage valve: Wide adjustment range, low vibration, low unbalanced force, simple structure, good interchangeability of sleeves, low cavitation, and low noise; suitable for applications with high pressure differences where low noise is required. Not suitable for media with high temperatures, high viscosity, or solid particles. 9. What are air-operated actuator types of on-air and off-air operation? What are its selection principles? Answer: A valve that gradually opens as the air pressure signal sent to the actuator increases is called a air-operated open type; conversely, it is called an air-operated closed type. The choice between gas-on and gas-off modes is primarily determined by the safety requirements of the production process. Generally speaking, when the valve is fully open, an open-type design is preferred in production processes or equipment where there is a high level of risk ； When the valve is fully closed, an air-operated type should be chosen for processes or equipment where there is a high level of risk ； www.cscyyb.com 10. Briefly describe the functions and main roles of electric actuators. Answer: The function of angular stroke is to receive a branch current signal of 0–10A from the controller, and convert it into the corresponding angular or linear displacement, in order to operate control mechanisms such as valves and dampers and achieve automatic control. The main types of electric actuators include angular stroke, linear stroke, and multi-turn types. Angle-travel electric actuators use motors as the driving element to convert the input DC current signal into corresponding angular displacement; such actuators are suitable for operating rotary control valves such as butterfly valves and dampers. Upon receiving an input DC current signal, the linear actuator causes the motor to rotate; the rotation is then reduced in speed via a reducer, and the resulting motion is converted into linear displacement, which is used to operate various control valves such as single-seat, double-seat, and three-way valves, as well as other linear control mechanisms. Multi-turn electric actuators are primarily used to open and close multi-turn valves such as gate valves and globe valves, and are generally employed for local operation as well as remote control.