Comprehensive Guide to Control Valves

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This post was last edited by yunrun on 2019-7-22 00:40. “Comprehensive Guide to Control Valves” comes from Yokogawa Electric in Japan, and covers topics such as control valve actuators, valve bodies, auxiliary equipment for actuators, the operation of control valves, flow characteristics of control valves, and the calculation of flow coefficients for these valves; it is very helpful for everyone to gain a thorough understanding of control valves. Comprehensive Guide to Control Valves: yunrun.com.cn/tech/1346.html In the field of process control, control valves are commonly used as devices for regulating process variables such as flow rate, pressure, temperature, and level. A control valve consists of an actuator and a valve body, and when used in combination with various auxiliary devices (such as positioners, solenoid valves, boosters, speed controllers, lockout valves, etc.), it can perform a variety of functions. Actuator: Depending on the operation signal as well as the direction of movement and flow characteristics when the driving source is removed, actuators and valve bodies can have various different structures; here only the representative ones are described. The actuators of control valves come in pneumatic, electric, and electro-hydraulic types; the pneumatic types are further divided into diaphragm and cylinder types. Based on the behavior when the operation signal and drive source are absent, actuators are classified into forward-type, reverse-type, directionless-type, and holding-type. In the positive type, the valve shaft rises when the operating signal and drive source are removed, while in the negative type it drops. http://yunrun.com.cn/upload/201704/24/201704240245313786.png http://yunrun.com.cn/upload/201704/24/201704240246030348.png Schematic diagram of the structure principle of the actuator. Valve body: There are more types of valve bodies than types of driving devices, and the choice is made based on the type of valve, its diameter, and the properties of the fluid. There are angular valves, three-way valves, diaphragm valves, gate valves, butterfly valves, ball valves, eccentric rotary valves, etc. Spherical valves are the most common type. The air pressure control signal is transmitted to the actuator, namely a cylinder-type or diaphragm-type pneumatic actuator, to cause the valve stem to move up and down. As shown in the figure below, the upward and downward movement of the valve stem changes the space between the valve plug and the valve seat, thereby altering the flow rate. Types and Features of Valves Type Features and Applications Single-seat ball valve Good control performance, with low leakage when the valve is fully closed. It can be widely used in water, steam, gases, etc., except for slurries or highly corrosive fluids. The two-seat ball valve uses a structure with two sets of seat assemblies connected to the valve plug in order to reduce the load on the actuation mechanism. It has certain limitations in application due to high leakage when the valve is fully closed and large face dimensions. Control valve: The valve plug inside the cylinder is modified to have a movable structure, in order to prevent vibration of the plug caused by excessive differential pressure. However, it should be noted that fibers and debris may get trapped between the cylinder and the valve plug. Butterfly valves are used for controlling low differential pressures such as those in furnaces and during gas combustion. There are also high-performance butterfly valves suitable for steam and medium to high pressure conditions. Ball valves are commonly used for ON/OFF operations such as pipeline switching. Sometimes, existing manual valves are also replaced with ball valves to achieve automated switching. Eccentric rotary valve with large capacity, suitable for slurries. The valve plug is bowl-shaped and susceptible to fluid biasing, resulting in limited application scope. Sanders valves are suitable for corrosive fluids and slurries, but have poor durability. Actuator auxiliary equipment Common actuator auxiliary equipment includes pneumatic valve positioners, electro-pneumatic valve positioners, electric valve positioners, solenoid valves, boosters, speed controllers, lockout valves, etc. A brief introduction is provided below. 1. Pneumatic valve positioner: A positioner is a device installed on the actuator to keep the valve opening in line with the signal from the controller. It is mainly used to avoid being affected by external force changes such as fluctuations in fluid pressure, thereby increasing the speed of the drive mechanism. Pneumatic instruments are also used to improve response when the regulator is separated from the valve. http://yunrun.com.cn/upload/201704/24/201704240126175973.png Principle of pneumatic valve positioners 2. Electro-pneumatic valve positioners Electro-pneumatic valve positioners are devices that convert electronic control signals of 4-20mA DC into pneumatic signals in order to adjust the opening degree of the valve. http://yunrun.com.cn/upload/201704/24/201704240127128473.png Principle of electric-pneumatic valve positioners 3. Electric valve positioners Electric valve positioners are designed to be used in conjunction with electric valves. A reversible motor is installed in the electric drive section, and the valve opening is fed back as a voltage signal to the electric valve positioner via a variable resistor (potentiometer). By using comparators Q1, Q2 and relays R1, R2, the forward, stop, or reverse operation of the reversible motor is controlled to keep the regulator signal Vi in agreement with the opening signal Vf. http://yunrun.com.cn/upload/201704/24/201704240128353630.png Principle of electric valve positioners 4. Solenoid valve: The solenoid valve functions as a switch for the air circuit. Solenoids are divided into 2-way solenoids, 3-way solenoids, 4-way solenoids, etc. Compared to direct fluid control, they are more commonly used in the drive circuits of on/off valves or for switching pneumatic signals. http://yunrun.com.cn/upload/201704/24/201704240129535817.png Principle of solenoid valves http://yunrun.com.cn/upload/201704/24/201704240130441755.png Examples of solenoid valve applications 5. Boosters: When the air capacity of a control valve is large or the distance over which the pneumatic signal needs to be transmitted is long, a booster is used to improve the response speed. http://yunrun.com.cn/upload/201704/24/201704240131358005.png Principle and structure of the voltage booster 6. Speed controller: The speed controller is a device installed between the regulator’s signal and the driving section, used to delay the regulator’s signal. It can also be used to prevent water hammer phenomena. The variable orifice plate/adjustment is composed of a needle valve, with the delay time set to the desired value. http://yunrun.com.cn/upload/201704/24/201704240132373317.png Principle and structure of the speed controller. 7. Locking valve: A locking valve is a small valve that allows the opening degree of the control valve to be maintained in case of a power outage or interruption in air supply. When designing, consider whether the lock valve returns to its original state when power is restored or the supply air pressure returns to normal, and select the sequence or type of lock valve. http://yunrun.com.cn/upload/201704/24/201704240140373473.png Locking valves and application examples. Flow characteristics of control valves: The main flow characteristics of control valves include linear, equal percentage, quick-open, and parabolic types. It is described in detail in the article \"Methods for Selecting the Flow Characteristics of Control Valves\" on this site, so it will not be repeated here. When installed in actual control processes, the valve differential pressure changes with variations in flow rate. That is, at low flow rates, the pressure loss in the piping section is small, resulting in a larger differential pressure across the valve; whereas at high flow rates, the differential pressure across the valve decreases. This valve characteristic, which is different from the inherent properties, is referred to as the effective flow characteristic. The internal valve of the quick-start feature is disc-shaped and is mainly used for opening/closing actions. http://yunrun.com.cn/upload/201704/24/201704240153503473.png The surface shape of the control valve’s spool determines the valve’s flow control characteristics, which are influenced by both the valve’s own flow properties and the combination of process piping, pumps, etc. The appropriate choice is made based on the proportion of pressure loss caused by valves in each control object and system, as shown in the table below. Control object: Proportion of pressure loss due to valves in the system; Flow characteristics of the valves; Flow control or level control. Below 40%: Fixed percentage; Flow control or level control. Above 40%: Linear; Pressure control or temperature control. Below 50%: Fixed percentage; Pressure control or temperature control. Above 50%: Linear. Since the pressure loss in the piping increases proportionally to the square of the flow rate, if the characteristics of the valve itself are linear, the differential pressure across the valve increases when the flow rate is low, while a slight opening of the valve results in a high flow rate. Conversely, when the flow rate is high, the differential pressure across the valve decreases, and the flow rate is not proportional to the degree of valve opening. To this end, by incorporating the characteristics of pipes and pumps, the goal of designing equal percentage characteristics is to achieve flow control that varies proportionally only to the valve opening, regardless of the flow rate. http://yunrun.com.cn/upload/201704/24/201704240155381286.png Piping systems and pressure loss; operation of control valves. The operation of control valves can be selected based on the combination of the actuation device and the valve body. The combination of the actuator and the valve body, as well as valve operation (example of a single-seat valve): Valve operation includes three modes: forward operation, reverse operation, and holding operation. The positive actuation mode of pneumatic drives such as diaphragm and cylinder types is a method of closing the valve by increasing the air pressure signal, also known as “AIR TO CLOSE”. The reverse operation mode is a method of opening the valve by increasing the air pressure signal, also known as “AIR TO OPEN” or “AIRLESS TO CLOSE”. The electrical control signal can be converted into a pneumatic signal via a positioner. In the event of an interruption in the control signal, a failure in the air supply, or a power outage, please consider the safety and rationality of the process when deciding whether to close or open the valve. For example, in a process where the amount of acid is controlled via valves when water is mixed with acid, it is safe and reasonable to close the acid control valve in cases of a broken electrical signal line, a leak in the air signal piping, an interruption in the air supply, or a power outage; therefore, reverse-acting valves should be used. Selection of the diameter of control valves: The valve diameter is chosen based on the properties of the fluid and the operating conditions, by determining the most suitable diameter through calculations. The capacity coefficient Cv for the required flow rate is calculated, and then the appropriate valve diameter is selected by comparing it with this Cv value. 1. Definition of the Cv value: When the differential pressure across the valve is 1 psi (0.07 kg/cm2 = 7 kPa), and the temperature of the water is at room temperature with a flow rate of 1 gallon per minute (3.785 liters), the value of the flow capacity coefficient Cv is defined as 1. 2. Calculation of Cv values: ① Calculation of Cv values for liquids; ② Calculation of Cv values for gases; ③ Calculation of Cv values for steam. 3. Dimensioning methods in the SI unit system: Definition of Av values and Cv values. However, the Cv value is currently more widely used. 4. Method for calculating the Cv value of valves: The Cv value is divided into required Cv and valve Cv. The Cv is required to be calculated based on the valve differential pressure and fluid conditions. The Cv of a valve refers to its flow capacity, which is determined by the valve’s shape and diameter. When selecting the valve diameter, choose a valve with a Cv value that is slightly higher than the required Cv value. No precise calculation is required for Cv. ①Calculation of Cv: When the fluid is a liquid. ② A simple method for estimating the Cv value of valves: The Cv value of a 1-inch ball valve is approximately 10. The Cv value varies in proportion to the square of the valve diameter multiple; therefore, the Cv value for a 2-inch valve is approximately 40. ③Calculation example: When we turn on the faucet to wash our hands, the flow rate is approximately 5 L/min (0.3 m3/h), and the water pressure is around 0.05 MPa; the square root of this value is about 0.22. Therefore, the Cv value for a valve with a diameter of 13 mm is approximately 3, while the Cv value for a valve with a diameter of 8 mm is approximately 1. This ensures that an adequate amount of water will be available. Tap water pipes typically use 13mm (1/2 inch) and 8mm (1/3 inch) pipes, which are clearly suitable diameters that allow for a good balance. The article is from “Changhui Instrument Network””