Process Engineering Division – Instrumentation and Automation Section – Daily Question – Question dated 2020-09-23

This post was last edited by Hao Liang Zhi Le on 2020-9-23 at 08:15. 1. Achieve signal conversion and power amplification； 2. Precise positioning of the valve opening ； 3. Divide the signal into sections to achieve segmented control ； 4. The valve characteristics can be changed.

Functions of valve positioners: 1. They are used to improve the quality and speed of operation of control valves, primarily thanks to the amplifier and sufficient air supply pressure. 　　2. Used to increase output force, mainly applicable in situations involving high pressure differences, high-pressure media, high temperatures, low temperatures, and large-diameter valves. 　　 　　3. Used for electro-pneumatic conversion; currently, most control valves are used in conjunction with computers, which output current signals of 4–20mA in most cases, and electro-pneumatic valve positioners can be used to meet these requirements. 　　4. Used for stepwise regulation; two control valves operate one set of equipment each – one valve receives a 4–12mA electrical signal, while the other receives a 12–20mA electrical signal. 　　5. Used to change flow characteristics. The inherent flow characteristics of the original valve can be adjusted using a positioner. For example: (1) The original quick-opening characteristic can be changed to a linear or equal percentage characteristic through the use of a positioner. 　 　　(2) The original linear characteristic can be changed to an equal percentage characteristic using a positioner. 　　(3) The originally approximate equal percentage characteristic can be turned into an equal percentage characteristic through the locator. 　 　　6. Used to convert two-position actions into proportional actions. 　 　　7. Used for the reverse operation of valves. Using a positioner, a gas-open valve can be turned into a gas-close valve, and a gas-close valve can also be turned into a gas-open valve. 　　8. It is used when the length of the pneumatic signal pipeline is very large, generally ≥150M.

Functions of a valve positioner: 1. Improving the steady-state characteristics of the valve. With the use of a valve positioner, even slight changes in the controller’s output signal can result in significant variations in the air pressure supplied to the diaphragm of the control valve. This allows for overcoming the friction associated with the valve stem and eliminating the effects of unbalanced forces on the control valve, thereby ensuring that the valve is positioned accurately according to the signals sent by the controller. After improving the steady-state characteristics, the control valve can be used in the following situations: ① Cases where adjustment of the valve position is required ； ②Applications with large diameters, high pressure differences, and other conditions resulting in significant unbalanced forces ； ③To prevent leakage, it is necessary to compress the stuffing box tightly, such as in situations involving high pressure, high temperature, or low temperature ； ④There are solid particles stuck in the process medium, or the viscosity is high. 2. Improving the dynamic characteristics of the valve: The actuator alters the original first-order lag characteristic of the valve, reduces the time constant, and turns it into a proportional characteristic. Generally speaking, when the pressure transmission pipeline exceeds 60 m, a valve positioner should be used. 3. Changing the flow characteristics of the valve: By altering the shape of the actuator feedback cam, it is possible to switch between linear, logarithmic, and quick-open flow characteristics for the control valve. 4. For proportional control, one controller is used to operate two or more control valves, enabling them to complete a full stroke of movement within a certain range of the signal. For example, the two control valves are made to move through their full stroke within signal ranges of 4–12 mA DC current and 12–20 mA DC current, respectively. 5. Valve positioners used for the reverse operation of valves come in positive and negative acting types. During positive action, as the input signal increases, the output air pressure also increases ； During reaction, the input signal increases while the output air pressure decreases. Using a reverse-acting locator can convert a air-operated open valve into an air-operated closed valve, and vice versa.

1. Implement signal conversion and power amplification； 2. Precise positioning of the valve opening ； 3. Divide the signal into sections to achieve segmented control ； 4. The valve characteristics can be changed.

1. Improve the positioning accuracy and reliability of control valves； 2. In situations where the pressure difference across the valve is high (△p>1MPa). By increasing the gas supply pressure to boost the output force of the actuator, it is possible to overcome the unbalanced force exerted by the liquid on the valve core and reduce stroke error.

It receives the output signal from the controller, and its output signal controls the movement of the valve. (1) Improve the linearity of the valve stem position (2) Increase the operating speed of the actuator

Functions of valve positioners: 1. They are used to improve the quality and speed of operation of control valves, primarily thanks to the amplifier and sufficient air supply pressure. 　　2. Used to increase output force, mainly applicable in situations involving high pressure differences, high-pressure media, high temperatures, low temperatures, and large-diameter valves. 　　 　　3. Used for electro-pneumatic conversion; currently, most control valves are used in conjunction with computers, which output current signals of 4–20mA in most cases, and electro-pneumatic valve positioners can be used to meet these requirements. 　　4. Used for stepwise regulation; two control valves operate one set of equipment each – one valve receives a 4–12mA electrical signal, while the other receives a 12–20mA electrical signal. 　　5. Used to change flow characteristics. The inherent flow characteristics of the original valve can be adjusted using a positioner. For example: (1) The original quick-opening characteristic can be changed to a linear or equal percentage characteristic through the use of a positioner. 　 　　(2) The original linear characteristic can be changed to an equal percentage characteristic using a positioner. 　　(3) The originally approximate equal percentage characteristic can be turned into an equal percentage characteristic through the locator. 　 　　6. Used to convert two-position actions into proportional actions. 　 　　7. Used for the reverse operation of valves. Using a positioner, a gas-open valve can be turned into a gas-close valve, and a gas-close valve can also be turned into a gas-open valve. 　　8. It is used when the length of the pneumatic signal pipeline is very large, generally ≥150M.

1. To improve the steady-state performance of the valve, by using a valve positioner, even slight changes in the controller’s output signal can result in significant variations in the air pressure supplied to the control valve’s diaphragm. Through the action of the nozzle and throttle plate system as well as the amplifier, these pressure changes help to overcome friction in the valve stem and eliminate the effects of unbalanced forces on the control valve, thereby ensuring that the valve is positioned accurately according to the signals sent by the controller. After improving the steady-state characteristics, the control valve can be used in the following situations: ① Cases where adjustment of the valve position is required ； ②Applications with large diameters, high pressure differences, and other conditions resulting in significant unbalanced forces ； ③To prevent leakage, it is necessary to compress the stuffing box tightly, such as in situations involving high pressure, high temperature, or low temperature ； ④There are solid particles stuck in the process medium, or the viscosity is high. 2. Improving the dynamic characteristics of the valve: The actuator alters the original first-order lag characteristic of the valve, reduces the time constant, and turns it into a proportional characteristic. Generally speaking, when the pressure transmission pipeline exceeds 60 m, a valve positioner should be used. 3. Changing the flow characteristics of the valve: By altering the shape of the actuator feedback cam, it is possible to switch between linear, logarithmic, and quick-open flow characteristics for the control valve. 4. For proportional control, one controller is used to operate two or more control valves, enabling them to complete a full stroke of movement within a certain range of the signal. For example, the two control valves are made to move through their full stroke within signal ranges of 4–12 mA DC current and 12–20 mA DC current, respectively. 5. Valve positioners used for the reverse operation of valves come in positive and negative acting types. During positive action, as the input signal increases, the output air pressure also increases ； During reaction, the input signal increases while the output air pressure decreases. Using a reverse-acting locator can convert a air-operated open valve into an air-operated closed valve, and vice versa.

1. To improve the steady-state performance of the valve, by using a valve positioner, even slight changes in the controller’s output signal can result in significant variations in the air pressure supplied to the control valve’s diaphragm. Through the action of the nozzle and throttle plate system as well as the amplifier, these pressure changes help to overcome friction in the valve stem and eliminate the effects of unbalanced forces on the control valve, thereby ensuring that the valve is positioned accurately according to the signals sent by the controller. After improving the steady-state characteristics, the control valve can be used in the following situations: ① Cases where adjustment of the valve position is required ； ②Applications with large diameters, high pressure differences, and other conditions resulting in significant unbalanced forces ； ③To prevent leakage, it is necessary to compress the stuffing box tightly, such as in situations involving high pressure, high temperature, or low temperature ； ④There are solid particles stuck in the process medium, or the viscosity is high. 2. Improving the dynamic characteristics of the valve: The actuator alters the original first-order lag characteristic of the valve, reduces the time constant, and turns it into a proportional characteristic. Generally speaking, when the pressure transmission pipeline exceeds 60 m, a valve positioner should be used. 3. Changing the flow characteristics of the valve: By altering the shape of the actuator feedback cam, it is possible to switch between linear, logarithmic, and quick-open flow characteristics for the control valve. 4. For proportional control, one controller is used to operate two or more control valves, enabling them to complete a full stroke of movement within a certain range of the signal. For example, the two control valves are made to move through their full stroke within signal ranges of 4–12 mA DC current and 12–20 mA DC current, respectively. 5. Valve positioners used for the reverse operation of valves come in positive and negative acting types. During positive action, as the input signal increases, the output air pressure also increases ； During reaction, the input signal increases while the output air pressure decreases. Using a reverse-acting locator can convert a air-operated open valve into an air-operated closed valve, and vice versa.