With the rapid development of China's chemical industry, especially the coal chemical industry, there are high requirements for the use of valves. This makes valve positioners a key accessory in the control system, whose function is to convert the signal output by the regulating equipment into a gas signal that drives the action of the regulating valve
1 Overview With the rapid development of China's chemical industry, especially the coal chemical industry, which has high requirements for the use of valves, valve positioners have become a key accessory in control systems. Their function is to convert the signal output by the regulating equipment into a pneumatic signal that drives the action of the regulating valve, improve the position control accuracy of the valve, and overcome the influence of stem friction and medium imbalance force, This ensures that the regulating valve is correctly positioned according to the signal of the regulator. Especially intelligent valve positioners have been widely used.
2. The specific structure and principle of the locator Valve positioners can be divided into three types: pneumatic, analog, and digital, with the latter two being widely used on site.
2.1 Working Principle of Traditional Electrical Valve Positioners Electrical valve positioners have undergone decades of development The characteristic of a conventional electric valve positioner is to use the force balance principle of the nozzle, baffle, feedback cam, magnetic steel, and spring using the lever principle, and then improve the performance of the regulating valve through a pneumatic amplifier. The opening position of the feedback rod feedback valve changes. When the electromagnetic torque generated by the input signal is equal to the torque generated by the feedback system of the locator, the force balance system of the locator is in a balanced state, and the locator is in a stable state. At this time, the input signal is proportional to the valve position. When the input signal changes, the balance state of the force balance system is broken, and the force of the magnetoelectric component is in an unbalanced state with the force generated by the feedback circuit caused by the change in valve stem position. This converts the baffle into a corresponding air pressure signal by causing small displacement changes to the nozzle. The nozzle baffle mechanism is composed of a variable throttle element composed of a nozzle and baffle, a constant throttle hole, and a back pressure chamber.
After the air source enters the nozzle baffle mechanism, the first thing it encounters is a constant throttle hole. The constant throttle orifice causes significant resistance to the airflow, allowing only a portion of the airflow to pass through. A portion of the airflow thatpasses through enters the back pressure chamber and is then discharged into the atmosphere through the nozzle. A baffle is installed vertically facing the nozzle. When the baffle is close to the nozzle, it increases the air resistance, making it difficult for the airflow to be discharged, and finally causing the pressure in the back pressure chamber to rise; When the baffle leaves the nozzle, the air resistance decreases, the airflow is discharged, and finally the pressure in the back pressure chamber decreases. The baffle causes a second resistance to the airflow passing through the constant orifice, which varies with the position of the baffle. The different spacing between the nozzle and baffle generates different back pressure. Due to the action of the nozzle and baffle, the power amplification is achieved through a pneumatic amplifier. Under the action of the pneumatic amplifier, the output pressure of the locator's air source changes, and the change in the pressure of the actuator's air chamber drives the actuator to move, positioning the valve stem in a new position and re corresponding to the input signal, achieving a new equilibrium state. By changing the structure of the feedback rod of the locator (such as the cam curve) during use, the positive and negative effects, flow characteristics, etc. of the regulating valve can be changed, achieving an improvement in the performance of the regulating valve.
2.2 The performance of intelligent electrical valve positioners has taken a significant leap compared to traditional valve positioners The intelligent electrical valve positioner has higher positioning accuracy, wider application range, and is more convenient and reliable to use. However, in specific applications, it is necessary to choose the type of locator reasonably from aspects such as meeting safety requirements, better control effect, matching with the regulating circuit, adapting to special environmental requirements, and extending service life, and set and adjust its functional parameters.
With the development of electronic technology, intelligent valve positioners have become the mainstream for valve positioners used in pneumatic actuators. It integrates self correction function, self diagnosis function, fault alarm function, valve position analog signal feedback function, and various feature correction functions. The menu operation is convenient for on-site installation and debugging.
The intelligent valve positioner uses HART protocol to complete the data communication function. At the same time, it uses pulse width modulation (PWM) technology to replace the lever force balance structure of magnetic steel and spring, uses piezoelectric valves to replace nozzles and baffles, and uses conductive plastic Potentiometer to replace feedback cams, so that the accuracy of the positioner can be improved. The specific approach is:
Firstly, the data communication function of the HART protocol has been added. The HART protocol uses FSK frequency shift keying signals based on the Bell202 standard, and overlays an audio digital signal with an amplitude of 0.5mA on a low-frequency 4-20mA analog signal for bidirectional digital communication. The data transmission rate is 1.2Mbp. 2200Hz is 11100Hz, which is 0. As the average value of FSK signal is 0, it does not affect the size of the analog signal transmitted to the control system, ensuring compatibility with existing analog systems.
Secondly, piezoelectric valves are used instead of nozzles and baffles. The leading element of conductive plastic Potentiometer instead of feedback cam piezoelectric valves is a piezoelectric flexible on-off valve, also known as a silicon micro control valve. Because of its small mass and very small switching inertia, it can perform very high switching frequency. Therefore, as a high-frequency pulse valve, it can control the pressure of the output air circuit and drive the actuator to achieve high valve positioning accuracy. Piezoelectric valve is a two position (or proportional) control valve made by utilizing the principle of bending deformation of ceramic sheets of functional piezoelectric ceramics under voltage. Controlling the action of the piezoelectric valve only requires providing sufficient voltage, and the electrical power consumption is almost zero. The valve position feedback element is a conductive plastic Potentiometer with simple structure, high precision and high reliability, which converts the linear or angular displacement of the actuator into a resistance signal, thus it can accurately detect the valve position and conveniently locate the zero position, full degree and valve flow characteristic curve of the valve.
Due to the use of new control components such as conductive plastics and piezoelectric valves, valve positioning can achieve high accuracy. Due to the use of microprocessors, the calibration and applicability of the positioner can be greatly improved.
Thirdly, pulse width modulation (PWM) technology replaces the lever force balance structure of magnetic steel and springs, and PWM technology refers to:
In the non powered state or when two piezoelectric valves are applied at a low level, the piezoelectric ceramic inlet valve closes and the exhaust valve opens. (Exhaust) ① The intake valve is electrically opened, and the opening increases with the increase of PWM duty cycle; ② The exhaust valve is electrically closed, and the opening decreases as the PWM duty cycle increases.
In the intake state, the two piezoelectric valves should be raised to open the intake valve and close the exhaust valve.
When the valve position is maintained, the intake valve should be lowered and the exhaust valve should be raised to close both the intake and exhaust valves.
When the deviation between the set valve position and the feedback valve position is significant, the intake valve widens the pulse, the intake valve opens the intake volume, the exhaust valve widens the pulse, and the exhaust valve is basically closed.
When the deviation is small, the inlet pressure electric valve narrows the pulse, the inlet valve opening decreases, the exhaust pressure electric valve widens the pulse, and the exhaust valve is basically closed.
When the deviation is basically 0, the intake pressure electric valve is at a low level, the exhaust pressure electric valve is at a high level, and the intake and exhaust valves are closed. Maintain pressure and valve position.
The microprocessor outputs PWM pulse signal with variable duty ratio, controls the opening of air inlet and exhaust pressure electric valves through the boost drive circuit, and changes the air inlet and Engine displacement volume.
3. Shortcomings of conventional locators Conventional locators are mostly based on the principle of mechanical force balance, which adopts a nozzle baffle mechanism with many movable parts and is easily affected by temperature fluctuations, external vibrations, and other disturbances, resulting in poor environmental resistance; The elastic coefficient of the spring can change in harsh environments, which can cause nonlinearity in the control valve and lead to a decrease in control quality; External vibration transmitted to the force balance mechanism can easily cause component wear and zero point and stroke drift, greatly affecting the accuracy of the locator;
Due to the characteristics of the nozzle itself, the actuator also requires a large amount of exhaust when in a stable state. If a large number of actuators are used, energy consumption will be high; And the nozzle itself is a potential source of failure;
When manually adjusting a conventional locator, it is impossible to use specialized equipment without isolating the control circuit, and the adjustment of zero and stroke affect each other, requiring repeated tuning. When nonlinearity is severe, it is even more difficult to adjust.
4. In the application of intelligent locators, reasonable selection and correct installation should be based on on-site control requirements, with key requirements to be noted Although intelligent locators are simple to use and powerful in functionality; Easy installation, automatic calibration, simple and flexible configuration; Equipped with intelligent communication and on-site display functions, it is convenient for maintenance personnel to inspect and repair the working condition of the locator; It has advantages such as rich self-diagnosis functions. However, in engineering applications, attention should still be paid to some issues to ensure its reliable operation, better control effect, and extended service life.
Because of the structural characteristics of the piezoelectric ceramic valve of the intelligent positioner, the Engine displacement of the positioner is very small. The nozzle and baffle system of traditional locators is continuous exhaust. The intelligent locator only exhausts when the output pressure is reduced, so it is in a non gas consuming state for most of the time, which can be ignored compared to traditional locators. Therefore, the quality requirements for the air source are relatively high, and the dust and moisture in the air source should be minimized as much as possible.
The selection of the 2/4 wire wiring method for the positioner is due to the high input impedance of the intelligent valve positioner, which increases with the increase of input current. When selecting an intelligent electrical valve positioner, it is necessary to verify the load capacity of the regulator's output control signal, which should be greater than 500 ohms, in order to ensure the normal operation of the positioner when the opening is large.
Reasonably set the action dead zone of the locator. The smaller the dead zone setting, the more frequent the movement of corresponding piezoelectric valves, feedback links, and other moving components. Sometimes, it can cause valve oscillation, increase mechanical wear, and affect the control quality of the locator and the lifespan of the regulating valve. Therefore, the dead zone setting of the locator should not be too small.
Reasonably setting the output change cycle of the control cycle regulator to around 1.0 seconds is more appropriate.
Selection of flow characteristics for intelligent locators. All intelligent locators have the function of flow characteristic selection and setting. However, in practical use, it should be reasonably determined based on the flow characteristics and specific process requirements of the equipped valve.
The on-site installation method in explosion-proof environments should comply with the installation specifications for explosion-proof electrical equipment.
The application under harsh on-site conditions can separate the valve position sensor from the intelligent locator for installation in some harsh on-site working conditions. Shielded cable shall be used for cable connection, and EMC filter shall be used in intelligent locator to suppress Confounding generated by harsh environment.
In summary, the role of locators in control systems is very important. With the development and improvement of technology, intelligent locators, with their unique advantages, will be the development direction of locators in the future and will be more widely applied in various fields of control. Our company mainly uses intelligent locators such as Mesorilan and Siemens Yamamoto. After 5 years of use, with reliable performance, especially in small range adjustment, the valve shows high accuracy in the locator, ensuring that the valve can timely reflect control signal changes during working conditions and drive valve action for quality adjustment.
