In the process of production, the control valve is the terminal of the control system. Once it fails, it will directly affect the safe operation of the device and have a significant impact on the production process. Using the intelligent control valve positioner can improve the flow characteristics and performance of the control valves. Through digital information, communication with the DCS or BUS equipment can improve the enterprise’s production control ability and guarantee the safe and stable production of the device.
1. Conventional positioners
1) conventional positioners are mostly mechanical force balance principle, they adopt nozzle baffle mechanism, more movable parts, easy to be affected by temperature fluctuation, external vibration, and can change other interference, poor environmental resistance; elastic spring coefficient, will cause the control valve nonlinearity, resulting in quality reduction; external vibration to force balance mechanism, easy to cause component wear and zero and stroke drift, also make the positioner challenging to work;
2) Because of the nozzle’s characteristics, the actuator also consumes a large amount of compressed air in a stable state. If the number of actuators is more, the energy consumption is large, and the nozzle itself is a potential fault source, which is easily blocked by dust or dirt particles. The positioner does not work correctly;
3) it is impossible to use special equipment without isolating the control loop when the conventional locator is manually adjusted, and the adjustment of zero points and stroke affects each other. It must be set repeatedly, time-consuming and laborious. It is more difficult to change when the nonlinearity is severe.
2. Composition and Principle of Intelligent Valve Positioner
2.1 Composition of intelligent/smart positioners
The intelligent valve positioner is a kind of valve positioner with HART communication protocol. It consists of three parts: microprocessor electronic controlled module, including HART communication module and local user interface switch, a piezoelectric valve of electric/pneumatic converter module, and valve position sensor.
2.2 Working principle of intelligent positioner
Two lines,4~20 mA signals, control the whole control loop. HART module sends out and receives digital information superimposed on 4~20 signals to realize bidirectional digital communication with the microprocessor. The signal of 4~20mA of analog quantity is transmitted to the microprocessor, which is compared with the feedback of the valve position sensor. The microprocessor carries on the control calculation according to the deviation size and the direction (the first level control), sends the electronic control instruction to the piezoelectric valve to make it open and close. The piezoelectric valve corresponds to the increment of the output pressure of the pneumatic amplifier according to the width of the control instruction pulse, and the output of the pneumatic amplifier is fed back to the internal control loop. The valve stroke is controlled by the change of air pressure in the actuator. When the control deviation is significant, the piezoelectric valve sends out a wide pulse signal, so that the positioner outputs a continuous signal, which is greatly changed to the signal pressure of the actuator to drive the valve quickly. When the valve reaches the required position (entering the dead zone), the piezoelectric valve has no pulse output and the positioner output remains zero, which makes the valve stable in a certain position.
3. Adjusting intelligent positioner
Through the local user interface set switch, can complete the positioner gain, positive and negative action, positioner characteristics and whether to allow automatic adjustment and other basic settings; without adding tools, can automatically or manually calibrate the positioner; and through the local user interface manual control button, manual control valve.
4. Other features of smart valve positioners
1) through a variety of combinations to indicate the operating state or warning conditions, with diagnostic, monitoring functions;
2) gas consumption is very small, in the stable state of 0.6 MPa, only 0.12 NM3/h, less than 8% of the conventional positioner, not sensitive to the change of gas source pressure;
3) using the same model can be used for both straight and angular travel, through the selection of dual-acting module, dual-acting piston cylinder actuator can be controlled;
4) two-way communication with locators using HART communication protocols;
5. Should pay attention to in practical use
5.1 Higher requirements for load-carrying capacity to regulate signals
The input impedance of the intelligent locator is high in the process of practical use. When the input signal is 20 mA, the minimum required value of the supply voltage is 12 VDC, and the load capacity is not less than 600Ω, otherwise the locator can not work properly.
5.2 Operational dead zones of locators shall be reasonably set
The smaller the dead zone of the positioner, the higher the positioning accuracy, which causes a misunderstanding that the smaller the dead zone, the better, but this will make the piezoelectric valve and feedback rod and other moving parts more frequent, sometimes cause valve oscillation, affecting the service life of the positioner and valve, so the dead zone setting of the positioner is not easy to be too small; after the positioner setting is changed, it must be readjusted before it can take effect;
5.3 Installation of valve positioner
An essential principle of positioner installation is that the positioner, stem, and feedback rod should form a closed-loop negative feedback.
After the positioner is installed, the stem and feedback rod are not connected, and the feedback rod is rotated by hand. If the stem action direction is opposite to the feedback rod action direction, the closed-loop negative feedback is formed. The feedback rod and stem are fixed to ensure that the positioner works in the best linear segment. Locator installation is not correct, will also increase its linear deviation.
5.4 Selection of flow characteristics of positioner
The flow characteristics of the control valves are determined by the processing characteristics of the valve core. If the process requirements are consistent with them, the output characteristics of the positioner should be linear output; in practical use, if the valve core characteristics do not conform to the process requirements, The valve with linear core can be changed by setting the output characteristics of the positioner.
5.5 Maintenance of positioners
When the positioner is damaged by different functional modules, if the locator can not be used, it is expensive to replace the locator as a whole. Because the control valves (stroke, etc.) using the locator are changed, the automatic adjustment may not meet the requirements. In this way, the control valve can be positioned accurately and with appropriate response speed, thus meeting the requirements of process control.