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Analysis of Intelligent Valve Positioner in Chemical Plant
Abstract: This article introduces the composition, working principle, characteristics and application of intelligent valve positioner. Describe the use of intelligent valve positioner in chemical control process to achieve high quality adjustment, increase the accuracy and stability of process control. Key words: intelligent valve positioner piezoelectric valve adjustment valve is the control system of the terminal, once it fails, it will directly affect the safe operation of the device, the production process is very large. The intelligent valve positioner can improve the flow characteristics and performance of the regulating valve, enhance the production control ability of the enterprise through the digital information communication with DCS or bus equipment, and provide the guarantee for the safe and stable production of the device. The following to the United States FLOWSERVE company Logix520 intelligent valve positioner application in our plant, for example, introduces intelligent valve positioner. 1. Conventional positioner deficiencies 1) Conventional positioner mostly mechanical balance principle, which uses the nozzle baffle mechanism, moving parts are more susceptible to temperature fluctuations, external vibration and other interference, poor resistance to the environment; spring elasticity Coefficients can change in harsh environments, causing non-linearity in the control valve, resulting in poor control quality. External vibration is transmitted to the force balance mechanism, which can easily cause component wear and zero and stroke drift, making the positioner difficult to work. 2) Its own characteristics, the actuator in a stable state also consume a lot of compressed air, if the use of a larger number of actuators, larger energy consumption; and the nozzle itself is a potential source of failure, easily blocked by dust or dirt particles, So that the positioner can not work properly; 3) When the conventional positioner manual adjustment requires the use of special equipment, it is impossible not to isolate the control loop, and zero point and stroke adjustment affect each other, and must be repeatedly set, time-consuming and laborious, nonlinear serious , It is more difficult to adjust. 2. Logix520 intelligent valve positioner composition and principle 2.1 Logix520 intelligent valve positioner The Logix520 intelligent valve positioner is a HART communication protocol with intelligent valve positioner, consists of three parts: microprocessor-controlled electronic control module, including HART communication module and local user interface switch; piezoelectric valve pneumatic / pneumatic converter module; valve position sensor. 2.2 Logix520 intelligent valve positioner works The entire control loop from the two-wire, 4 ~ 20mA signal control. The HART module sends and receives digital information superimposed on a 4-20 mA signal to enable bi-directional digital communication with the microprocessor. Analog 4 ~ 20mA signal is passed to the microprocessor, compared with the feedback valve position sensor, the microprocessor according to the size and direction of bias control calculation (a control), the piezoelectric valve to issue electronic control instructions to make it To open, close action. The piezoelectric valve corresponds to the increment of the output pressure of the pneumatic amplifier according to the width of the control command pulse, and the output of the pneumatic amplifier is fed back to the inner control circuit again for comparison operation with the microprocessor (secondary control) Two levels control the output signal to the implementing agency, and the change of the air pressure in the executing agency controls the valve travel. When the control deviation is large, the piezoelectric valve sends a wide pulse signal, the positioner output a continuous signal, a substantial change to the actuator signal pressure to drive the valve fast action; as the valve close to the required position, the command required The difference between the position and the measured position becomes smaller, the piezoelectric valve outputs a pulse signal with a smaller pulse width, intermittently and slightly changes to the signal pressure of the actuator, and the actuator moves toward the new command position gently. When the valve reaches the required position (into the dead zone), the piezoelectric valve without pulse output, the retainer output remains at zero, so that the valve is stable at a certain position does not move. 3. Logix520 Smart Locator Tuning The local user interface DIP settings switch completes the basic settings of the positioner gain, positive and negative, locator characteristics, and whether auto-tuning is allowed or not; without additional tools, Automatically or manually calibrate the positioner; and manual control of the control valve with manual control of the button via the in-place user interface. 3.1 Autotuning Set the "Quick_cal" DIP switch to "Auto", hold down the "Quick_cal" button on the positioner's local keypad for about three seconds, the positioner closes the valve and registers 0%, then opens the valve To stop and register 100% position, repeated twice, in the process, the positioner to measure the positioning speed in both directions to determine the minimum positioning increment (second pass in the process of 50% slightly stop the test valve Bit midpoint offset) during which the status indicators on the panel will flash in the order of "YRGG", indicating that the calibration is in progress. When the indicator returns to the changing sequence from the green light, the calibration is done automatically. 3.2 Manual Adjustment Set the "Quick_cal" DIP switch to "jog", the calibration process will close the valve at the beginning, the zero position will be automatically set at the valve seat, and the range will need to be manually set by the user. When the status indicator on the panel Press "YRRG" to flash, use "Jog ↑" button to manually adjust the valve to about 100% position, and press both "Jog" buttons at the same time, the valve will switch the valve. When the status indicator on the panel is again When blinking in the "YRRG" sequence, use the "jog ↑" button to adjust the valve to the exact 100% again, then press both "Jog" buttons again to register the 100% position during the calibration , Do not need to operate again. When the indicator returns to the green light from the beginning of the order, to complete the calibration. This feature makes the adjustment work convenient and quick, but also adjust the linear, high precision, moderate response speed and dead zone, good stability. 4. Logix520 other features of the valve positioner 1) The local panel with red, yellow and green light-emitting diodes, through a variety of combinations indicate operating conditions or warning conditions, with diagnostic and monitoring functions; Basic meaning: Any flashing sequence beginning in green indicates normal operating mode, no internal problems, errors and alarms; any flashing sequence beginning with a yellow light indicates a special calibration or test condition; The flashing sequence begins to indicate that there is an operational problem or malfunction. 2) Air consumption is very small, only 0.12NM3 / h at 0.6 MPa steady state, less than 8% of the conventional positioner; insensitive to changes in air pressure; 3) The same model can be used both for straight travel and In the corner trip; through the optional double-acting module, you can achieve control of double acting piston cylinder actuator; 4) "closed" function default starting wind pressure to ensure that the implementing agencies on the seat suitable positioning pressure, so that the control valve In different conditions to ensure zero zero "close"; 5) using HART communication protocol, with the locator for two-way communication; In actual use should pay attention to the problem 5.1 on the regulation signal with a high load capacity requirements In actual use, due to Logix520 locator input impedance is high, when the input signal is 20mA, the minimum required supply voltage 12VDC, with load capacity of not less than 600Ω, otherwise the positioner can not work; minimum input current is not less than 3.6mA, to ensure its performance. 5.2 Locator should be set reasonable action Dead zone locator The smaller the dead zone setting, the higher the positioning accuracy, which gives people a misunderstanding that the dead zone is smaller the better, but this will make the piezoelectric valve and the feedback lever and so on Moving parts of the more frequent movements, and sometimes cause valve oscillation, affecting the life of the locator and the valve, so the locator is not easy to set the dead zone is too small; locator settings change, you must re-adjust to take effect after; 5.3 Logix520 positioning Installation of the positioner installation An important principle is that the positioner, the stem, the feedback rod to form a three-part closed-loop negative feedback. Installation can be tested as follows: After the positioner is installed, the stem and the feedback lever is not connected, turn the feedback lever by hand, if the direction of action of the valve stem and the action of the feedback lever are opposite, then the closed-loop negative feedback has been formed; at this time, The valve position is at 50% with the feedback lever in a horizontal position, and then the feedback lever and stem are fixed, which ensures that the positioner operates at the optimum linearity. Locator installation is not flat, it will increase its linear deviation. 5.4 Logix520 locator flow characteristics of the selection Flow characteristics of the control valve is determined by the processing characteristics of the spool, and if the process requirements consistent with it, the output characteristics of the locator should choose a linear output; in practice, if the spool characteristics And process requirements do not match, you can change the output characteristics of the locator to set the overall flow characteristics of the valve, such as the valve spool can be linear characteristics of the valve, the locator output characteristics set equal percentage characteristics, you can have Linear spool valve into equal percentage flow characteristics of the valve to use. 5.5 Logix520 locator maintenance Locator Different functional modules are damaged, resulting in the locator can not be used, if the overall replacement cost is high; then you can use the module without trouble to reposition the locator, but after assembly according to different Regulator valve to be reset, the use of the regulator valve (stroke, etc.) has changed, the use of automatic adjustment may not meet the requirements, then you can manually adjust to determine its travel, and then automatically adjust the calibration. This allows the valve positioning precision, with the appropriate response speed, so as to meet the process control requirements, but also save a lot of money. 6. Logix520 valve positioner in a plant in the practical application 1) a methanol plant C-203A / B / C oxygen pressure unit violent shock, the return line PCV-2008A / B / C control valve, the use of conventional positioner, nozzle Bezel shortly after wear occurs, there is drift when the zero range, frequently damaged locator, poor quality of process control, endangering the safety of production; Logix520 locator, due to its fully sealed structure, few moving parts, the force conversion process is not Mechanical transmission, eliminating the interference caused by vibration, so that this problem will be resolved, greatly reducing the amount of maintenance, saving money, to ensure the quality of process control and safe operation of the device. 2) a device gasifier waste pot level LCV-2003A / B control valve, the use of conventional locator before the transformation, installed near the gasifier, the working environment temperature is high (about 80 ℃), the medium state is high temperature and pressure 304 ℃, 8MPa), in order to prevent media leakage, the packing is pressed tight, resulting in slow sluggish stem action, the valve position changes step by step, the stability is poor, a greater impact on the process control. And the operation is difficult, the instrument maintenance and more; using Logix520 locator, the locator through the two-stage control, combined with the main gas circuit connected to the piezoelectric valve can release a very short control pulse, the output to the membrane head The signal is more accurate and stable, and the valve position changes smoothly and accurately. The "valve stability" DIP switch on the local interface of the positioner can also be set to "Hi Friction" in "Hi Friction" and "Lo Friction" To reduce the valve stem to withstand high friction and unbalanced force caused the impact of the basic elimination of these problems. 3) In April 2005, when the meter maintainer inspected a unit hydrogenation reactor level control valve LV-1501B, the status indicator of the Logix520 valve positioner showed "RYYY" to judge and check the adjustment Valve head leaks, timely processing, to avoid the malfunction of the control valve endanger the safety device production; the same inspection or maintenance personnel found that the TV-0706B control valve Logix520 valve positioner status indicator shows "RYYR" by This check valve output becomes smaller, regulating valve supply pressure is insufficient, can not meet the travel requirements. Maintenance personnel found that the pressure relief valve inspection damaged, timely replacement of the pressure relief valve and the required pressure to restore the source to avoid the operation due to the adjustment is not in place and affect the process control and product quality. 7. Conclusion Due to the microprocessor and new components, intelligent locator performance has been greatly improved, wider range of applications, the use of more simple and reliable. There are many manufacturers of intelligent valve positioner, such as SIEMENS PS2, FISHER's DVC5000, YAMATAKE's AVP100, etc. are similar in function, performance advantages and disadvantages, but also in the further development of software features to make it fully used , Improve the quality of process control, enhance the level of enterprise management.