[Key words] intelligent valve positioner, impedance matching INTRODUCTION Intelligent valve positioner is a new generation of products, its working principle, structural characteristics, compared with the conventional electric valve positioner is very different, so in use encountered some new problems In combination with the successful application and typical failures of the smart valve positioners manufactured by more than 100 FISHER companies and more than 20 NELES companies in our unit melamine, we discussed the following issues of intelligent (analog signal superimposed digital signal) valve positioners.
1. Structure Principle A new generation of intelligent valve positioner is based on the microprocessor read input signal (mA), the position sensor position signal, and the pressure sensor output pressure signal by the operation processing, resulting in a drive signal, the actuator action. Now choose to compare the representative of the slider-type DVC5000 and rotating ND800 to explain in detail.
2. The DVC5000 digital valve controller has a separate modular base that can be easily replaced in the field without having to disconnect the field wire or conduit. This module base includes several sub-modules: I/P converters; PWB (printed circuit board) components; pneumatic repeaters; indicator tables. Module bases can be reassembled by swapping sub-modules. The FIELDVUE series of digital valve controllers accepts input signals and electrical energy through a pair of twisted-pair wires that enter the terminal block. The input signal also arrives at the PWB component submodule, where it is appended with many parameters, such as node coordinates in the multi-section linearization. Limits and other values. The PWB component submodule then signals the I/P converter submodule. The I/P converter converts the input signal into a pressure signal. The pressure signal is sent to a pneumatic repeater, amplified and sent to the actuator as an output signal. The output signal can also be sensed by a pressure sensitive element placed on the PWB assembly sub-module. Diagnostic information for valve actuators. The valve stem and actuator stem position is used as an input signal to the PWB submodule, which is used as a feedback signal for the digital valve controller. At the same time as the input signal value, if there is a deviation, the microprocessor generates a new current value through the calculation process. The deviation is corrected until the valve position signal of the regulating valve matches the input signal and the deviation is zero.
Intelligent valve positioner has a wealth of software functions, such as automatic adjustment of zero and full scale; configuration required valve characteristics, coupled with dedicated software such as FISHE 's intelligent positioner's VAVLELINK software; NELE 's intelligent positioner's VAVLEMANAGE TM , software nine With advanced dynamic diagnostics, the mechanical performance indicators of the valve are combined with the indicators of the control system to determine the valve damage as shown in Figure III.
Conventional electric locators generally have an input impedance of 250 ohms, and the smart locators have input impedances greater than 250 ohms. The brands have different brands, different models, different impedances, and different expressions. The greater the impedance, the greater the DCS carrying capacity, which will affect the system operation. , In our factory melamine device loop linkage adjustment found that the DCS output card can only drive the NELES ND8221/S1 positioner control valve moves only 28% of the stroke, the specific solution, replace the drive larger security (internal resistance Smaller) The reduction of the safety voltage in the regulation loop increases the output power of the smart positioner. The input impedances of smart valve positioners of some well-known brands are listed below.
5. Valve position detection of device and transmission intelligent valve positioner
The precise valve position signal is an important parameter of the control system and is an important parameter that constitutes an advanced dynamic diagnosis function. Therefore, high-precision measurement and transmission of valve position detection are required. Various brands and models have different valve position detection devices and different feedback transmission signals. See Table 2 for details.
6 Intelligent Valve Positioner Intelligent Features
1.2.1 Real-time information control, increased security and reduced expenses
1 ) Improve control: Bidirectional digital communication brings you the information of the current situation of the valve. You can rely on the valve work information to make a management decision on the process control in a systematic way to ensure timely control.
2 ) Improve security: You can use the communicator, PC , or system workstation to select information from field junction boxes, terminal blocks, or in a safe area such as a control room , minimizing your chances of facing a hazardous environment, and not having to be in person on site.
3 ) Protecting the environment: The valve leakage detector or limit switch can be connected to the auxiliary terminal of the intelligent digital valve controller, so as to avoid additional site wiring. If the limit is exceeded, the meter will alarm.
4 ) Save on hardware expenses: When FIELDVUE series digital valve positioners are used in integrated systems, FIELDVUE digital valve controllers can save hardware and installation costs by replacing the regulators. The FIELDVUE series of digital valve controllers saves investment in wiring investment, terminals, and I/O requirements by 50% . At the same time, FIELDVUE meters use two-wire power supply, which does not require separate and high-priced power supply wires. They replace the existing analog instruments fitted to the valves, saving the high cost of laying separate power lines and signal lines.
7. Intelligent valve positioner common faults and on-site processing
( 1 ) The change of the control signal and the non-actuation of the regulating valve (excluding the valve body factor) is a common fault in the operation of the intelligent positioner DVC5000 . The key measures are to check whether the valve position transmitter operates freely, electrical continuity, etc. ( 2 ) Valve position Difficult to control, small signal does not move, when the signal is not fully open or not fully closed, after repeated adjustment is still not normal, the replacement of the new positioner is still not normal, the replacement of the new positioner is still not normal, and finally found the ND800 intelligent The feedback lever of the positioner and the internal signal conversion portion of the positioner are non-contact inductively connected. The feedback lever can be rotated 360 degrees to cause a 180- degree difference between the feedback lever and the valve stem connection , and the re-installation of the adjustment work is normal ( 3 ) the small opening degree oscillation. The operation of the regulating valve at a small opening easily causes oscillation. The solution is to avoid the small opening work, flow closed type reflow open, more trouble. The smart locator solves this problem more easily. Simply set the parameters and the oscillation is greatly improved. Now take DVC5000 as an example to illustrate the use of HART275 communicator to adjust the DVC5000 positioner on-line status, select STABILIZE/OPTIMIZE with shortcut keys , select DECREASE RESPONSE (decrease response sensitivity 0 , select lower setting parameter group, improve ( 4 ) Smart valve positioner and HART275 type communicator cannot communicate normally.Common reasons to check if the effective voltage in the field is greater than 12V , if the output impedance is lower than 250 ohms, if the wire capacitance is too high, the input signal is less than 4mA .
8. Application and Maintenance
DVC5000 intelligent positioner is installed in April 2002 for use in our devices use melamine to more than 100. It has been configured and verified using a communicator. Its linearity can reach 99% . The zero point and range and hysteresis can be controlled within the range of precision requirements. The control is extremely stable and the ability to resist interference is particularly strong. , fully meet the requirements of process control.
The FIELDVUE locator has minimal maintenance and is essentially maintenance free. Its field adaptability is exceptionally strong. However, in order to ensure long-term and stable operation, instrument personnel should do the following work.
1 ) In order to ensure a good working environment and prevent accidental damage, the working environment around the positioner should be regularly checked. At the same time, it ensures the stability and cleanliness of its working gas source, and reduces instrumental fluctuations and failures caused by external factors.
2 ) The instrumentation personnel should check the leakage and working condition of the valve and positioner every week to eliminate hidden dangers in time. Monthly use the communicator to check the characteristic curve of the positioner, check the parameters such as zero point, range, linearity and hysteresis, and optimize and adjust it to ensure its working quality.
3 ) Regularly check and maintain the regulating valve to ensure the working quality of the valve. At the same time, the parameters of the DCS regulating control loop are optimized to ensure coordination and stability with the positioner.
4 ) Due to DCS and other reasons, its field bus and software functions have not been fully developed and utilized, and intelligent maintenance and diagnostic functions cannot be completely used, but the daily maintenance volume is greatly reduced.
Through the use of melamine devices in the past two years, the intelligent valve controller has stable performance and easy adjustment.
2. Performance
Because the intelligent valve positioner is a new generation of microprocessor-based products with high precision valve position signal sensor output pressure sensor, etc., and thus has a high control accuracy ( 0.5 - 1% ), (regular electrical valve positioning The accuracy is only 2-5% ), and it has long-distance configuration, debugging, diagnosis, data management and other operations. After more than one year of operation, it has been compared with the performance, usage, and cost performance of ordinary valve positioners, as shown in Table 1 .
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