Analysis of the application of intelligent valve positioner in chemical plant Electric and gas valve positioner

Analysis of the application of intelligent valve positioner in chemical plant Electric and gas valve positioner

Abstract: The composition, working principle, characteristics and application of intelligent valve positioner are introduced. In the process of chemical control, intelligent valve positioner can be used to achieve precision regulation and increase the accuracy and stability of process control.
Key words: intelligent valve positioner piezoelectric valve adjustment
Regulating valve is the terminal of the control system, once its failure, will directly affect the safe operation of the device, the impact on the production process is very great. The use of intelligent valve positioner can improve the flow characteristics and performance of the regulating valve, and digital information communication can be carried out with DCS or bus equipment to improve the production control ability of enterprises, and provide guarantee for the safe and stable production of the device. The following to the United States FLOWSERVE company production of Logix520 intelligent valve positioner in my factory application as an example, to introduce the intelligent valve positioner.
1. The shortcomings of conventional locator
1) Conventional positioner is mostly mechanical force balance principle, it uses nozzle baffle mechanism, movable parts are more, easy to be affected by temperature fluctuations, external vibration and other disturbances, poor environmental resistance; The spring elasticity coefficient can change in the harsh environment, will cause the control valve nonlinear, resulting in the quality of control; External vibration to the force balance mechanism, easy to cause component wear and zero and stroke drift, but also make the locator difficult to work;

2) Due to the characteristics of the nozzle itself, the actuator also needs to consume a large amount of compressed air in the stable state. If a large number of actuators are used, the energy consumption is larger; And the nozzle itself is a potential source of failure, easily blocked by dust or dirt particles, so that the locator can not work properly;
3) conventional locator manual adjustment needs to use ** equipment, not isolated control loop is impossible, and the zero point and stroke adjustment affect each other, must be repeatedly set, time-consuming and laborious, nonlinear serious, it is more difficult to adjust.
2. Composition and principle of Logix520 intelligent valve positioner
2.1 Composition of Logix520 Intelligent Valve positioner
Logix520 intelligent valve positioner is a kind of intelligent valve positioner with HART communication protocol, which consists of three parts: microprocessor electronic control module, including HART communication module and local user interface switch; Piezoelectric valves for electrical/pneumatic converter modules; Valve position sensor.
2.2 Working principle of Logix520 intelligent valve positioner
The whole control loop is controlled by two lines, 4 ~ 20mA signal. The HART module sends and receives the digital information superimposed on the 4 ~ 20mA signal to realize the bidirectional digital communication with the microprocessor. The analog amount of 4 ~ 20mA signal to the microprocessor, compared with the feedback of the valve position sensor, the microprocessor according to the deviation of the size and direction of the control calculation (a control), to the piezoelectric valve issued electronic control instructions to make it open and close action. Piezoelectric valve according to the control instruction corresponds to the width of pulse pneumatic amplifier output pressure increment, pneumatic amplifier output and is at the same time feedback to the control circuit, comparing with the result of the microprocessor operation again (secondary control), through two levels of control output signal to the actuator, the change of air pressure in the actuator controls the valve stroke. When the control deviation is large, the piezoelectric valve sends out a wide pulse signal, so that the positioner output a continuous signal, a large change to the actuator’s signal pressure to drive the valve fast action; As the valve approaches the required position, the difference between the command position and the measured position becomes smaller, and the piezoelectric valve outputs a pulse signal with a small pulse width, which changes the signal pressure of the actuator intermittently and in a small amplitude, so that the actuator moves gently when approaching the new command position. When THE valve REACHES the required position (entering the dead zone), the piezoelectric valve has no pulse output, the positioner output remains zero, so that the valve is stable in a certain position.
3. Adjust the Logix520 intelligent locator

Through the local user interface DIP setting switch, you can complete the gain, positive and negative effects of the locator, locator characteristics and whether to allow automatic adjustment and other basic Settings; The positioner can be calibrated automatically or manually without adding tools; And can be through the local user interface manual control button, to achieve manual control of the valve.
3.1 Automatic Adjustment

Set the “Quick_cal” DIP switch to “Automatic”, press and hold the “Quick_cal” button on the ground boundary panel of the locator for about three seconds, and the locator will fully close the valve and register the 0% position. Then, open the valve to stop and register the 100% position, and repeat twice. During this process, the locator will measure the positioning speed in both directions. To determine the ** small positioning increment (during the second pass, the midpoint shift of the detection valve position is slightly stopped at 50%), during which the status indicator light on the panel will flash in the order of “Y-R-G-G”, indicating that the calibration is in progress. The calibration is done automatically when the indicator lights return to the change sequence from green.

3.2 Manual Adjustment

Setting the “Quick_cal” DIP switch to “slow forward” will close the valve at the beginning of the calibration process, with the zero position automatically set at the seat and the range manually set by the user; When the status indicator on the panel flashes in the order of “Y-R-R-G”, use the “Slow Forward ¡ü” button, manually adjust the valve to about 100% position, and then press two “Slow forward” buttons at the same time, the valve will carry out switching trip. When the status indicator on the panel flashes in the order of “Y-R-R-G” again, use the “Slow Forward ¡ü” button, Adjust THE VALVE AGAIN TO 100% ACCURATE, THEN PRESS BOTH “SLOW FORWARD” BUTTONS AGAIN TO REGISTER 100% POSITION, AND NO FURTHER OPERATION IS REQUIRED WHILE THE CALIBRATION IS COMPLETED. The calibration is completed when the indicator lights return to the sequence from green.

This function makes the adjustment work convenient and fast, and the linearity of the adjustment is good, the precision is high, the response speed and the dead zone is moderate, the stability is good.

4. Additional features of the Logix520 valve positioner

1) The local panel is equipped with three red, yellow and green leds, which indicate the operation state or warning condition through a variety of combinations, with diagnostic and monitoring functions; The basic meaning of the three LCD flash sequence combination: any flash sequence starting with green indicates that it is in normal operation mode without internal problems, errors and alarms; Any flashing sequence starting with a yellow light indicates that it is in a special check or test state; Any flashing sequence that begins with a red light indicates an operational problem or malfunction.

2) The gas consumption is very small. At the steady state of 0.6mpa, * is 0.12nm3 /h, which is less than 8% of the conventional locator; Insensitive to the change of air source pressure;

3) The same model can be used for both straight stroke and angular stroke; By selecting the double acting module, the double acting piston cylinder actuator can be controlled.

4) The “close” function sets the initial wind pressure by default to ensure the appropriate positioning pressure of the actuator on the seat, so that the regulating valve can ensure zero “close off” under different working conditions;

5) Use HART communication protocol to carry out two-way communication with the locator;

5. In the actual use should pay attention to the problem

5.1 There is a high requirement for the load capacity of the regulation signal

In actual use, due to the high input impedance of Logix520 positioner, when the input signal is 20mA, the minimum required value of supply voltage is 12VDC, with load capacity is not less than 600¦Ø, otherwise the positioner can not work normally; ** The minimum input current is not less than 3.6mA to ensure the performance.

5.2 The dead zone of the positioner should be set reasonably

The smaller the locator dead zone setting, the higher the positioning accuracy, which gives people a mistake, that the smaller the dead zone is the better, but this will make the piezoelectric valve and feedback rod and other moving parts more frequent action, sometimes cause valve oscillation, affect the service life of the positioner and valve, so the positioner dead zone setting is not easy to be too small; After the Settings of the locator are changed, they must be adjusted to take effect.

5.3 Installing the Logix520 Locator

An important principle for the installation of the positioner is that the positioner, the valve stem and the feedback lever should form a closed-loop negative feedback.
When installing the positioner, the valve stem and the feedback rod are not connected. Rotate the feedback rod by hand. If the action direction of the valve stem is opposite to the action direction of the feedback rod, it indicates that the closed-loop negative feedback has been formed. At this time to the regulator valve position at 50%, and make the feedback rod in a horizontal position, and then the feedback rod and valve stem fixed, so as to ensure that the positioner works in a good linear section. The locator installation is not straight, will also increase its linear deviation.
5.4Logix520 locator flow characteristics selection

Regulating valve flow characteristics are determined by the processing characteristics of the spool, if the process requirements are consistent with the locator output characteristics should choose linear output; In actual use, if the valve core features do not tally with the technological requirements, through the locator output characteristic can be set up to change the whole flow characteristics of the valve, such as for the linear characteristic of regulating valve, the valve core can be through the positioner output characteristic is set to equal percentage features, can turn with linear valve spool valve into equal percentage flow characteristic of the valve to be used.

5.5Logix520 locator maintenance

When different functional modules of the positioner are damaged, the positioner can not be used, if the whole replacement, the cost is high; Then can use trouble-free module to reassemble the locator, but after assembly have to be reset according to the different regulating valve, due to the use of the locator’s regulator (travel, etc.) has changed, the use of automatic calibration can can not meet the use requirement, then we can manually set-up determine its stroke, and then use automatic calibration calibration. This can make the regulating valve positioning fine, with a suitable response speed, so as to meet the requirements of process control, but also save a lot of money.

6. The practical application of Logix520 valve positioner in a factory

1) The C-203A/B/C oxygen compressor unit of a methanol unit was violently shaken, and the PCV-2008A/B/C regulating valve on the reflux pipeline was used with conventional positioner, and the nozzle baffles were worn out soon, the zero range drifted, the positioner was frequently damaged, and the process control quality was extremely poor, endangering the safe production; Using Logix520 positioner, due to its fully sealed structure, movable parts are few, force conversion process without mechanical transmission, eliminate the interference caused by vibration, so that the problem is resolved, reduce the maintenance amount, save money, ensure the quality of process control and safe operation of the device.

2) a device gasifier waste boiler liquid level LCV – 2003 – a/B valve, using conventional locator before modification, and installed near the gasifier, work environment, high temperature (80 ¡æ), state of medium to high temperature and high pressure (304 ¡æ, 8 mpa), to prevent medium leakage, will be tighter packing, and lead to stem movement hysteresis, slow step change valve position, The stability is poor and has great influence on the process control. And the operation is difficult, instrument maintenance volume; After using Logix520 positioner, the positioner can release a very short control pulse through two stages of control, coupled with the piezoelectric valve connected with the main control air path, so that the signal output to the membrane head is more accurate and stable, and the change of valve position is smooth and fine. The “Valvestability” DIP switch can also be applied to “HiFriction” and “LoFriction” in the locator interface to eliminate the effects of high friction and unbalanced forces on the valve stem.

3) In April 2005, when the meter maintenance personnel inspected the liquid level regulating valve LV-1501B in the hydrogenation reactor of a device, it was found that the status indicator light of Logix520 valve locator showed “R-Y-Y-Y”. Thus, it was judged and checked that the membrane head of the regulating valve was leaking, and timely treatment was carried out to avoid the safety production of the device due to the failure of the regulating valve. Also, maintenance personnel found that the status indicator light of Logix520 valve positioner of TV-0706B regulating valve shows “R-Y-Y-R”, which checks out that the output of the reducing valve becomes smaller, and the gas supply pressure of the regulating valve is insufficient, which cannot meet the travel requirements. After inspection, maintenance personnel found that the reducing valve was damaged, and replaced the reducing valve in time and restored the air source pressure according to rated requirements, so as to avoid the influence of process control and product quality due to improper adjustment during operation.