I. introduction of intelligent valve locator
Process control in petroleum, chemical, electric power, metallurgy and other departments have a wide range of applications, and process control system generally consists of hundreds or even thousands of control circuit, a control circuit will create a disturbance, accept or from within a decisive impact on process variables. The interaction between different paths also produces disturbances that affect the process variables. Various sensors and transmitters collect information about the process variables, and the controller takes the information and processes it, so that the process variable returns to its normal range after the load disturbance occurs. All measurement, comparison and calculation work must be performed by the terminal control element to implement the control strategy selected by the controller. The most commonly used terminal control element in the control loop is the control valve. The control valve regulates the flow of fluid, such as gas, steam, water, or chemical mixtures, to compensate for the load disturbance and to make the controlled process variables as close to the required set points as possible.
Valve positioner can provide high positioning accuracy and a more rapid response to process interference when used together with digital control system. Based on microprocessor locator, provides the same dynamic performance with ordinary secondary locator, and monitoring and diagnosis function, a valve helps to ensure that the original excellent properties not down with use. The positioner can be divided into three types: pneumatic, analog and digital. The digital localizer is divided into three kinds: digital non-communication type, HART type and field bus type.
The performance and working principle of intelligent locator
2. 1 features and structure
Main features of smart valve locator:
(1) high output force and movement speed:
(2) high accuracy (minimum stroke resolution can be reached 0. 05%)
(3) simple installation and high automation adjustment;
(4) almost free maintenance operation, which means saving time and convenient application;
(5) manual and automatic calibration function with zero and travel range;
(6) having optional or programmable output characteristics:
(7) have a strong self-diagnosis function;
(8) less energy consumption relative to conventional valve locator;
(9) the setting value and control variable limit value can be selected;
(10) the dead zone setting of the regulating valve can be carried out;
(11) online adaptive program.
(12) the embedded parameters can provide many functions;
(13) the locator can have a flexible and simple configuration
(14) the impact of changes in temperature and compressed air pressure is minimal
Intelligent electric valve positioner compared with the transmitter, has obvious different: one is the external 4-20 ma analog signal as control signal of valve position, at the same time, the 4-20 ma analog current as intelligent electric valve positioner power source, so the intelligent electric valve positioner is extremely harsh to the requirement of low power consumption; 2 it is primarily through intelligent transmitter HART digital communication interface, at the same time send out 4-20 ma analog signal is measured, generally available coupling AD421 chip to realize, while intelligent electric valve positioner by HART digital communication interface card, at the same time, receives input 4-20 ma analog signal valve position control. Many vendors embed various discrete and analog sensors in DVC. For example, Fisher DVC of Emerson process management consists of a discrete output (DO) and four discrete inputs (DI), which are functional modules conforming to the foundation's fieldbus (FF) standard. In addition to meet the DI FF standard and D0 function condition, Fisher also use a proximity sensor instead of similar standard DI limit switch (set the proximity sensor is one of the advantages of can know the final control equipment in the process of application of 0 and 100% of the position, such as implementing emergency shutdown). DVC USES embedded sensors and microprocessors to manage diagnostic test procedures and digital communication with the main system to complete dynamic scanning and advanced diagnostics. Its diagnostic procedures include: the final tracking control equipment parameters, to monitor the total cumulative displacement and conversion number (cycle) DVC normal state parameter testing, to provide storage, microprocessor and/or the alarming signal from the sensor problem, and to allow actuator alarm, such as the problem is more serious, the closed cell or process; Send out the final control equipment alarm signal; When the expected high/low limit displacement deviation and displacement exceeds preset cumulative displacement and cycle index, and the dynamic error range test, in order to check the hysteresis of the final control equipment and dead zone and rotary characteristics; Drive signal and output signal test, change the sensor set point at a controllable rate and draw the operation curve of the final control equipment to determine the dynamic characteristics; Predetermined final control equipment maintenance and feature graphics testing. Process management digital smart locator is a digital instrument used for process control based on the foundation fieldbus technology (FF) and with interoperability, full digital communication and micro-processing technology. Besides having to convert the digital signal to pneumatic signal to drive the basic function of the pneumatic actuators, more with FF bus protocol, easy to provide operations and control in the process of regulating valve and a wide range of equipment information locator, and the information transmitted on a machine, make the operation personnel, maintenance personnel and engineers can timely grasp the running state of the equipment. The positioner can be installed on the pneumatic actuator of the pass-through regulating valve and the rotary regulating valve, and flexible selection according to the design of the control system. The basis mainly includes control function and diagnostic function. Standard control function (SC) for routine control; Field bus logic control function (FL) is used to achieve logical control. Field bus diagnostic function (FD) can be accepted by any host system with FF protocol; Advanced diagnostic and operational diagnostic functions are used for related equipment management system software (such as Fisher's AMS ValveLink software) to provide visual cues and alarm information. FF bus modular structure is used in digital intelligent positioner built-in modules including resource block (the resource block), transfer (transducerblock) and function blocks of (a function block) transmission block for analog output (AO) and electric/gas converter (I/P), pneumatic components, such as distance sensor locator internal hardware connection. Functional blocks provide functions for routine control and logic control. Conventional control blocks include A0 (analog output), PID and ISEL (input selection) function blocks. A0 can receive output signals from other functional blocks (such as PID) and convert to actuator control signals to transport blocks. PID control function of PID control loop. ISEL provides 4 input selection function and the qualified signal as input signal of PID function block. Logic control blocks include D1 (discrete input) and DO (discrete output) function blocks. DI receives a DI signal from the field instrument and sends the signal to other functional modules. Used as limit switch and valve position detection. Do to output the switch volume set value to the specified I / 0 channel and generate the output signal, providing the function of regular open/close control and 5% incremental positioning.
2. 2 working principle
The digital intelligent locator adopts modular components, and the main modules include three submodules, such as main control board, electric/gas converter and pneumatic components. The main module can replace its working principle diagram as shown in the picture above in the scene environment without the need to remove/connect the cable and the pneumatic pipe.
Digital intelligent positioner working principle diagram input signal through the twisted pair transmission to master control board microprocessor, after operation by the I/P to convert the digital signal to pneumatic signal, drive actuator to valve stroke a given position.
When the input signal increases, the output signal of the I/P converter increases and drives the actuator through the pneumatic component to drive the stem downward movement.
At the same time, even on the valve stem stroke measurement sensor, the position of the valve stem of feedback signal to the main control board microprocessor and comparing with the given signal, balance until the stem is in the requirements of the position.
However, the input signal is reduced by an hour, the output signal of the I/P converter is reduced, and the actuator is driven by the pneumatic component to drive the stem upward.
At the same time, the stroke detection sensor on the stem can feed the position signal of the stem to the microprocessor on the main control board and compare with the given signal until the stem is in a new balance position.
Pneumatic signal line pressure sensor used to detect whether there is leakage, leakage, if any, is the responsibility of the pressure sensor to detect the warning or alarm information by the microprocessor and inform through digital communication operator or maintenance staff handle in time.
Due to the digital intelligent positioner used microprocessor technology and new type of sensor technology and fieldbus technology, make it more than just a valve positioner, but intelligent digital controller and valve positioner integration of new intelligent digital controller. It greatly improves the precision of the control loop terminal actuator a control valve to reach 0. 5%, greatly improved the control quality of the control loop. At the same time, it moves the control function down to the field instrument, and the risk of control system is completely dispersed.
The example of significant application progress for DVC technology is part of the travel test for emergency shutdown (ESD) valves. Some companies increase profits by extending the running time of the continuous process. Extension of the process run time including the safety instrumentation system (SIS) requires changing the frequency of the security system test and using the emergency shutdown (ESD) valve part of the stroke test. Partial stroke test can help the operator ensure that the ESD valve within the security system range can operate as required. The main system DVC, which will have both board diagnostic programs and available communication performance data, is used as an ESD valve to make it part of the partial travel test solution to meet the general requirements.
By providing analysis of valve performance degradation, the digital smart locator is a great help for predictive maintenance, which is important for critical valves in the security system. Some digital smart locators do not release the actuator pressure completely when a partial stroke action test is performed, if for some reason the valve is stuck. You can ensure that the valve is not shut off when the card is unstuck. The digital smart locator will give up the test and issue an alarm, telling the operator that the valve is stuck.
The application of HART protocol in intelligent locator
HART interface card connects the intelligent electric valve locator and the HART host equipment to realize the remote operation of the intelligent electric valve locator.
3. 1 the overall structure of the interface card and valve locator
There are two main types of communication modes of HART protocol equipment: one is question and answer; The other is group. Use the question-and-answer mode when parameter configuration.
The intelligent electrical valve locator is mainly in control of the actuator's itinerary during normal operation, so the state of the actuator's schedule and the intelligent electric valve locator is the most important. In the design, the two kinds of information are taken into a group mode so that the host can detect the state of the intelligent electric valve locator in the field and the valve position controlled by it.
In software design, the intelligent electric valve locator adopts the interrupt mode to receive the host signal. When the host message arrives, the data sent by the host is received by triggering the serial interrupt. Since the data is transmitted at 1, 200 bit/s, each character needs 9. 167ms time, relative to the execution speed of the instruction (1. The 8432MHz oscillator is much slower. In order to improve CPU utilization, in the serial interrupt handler, every character is interrupted to return. Let the CPU handle other transactions. After the data has been received, the data will be processed, then the data will be processed according to the correct error and command of the data, and then the response frame will be sent to the host.
3. 2. Communication of the locator
L have two timers, we use the timer to do overtime management, whenever field instruments to receive a single character, 1 to start the timer timing (no more than one character at a time of transmission time and the time interval between the sum). When the timer is off, it is not yet received the next character that triggers the timer l interrupt. This handler is mainly reinitialized to receive new data. If a character is received within a timed time, the timer l should be initialized to receive new characters. The receiving process flow is shown in figure 7. In the group mode, the electrical valve locator sends HART data to the host regularly, and the timing time is BT= 75ms.
4. Problems and solutions in the debugging process
Field use valve positioner is very wide range, a pneumatic valve positioner, electric valve positioner, a thin film actuator valve positioner, a piston actuator valve positioner, strong balance type displacement of valve positioner, a balanced valve positioner, valve positioner is widely used, in the process of production, hard to avoid can appear all sorts of fault, to ensure the quality, and safe production, must be timely eliminate all fault locator may be produced.
4. 1. Cause analysis of fault
To eliminate the failure of valve locator, it is necessary to correctly determine which part of the valve locator and which element has failed. Usually has the following two kinds of failure analysis: one is according to the transfer function of valve positioner, to every tache of the valve positioner, each element analysis, the overhaul is not applicable on the spot, but for the analysis of the problems, but very effective; The second is to analyze and judge the phenomenon of failure according to the troubleshooting. This method is suitable for on-site maintenance. The following causes of common failure of valve locator are analyzed as follows:
4. 1. 1 valve locator has signal input, but no output pressure signal
(1) the electric/gas positioner, the armature and the coil frame have a foreign body.
(2) constant throttle hole plugging.
(3) nozzle baffle with bad or nozzle baffle damage.
(4) damage to the diaphragm in the amplifier (metal diaphragm or rubber diaphragm).
(5) faulty air connections (including amplifiers).
(6) the input signal of the electric/gas positioner is negative.
(7) diode opening or wiring in the input junction box of the locator.
(8) the size of the air source pressure is not required.
(9) the amplification of the amplifier is too large.
(10) the installation of the magnetic steel polarity of the electric/gas locator is different.
(11) the prestressing force of the amplifier is overweight.
(12) slide valve in the slide valve is killed by foreign body card.
(13)& ldquo: manual/automatic & rdquo: switch position is not correct (non-manual position and non-automatic position).
(14) electrical/gas positioner input signal short circuit. & lsquo;
(15) balance spring installation, debugging is not good.
