Application of the hottest PLC in boiler slagging

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Application of PLC in the boiler slag removal renovation project of power plant

our company carried out the boiler slag removal renovation in the summer of 2005. It is planned to set a scraper conveyor under the two accident slag discharge outlet and normal slag discharge outlet of the slag cooler on each side of each boiler. The bottom slag collected by the scraper conveyor is sent to the bucket elevator, which is broken by the slag crusher and then sent to the original pneumatic conveying system and sent to the slag bin

the control range of the system includes the control of all equipment from the normal outlet and accident outlet of the slag cooler to the inlet of the raw pneumatic conveying system

I. system description

a chain bucket conveyor and a scraper conveyor are set under each slag cooler slag discharge port, two accident slag discharge ports and normal slag discharge ports to send the bottom slag outside the slag cooler. The bucket elevator lifts the crusher at the top of the slag bucket. After being crushed by the crusher, it enters the pneumatic conveying system and is sent to the slag bin by the pneumatic conveying system

in order to ensure the positive pressure of the operation of the slag cooler and that the hot air will not be discharged from the accident slag discharge port, a differential pressure detection device is installed in the first and second rooms of the slag cooler to control the plug-in door switch of the accident slag discharge port with the differential pressure signal, so as to control the material height of the bottom slag in the slag cooler, so as to ensure that the hot air in the slag cooler will not be discharged from the accident slag discharge port

in order to ensure that the bottom slag in the slag cooler can be completely discharged from the normal slag outlet, the normal slag outlet of the original dn420 is expanded to dn550. The middle slag hopper is installed under the normal slag discharge port, and the high and low material level meters are set on the middle slag hopper to control the material level in the middle slag hopper with the material level meter, so as to ensure that the hot air in the slag cooler will not be discharged to the subsequent mechanical conveying system. The outlet of the intermediate slag hopper is equipped with a gate and an electric feeder to feed the subsequent mechanical conveying system evenly from the normal slag discharge port

considering the high temperature of the bottom and coated peek layer (PEEK) slag discharged from the accident slag discharge port, in order to ensure the safety and reliability of the bucket elevator, each set of system bucket elevator is equipped with two sets, one for operation and one for standby; The outlet of the bucket elevator is equipped with a local emergency slag discharge port to ensure local slag discharge in case of subsequent pneumatic conveying system failure

in view of the above situation, the current problems can be solved by using PLC (programmable controller) control system. The working principle block diagram of the system is as follows:

there are three working modes for system operation: "remote automatic", "remote manual" and "local manual". "Remote automatic" mode is the main normal operation mode. According to the conditions that the system meets the automatic sequential operation, the whole slag removal process is completed on the operator station (touch screen in the control room). During the automatic sequence execution, any fault or operator interruption signal can interrupt the running program and return to the safe state, and the fault of program interruption or operator's instructions will be displayed on the touch screen in real time. After the fault is eliminated, the automatic control can be started after it is confirmed to be correct. The system has rich protection and fault interfaces for operators to operate and analyze. The "remote manual" mode allows the operator to touch each controlled object on the touch screen. There are permission conditions for remote control operation to prevent operators from misoperation. In the remote manual mode, the system provides rich help, operation guidance and feedback information to guide the operation of operators to prevent misoperation. "Local manual" mode is that the operator operates the controlled object through the local control box, and there is mutual interlocking between the local operation and the remote programmed operation

1. Characteristics and composition of PLC control system

plc has long been widely used in the field of modern industrial control. In terms of the control function of PLC, it has the advantages of preciseness, convenience, easy programming, easy installation and high reliability. It has strong versatility and wide adaptability, especially in the field of logic control such as digital input/output. PLC has abundant logic control instructions and advanced application instructions. It provides high-quality hardware, high-level system software platform and easy to learn and program application software platform. In addition, PLC not only has its own network system, but also has open I/0 and communication interface, so it is easy to establish network and realize remote access

s series produced by Siemens is adopted for PLC. Since the PLC system on site is far away from the upper computer in the control room (about 800 meters), the communication system needs to be equipped with RS-485 repeaters in pairs to ensure the stability of system operation

(1) system structure and hardware configuration

according to the control requirements, CPU module adopts CPU314, digital input (DI) adopts sm321 module, digital output (do) adopts sm322 module, analog input (AI) adopts sm331 module, analog output (AO) adopts SM332 module and im365 and other modules, im365 realizes rack expansion, and the upper computer adopts easyview MT510T true color touch screen for display and control, The process flow, measurement parameters, control mode, sequential operation status and control object status of the whole dry slag system can be clearly displayed on the touch screen. When the parameter exceeds the limit alarm or the control object fails or the status changes, they are displayed in different colors, so that the operator can understand the operation of the system at a glance and adjust the system parameters in real time according to the process requirements

(2) the function realization of the control system

plc programming is directly related to whether the bottom slag system can work normally, and the key of programming is the programmer's understanding of the process system and the flexible application of programming technology. Therefore, in the program design, the characteristics of the gas supply pressure regulation system are first considered, and the program design is refined, divided into multiple program modules, and modular programming is implemented. In this way, it is convenient to add or delete program modules, facilitate the adjustment of process on site, and improve different program modules according to the controllability of supporting equipment

plc programming software adopts SIMATIC STEP7 V6 software platform of Siemens company to complete hardware configuration, address and station address allocation and compile the control program of the whole production process. The upper computer monitoring software adopts the domestic software Kingview, all of which adopt the Chinese interface, which is convenient for the development and operation of the system. The system runs on the Windows2000 Chinese platform, which can realize the comprehensive monitoring of the production process, form a historical record of important parameters, and display them to the operators in the form of statements or curves. Through the VB language script, the historical trend and real-time trend of important parameters can be displayed on the upper computer in the main control room, and the manual automatic switching, operation, high and low limit reporting of interlocking regulation can be realized. The famous luxury brand mobiado in Russia once launched the display and accumulation of alarm and flow data called Grand 350 pioneer in 2010, meeting the scheduling needs of high productivity

(3) field display

MCC screen is used for parameter control on site, and the touch screen program is completed by configuration software. The man-machine interface adopts Chinese menu, which has friendly interface, convenient operation and strong functions. It is mainly used for the display and operation of field pressure, flow and valve position. It can be used as the basis for operators to operate on site

second, the system realizes the automatic control and monitoring of the slag removal system

mainly includes the following functions:

(1) flexible operation mode and powerful system control function: the system can realize the upper computer operation, control cabinet operation and local manual operation

(2) alarm function: when the temperature exceeds the process requirements, the over limit alarm can be realized on site and locally

(3) simple and convenient parameter setting: the pressure setting value, P, I, D and other parameters of pressure regulation can be set in the upper computer

1. System control function

(1) process control function:

1) the system realizes PID automatic adjustment control of bed pressure

2) linearize, filter and convert the collected analog signal into engineering units

3) the temperature and pressure compensation of flow signal is realized, and the measurement accuracy of the instrument is improved

(2) logic control

interlocking logic control realizes on/off control, logic control and user-defined function blocks, etc. The system can realize solenoid valve control, parameter out of limit alarm and other functions

(3) human machine interface

hmi system includes main process screen, subsystem screen, which is intuitive and rich, and has PID adjustment and display functions, including the real-time trend of process quantity change trend

III. software design

according to the specific situation of the system, the following aspects are important to consider in the software design process of the PLC system:

(1) data acquisition and engineering quantity conversion

(2) P then expensive methods similar to "tearing tape" must be used to peel off the ID algorithm of each layer

(3) flow temperature and pressure compensation calculation and flow accumulation calculation

ladder diagram (ladder) programming is used for the logic control in the system, Intuitive and convenient; For the PID loop control flow temperature and pressure compensation calculation and the cumulative calculation of flow, the statement table (STL) programming is used, which is compact and flexible. PID regulation is the most important control program in the system, so the PID algorithm is introduced in particular

1. PID algorithm

step7 provides two commonly used PID algorithms: continuous PID (fb41) and discrete PID (fb42). According to the actual requirements, fb41 is selected. In Kingview, the drawing function is used to simulate the operation panel of a PID regulator to complete the functions of manual/automatic switching, given value input, manual output value input, PID parameter (proportional coefficient, integral time) input in PID regulation and control

the output of PID algorithm is actually proportional (P), integral (I) Sum of three parts of differential (d):

Mn = MPN + min + MDN

MPN = gain amount (SPN PVN)

MPN = gain amount TS/Ti amount (SPN PVN) + MX

MDN = gain amount TD/TS amount (pvn-1-pvn)

film thickness measurement method (method a) gb/t 1764 ⑺ 9 (89)

Mn: output value at the nth sampling time

mpn: the proportional effect at the nth sampling time is proportional to the deviation

min: the integration function at the nth sampling time can eliminate the static error and improve the control quality

mdn: the differential effect at the nth sampling time can be adjusted according to the change rate of the difference, and the overshoot can be suppressed

spn: the set value at the nth sampling time

pvn: the process value at the nth sampling time

mx: the integral function at the n-1st sampling time, which will be refreshed automatically after each sampling calculation

gain: loop gain, P parameter

ti: integral time constant, i.e. I parameter

ti: differential time constant, i.e. D parameter

ts: sampling time

from the above formula, it can be seen that the parameter p (gain) is proportional to the action of P, I and D, which determines the sensitivity of the PID loop, that is, the speed of adjustment; The larger the I parameter is, the weaker the integral action is, while the larger the D parameter is, the stronger the differential action is. PID parameters cannot be determined by theoretical calculation alone. The only measurement standard is the accuracy and stability of the controlled parameters (pressure). Therefore, in the actual debugging, we refer to the real-time curve of the controlled parameters, observe and analyze repeatedly, so as to achieve the best control effect. (end)

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