Application of the hottest material level sensor i

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Application of material level sensor in thermal power plant

material level measurement is a branch of material level measurement. The material level of metal accessories of material wear testing machine refers to the interface position between the liquid, powdery solid and gas in the storage container or industrial production equipment, which is divided into liquid level, material level and boundary level sensor according to the specific purpose

at present, China mainly uses raw coal as the fuel for power generation, and most power plant boilers use pulverized coal to feed the boiler. For direct fired boilers, the level of coal bunker is related to the normal operation of the boiler and even the power generation system. The bunker level is too full and overflows, causing a coal caving accident; Low bunker level or emptying will cause unstable combustion and even fire-fighting shutdown. 2. Polycarbonate PC accident. For the boiler with medium storage feeding, there are both coal bunker and powder bunker, so the control of powder level in coal bunker is particularly important. Pulverized coal bunker is the transfer station of fuel. Pulverized coal is transmitted by air. The high-temperature gas makes pulverized coal enter the bunker and have a certain "base temperature", which is generally about 70 ° C. its function is to make pulverized coal discrete. However, this temperature makes the moisture in the pulverized coal evaporate rapidly and be discharged out of the silo by the moisture absorption pipe. The pulverized coal will become more and more dry. This kind of pulverized coal is very easy to collect heat, and the final result of heat collection is combustion. The combustion intensifies the heat collection around and even in the silo, and the vicious cycle starts again and again. In this way, if it cannot be effectively controlled in time, the result will be to burn a large amount of pulverized coal for nothing. According to relevant data, spontaneous combustion pulverized coal accounts for about 0.5% of the total coal consumption for power generation. In addition, the loss of coal bunker pulverized coal explosion is greater. Over the years, coal bunker pulverized coal explosion accidents often occur, causing huge losses to thermal power plants. At present, the most economical and applicable method is to monitor the coal level and powder level of the coal bunker through a reliable material level sensor, so that it is always in the best transfer and suitable control state, which is the primary guarantee for the safe operation of thermal power generating units. Aluminum spot welding machine

II. Principle and characteristics

at present, the material level sensors used in thermal power plants mainly include heavy hammer type, nuclear radiation type, ultrasonic type and capacitance type

1. Heavy hammer level sensor

heavy hammer level sensor is composed of servo motor, steel wire rope suspended with heavy hammer, level sending device and display instrument with microcomputer. After starting, the microcomputer sends a hammer lowering signal, and the servo motor rotates to lower the hammer. When the hammer touches the material level, the transmitter sends a signal to the microcomputer to stop the hammer from falling and send a hammer raising signal. The servo motor reverses to raise the hammer and send the material level signal value to the display meter. After the heavy hammer rises to the top of the silo, the motor stops running, and the above action is repeated after a period of time delay. The display instrument also has devices such as upper and lower limit alarm sending of material level. The heavy hammer level sensor can solve the measurement problem within a certain range, and the typical measurement range can reach 60m. Moreover, this measurement has nothing to do with steam and dust, and has high measurement accuracy

2. Nuclear radiation level sensor

the g-rays emitted by the radioactive sources Co-60 (half-life 5.26 years) and Cs-137 (half-life 32.2 years) can penetrate the container wall and the materials in the container. A g-ray receiver is installed at the lower side of the silo. With the change of the height of the material level, the intensity of the g-ray passing through the material layer is also different. The receiver detects the intensity of the emitted g-ray and displays the height of the material level through the display instrument

this is a non-contact measurement method, which does not need to pierce the container to damage the container, so it is suitable for the measurement of dangerous goods, high temperature and high pressure and other harsh environments. Although g-ray is harmful to human body, it is not dangerous for a limited dose under proper protection

3. Ultrasonic level sensor

an ultrasonic generator and receiver are installed on the top of the silo facing the material surface. The ultrasonic wave emitted by the generator is reflected after it is shot to the material surface through the air layer, and part of the reflection is received by the receiver. The height of the material level can be calculated by multiplying the time from ultrasonic wave transmission to reception by the speed of sound. Since the air temperature will affect the propagation speed of sound waves, it is also necessary to measure the air temperature to correct the sound speed. The ultrasonic level sensor is suitable for measuring the level of block material with large particle size

4. Capacitive level sensor

capacitive level sensor works by measuring the capacitance between the probe and the inner wall of the container, between the two probes or between the probe and the concentric measuring tube, using the principle that the capacitance between electrodes is proportional to the level when the dielectric constant of the material is constant

the capacitive level sensor is characterized by no movable parts and has nothing to do with the density of the material, but it requires that the dielectric constant of the material is greatly different from that of the air. The changing dielectric constant should be compensated during continuous measurement, and a high-frequency circuit is required

energy saving and material saving have become the focus of the extrusion processing industry. Most of the ultrasonic and nuclear radiation level sensors are imported products from abroad, such as the ultrasonic level meters of Kay ray and magnetrol, with an accuracy of 0.25; The minimum blind area of du212 and du213 of German e + H (address + Hauser) company is 0.7m, which cannot be used within this range. The longest distance is limited by sound power, and only the material level within 40m can be measured; The QG type of German e + H company is a nuclear radiation level sensor. The Co-60 or Cs-137 of the emission source is encapsulated in a steel protective cover filled with lead. This cover is equipped with a window that can be opened and closed with sliding appearance, which can be closed when not in use to avoid radiation hazards. The receiver is a tubular structure with a length of 100 ~ 1500mm. It is installed at the position corresponding to the emission source. If the g-ray divergence angle, distance and receiver cooperate with each other, they can be effectively detected in the whole range

III. application

the Institute of automatic control of Shenyang Electric Power College has developed a heavy hammer level sensor and applied it to the level measurement of pulverized coal bunker of 670t/h boiler

(I) composition principle

the model of the heavy hammer level sensor is se-2, also known as intelligent level meter, which is mainly composed of detector and controller. The detector is composed of switched reluctance motor, transmission mechanism and weight (also known as probe), and the height of material level is reflected by the stroke of weight. The controller is composed of single chip microcomputer (8098), display, power converter and panel. The panel adopts PVC film, with nixie tube display, touch keys and indicator lights. Use software to control the lifting process of the heavy hammer in the detector

the weight is fixed on the steel wire rope of the switched reluctance motor, and depends on its own gravity to vertically drop a certain unit height with the winding wheel, so as to judge whether the weight touches the pulverized coal surface by the magnitude of the lifting current. If the lifting current is very small, it means that the hammer has been in contact with pulverized coal. At this time, a pulse signal is sent, and the motor reverses to the initial position, so that the hammer can be lifted to the top of the bin and reset, so as to prevent the hammer from being submerged by coal and powder. If there is little coal or powder, the heavy hammer will continue to fall, and the signal will be collected in sections until it touches the coal or powder level, or the heavy hammer will fall to the limit position of coal or powder level, and then the heavy hammer will be raised to the reset position and give an alarm, the limit switch will be closed, the motor will be powered off, one detection will be completed, and wait for the next detection instruction

the secondary instrument with single chip microcomputer (8098) as the main control unit receives the revolution signal of the reluctance motor transmitted by the photoelectric switch, and displays the material level digitally after data processing

(II) characteristics

se-2 heavy hammer level sensor has the following main characteristics:

① it can operate automatically and continuously during the feeding process, and is not affected by the collapse, buried pressure and impact of materials

② it has high structural reliability and can adapt to the harsh environment with high temperature, large dust and dense smoke

③ it has two functions: manual detection at any time and automatic timing detection

④ it has the function of upper and lower limit alarm and circular display of multi-point material level height

⑤ it has the recovery and processing function of recoverable faults, heavy hammer reset of unrecoverable faults and display and alarm of fault signals

⑥ for unrecoverable heavy hammer failure, the function of forced lifting and lowering of heavy hammer can be used to deal with

⑦ simple operation, safe use and high measurement accuracy

(III) application effect

se-2 heavy hammer level sensor has achieved good application effect after debugging and nearly two years of production operation:

1. It changes the backward state of bunker level detection and improves the monitoring level of level control

2. The history of manual shaking and floating measurement of pulverized coal level has been completely ended, and the workers have been relieved from the heavy manual labor, and the working conditions have been improved

3. It provides reliable material level data for the pulverizing system of thermal power plants and ensures the safe and economic operation of the coal supply system

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