Brief analysis of the application principle of material level sensor

At present, China mainly uses raw coal as fuel for power generation. Most power plant boilers use coal powder to feed the boiler. For direct-fired boilers, the level of the coal bunker is related to whether the boiler or even the power generation system can operate normally. The overflow of the coal bunker is too full, resulting in a coal spill accident; a low level of the coal bunker or emptying can cause a major accident, such as burning instability or even fire extinction. For the boilers with medium storage, there are both coal bunkers and powder bunkers. The control of the coal bunker powder level is particularly important. The pulverized coal bunker is a transfer station for fuel. The pulverized coal is transported by air. The high-heat gas makes pulverized coal enter the coal bunker with a certain “base temperature”, generally around 70° C., and its role is to make pulverized coal. There is a certain degree of dispersion.

However, this temperature causes the moisture in the pulverized coal to rapidly evaporate and is discharged outside the bin by the moisture absorption tube. The pulverized coal will be more and more dry. This pulverized coal is extremely easy to collect heat, and the ultimate result of the heat collection is combustion. Burning aggravates the heat collection in the vicinity and even in the warehouse. It is a cycle of vicious reincarnation. If it cannot be effectively controlled in time, the outcome will be to burn off a large amount of pulverized coal in vain. According to relevant information, self-fired coal meal accounts for about 0.5% of the total coal used for power generation. In addition, the explosion of pulverized coal in the coal bunker is even greater. Over the years, pulverized coal explosions in coal bunkers have frequently occurred, causing huge losses to thermal power plants. At present, the most economical and most suitable method is to monitor the coal level and the powder level of the coal bunker through a reliable level sensor, so that it is always in the best state of proper transfer control. This is the primary guarantee for the safe operation of thermal power generating units.

Principles and Features The material level sensors currently used in thermal power plants mainly include hammer type, nuclear radiation type, ultrasonic type, and capacitive type.

1. Heavy hammer type material level sensor The heavy hammer type material level sensor consists of a servo motor, a wire rope suspended with a hammer, a material position sending device and a display instrument with a microcomputer. After starting, the microcomputer sends a drop hammer signal, and the servo motor rolling down puts down the weight. When the heavy hammer meets the material surface, the sender sends a signal to the microcomputer so that the heavy hammer stops falling and sends out the rising hammer signal. The servo motor is reversed. Raises the weight and sends out the material level signal to the display meter. After the weight has risen to the top of the warehouse, the engine stops and the above action is repeated after a delay. Display device on the material level, the lower limit alarm signaling and other devices. Heavy hammer level sensor can solve the measurement problem within a certain range. The typical measurement range is up to 60M, and this measurement has nothing to do with steam and dust, and it has a high measurement accuracy.

2. Nuclear radiation level sensor The G-ray emitted from CO-60 (half-life 5.26 years) and CS-137 (half-life 32.2 years) can penetrate the container walls and the contents of the container. A G-ray receiver is installed on the underside of the silo. Following the change of the height of the material plane, the intensity of the G-rays passing through the material layer is also different. The receiver detects the intensity of the injected G-rays and displays the height of the material level through the display meter. .

This is a non-contact measurement method that eliminates the need to pierce the container and damage the container. Therefore, it can be used in hazardous environments and in harsh environments such as high temperature and high pressure. Although G-rays have a harmful effect on the human body, for a limited dose, there is no danger under proper protection.

3. The ultrasonic type material level sensor is equipped with an ultrasonic generator and a receiver at the top of the silo opposite to the material surface. Ultrasonic waves emitted by the generator are reflected by the air layer and then reflected by the receiver. A part of the reflection is received by the receiver. The height of the material level can be calculated by multiplying the ultrasonic radiation time until reception and multiplying the sound velocity. Because the air temperature affects the speed of sound waves, it is also necessary to measure the air temperature to correct the speed of sound. The ultrasonic material level sensor is suitable for measuring the material level of the block material with a larger particle size.

4. Capacitance level sensor Capacitive level sensor is used to measure the capacitance between the probe of the container and the inner wall of the container, between two probes or between the probe and the concentric measuring tube. When the material dielectric constant is constant, the capacitance between the electrodes is proportional to the The principle of material level work.

Capacitive level sensor is characterized by no moving parts, and has nothing to do with the material density, but requires a large difference between the dielectric constant of the material and the air dielectric constant, the change of the dielectric constant to be compensated in the continuous measurement, and need to use High frequency circuit.

Ultrasonic and nuclear radiation type level sensors are mostly imported products from abroad, such as ultrasonic level gauges from KAY-RAY and MAGNITROL, with an accuracy of 0.25; German E+ H (ENDRESS + HAUSER) company's DU212 and DU213, the minimum dead zone is 0.7M, this range can not be used within the range of the most distant by the sound power limit, can only measure the material level within 40M; Germany E + H company's QG type is The nuclear radiation level sensor, the emission source CO-60 or CS-137 is encapsulated in a lead-proof steel protective cover. This cover is equipped with a window that can be opened and closed. It is closed when not in use to avoid radiation hazards. The receiver is a tubular structure, length 100 ~ 1500MM, it is installed in the position corresponding to the emission source, if the G-ray divergence angle, spacing and receiver of the three interact with each other, can effectively detect the entire range.

The heavy hammer type material level sensor was developed by the Institute of Automatic Control of Shenyang Electric Power College, and it was applied to the level measurement of a 670T/H boiler pulverized coal bunker.

1. Construction Principle The hammer type level sensor is model SE-2, also called smart level meter, which is mainly composed of detector and controller. The detector is composed of a switched reluctance motor, a transmitter structure and a hammer, also known as a probe. The height of the material is reflected by the weight stroke. The controller consists of a single-chip microcomputer (8098), a display, a power converter, and a panel. The panel is made of PVC foil and has a digital tube display, touch keys and indicator lights. The software controls the lifting and lowering of the weights in the detector.

The weight is fixed on the wire rope of the switched reluctance motor, and depends on its own gravity with the reel vertically down by a certain unit height, and the size of the promotion current determines whether the heavy hammer touches the surface of the coal powder. If the promotion current is very small, it means that the heavy hammer has come into contact with the pulverized coal. At this time, a pulse signal is sent out and the motor is inverted to the initial position, so that the heavy hammer is promoted to the top of the bin to prevent the heavy hammer from being buried by coal and powder. If coal or powder is scarce, the weight descends and the signal is collected in segments until it reaches the coal or powder level, or the weight falls to the coal and powder limit position, and then the heavy hammer refers to the reset position and alarms. The position is closed and the motor is de-energized. Once the test is completed, it waits for the next test command.

The secondary instrument with the single-chip microcomputer (8098) as the main control unit receives the magneto-reluctance revolution signal transmitted by the photoelectric switch, and the material level is digitally displayed after data processing.

2. Characteristics The main features of the SE-2 heavy hammer level sensor are as follows:

1 It can be automatically and continuously operated during the feeding process, and it is not affected by the material's collapse pressure and impact;

2 It has high structural reliability and adapts to harsh environments with high temperature, large dust and strong smoke.

3 has two functions of manual detection and automatic timing detection at any time;

4 with the upper and lower limit alarm function and round display multi-level material height function;

5Recovery processing function of recoverable faults, reset of heavy hammers with unrecoverable faults, and alarm display of fault signals;

6 pairs of irrecoverable hammer failure can be dealt with by the function of forced promotion and decentralization of the weight;

7 is easy to operate, safe to use, and has high measuring accuracy.

3. Application Effect SE-2 heavy hammer level material sensor has achieved good application effect through debugging and production operation in the past two years:

1. Changed the backward status of coal bunker material level detection and improved the monitoring level of material level control;

2. Completely finished the history of manually measuring the level of pulverized coal powder, which freed the workers from heavy manual labor and improved the working conditions;

3. Provided reliable fuel level data for the coal-fired power plant's pulverizing system to ensure the safety and economic operation of the coal-fired supply system.