I. INTRODUCTION Feed level measurement is a branch in level measurement. Level refers to the position of the interface between the liquid, powdered solid and gas in a storage container or industrial production equipment. It is divided into liquid level, material level and boundary level sensors according to specific purposes.
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. Pulverized coal is transported by air. The high-heated gas allows pulverized coal to enter the coal bunker with a certain “base temperatureâ€, 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 final result of the heat collection is burning. Burning has aggravated the heat collection around and even in the warehouse. It is a vicious cycle. If it cannot be effectively controlled in time, the result will be a large amount of pulverized coal. According to relevant information, self-fired coal meal accounts for approximately 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.
II. 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 Level Sensor The heavy hammer type level sensor consists of a servo motor, a wire rope suspended with a hammer, a material level sending device, and a display instrument with a microcomputer. After starting, the microcomputer sends a drop hammer signal, and the servo motor rotates to put down the weight. After the hammer hits the material surface, the sender sends a signal to the microcomputer so that the heavy hammer stops falling and sends out the rise hammer signal. The servo motor reverses to make the weight heavy. The hammer rises and emits the level signal value to the display meter. The motor stops after the weight has risen to the top of the bin, 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. Typical measurement range is up to 60m, and this kind of measurement has nothing to do with steam and dust, and has high measurement accuracy.
2. Nuclear radiation type level sensor radiation source Co-60 (half-life 5.26 years) and Cs-137 (half-life 32.2 years) emit g-rays that can penetrate the vessel walls and the contents of the vessel. A g-ray receiver is installed on the underside of the silo. As the height of the material plane changes, the intensity of the g-ray beam passing through the material layer is also different. The receiver detects the intensity of the injected g-ray and displays the material level through the display meter. height.
This is a kind of non-contact measurement method. It does not need to pierce the container and damage the container, so it is suitable for the measurement of dangerous goods and harsh environments such as high temperature and high pressure. G-rays do have harmful effects on the human body, but for limited doses, there is no danger under proper protection.
3. The ultrasonic level sensor is equipped with an ultrasonic generator and 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 reflected on the material surface. A part of the reflection is received by the receiver. The height of the material level can be calculated by multiplying the time elapsed from the ultrasonic wave to the receiving and multiplying the sound velocity. Since the air temperature affects the speed of sound waves, the air temperature must be measured to correct the speed of sound. The ultrasonic level sensor is suitable for measuring the material level of a larger particle size.
4. Capacitive level sensor The capacitive level sensor is the capacitance between the probe of the measuring vessel and the inner wall of the vessel, between the two probes or between the probe and the concentric measuring tube. When the dielectric constant of the material is constant, the inter-electrode capacitance is proportional to the material level. The principle works.
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 of Kay-Ray and Magnitrol, with an accuracy of 0.25; Germany E+ DU212 and DU213 of H (Endress+Hauser) company, the minimum blind zone is 0.7m, which cannot be used within this range. The longest distance is limited by the sound power and can only measure the material level within 40m; the QG type of German E+H company is The nuclear radiation level sensor, the transmitter source Co-60 or Cs-137 is encapsulated in a lead-covered 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 with a length of 100-1500mm. It is installed at a position corresponding to the emission source. If the g-ray divergence angle, distance and receiver are matched with each other, it can be effectively detected in the entire range.
Third, the Shenyang Electric Power Institute Institute of Automatic Control developed a heavy-weight material level sensor, and it is applied to the 670T / h boiler pulverized coal bunker level measurement.
(A) The principle of the composition of the hammer type material level sensor SE-2, also known as smart level meter, which consists of detectors and controllers. The detector is composed of a switched reluctance motor, a transmission mechanism and a weight, also called a probe, and the height of the material level 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 to decrease vertically with a certain unit height of the reel, so as to judge whether the heavy hammer touches the surface of the coal powder by increasing the magnitude of the current. If the lift 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 reversed to the initial position so that the weight is lifted to the top of the bin to prevent the heavy hammer from being buried by coal and powder. If there is little coal or powder, the weight continues to drop, and the signal is collected in segments until it reaches the coal or powder level, or the weight falls to the limit position of coal and powder, and then the heavy hammer refers to the reset position and alarms. The bit switch is closed, the motor is de-energized, and one test is completed, waiting for the next test command.
The secondary instrument with the single-chip microcomputer (8098) as the main control unit receives the reluctance motor revolution signal transmitted by the photoelectric switch and carries out digital display of the material level after data processing.
(b) 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 upper and lower limit alarm function and cycle display multi-level material height function;
5Recovery processing function of recoverable faults, reset of heavy hammers with unrecoverable faults, and display alarm of fault signals;
6 pairs of irrecoverable hammer failure can be dealt with by the function of forcibly lifting and lowering the weight;
7 easy to operate, safe to use, high measurement accuracy.
(III) Application effect
SE-2 heavy hammer level sensor has been applied in production and operation in the past two years, and has achieved good results:
1. Changed the backward status of coal bunker level detection and improved the level of material level control monitoring;
2. The history of manual measurement of pulverized coal powder by artificial shaking was completely ended, workers were relieved from heavy manual work, and working conditions were improved;
3. Provides reliable material level data for the coal-fired power plant's pulverizing system, ensuring the safety and economical operation of the coal-fired supply system.
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