Material level sensor and its application in Shenyang Thermal Power Plant
chrispeng [ad_1]
I. Introduction
Level measurement is a branch of level measurement. Level refers to the location of the interface between liquids, powdered solids, and gases in storage containers or industrial production equipment. According to specific uses, it is divided into liquid level, material level and boundary level sensors.
At present, my country mainly uses raw coal as the fuel for power generation, and most power plant boilers use pulverized coal to feed the boilers. For the boiler with direct-blowing feed, the level of the coal bunker is related to the normal operation of the boiler and even the power generation system. The level of the coal bunker is overfilled and overflowing, causing coal accidents; if the level of the coal bunker is too low or empty, it will cause instability of combustion and even major accidents such as fire suppression and shutdown.
For boilers with medium storage type feed, there are both coal bunker and powder bunker. The control of the coal bunker powder level is particularly important. The pulverized coal silo is a transfer station for fuel. The pulverized coal is transported by air. The high-heat gas causes the pulverized coal to enter the coal silo to have a certain “base temperature”, generally around 70°C. Its function is to make the pulverized coal There is a certain degree of discreteness. However, this temperature makes the moisture in the pulverized coal evaporate quickly and is discharged out of the warehouse by the moisture absorption tube. The pulverized coal will become more and more drier. This pulverized coal is very easy to collect heat, and the final result of the heat collection is combustion. Combustion intensifies the heat collection around and even in the warehouse, and it repeats itself in a vicious circle. If it cannot be effectively controlled in time, the result will be the burning of a large amount of pulverized coal in vain. According to relevant reports, self-igniting pulverized coal accounts for about 0.5% of the total coal consumption for power generation. In addition, the loss of pulverized coal explosion in coal bunkers is even greater. Over the years, coal pulverized coal explosion accidents have often occurred, causing huge losses to thermal power plants. At present, the most economical and most 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 transit and control state, which is the primary guarantee for the safe operation of thermal power generating units.
2. Principles and characteristics
The material level sensors currently used in thermal power plants are mainly hammer type, nuclear radiation type, ultrasonic type and capacitive type.
1.Weight type material level sensor
The weight type material level sensor is composed of a servo motor, a steel wire rope with a heavy hammer, a material level transmitter and a display instrument with a microcomputer. After starting, the microcomputer sends out a hammer drop signal, and the servo motor rotates to lower the weight. When the weight hits the material surface, the transmitter sends a signal to the computer to stop the weight drop and send a rising hammer signal. The servo motor reverses to make the weight drop. The hammer rises and sends out the material level signal value to the display instrument. After the heavy hammer rises to the top of the silo, the motor stops, and then repeats the above action after a delay. There are also devices such as upper and lower limit alarms on the display instrument. The weight-type material level sensor can solve the measurement problem within a certain range. The typical measurement range can reach 60m, and 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 (with a half-life of 5.26 years) and Cs-137 (with a half-life of 32.2 years) can penetrate the walls of the container and the materials in the container. A g-ray receiver is installed on the lower side of the silo. As the height of the material level changes, the intensity of the g-ray after passing through the material layer is also different. The receiver detects the intensity of the g-ray injected and displays the material level through the display instrument. high.
This is a non-contact measurement method that does not require perforation on the container to damage the container, so it is suitable for measurement in harsh environments such as dangerous goods and high temperature and high pressure. g Radiation has harmful effects on the human body, but for a limited dose, there is no danger 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 hits the material surface through the air layer, and a part of the reflection is received by the receiver. The material level height can be calculated by multiplying the time from ultrasonic transmission to reception by the speed of sound. Since the air temperature will affect the speed of sound waves, it is necessary to measure the air temperature to correct the speed of sound. Ultrasonic level sensor is suitable for measuring the bulk material level with larger granularity.
4.Capacitive level sensor
Capacitive material level sensors use the capacitance between the probe of the measuring container and the inner wall of the container, between the two probes or between the probe and the concentric measuring tube, and use the principle that the interelectrode capacitance is proportional to the material level when the dielectric constant of the material is constant. .
The characteristic of capacitive level sensor is that it has no moving parts and has nothing to do with the density of the material, but the dielectric constant of the material is required to be very different from the dielectric constant of air. The changed dielectric constant must be compensated during continuous measurement, and it needs to be used High frequency circuit.
Ultrasonic and nuclear radiation level sensors are mostly imported products, such as the ultrasonic level gauges of Kay-Ray and Magnitrol in the United States, with an accuracy of 0.25; German E+ H (Endress+Hauser) DU212 and DU213, the smallest blind zone is 0.7m, can not be used within this range, the farthest distance is limited by the sound power, can only measure the material level within 40m; Germany E+H company’s QG type is Nuclear radiation level sensor, the emission source Co-60 or Cs-137 is enclosed in a lead-filled steel protective cover, this cover is equipped with an openable and closable window, which is closed when not in use to avoid radiation hazards. The receiver is a tubular structure with a length of 100 to 1500mm. It is installed at a position corresponding to the emission source. If the g-ray divergence angle, distance and the receiver cooperate with each other, it can be effectively detected in the entire range.
Three, application
The Automatic Control Institute of Shenyang Electric Power College has developed a heavy hammer material level sensor and applied it to the level measurement of the 670T/h boiler pulverized coal storage.
(1) Principle of composition
The model of the weight-type material level sensor is SE-2, also called an intelligent material level meter, which is mainly composed of a detector and a controller. The detector consists of a switched reluctance motor, a transmission mechanism and a heavy hammer (also called probe), and the stroke of the heavy hammer reflects the height of the material level. The controller is composed of a single chip microcomputer (8098), a display, a power converter and a panel. The panel is made of PVC film, with a digital tube display, touch buttons and indicator lights. Use software to control the lifting process of the heavy hammer in the detector.
The heavy hammer is fixed on the steel wire rope of the switched reluctance motor, and depends on its own gravity to drop a certain unit height vertically with the reel to determine whether the heavy hammer touches the surface of the pulverized coal by the magnitude of the lifting current. If the lifting current is very small, it means that the heavy hammer has contacted the pulverized coal. At this time, a pulse signal is sent out and the motor reverses to the initial position, so that the heavy hammer is lifted to the top of the silo and reset 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 sections until it touches the coal or powder level, or the weight drops to the limit position of the coal or powder level, and then lifts the weight to the reset position and alarms. The position switch is closed, the motor is powered off, one test ends, and the next test command is waited.
The secondary instrument with a single-chip microcomputer (8098) as the main control unit receives the reluctance motor revolution signal sent by the photoelectric switch, and performs digital display of the material level after data processing.
(Two) characteristics
The main characteristics of SE-2 heavy hammer type material level sensor are as follows:
① It can run automatically and continuously during the feeding process, and is not affected by the collapse and buried pressure and impact of the material;
② Its structural reliability is high, and it can adapt to the harsh environment of high temperature, heavy dust and dense smoke;
③It has two functions: manual detection at any time and automatic timing detection;
④With upper and lower limit alarm function and loop display function of multi-point material level height;
⑤It has the recovery processing function of recoverable faults, the heavy hammer resetting of unrecoverable faults and the display and alarm of fault signals;
⑥For unrecoverable heavy hammer failures, the functions of forcibly raising and lowering the heavy hammer can be used to deal with;
⑦Easy to operate, safe to use and high measuring accuracy.
(3) Application effect
The SE-2 heavy hammer material level sensor has been debugged and has been in production for the past two years, and has achieved good application results:
1. Changed the backward state of coal bin material level detection and improved the monitoring level of material level control;
2. Completely ended the history of manual shaking and bleaching to measure the level of pulverized coal, freeing workers from heavy manual labor and improving working conditions;
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-fired supply system.
[ad_2]
