Level is an important process parameter in industrial production, which not only reflects material balance but also is a key factor to ensure safe production. The ultrasonic level meter adopts non-contact measurement and uses the attenuation, penetration ability, and acoustic impedance of ultrasonic waves in gas, liquid, or solid to measure the material level of the medium, which is widely used. In this article, taking the instrument in a waste incineration project as an example, the characteristics, selection, and installation precautions of the ultrasonic level meter are described and studied in detail so that it can be used more flexibly in the work.
Level measurement can be seen everywhere in production and life. The level meter is like the human eye, telling us the height or position of liquid and solid in the storage container or production equipment. Based on different measuring principles, there are many kinds of level measuring instruments. In the engineering design, appropriate level measuring instruments shall be selected based on the principles of advanced and reliable technology, economic suitability, and convenient management.
An ultrasonic level meter is a non-contact measurement that uses acoustic reflection to measure the level. There are no movable parts inside the instrument, which is characterized by reliable operation, simple installation, long service life, convenient maintenance, etc., and has a relative price advantage. It is suitable for the petroleum, chemical industry, tap water, sewage treatment, coal mine, steel, garbage incineration, food processing, and many other industries.
According to the principle and characteristics of the ultrasonic level meter, combined with the actual design cases, this paper introduces and studies in detail the technical parameters of the ultrasonic level meter and the matters needing attention in the field installation process.
The ultrasonic level sensor is based on the principle that sound waves touch the liquid level (or material level) to generate reflected waves, measure the time difference between the reflected wave and the reflected wave, and then calculate the height of the material level for continuous measurement.
The velocity of sound in the medium is known as c, and the time difference between echo and transmitted pulse is t, according to the formula D=c × T/2, the distance D between the sensor and the object surface can be calculated.
The measuring principle of the ultrasonic level meter is shown in the figure below, and the calculation formula is L=H-D.
Where: H is the empty tank distance, F is the range (full tank distance), L is the level, BD is the measurement blind area, and D is the distance between the sensor diaphragm and the object surface.
The ultrasonic level meter is applicable to the measurement of regular solid, liquid, and other normal-pressure or low-pressure media. An Anticorrosion probe shall be selected to measure corrosive media, and an explosion-proof device shall be added to measure flammable and explosive media.
An ultrasonic level meter is generally not applicable to medium with large temperature changes and uneven liquid levels; Ultrasonic transmission requires a medium, so ultrasonic level meter cannot be used for a vacuum tank; Steam and dust will hinder the transmission of sound waves, so the ultrasonic level meter cannot be used in the situation with more steam and dust.
In engineering design, instrument selection is very important. It is not only necessary to ensure the normal use of the instrument under actual working conditions, but also to consider the project cost. The following describes the detailed selection process in combination with a waste incineration project. The treatment capacity of this project is 800 t/d, and two incinerator grate heaters and two waste heat boilers with a treatment capacity of 300 t/d are also provided. The superheated steam generated by the waste heat boiler is generated by two 9MW steam turbine generators to realize the comprehensive utilization of energy.
During model selection, complete the model selection of instruments according to the data provided by the process discipline and in combination with the model selection samples of instrument manufacturers. The parameters of an ultrasonic level meter include: measuring range, power supply, process connection, flange material, shell material, signal output, protection and explosion-proof grade, installation mode, accuracy grade, etc.
The ultrasonic level meter is generally applicable to a temperature of - 40~100 ℃ and pressure below 0.3 MPa. From the process parameters provided by the process discipline, it can be seen that these liquid-level instruments are used in situations where the measuring medium is cooling water, municipal water, and other water, and it can be judged from the temperature and pressure that ultrasonic liquid level gauges can be used in these situations.
When filling in the accuracy, pay attention to the temperature of the medium. Because in different media, different temperatures, ultrasonic sound velocity is different. The gas composition has the greatest influence on measurement accuracy. For example, the sound speed of the sound waves in air and nitrogen at 20 ℃ is 343 m/s and 349 m/s respectively. Taking the cooling tank and recovery tank as an example, the actual depth is 2 m. It takes 0.005 83 s to complete a complete sound wave transmission and reception in air, and 0.005 73 s in nitrogen.
If the calibration is based on nitrogen, it will result in an error of 1.7%. When sound waves propagate in the air, the sound speed will increase by 0.6 m/s for every 1 ℃ increase in air temperature. Therefore, the ultrasonic liquid level measurement system with temperature compensation is generally selected during model selection. The instrument manufacturer inputs the experienced sound velocity of the corresponding medium during installation and commissioning or conducts actual adjustment to calculate the sound velocity, so as to achieve accurate level measurement.
Inserted equipment length: when the ultrasonic level meter transmits an ultrasonic pulse, it cannot detect the transmitted echo at the same time. Since the transmitted ultrasonic pulse has a certain time width, and the sensor still has residual vibration after transmitting the ultrasonic, during which the transmitted echo cannot be detected, a short distance from the probe surface downward cannot be detected normally, and this distance is a blind area. For example, if the blind area is 40 cm when the distance between the liquid level and the probe is less than 40 cm, it will be impossible to measure.
Therefore, when selecting the instrument, it must be noted that the highest liquid level cannot extend to the measurement blind zone of the instrument. Relevant data affecting the insertion depth will be noted in the specification, such as the length of the pipeline-interface and the thickness of the concrete cover plate. This item shall be determined by the manufacturer. Because different manufacturers have different appearance forms of instruments, the manufacturer shall determine it on the premise of meeting the measurement requirements after integrating the measurement range and the blind area of the instrument itself. When the instrument is reviewed, it will be rechecked to avoid blind areas.
With the continuous revision of the process flow chart, it is necessary to check whether the size and material of the processing pipeline have changed. The instrument production cycle is long. In order to ensure the smooth implementation of the project, the parameters must be filled carefully and carefully to ensure that the instrument can meet the measurement requirements of working conditions.
To achieve an accurate measurement, it is necessary not only to select the correct instrument parameters but also to ensure the correct installation method, otherwise, the advantages of the selected instrument cannot be played. The following points shall be noted during the installation of the ultrasonic level meter.
Different manufacturers have different requirements for the distance between the sensor and the tank wall. For example, in the Meacon sample, the sensor is required to be installed at 1/2 or 1/3 of the tank top radius (provided that a certain distance from the tank wall is met). In actual design, the principle to follow is r>L × tan( α/ 2), where L is the distance from the probe surface to the lowest level, α Is the beam angle and r is the distance from the probe centerline to the tank wall. As long as this principle is met, the measurement will not be affected.
Within the beam angle range of the ultrasonic level meter, the material inlet, heating coil, ladder, etc. shall be avoided to prevent the beam from hitting other targets other than the medium, causing too much ultrasonic interference, or detecting false material levels.
For a conical vessel with a flat top, the best installation position of the instrument is the center of the top of the vessel, so that the bottom of the vessel can be measured.
When installing the ultrasonic level meter, it should be noted that the distance between the probe head radiation surface and the level interface should be greater than the blind area of the level meter. If it cannot meet the requirements, the following two methods can be considered: the first method is to heighten the instrument for installation; The second is to add a piping nozzle. For the maximum nozzle length L and nozzle diameter D, different manufacturers have different requirements. According to previous experience, the nozzle length is about 80 mm for a diameter of 50 mm; 80 mm diameter, nozzle length approx. 240 mm; 100 mm diameter, nozzle length approx. 300 mm. The nozzle shall be free of edges and welding points, and the part extending into the tank shall be free of burrs. The inner wall shall be smooth. To reduce interference factors, the lower edge of the nozzle shall have a 45 ° angle.
If the liquid level fluctuates greatly, or there are floating balls, objects that block the sound wave, etc., a guided wave tube can be added to the container, and the guided wave tube extends directly into the bottom of the liquid level so that the sound wave only propagates in the guided wave tube, and the measurement is stable and reliable. Select the appropriate size of the waveguide tube according to the size of the probe, and ensure that the inner wall of the waveguide tube is smooth and straight, and there is no wall-hanging phenomenon when the liquid level rises or falls, so as to prevent ultrasonic interference caused by the beam hitting the casing wall, or even the actual liquid level cannot be detected normally. The length of the waveguide tube is from the flange opening to the bottom of the container, and a gap or small hole is cut on the side to ensure that there is a consistent liquid level inside and outside the waveguide tube.
The connection between the sensor and the control and display electronic unit is generally coaxial cable, which shall not exceed the length specified by the manufacturer. The cable shall be laid in a separate bridge of the instrument discipline, isolated from other signal-type instrument cables and power lines, and properly grounded and protected to reduce interference during transmission.
If the process discipline requires double liquid level measurement, it is prohibited to select two ultrasonic level meters, because the two signals will interfere with each other and cause measurement errors. During installation, ensure that the sensor is perpendicular to the material surface, so as to maximize the recovery of reflected waves.
The ultrasonic level meter belongs to non-contact measurement, which avoids the corrosion of the instrument by the medium. It has fast response speed, high measurement accuracy, simple later maintenance, low cost, and a relatively low failure rate.
Under the condition that the ultrasonic level meter can meet the measurement requirements, some owners or process patent manufacturers will also choose the air-blowing pressure level meter. Compared with the air-blowing pressure level gauge, the ultrasonic level gauge also has the following advantages.
The ultrasonic level meter has a simple structure, moderate price, reliable work, and convenient installation, and is especially suitable for liquid level measurement of the atmospheric storage tank, clean water tank, and sewage tank.
In order to achieve an accurate measurement, four aspects must be achieved.