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1、附录CLiquid Level MeasurementChemical industry with the progress of science and technology got rapid development, in order to make the equipment safe and reliable operation, and can be utmost to improve product quantity and quality, to effectively operate automatically and process adjustment, realize
2、the scientific management for the enterprise to provide the necessary material and energy of accurate data, must be in production in the process of pressure, flow, level, temperature, chemical composition of various parameters such as timely, accurate measurement.Generally the process of containers,
3、 tanks, towers, tank of liquid surface position level, slot bucket, cans, and warehouse ChuFa solid block, the accumulation of particles, powder height and surface position is called a material level; Two not fluid boundary between the height and the liquid and solid separation height is called inte
4、rface position, the position of assembly level, material level, liquid level interface.Material level measurement to correctly measure the container storage capacity or quality, know container level of high and low and adjust at any time, in the material level to maintain the required height.Materia
5、l level measurement in chemical process, not only under normal temperature, atmospheric pressure and ordinary medium, material level, liquid level measurement of the interface, but also often meet high temperature, low temperature, high pressure, inflammable and explosive (negative temperature). Vis
6、cosity and foam, boiling dielectric material level measurement problems.In order to adapt to the characteristics of chemical objects, to meet the requirements of the chemical process material level measurement, there are many material level measurement and instrumentation.1. Ultrasonic liquid level
7、meter and radar material level meter(1) ultrasonic liquid level meterMaterial level measurement process, the ultrasonic signal, issued by the ultrasonic probe, the liquid or solid material surface after launch back, by the same sensor receiving, ultrasonic measurement of the whole running time, so a
8、s to realize the measurement of material level, acoustic velocity transmission of the ultrasonic pulse, from transmitting to receiving to ultrasonic pulse need time interval and the transducer to the surface of the measured medium is proportional to the distance.Emit ultrasonic level meter pulse, no
9、t the echo of detection at the same time. Due to the launch of the ultrasonic pulse width has a certain time, and after launch the ultrasonic sensor and aftershocks, during cannot detect the echo, so from the start down the surface of the probe a short distance cant normal inspection, this distance
10、is called blind spots. Was the highest level such as into the blind area, detect instrument will not be able to correctly, there will be error, if necessary, heightened level meter can be installed.Ultrasonic liquid level meter also has shortcomings, the ultrasonic transmission by gas interference,
11、level surface bubbles, corrugated, etc, the ultrasonic transmission by gas interference, level surface bubbles, corrugated, etc., the ultrasonic reflection effect. Material is volatile, liquid gas entrapment, the container by vibration (e.g., motors) are installed on the container cant use ultrasoni
12、c liquid level meter.(2) radar material level meterRadar level gauge USES launch - reflection - receive work mode. Radar level meter antenna emit electromagnetic wave, the wave reflected by the measured object surface, again by the antenna, the electromagnetic wave from transmitting to receiving tim
13、e is proportional to the distance to the liquid surface. In practice, there are two kinds of radar level gauge means the frequency modulation continuous wave and pulse wave. Liquid level meter adopts frequency modulation wave technology, power consumption is big, the four wire system, should be used
14、 in electronic circuit is complicated. And the radar pulse wave technology level gauge, low power consumption, easy to implement essential safety, high precision, wider application scope.Radar level gauge measuring the electromagnetic waves can pass through a vacuum, dont need a transmission medium,
15、 is not affected by air, steam, groove volatile fog effect, can be used for such as coarse benzene volatile medium level measurement.Radar level meter can be used for almost all of the liquid level measurement. Electromagnetic wave on the surface level when firing, signal attenuation, when the signa
16、l attenuation through the hours, can lead to radar level meter cannot detect enough electromagnetic wave signal, the conductive medium can be a very good reflection of electromagnetic wave, the dielectric constant is greater than 1.5 non conductive medium dielectric constant was 1.0 (air) will also
17、be able to guarantee sufficient reflection wave, dielectric constant, the greater the reflection signal. In practice, almost all medium can reflect enough of reflection wave.(3) radar level gauge, ultrasonic liquid level meterUltrasound and radar is the main measuring principle is different, lead to
18、 their different application occasions, radar is given that the dielectric constant of the measured substance, which is given that the density of the material being measured, so the low dielectric constant material radar measurements will be effect to sell at a discount greatly, the solid matter in
19、general, push harder, another wave of different means of transmission components, such as ultrasonic wave was launched by the vibration of the piezoelectric material, so it is can be used in higher pressure or negative pressure of the situation, usually only in the atmospheric pressure containers. T
20、he radar can be used in the process of high pressure tank. Radar emission Angle is bigger than ultrasound, small containers or elongated dont recommend using non-contact radar, generally recommend a director radar. Finally is the temperature on the accuracy, of course, must be higher than the ultras
21、onic, radar must use high resolution radar in the tank, without ultrasonic. As for the price, usually ultrasonic, lower than the radar in a wide range of course some ultrasonic price is also high. Such as the range of 6-70 meters, the preferred ultrasonic liquid level meter. Pay attention to the sti
22、rring groove, groove wall adhesion can produce interference of the reflected wave, level measurement, the installation is to choose the appropriate installation position, so as to avoid the interference of these factors.Level two categories, including liquid level and material level. Liquid level an
23、d liquid level indicator and two continuous level measurement. Liquid level indicator is the level of several fixed position measuring, used in liquid level of upper and lower limit alarm, etc. Continuous level measurement is to measure liquid level continuously, it is widely used in many fields suc
24、h as petroleum, chemical industry, food processing, have very important significance. For more than 20 kinds of continuous level measurement method for comparative analysis. Glass tube, glass plate, double color water method.Manual gauging glass tube method: this method is using the principle of com
25、municating vessels, level scale read directly from the instructions. Glass plate method: glass which can be installed through connectors can also be installed in the container wall openings, and series a few piece of glass to increase the range. Numerical scale read directly from the glass plate lev
26、el.Double color gauge method: this method using the optical principle, make water shows green, and make the water vapor shows red, which indicate the water level.Artificial gauging method: the method is used for measuring oil tank liquid level. When measuring, the surveyor gauge Surveyor to volume o
27、f the oil dipstick into oil, and filed when feet lead contact with the tank bottom oil dipstick. According to the quantity of oil traces on the oil dipstick, read out the oil level height; End according to the quantity of oil dipstick test cream color change to determine the height of the water cush
28、ion layer, to determine high oil and high water above 4 methods are artificial measurement method, measurement is simple, the advantages of high reliability, intuitive, and low cost.Blowing method, differential method, HTG blowing method: because of the blow pipe approximation is equal to the fluid
29、column pressure in the static pressure, thus P = rho gH type of rho - liquid density; H - level. So the liquid level is measured by the static pressure P can be H. Strong method suitable for measuring the corrosive gas blowing, have suspended material content in the liquid, and is mainly used in the
30、 measurement accuracy is not high.Differential pressure method: the open container or atmospheric vessels, valves and gas phase pressure pipeline saves. Based on the relationship between differential pressure and liquid level P = P2 - P1 = rho gH type: P - positive and negative pressure chamber pres
31、sure transmitter; P2 and P1 - led tube pressure; H - level. Differential pressure transmitter will pressure difference transformation for 4 20 mA dc signal. If the pressure in the measuring range lower limit of the corresponding output signal is greater than or less than 4 mA, are all need to adjust
32、 the migration zero migration technology such as spring, is equal to 4 mA. HTG method: this method is applied to differential pressure oil tank liquid level measurement.Buoy method: this method USES the middle maglev barrel with holes as liquid level sensor. Stainless steel casing from buoy hole thr
33、ough the middle, between fixed to the roof and bottom. Liquid level change driven maglev hollow tube (permanent magnet) hides inside can move up and down the casing, and attract the casing of magnet can move up and down the casing wall, secondary instrument | instrument according to the moving magne
34、t volume to calculate the liquid level.Floating ball method: this method is using the lever principle of work. Floating ball with liquid level change and around the axis of rotation, the pointer on the drive shaft rotation, and at the other end of the lever balance weight balance, at the same time i
35、ndicates the level value on the dial. Floating ball method have ball float within and outside the floating ball type two kinds. Floating ball method is mainly used for high temperature measurement, the viscosity of the liquid level, but the range is smaller.Servo method: this method USES volatile in
36、tegral circuit, eliminate jitter, prolong life, improve the precision of level measurement. The modern servo liquid meter measuring accuracy is higher, has 40 m range less than the accuracy of 1 mm, and generally has the function of measuring density distribution and average density.Sinking method:
37、the location of the sinking as the change of liquid level change, but the variation is not equal and level variation. Torque tube sinking method: the torque tube core shaft torsional Angle changes, secondary instrument according to the torsional Angle variation to calculate the liquid level above fi
38、ve kinds of methods are using the buoyancy principle to work.Such as R, R, l, is a fixed value, as long as the use of epsilon 1, epsilon 2, CH can calculate the liquid level H. Measurement of conductive liquid capacitance method principle, the formula is slightly. Capacitive liquid level meter price
39、 is low, easy to install and can be applied to occasions of high temperature and high pressure. Capacitive liquid level instrument measurements but repetitive accuracy is low, need regular maintenance and calibration again, working life is not very long.This method is especially suitable for the mea
40、surement of conductive liquid, sensitive device resistance characteristics, its resistance along with the change of liquid level change, so the resistance value is transmitted to the secondary circuit level. Electrical parameters of the liquid level sensor method to measure the liquid level changes.
41、Magnetostrictive method, ultrasonic method, modulating optical method, microwave method Magnetostrictive method: the method is used for measuring oil tank liquid level has two float, respectively used to detect oil and gas and oil/water interface. Each float in possession of a set of permanent magne
42、ts, used to generate a fixed magnetic field. Measurement, level gauge head emit low enquiry pulse current, the magnetic field generated from the current conduction down the waveguide. When electric current magnetic field is met, and float a return impulse (also called waveguide distortion pulse). As
43、k pulse and the back pulse time lag between the corresponding to the height of the oil-water interface and the interface of oil and gas. Easy to install, magnetostrictive liquid level meter measuring accuracy is high, but the liquid density and temperature changes can lead to a measurement error, fl
44、oat along the outside of the waveguide tube move up and down, are easy to be jammed.Ultrasonic method: antenna (mostly aperture antenna, also has a planar antenna) radiation, after liquid surface reflection is the antenna, and then calculated by the secondary circuit, transmitting and receiving sign
45、als of liquid level difference. The liquid level meter adopts triangle wave form of frequency modulation, and through to the transmitting and receiving signal mixing after get the difference between the signal analysis, microwave transmission time, thus calculate the liquid level. Microwave speed by
46、 the transmission medium, the influence of temperature, pressure, liquid dielectric constant is small, but the liquid interface fluctuations, on the surface of the dielectric constant of bubbles, liquid medium has a great influence on the microwave reflection signal strength. When the pressure in ex
47、cess of the prescribed value, the pressure will have a significant impact on level measurement accuracy. For liquid dielectric constant is less than the stipulated value, most of the radar level gauge need to adopt waveguide, but rust, bend waveguide and tilt will affect the accuracy of measurement.
48、 Sky high h, for example, to 20 m, guided wave tube and vertical direction Angle alpha just over 0.573 , is caused by the liquid level h will be more than 1 mm error, thus proved that the tilted Angle alpha (in degrees) is small, h meet:Radar level gauge is particularly suited to high pollution and
49、high viscosity of the product, such as asphalt, etc. Repeat accuracy of radar level gauge measuring is higher, need regular maintenance and calibration, measurement accuracy is higher, but the price is higher, difficult measurement of oil-water interface.Modulating optical method and microwave method is similar, only by phase or frequency modulation of optical signal instead of microwave signal. But light signals are greatly influenced by steam, oil, steam, and the liquid level fluctuation is very sensitiv