Application of the hottest mass flowmeter in chemi

2022-08-08
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The application of thermal mass flowmeter in chemical industry

I. Introduction

calorimetric gas mass flowmeter is dedicated to the measurement of single component gas or fixed proportion mixed gas. At present, it has been widely used in petroleum, chemical industry, semiconductor, medical instruments, bioengineering, combustion control, gas distribution, environmental monitoring, precision instruments, scientific research, measurement, food, metallurgy, aerospace and other fields

calorimetric mass flowmeter is used for precise measurement and automatic control of gas mass flow. Adopt standard input and output signal 4 If the time number of heating sample is placed in the box, computer centralized control can be realized. There are many forms of applications in Daqing Petrochemical Company. For example, the hydrogen flow meter ft-121a/b of polypropylene unit adopts Brooks calorimetric flowmeter with ranges of 1.45kg/h and 9.5kg/h

compared with the traditional flowmeter, it does not need to be equipped with temperature and pressure transmitters, and it can directly measure the mass flow without temperature and pressure compensation (in the standard state, 0 ℃, 101.325kpa)

when the gas is used as the control variable in the production process (such as combustion, chemical reaction, ventilation and exhaust, product drying, etc.), the molar number of the gas can be controlled by using the mass flow controller for direct measurement

so far, there is no better technology than using mass flow controller if we want to maintain quantitative gas mixing as mixing or batching, or to optimize the chemical reaction process. The mass flow controller can continuously adjust and control the flow, and the cumulative flow can also be obtained through the display instrument

thermal mass flowmeter is also a better instrument for the tightness detection of pipeline systems and valves, which directly displays the amount of air leakage. Mass flowmeter has high performance price ratio, easy installation and simple operation. Using mass flowmeter and mass flow controller products is one of the most reasonable choices

because the sensor of this kind of mass flowmeter is a thermal principle, if the gas is not dry gas, it will affect the heat transfer efficiency, thus affecting the output signal and measurement accuracy of the sensor

II. Basic principle

thermal mass flowmeter can be divided into contact type and non-contact type according to different heat sources and temperature measurement methods

1. contact thermal mass flowmeter

the heating element and temperature measuring element of this mass flowmeter are placed in the pipe of the measured fluid and are in direct contact with the fluid. It is often called Thomas flowmeter, which is suitable for measuring the large mass flow of gas Its structural principle is shown in Figure 1. Because the heating and measuring elements are in direct contact with the measured fluid, the elements are vulnerable to fluid corrosion and wear, affecting the measurement sensitivity and service life of the instrument. It is not suitable for measuring high flow rate and corrosive fluid, which is the disadvantage of contact type

Figure 1: structural principle of contact thermal mass flowmeter

l, 3 - thermocouple; 2. A heater; 4. Power meter

2. Non contact thermal mass flowmeter

the heating and temperature measuring elements of this flowmeter are placed outside the fluid pipe, which are not in direct contact with the measured fluid, overcoming the shortcomings of contact type. The thermal micro flow type (a typical structure of non-contact mass flowmeter) is shown in Figure 2. The measuring tube of the instrument is a thin-walled small-diameter nickel tube. Platinum resistance wires 3 and 5 are wound on both sides of the nickel tube as the temperature measuring coil and as the two arms R1 and R2 of the measuring bridge. Manganese copper wire plus ring 4 is wound between the two temperature measuring coils as the heater of the instrument. When the fluid is static, because the temperature measuring coil is symmetrically installed on both sides of the heater and the resistance value is equal (about 100 each), the measuring bridge is in a balanced state. However, when the fluid flows through the temperature measuring resistance in the nickel tube, the temperature field of the heater is destroyed, and the two temperature measuring coils are in different temperature fields, resulting in changes in the resistance value. The resistance values of the two temperature measuring coils are different, which destroys the balance of the bridge. According to the principle of bridge balance, the change of resistance value measured by galvanometer 8 can obtain the mass flow M

non contact thermal mass flowmeter

1 - measuring tube; 2 - isothermal shell; 3 - temperature measuring coil; 4 - turn

7 - zero resistance; 8 - galvanometer

thermal micro flowmeter is suitable for measuring the micro mass flow of liquid and gas. It can measure the micro liquid flow of 0-100cm3/h and the micro gas flow of about 10L/h

in order to simplify the structure, some products cancel the heater and only use two measuring resistors as heating elements and temperature measuring elements. In this design, due to thermal inertia, the reflection speed of the instrument is relatively small and the sensitivity is low; The temperature change of the measured fluid affects the accuracy of the instrument indication

in order to improve the flow measurement range of the non-contact thermal mass flowmeter, a boundary layer mass flowmeter is designed, which uses the heat conduction of the boundary layer of the measuring fluid close to the pipe wall to reflect the flow. In this way, the flow is generally measured by using the heat given by the heater controlling the outer wall of the pipe to keep the temperature difference inside and outside the boundary layer constant, and then the mass flow is reflected according to the measurement of the heat source

at present, thermal mass flowmeters are developing rapidly: mass flowmeters, boundary layer mass flowmeters, split flow thermal capillary mass flowmeters, and thermal mass flowmeters using IC substrate technology

III. practical application and maintenance

four thermal micro mass flowmeters are used in our polyolefin plant because it is necessary to adjust the injection amount of hydrogen, oxygen, etc. to control the molecular weight of resin. According to the actual operation of thermal micro mass flowmeter in recent years, its reliability and accuracy are analyzed as follows:

(I) reliability

1 Mean time between failures (MTBF)

the mean time between failures (MTBF) is used to measure the reliability of equipment. The fault here refers to the obvious display error, which exceeds the accuracy requirements of the instrument. Since a single device has not experienced two failures, it can be assumed that one instrument has experienced one failure in four times of the time

mtbf = working time/number of failures

=4 × six × three hundred and sixty-five × 24/1

≈2.1 × 105(h)

2. Mean time to repair (MTTR)

take the micro flowmeter with tag No. ft-201a as an example: due to the problem of the internal detection element of the sensor, it took 2 months to return to the manufacturer for repair and calibration. Therefore, the average repair time of thermal micro mass flowmeter is greatly affected by external factors

(II) accuracy

at the same time

all thermal micro quality Jinan new era Gold Testing Instrument Co., Ltd. has the technical knowledge of each model of the experimental machine. We have the measurement error of the flowmeter after working for a period of time to make a practical comparison. The maximum error between the flow measurement value and the standard calibration reading is not greater than 1%. From the actual instrument measurement value and calibration, it has met the production needs

(III) equipment maintenance

our factory is located in high cold areas, and the requirements for the low temperature resistance of application equipment are extremely strict. Although the nominal operating temperature range of some thermal micro mass flowmeters can reach - 40 ℃ ~ 80 ℃, it is not ideal in practical application. Due to the influence of external temperature factors, the heat loss generated by the heating element is too large to heat the measured medium normally, or the external temperature is too high to affect the normal heating of the medium. At this time, the indication of the meter is too high or there is no indication of overtravel. According to our practical application experience, it is considered that this type of table is more suitable to work at 0 ℃ ~ 55 ℃. The polypropylene plant has experienced equipment failure caused by low temperature, and the problem was solved after the corresponding heat tracing pipeline and instrument insulation box were laid

after the equipment is installed in place and commissioned, it only needs routine operation inspection and dust removal cleaning, without special maintenance

IV. conclusion

thermal mass flowmeter is increasingly applied to hydrogen, oxygen, ammonia, gas, etc. in various places such as semiconductor industry epitaxial diffusion, petrochemical micro reaction devices, coating process, optical fiber manufacturing, heat treatment quenching furnace, etc. This is a major theme of the three-day China International Engineering Plastics Industry Innovation Conference on flow measurement and control, And solid argon evaporation in solid gas refrigeration and leakage measurement in valve manufacturing. It has also been widely used in laboratory instruments such as gas chromatographs and gas analysis instruments, and has attracted the attention of more and more professionals, with broad development prospects. (end)

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