Vortex flow meter: the core technology of gas flow measurement
2024-11-06
There are many types of gases, including those used as energy sources, such as natural gas, gas, hydrogen and compressed air; There are also industrial raw gas, such as oxygen, nitrogen, carbon dioxide, hydrogen sulfide, methane, chlorine, etc.; There are also some enterprises that discharge the exhaust gas generated after combustion and chemical reaction into the atmosphere. Whether it is energy gas, raw material gas, or waste gas, flow measurement or detection is needed in the transportation process.
There are many meters for gas flow measurement, in all kinds of flow meters, in addition to electromagnetic flow meters, the rest of the classes can almost measure gas flow, the current application of more differential pressure type, fluid vibration type, turbine type, ultrasonic flowmeter. Measuring gas flow with vortex flow meter is the main field of its application.
Compressed air can be regarded as a secondary energy source as an energy-carrying working medium. When the required working pressure is low, it can be generated by a fan. When the required working pressure is high, it is generated by the air compressor. In production, measuring the air volume of the fan and the flow rate of compressed air is an important means of energy management for enterprises. As the measured body, the air is a relatively clean, non-corrosive medium, although the air contains a certain amount of water and acid gas, but the vortex flowmeter with stainless steel as the liquid material, is completely usable, will not affect the measurement. Measure compressed air flow,.jpg)
There are several issues worth noting:
(1) Vibration effect.
Compressed air is produced by air compressors or high pressure fans. These devices have varying degrees of vibration during operation, and sometimes the vibration is relatively strong, and this vibration will be transmitted through the connecting pipe. This influence factor should be fully considered when selecting and determining the installation position of vortex flowmeter. In various detection methods, due to different detection components, the anti-vibration performance of the instrument is also different. The vibration resistance of vortex flowmeters using the principle of detecting speed changes is better, such as the vibration resistance of ultrasonic and thermal vortex flowmeters can reach 2g. The vortex flowmeter with force detection mode is more sensitive to resistance. In recent years, the progress in the design of force detection components and signal processing technology has significantly improved the vibration resistance, which can reach 0.5g to 1g. In the selection of vortex flowmeters, for pressurized gases, such as compressed air, due to the increase in density, the lower limit of flow rate can be reduced accordingly. Each manufacturer gives a curve or empirical formula of density and lower flow rate in the selection sample or the instruction manual.
(2) Pulsating flow effect
Most of the gas output by fans and compressors contains a certain component of the pulsation. The pulsation frequency and amplitude of the gas output by Roots fan are related to the speed of the waist wheel and the constant displacement volume, and the pulsation frequency is usually 100-200Hz; The reciprocating compressor output gas frequency is low, generally only a few Hertz. In addition, some gas equipment, such as air hammers, pneumatic tools, etc., will also cause airflow pulsation, and the frequency and amplitude of this pulsation are random. As mentioned above, the pulsating flow has an obvious effect on the stability of the Karman vortex street, and in severe cases, the vortex frequency may be "locked". There has been a "locking" phenomenon in the gas test device, which is used to debug the shunt rotor flowmeter device. A DN50 stress vortex flowmeter is installed on the pipeline. Roots fan is used as the air source upstream, and DN50 gas turbine is installed farther downstream (about 10m) for flow monitoring. The flow rate is controlled by the downstream valve.
In the test, the following phenomenon appears: when the downstream valve is enlarged to a certain opening, the frequency of the output signal of the vortex flowmeter no longer increases with the increase of the flow rate, and remains unchanged at about 200Hz. In this case, the gain and trigger sensitivity of vortex flowmeter preamplifier are not adjusted. At first, it was thought that it was 50Hz or frequency doubling interference, but strengthening the shielding and grounding did not work. It is also thought that the power supply l00Hz or frequency doubling interference, measuring the ripple of the DC power supply, the ripple is very small, and the replacement of the DC power supply will not help. At the same time, it is also observed that the signal frequency of the output of the turbine flowmeter is not stable, the waveform is jitter, and the frequency meter indicates a large change. In order to find out the reason and clarify the problem, the installation position of vortex flowmeter and turbine flowmeter is changed. Results The output signal frequency of the turbine flowmeter in the upstream had a large jump. The waveform was observed by oscilloscope, and the waveform jitter was serious and the pulse width was uneven. Looking at the downstream vortex flowmeter, the output waveform is no longer as "stable" as seen earlier, and the frequency displayed by the frequency meter beats between 195 and 220Hz, indicating that the vortex flowmeter is no longer locked in the original frequency. According to the above tests, the reason for this phenomenon is completely caused by the air pulsation sent by the fan. During the test, the maximum speed of Roots fan was checked. The speed of the fan was 2950r/min, the air volume was 20m3/min, and the wind pressure (total pressure) was 49kPao. According to the speed of the fan and the principle of Roots fan, the fan rotated once, and the waist wheel discharged 4 working volumes of air volume. From this, it can be calculated that the number of changes in the output air volume of the fan in one second is (2950 x 4)/ 602196.7, that is, the frequency of the output air pulsation of the fan is 196.7 Hz. In this way, it is proved that the phenomenon that the output frequency of the vortex flowmeter is locked at about 200Hz is completely the result of the output airflow pulsation of the Roots fan.
After the above phenomenon occurred, the adjusting device of Roots fan was reformed. The implementation method is to add a pulsation damping container at the outlet of Roots fan, as shown in Figure 15-5(b). Several partitions are added to the container, and the air flow around the container through several partitions. Due to the damping effect of the gas volume and gas resistance of the container, the pulsation amplitude of the gas flow is effectively attenuated. After the modification of the meter adjustment device, the influence of the air source pulsation is eliminated, and the above phenomenon no longer occurs. This reminds us that when vortex flowmeter is used to measure pulsating flow, effective measures should be taken to attenuate the fluid pulsation to a certain range, so as not to affect the normal operation of vortex flowmeter.
2. Other compressed gas flow measurement
In industry, oxygen, hydrogen, carbon dioxide, nitrogen and other compressed gas flow measurement is also a more demanding field. The advantages of using vortex flowmeter to measure the flow of these compressed gases are higher accuracy, less pressure loss, wider range, and less on-site installation and maintenance.
There is no difference between measuring the compressed gas flow with the vortex flowmeter and measuring the compressed air flow, just because the use, nature and production equipment of these gases are different, so in the design of the measurement system, the selection of the instrument range, the determination of the installation position, etc., some specific problems should be considered in combination with the characteristics of the production process and equipment.
(1) Problems that should be noted in oxygen flow measurement
① The power frequency interference problem of the measurement site. Most of the oxygen is produced by air division oxygen generator, in the oxygen production process, large synchronous motor, pipeline gas heating, etc., all use a strong current, 50Hz power frequency electromagnetic field may be induced to the pipeline of oxygen production equipment, once it affects the instrument, it may cause strong interference to the instrument. Resulting in no oxygen flow state, the instrument output false signals. Therefore, choosing the appropriate installation position, strengthening the shielding of the instrument and reasonable grounding measures are the key issues that should be considered when the instrument is installed.
② Measurement sensitivity problem. In order to produce safety, reduce or avoid the impact of static electricity, the flow rate of oxygen in the pipeline can not be very high. For example, an air liquefaction plant produces oxygen with a flow rate of 3500-5800nm3 /h, a temperature of 65 ° C, a working pressure in the pipe of 0.025MPa, and a transmission pipe with an inner diameter of 400mm. According to the above data, the volume flow rate converted to the working state is 3239-5638m3 /h, and the oxygen flow rate is 7.2-11.9m /s. This flow range is basically at the low end of the range for vortex flowmeters. Therefore, when the instrument is put into operation, the gain and trigger level of the vortex flowmeter preamplifier should be adjusted to a suitable level according to the field situation. Because the density of oxygen is higher than that of air, the signal amplitude is larger than that of air at the same flow rate, and the impact of field interference is considered, so the sensitivity of the vortex flowmeter can be appropriately reduced.
3. Oil ban. The instrument for measuring oxygen is absolutely prohibited from oil, which is a matter related to the safety of personnel and equipment. Due to lack of experience, a company provided a vortex flowmeter to measure oxygen flow, the oil ban was not complete when the factory was blocked by the oxygen factory, and the carbon tetrachloride was re-cleaned and degreased before installation.
(2) Problems that should be noted in hydrogen flow measurement
Hydrogen is a less dense gas, with a density of 0.0838kg/m3 in the standard state (101.325 kPa.20%), which is only 1/14 of the air in the same state. And because of the nature of hydrogen is very active, so in the industrial use of hydrogen, people are also very careful about it. In the design of the hydrogen pipeline, the flow rate of hydrogen is limited to a low, which brings trouble to flow measurement.
Because many enterprises have a cautious attitude towards hydrogen flow measurement, vortex flowmeters are rarely used in hydrogen flow measurement. The following problems should be paid attention to when using vortex flowmeter to measure hydrogen.
Measurement sensitivity issues. The density of hydrogen is very small and the flow rate is not high, so the vortex signal is very weak. Under the same state and flow rate, the signal strength is only 1/14 of that of air. Therefore, the sensitivity of the preamplifier should be adjusted to a higher level. The problem with the increased sensitivity of the preamplifier is that it is very easy to cause all kinds of interference in the environment. In order to avoid the intrusion of various interferences, the installation of the instrument should choose the appropriate installation position, pay attention to the shielding and grounding of the instrument.
Explosion protection issues. Hydrogen is flammable and explosive gas, and the required explosion-proof level is also very high, and this problem cannot be ignored when selecting the table. The problem of velocity limit for hydrogen flow measurement. After consulting relevant foreign personnel and the specific practice of domestic enterprises, it is believed that: "In the future, when we encounter low-pressure hydrogen flow measurement, we can ignore the consideration of flow rate, and we need to focus on the requirements of pressure drop and accuracy." This conclusion provides a new idea for using vortex flowmeter to measure hydrogen flow. Under the premise of allowing pressure loss, the measuring pipe diameter can be reduced to increase the flow rate, so that the amplitude of the vortex signal can be increased, so that the vortex flowmeter is in a better measuring range and the flow measurement accuracy can be improved. According to the field conditions, it is a better choice to use the reduced vortex flowmeter to measure the hydrogen flow.
If you want to know more about the flow meter or flow meter selection, please consult the Aister flow meter manufacturer email: sales@aistermeter.com for help.
There are many meters for gas flow measurement, in all kinds of flow meters, in addition to electromagnetic flow meters, the rest of the classes can almost measure gas flow, the current application of more differential pressure type, fluid vibration type, turbine type, ultrasonic flowmeter. Measuring gas flow with vortex flow meter is the main field of its application.
Compressed air can be regarded as a secondary energy source as an energy-carrying working medium. When the required working pressure is low, it can be generated by a fan. When the required working pressure is high, it is generated by the air compressor. In production, measuring the air volume of the fan and the flow rate of compressed air is an important means of energy management for enterprises. As the measured body, the air is a relatively clean, non-corrosive medium, although the air contains a certain amount of water and acid gas, but the vortex flowmeter with stainless steel as the liquid material, is completely usable, will not affect the measurement. Measure compressed air flow,
(1) Vibration effect.
Compressed air is produced by air compressors or high pressure fans. These devices have varying degrees of vibration during operation, and sometimes the vibration is relatively strong, and this vibration will be transmitted through the connecting pipe. This influence factor should be fully considered when selecting and determining the installation position of vortex flowmeter. In various detection methods, due to different detection components, the anti-vibration performance of the instrument is also different. The vibration resistance of vortex flowmeters using the principle of detecting speed changes is better, such as the vibration resistance of ultrasonic and thermal vortex flowmeters can reach 2g. The vortex flowmeter with force detection mode is more sensitive to resistance. In recent years, the progress in the design of force detection components and signal processing technology has significantly improved the vibration resistance, which can reach 0.5g to 1g. In the selection of vortex flowmeters, for pressurized gases, such as compressed air, due to the increase in density, the lower limit of flow rate can be reduced accordingly. Each manufacturer gives a curve or empirical formula of density and lower flow rate in the selection sample or the instruction manual.
(2) Pulsating flow effect
Most of the gas output by fans and compressors contains a certain component of the pulsation. The pulsation frequency and amplitude of the gas output by Roots fan are related to the speed of the waist wheel and the constant displacement volume, and the pulsation frequency is usually 100-200Hz; The reciprocating compressor output gas frequency is low, generally only a few Hertz. In addition, some gas equipment, such as air hammers, pneumatic tools, etc., will also cause airflow pulsation, and the frequency and amplitude of this pulsation are random. As mentioned above, the pulsating flow has an obvious effect on the stability of the Karman vortex street, and in severe cases, the vortex frequency may be "locked". There has been a "locking" phenomenon in the gas test device, which is used to debug the shunt rotor flowmeter device. A DN50 stress vortex flowmeter is installed on the pipeline. Roots fan is used as the air source upstream, and DN50 gas turbine is installed farther downstream (about 10m) for flow monitoring. The flow rate is controlled by the downstream valve.
In the test, the following phenomenon appears: when the downstream valve is enlarged to a certain opening, the frequency of the output signal of the vortex flowmeter no longer increases with the increase of the flow rate, and remains unchanged at about 200Hz. In this case, the gain and trigger sensitivity of vortex flowmeter preamplifier are not adjusted. At first, it was thought that it was 50Hz or frequency doubling interference, but strengthening the shielding and grounding did not work. It is also thought that the power supply l00Hz or frequency doubling interference, measuring the ripple of the DC power supply, the ripple is very small, and the replacement of the DC power supply will not help. At the same time, it is also observed that the signal frequency of the output of the turbine flowmeter is not stable, the waveform is jitter, and the frequency meter indicates a large change. In order to find out the reason and clarify the problem, the installation position of vortex flowmeter and turbine flowmeter is changed. Results The output signal frequency of the turbine flowmeter in the upstream had a large jump. The waveform was observed by oscilloscope, and the waveform jitter was serious and the pulse width was uneven. Looking at the downstream vortex flowmeter, the output waveform is no longer as "stable" as seen earlier, and the frequency displayed by the frequency meter beats between 195 and 220Hz, indicating that the vortex flowmeter is no longer locked in the original frequency. According to the above tests, the reason for this phenomenon is completely caused by the air pulsation sent by the fan. During the test, the maximum speed of Roots fan was checked. The speed of the fan was 2950r/min, the air volume was 20m3/min, and the wind pressure (total pressure) was 49kPao. According to the speed of the fan and the principle of Roots fan, the fan rotated once, and the waist wheel discharged 4 working volumes of air volume. From this, it can be calculated that the number of changes in the output air volume of the fan in one second is (2950 x 4)/ 602196.7, that is, the frequency of the output air pulsation of the fan is 196.7 Hz. In this way, it is proved that the phenomenon that the output frequency of the vortex flowmeter is locked at about 200Hz is completely the result of the output airflow pulsation of the Roots fan.
After the above phenomenon occurred, the adjusting device of Roots fan was reformed. The implementation method is to add a pulsation damping container at the outlet of Roots fan, as shown in Figure 15-5(b). Several partitions are added to the container, and the air flow around the container through several partitions. Due to the damping effect of the gas volume and gas resistance of the container, the pulsation amplitude of the gas flow is effectively attenuated. After the modification of the meter adjustment device, the influence of the air source pulsation is eliminated, and the above phenomenon no longer occurs. This reminds us that when vortex flowmeter is used to measure pulsating flow, effective measures should be taken to attenuate the fluid pulsation to a certain range, so as not to affect the normal operation of vortex flowmeter.
2. Other compressed gas flow measurement
In industry, oxygen, hydrogen, carbon dioxide, nitrogen and other compressed gas flow measurement is also a more demanding field. The advantages of using vortex flowmeter to measure the flow of these compressed gases are higher accuracy, less pressure loss, wider range, and less on-site installation and maintenance.
There is no difference between measuring the compressed gas flow with the vortex flowmeter and measuring the compressed air flow, just because the use, nature and production equipment of these gases are different, so in the design of the measurement system, the selection of the instrument range, the determination of the installation position, etc., some specific problems should be considered in combination with the characteristics of the production process and equipment.
(1) Problems that should be noted in oxygen flow measurement
① The power frequency interference problem of the measurement site. Most of the oxygen is produced by air division oxygen generator, in the oxygen production process, large synchronous motor, pipeline gas heating, etc., all use a strong current, 50Hz power frequency electromagnetic field may be induced to the pipeline of oxygen production equipment, once it affects the instrument, it may cause strong interference to the instrument. Resulting in no oxygen flow state, the instrument output false signals. Therefore, choosing the appropriate installation position, strengthening the shielding of the instrument and reasonable grounding measures are the key issues that should be considered when the instrument is installed.
② Measurement sensitivity problem. In order to produce safety, reduce or avoid the impact of static electricity, the flow rate of oxygen in the pipeline can not be very high. For example, an air liquefaction plant produces oxygen with a flow rate of 3500-5800nm3 /h, a temperature of 65 ° C, a working pressure in the pipe of 0.025MPa, and a transmission pipe with an inner diameter of 400mm. According to the above data, the volume flow rate converted to the working state is 3239-5638m3 /h, and the oxygen flow rate is 7.2-11.9m /s. This flow range is basically at the low end of the range for vortex flowmeters. Therefore, when the instrument is put into operation, the gain and trigger level of the vortex flowmeter preamplifier should be adjusted to a suitable level according to the field situation. Because the density of oxygen is higher than that of air, the signal amplitude is larger than that of air at the same flow rate, and the impact of field interference is considered, so the sensitivity of the vortex flowmeter can be appropriately reduced.
3. Oil ban. The instrument for measuring oxygen is absolutely prohibited from oil, which is a matter related to the safety of personnel and equipment. Due to lack of experience, a company provided a vortex flowmeter to measure oxygen flow, the oil ban was not complete when the factory was blocked by the oxygen factory, and the carbon tetrachloride was re-cleaned and degreased before installation.
(2) Problems that should be noted in hydrogen flow measurement
Hydrogen is a less dense gas, with a density of 0.0838kg/m3 in the standard state (101.325 kPa.20%), which is only 1/14 of the air in the same state. And because of the nature of hydrogen is very active, so in the industrial use of hydrogen, people are also very careful about it. In the design of the hydrogen pipeline, the flow rate of hydrogen is limited to a low, which brings trouble to flow measurement.
Because many enterprises have a cautious attitude towards hydrogen flow measurement, vortex flowmeters are rarely used in hydrogen flow measurement. The following problems should be paid attention to when using vortex flowmeter to measure hydrogen.
Measurement sensitivity issues. The density of hydrogen is very small and the flow rate is not high, so the vortex signal is very weak. Under the same state and flow rate, the signal strength is only 1/14 of that of air. Therefore, the sensitivity of the preamplifier should be adjusted to a higher level. The problem with the increased sensitivity of the preamplifier is that it is very easy to cause all kinds of interference in the environment. In order to avoid the intrusion of various interferences, the installation of the instrument should choose the appropriate installation position, pay attention to the shielding and grounding of the instrument.
Explosion protection issues. Hydrogen is flammable and explosive gas, and the required explosion-proof level is also very high, and this problem cannot be ignored when selecting the table. The problem of velocity limit for hydrogen flow measurement. After consulting relevant foreign personnel and the specific practice of domestic enterprises, it is believed that: "In the future, when we encounter low-pressure hydrogen flow measurement, we can ignore the consideration of flow rate, and we need to focus on the requirements of pressure drop and accuracy." This conclusion provides a new idea for using vortex flowmeter to measure hydrogen flow. Under the premise of allowing pressure loss, the measuring pipe diameter can be reduced to increase the flow rate, so that the amplitude of the vortex signal can be increased, so that the vortex flowmeter is in a better measuring range and the flow measurement accuracy can be improved. According to the field conditions, it is a better choice to use the reduced vortex flowmeter to measure the hydrogen flow.
If you want to know more about the flow meter or flow meter selection, please consult the Aister flow meter manufacturer email: sales@aistermeter.com for help.
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