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Principles and methods for selecting flow meters
1、 The principle of selecting a flow meter is to first have a deep understanding of the structural principles and fluid characteristics of various flo
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1、 Principles for selecting flow meters
The principle of selecting a flow meter is first to have a profound understanding of the structural principles and fluid characteristics of various flow meters, and to choose based on the specific situation on site and the surrounding environmental conditions. We also need to consider economic factors. In general, the selection should be made from the following five aspects:
① Performance requirements for flow meters;② Fluid characteristics;③ Installation requirements;④ Environmental conditions;⑤ The price of the flow meter.
1. Performance requirements for flow meters
The performance of flow meters mainly includes: measuring flow rate (instantaneous flow rate) or total flow rate (cumulative flow rate); Accuracy requirements; Repeatability; Linearity; Flow range and range degree; Pressure loss; Output signal characteristics and response time of flow meter, etc.
(1) Measuring traffic or total volume
Flow measurement includes two types, namely instantaneous flow and cumulative flow. For example, measuring the total amount of crude oil in distribution station pipelines for trade transfer or continuous proportioning production or process control in petrochemical pipelines, or occasionally supplemented by observation of instantaneous flow. In some workplaces, instantaneous flow measurement is required to control flow. Therefore, the selection should be based on the needs of on-site measurement. Some flow meters, such as volumetric flow meters and turbine flow meters, use mechanical counting or pulse frequency output to directly obtain the total amount. They have high accuracy and are suitable for measuring the total amount. If equipped with corresponding signaling devices, they can also output the flow rate. Electromagnetic flow meters, ultrasonic flow meters, etc. are used to measure fluid flow velocity and derive flow rate. They have fast response and are suitable for process control. If equipped with an accumulation function, the total amount can also be obtained.
(2) Accuracy
The specification for the accuracy level of a flowmeter is within a certain flow range. If it is used under a specific condition or within a relatively narrow flow range, such as only changing within a small range, its measurement accuracy will be higher than the specified accuracy level. If a turbine flowmeter is used to measure the distribution of oil products in barrels, when the valve is fully opened, the flow rate is basically constant, and its accuracy may increase from level 0.5 to level 0.25.
The accuracy level is generally determined based on the maximum allowable error of the flowmeter. The flow meter instructions provided by each manufacturer will be included. It is important to note whether the percentage of error refers to relative error or citation error. Relative error is the percentage of the measured value, commonly expressed as "% R". Reference error refers to the percentage of the upper limit or range of the measurement, commonly used as "% FS". Many manufacturing manuals do not specify this. For example, float flowmeters generally use reference error, and some models of electromagnetic flowmeters also use reference error.
If a flow meter is not simply used to measure the total amount, but is applied in a flow control system, the accuracy of detecting the flow meter should be determined based on the overall system control accuracy requirements. Because the entire system not only has errors in flow detection, but also includes errors in signal transmission, control regulation, operation execution, and various influencing factors. For example, there are about 2% in the operating system
It is uneconomical and unreasonable to determine an excessively high accuracy (above 0.5 level) for the measuring instruments used due to the hysteresis. As far as the instrument itself is concerned, the accuracy between the sensor and the secondary instrument should also be appropriately matched. For example, if the designed average speed tube error is between ± 2.5% and ± 4% without actual calibration, it is not meaningful to equip it with a high-precision differential pressure gauge of 0.2% to 0.5%.
Another issue is that the accuracy level specified for the flowmeter in the calibration regulations or manufacturer's instructions refers to the maximum allowable error of the flowmeter. However, due to the influence of environmental conditions, fluid flow conditions, and dynamic conditions on the flow meter during on-site use, some additional errors may occur. Therefore, the flow meter used on site should be a combination of the maximum allowable error and additional error of the instrument itself, and this issue must be fully considered. Sometimes, the error within the usage environment on site may exceed the maximum allowable error of the flow meter.
(3) Repeatability
Repeatability is determined by the principle of the flowmeter itself and the manufacturing quality, and is an important technical indicator during the use of the flowmeter, closely related to the accuracy of the flowmeter. In general, the metrological performance requirements in the verification regulations not only specify the accuracy level of the flowmeter, but also specify the repeatability. The general rule is that the repeatability of the flowmeter shall not exceed 1/3 to 1/5 of the maximum allowable error specified in the corresponding accuracy level.
Repeatability is generally defined as the consistency of multiple measurements in the same direction for a certain flow rate value in a short period of time, under constant environmental conditions and medium parameters. However, in practical applications, the repeatability of flow meters is often affected by changes in fluid viscosity and density parameters, and sometimes these parameter changes have not reached the level that requires specialized correction, leading to the misconception that the flow meter has poor repeatability. Given this situation, a flow meter that is insensitive to changes in this parameter should be selected. For example, a float flowmeter is easily affected by fluid density, while a small-diameter flowmeter may not only be affected by fluid density, but also by fluid viscosity; The viscosity effect of turbine flowmeter when used in high viscosity range; Some ultrasonic flow meters that have not been corrected may be affected by fluid temperature and other factors. If the output of the flowmeter is non-linear, this effect may be more prominent.
(4) Linearity
The output of a flowmeter mainly includes two types: linear and nonlinear square root. Generally speaking, the nonlinear error of a flowmeter is not listed separately, but is included in the error of the flowmeter. For flow meters with a generally wide flow range and pulse output signal used for total accumulation calculation, linearity is an important technical indicator. If a single instrument coefficient is used within its flow range, the accuracy of the flow meter will be reduced when the linearity is poor. For example, if a turbine flowmeter uses an instrument coefficient within a flow range of 10:1, its accuracy will be lower when the linearity is poor. With the development of computer technology, its flow range can be segmented, and the flow instrument coefficient curve can be fitted using the least squares method to correct the flowmeter, thereby improving its accuracy and expanding its flow range.(5) Upper limit traffic and traffic range
The upper limit flow rate is also known as the full flow rate or maximum flow rate of the flowmeter. When choosing the diameter of a flowmeter, we should configure it according to the flow range of the pipeline being tested and the upper and lower limits of the selected flowmeter, rather than simply matching it according to the diameter of the pipeline.
Generally speaking, the maximum flow velocity of the designed pipeline fluid is determined based on the economic flow velocity. If the selection is too low and the pipe diameter is too thick, the investment will be large; If the transmission power is too high, it will increase the operating cost. For example, low viscosity liquids such as water have an economic flow rate of 1.5-3m/s, while high viscosity liquids have an economic flow rate of 0.2-1m/s. Most flow meters have an upper limit flow rate that is close to or higher than the pipeline's economic flow rate. Therefore, when selecting a flowmeter with the same diameter as the pipeline, there are more options available, making installation more convenient. If they are not the same, they will not differ too much. Generally, the specifications of adjacent upper and lower levels can be connected using a reducer.
In the selection of flow meters, attention should be paid to different types of flow meters, whose upper limit flow rate or upper limit flow rate varies greatly due to the limitations of the measurement principles and structures of their respective flow meters. Taking liquid flow meters as an example, the flow rate of the upper limit flow is the lowest for glass float flow meters, generally between 0.5-1.5m/s, for volumetric flow meters between 2.5-3.5m/s, for vortex flow meters between 5.5-7.5m/s, and for electromagnetic flow meters between 1-7m/s, even reaching 0.5-10m/s.
The upper limit flow rate of liquid also needs to consider not causing cavitation due to high flow rate. The location where cavitation occurs is generally at the position with the highest flow rate and lowest static pressure. In order to prevent the formation of cavitation, it is often necessary to control the minimum back pressure (maximum flow rate) of the flowmeter.It should also be noted that the upper limit value of the flow meter cannot be changed after ordering, such as volumetric flow meters or float flow meters. Once a differential pressure flowmeter, such as a throttling device orifice plate, is designed and determined, its lower limit flow rate cannot be changed. The upper limit flow rate can be changed by adjusting or replacing the differential pressure transmitter. For example, for certain models of electromagnetic or ultrasonic flow meters, some users can reset the flow limit value themselves.
(6) Range degree
The range is the ratio of the upper limit flow rate to the lower limit flow rate of the flowmeter, and the larger the value, the wider the flow range. Linear instruments have a wide range, generally 1:10. The range of nonlinear flow meters is relatively small, only 1:3. Flow meters commonly used for process control or trade handover accounting should not be chosen if a wide flow range is required.
At present, some manufacturers promote the wide flow range of their flow meters by setting the upper limit flow rate very high in the user manual, such as increasing the liquid flow rate to 7-10m/s (usually 6m/s); Gas is increased to 50-75m/s (usually 40-50 m/s); In fact, such a high flow rate is not necessary. In fact, the key to a wide range is having a lower lower limit flow rate to meet measurement needs. So a wide range flowmeter with a low lower flow rate is more practical.
(7) Pressure loss
Pressure loss generally refers to the unrecoverable pressure loss caused by the static or active detection elements or changes in flow direction installed in the flow channel of a flow sensor, which varies with the flow rate and can sometimes reach tens of kilopascals. Therefore, the flow meter should be selected based on the maximum allowable pressure loss determined by the pumping capacity of the pipeline system and the inlet pressure of the flow meter. Improper selection can limit fluid flow and result in excessive pressure loss, which can affect flow efficiency. Some liquids (high vapor pressure hydrocarbon liquids) should also be aware that excessive pressure drop may cause cavitation and liquid-phase vaporization, reducing measurement accuracy and even damaging flow meters. For example, flow meters used for water transmission with a diameter greater than 500mm should consider the increased pumping costs caused by excessive energy loss due to pressure loss. According to relevant reports, flow meters with high pressure losses often incur pumping costs for measurement that exceed the purchase cost of low pressure loss and expensive flow meters in recent years.
(8) Output signal characteristics
The output and display quantity of the flowmeter can be divided into:
① Flow rate (volume flow rate or mass flow rate); ② Total amount; ③ Average flow velocity; ④ Point flow velocity. Some flow meters output analog quantities (current or voltage), while others output pulse quantities. Analog output is generally considered suitable for process control and is more suitable for connection with control circuit units such as regulating valves; Pulse output is more suitable for total and high-precision flow measurement. The pulse output of long-distance signal transmission has higher transmission accuracy than analog output. The output signal method and amplitude should also have the ability to adapt to other devices, such as control interfaces, data processors, alarm devices, circuit breakers, and data transmission systems.
(9) Response time
Attention should be paid to the response of flow meters to flow step changes when applied to pulsating flow scenarios. Some usage scenarios require the flow meter output to follow the changes in fluid flow, while others require a slower response output to obtain a comprehensive average value. Instantaneous response is often expressed as a time constant or response frequency, with values ranging from milliseconds to seconds for the former and below several hundred Hz for the latter. The use of display instruments may significantly prolong response time. It is generally believed that when the flow rate of a flowmeter increases or decreases, asymmetric dynamic response will accelerate the increase of flow measurement errors.
The principle of selecting a flow meter is first to have a profound understanding of the structural principles and fluid characteristics of various flow meters, and to choose based on the specific situation on site and the surrounding environmental conditions. We also need to consider economic factors. In general, the selection should be made from the following five aspects:
① Performance requirements for flow meters;② Fluid characteristics;③ Installation requirements;④ Environmental conditions;⑤ The price of the flow meter.
1. Performance requirements for flow meters
The performance of flow meters mainly includes: measuring flow rate (instantaneous flow rate) or total flow rate (cumulative flow rate); Accuracy requirements; Repeatability; Linearity; Flow range and range degree; Pressure loss; Output signal characteristics and response time of flow meter, etc.
(1) Measuring traffic or total volume
Flow measurement includes two types, namely instantaneous flow and cumulative flow. For example, measuring the total amount of crude oil in distribution station pipelines for trade transfer or continuous proportioning production or process control in petrochemical pipelines, or occasionally supplemented by observation of instantaneous flow. In some workplaces, instantaneous flow measurement is required to control flow. Therefore, the selection should be based on the needs of on-site measurement. Some flow meters, such as volumetric flow meters and turbine flow meters, use mechanical counting or pulse frequency output to directly obtain the total amount. They have high accuracy and are suitable for measuring the total amount. If equipped with corresponding signaling devices, they can also output the flow rate. Electromagnetic flow meters, ultrasonic flow meters, etc. are used to measure fluid flow velocity and derive flow rate. They have fast response and are suitable for process control. If equipped with an accumulation function, the total amount can also be obtained.
(2) Accuracy
The specification for the accuracy level of a flowmeter is within a certain flow range. If it is used under a specific condition or within a relatively narrow flow range, such as only changing within a small range, its measurement accuracy will be higher than the specified accuracy level. If a turbine flowmeter is used to measure the distribution of oil products in barrels, when the valve is fully opened, the flow rate is basically constant, and its accuracy may increase from level 0.5 to level 0.25.
The accuracy level is generally determined based on the maximum allowable error of the flowmeter. The flow meter instructions provided by each manufacturer will be included. It is important to note whether the percentage of error refers to relative error or citation error. Relative error is the percentage of the measured value, commonly expressed as "% R". Reference error refers to the percentage of the upper limit or range of the measurement, commonly used as "% FS". Many manufacturing manuals do not specify this. For example, float flowmeters generally use reference error, and some models of electromagnetic flowmeters also use reference error.
If a flow meter is not simply used to measure the total amount, but is applied in a flow control system, the accuracy of detecting the flow meter should be determined based on the overall system control accuracy requirements. Because the entire system not only has errors in flow detection, but also includes errors in signal transmission, control regulation, operation execution, and various influencing factors. For example, there are about 2% in the operating system
It is uneconomical and unreasonable to determine an excessively high accuracy (above 0.5 level) for the measuring instruments used due to the hysteresis. As far as the instrument itself is concerned, the accuracy between the sensor and the secondary instrument should also be appropriately matched. For example, if the designed average speed tube error is between ± 2.5% and ± 4% without actual calibration, it is not meaningful to equip it with a high-precision differential pressure gauge of 0.2% to 0.5%.
Another issue is that the accuracy level specified for the flowmeter in the calibration regulations or manufacturer's instructions refers to the maximum allowable error of the flowmeter. However, due to the influence of environmental conditions, fluid flow conditions, and dynamic conditions on the flow meter during on-site use, some additional errors may occur. Therefore, the flow meter used on site should be a combination of the maximum allowable error and additional error of the instrument itself, and this issue must be fully considered. Sometimes, the error within the usage environment on site may exceed the maximum allowable error of the flow meter.
(3) Repeatability
Repeatability is determined by the principle of the flowmeter itself and the manufacturing quality, and is an important technical indicator during the use of the flowmeter, closely related to the accuracy of the flowmeter. In general, the metrological performance requirements in the verification regulations not only specify the accuracy level of the flowmeter, but also specify the repeatability. The general rule is that the repeatability of the flowmeter shall not exceed 1/3 to 1/5 of the maximum allowable error specified in the corresponding accuracy level.
Repeatability is generally defined as the consistency of multiple measurements in the same direction for a certain flow rate value in a short period of time, under constant environmental conditions and medium parameters. However, in practical applications, the repeatability of flow meters is often affected by changes in fluid viscosity and density parameters, and sometimes these parameter changes have not reached the level that requires specialized correction, leading to the misconception that the flow meter has poor repeatability. Given this situation, a flow meter that is insensitive to changes in this parameter should be selected. For example, a float flowmeter is easily affected by fluid density, while a small-diameter flowmeter may not only be affected by fluid density, but also by fluid viscosity; The viscosity effect of turbine flowmeter when used in high viscosity range; Some ultrasonic flow meters that have not been corrected may be affected by fluid temperature and other factors. If the output of the flowmeter is non-linear, this effect may be more prominent.
(4) Linearity
The output of a flowmeter mainly includes two types: linear and nonlinear square root. Generally speaking, the nonlinear error of a flowmeter is not listed separately, but is included in the error of the flowmeter. For flow meters with a generally wide flow range and pulse output signal used for total accumulation calculation, linearity is an important technical indicator. If a single instrument coefficient is used within its flow range, the accuracy of the flow meter will be reduced when the linearity is poor. For example, if a turbine flowmeter uses an instrument coefficient within a flow range of 10:1, its accuracy will be lower when the linearity is poor. With the development of computer technology, its flow range can be segmented, and the flow instrument coefficient curve can be fitted using the least squares method to correct the flowmeter, thereby improving its accuracy and expanding its flow range.(5) Upper limit traffic and traffic range
The upper limit flow rate is also known as the full flow rate or maximum flow rate of the flowmeter. When choosing the diameter of a flowmeter, we should configure it according to the flow range of the pipeline being tested and the upper and lower limits of the selected flowmeter, rather than simply matching it according to the diameter of the pipeline.
Generally speaking, the maximum flow velocity of the designed pipeline fluid is determined based on the economic flow velocity. If the selection is too low and the pipe diameter is too thick, the investment will be large; If the transmission power is too high, it will increase the operating cost. For example, low viscosity liquids such as water have an economic flow rate of 1.5-3m/s, while high viscosity liquids have an economic flow rate of 0.2-1m/s. Most flow meters have an upper limit flow rate that is close to or higher than the pipeline's economic flow rate. Therefore, when selecting a flowmeter with the same diameter as the pipeline, there are more options available, making installation more convenient. If they are not the same, they will not differ too much. Generally, the specifications of adjacent upper and lower levels can be connected using a reducer.
In the selection of flow meters, attention should be paid to different types of flow meters, whose upper limit flow rate or upper limit flow rate varies greatly due to the limitations of the measurement principles and structures of their respective flow meters. Taking liquid flow meters as an example, the flow rate of the upper limit flow is the lowest for glass float flow meters, generally between 0.5-1.5m/s, for volumetric flow meters between 2.5-3.5m/s, for vortex flow meters between 5.5-7.5m/s, and for electromagnetic flow meters between 1-7m/s, even reaching 0.5-10m/s.
The upper limit flow rate of liquid also needs to consider not causing cavitation due to high flow rate. The location where cavitation occurs is generally at the position with the highest flow rate and lowest static pressure. In order to prevent the formation of cavitation, it is often necessary to control the minimum back pressure (maximum flow rate) of the flowmeter.It should also be noted that the upper limit value of the flow meter cannot be changed after ordering, such as volumetric flow meters or float flow meters. Once a differential pressure flowmeter, such as a throttling device orifice plate, is designed and determined, its lower limit flow rate cannot be changed. The upper limit flow rate can be changed by adjusting or replacing the differential pressure transmitter. For example, for certain models of electromagnetic or ultrasonic flow meters, some users can reset the flow limit value themselves.
(6) Range degree
The range is the ratio of the upper limit flow rate to the lower limit flow rate of the flowmeter, and the larger the value, the wider the flow range. Linear instruments have a wide range, generally 1:10. The range of nonlinear flow meters is relatively small, only 1:3. Flow meters commonly used for process control or trade handover accounting should not be chosen if a wide flow range is required.
At present, some manufacturers promote the wide flow range of their flow meters by setting the upper limit flow rate very high in the user manual, such as increasing the liquid flow rate to 7-10m/s (usually 6m/s); Gas is increased to 50-75m/s (usually 40-50 m/s); In fact, such a high flow rate is not necessary. In fact, the key to a wide range is having a lower lower limit flow rate to meet measurement needs. So a wide range flowmeter with a low lower flow rate is more practical.
(7) Pressure loss
Pressure loss generally refers to the unrecoverable pressure loss caused by the static or active detection elements or changes in flow direction installed in the flow channel of a flow sensor, which varies with the flow rate and can sometimes reach tens of kilopascals. Therefore, the flow meter should be selected based on the maximum allowable pressure loss determined by the pumping capacity of the pipeline system and the inlet pressure of the flow meter. Improper selection can limit fluid flow and result in excessive pressure loss, which can affect flow efficiency. Some liquids (high vapor pressure hydrocarbon liquids) should also be aware that excessive pressure drop may cause cavitation and liquid-phase vaporization, reducing measurement accuracy and even damaging flow meters. For example, flow meters used for water transmission with a diameter greater than 500mm should consider the increased pumping costs caused by excessive energy loss due to pressure loss. According to relevant reports, flow meters with high pressure losses often incur pumping costs for measurement that exceed the purchase cost of low pressure loss and expensive flow meters in recent years.
(8) Output signal characteristics
The output and display quantity of the flowmeter can be divided into:
① Flow rate (volume flow rate or mass flow rate); ② Total amount; ③ Average flow velocity; ④ Point flow velocity. Some flow meters output analog quantities (current or voltage), while others output pulse quantities. Analog output is generally considered suitable for process control and is more suitable for connection with control circuit units such as regulating valves; Pulse output is more suitable for total and high-precision flow measurement. The pulse output of long-distance signal transmission has higher transmission accuracy than analog output. The output signal method and amplitude should also have the ability to adapt to other devices, such as control interfaces, data processors, alarm devices, circuit breakers, and data transmission systems.
(9) Response time
Attention should be paid to the response of flow meters to flow step changes when applied to pulsating flow scenarios. Some usage scenarios require the flow meter output to follow the changes in fluid flow, while others require a slower response output to obtain a comprehensive average value. Instantaneous response is often expressed as a time constant or response frequency, with values ranging from milliseconds to seconds for the former and below several hundred Hz for the latter. The use of display instruments may significantly prolong response time. It is generally believed that when the flow rate of a flowmeter increases or decreases, asymmetric dynamic response will accelerate the increase of flow measurement errors.
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