By analyzing the principle of flow measurement, on-site conditions and measurement requirements, the flow meter is properly selected so that the performance of the selected meter meets the requirements of actual operating parameters and the purpose of accurate metering can be achieved. This article combines the author's work practices for several years and talks about the selection of flow measurement instruments.
1. First, determine whether it is necessary to install a flow meter. If you only want to know whether the fluid in the pipeline is flowing or not, and observe its general flow, then using a flow window or flow indicator can achieve this goal at a lower cost. Because of its simple structure, the domestic flow meter manufacturing industry pays insufficient attention to the window viewing and flow indicators, and only a few companies provide products. On the other hand, from the introduction of foreign petrochemical, steam turbine, and other complete sets of equipment, more flow is installed on the station. Peeper or indicator.
If the measurement requirements are high, you need to install a flow meter. Site conditions should be fully utilized. Flow sensors do not have to be specially installed. The bends in the pipe system can be used, as can the pressure drop of other turbulent pipe sections (eg T-tubes). The same purpose can usually be achieved. The relationship between flow rate and differential pressure can be checked online by making some simplifying assumptions (such as bends) or using an external clamped portable ultrasonic flowmeter to check online at an appropriate location upstream (or downstream) of the resistance section.
2. Analyze factors Collect all types of instrument samples, technical data, and selection manuals to fully understand the specification performance of various types of instruments. Then, according to performance requirements and instrument specifications, fluid characteristics, installation sites, environmental conditions, and economical application of five factors, According to the questions raised later, one by one analysis, list comparison. Different measurement objects have their own measurement purpose and have different focuses on the performance of the instrument. For example, business accounting and storage and transportation require high accuracy; continuous measurement process control usually requires good reliability and repeatability, sometimes requires a wider measurement range, and places measurement accuracy requirements on a secondary level; Better than production, we hope to have good accuracy. The order of consideration generally begins with "performance requirements and instrument specifications", with due consideration of other factors. If it is considered that "economic factors" are the main factors (such as low pumping costs for large pipe transmission requirements and small measurement errors for trade accounting requirements), other factors are secondary factors.
3. Accuracy In fluid flow measurement, if it is not only the total amount of measurement but it is used in a flow control system, the accuracy of the detection instrument must be determined under the overall system control accuracy requirement because the entire system not only has the error of flow detection. It also contains errors and various influencing factors such as signal transmission, control adjustment, and operation execution. For example, the operation execution link often has a return difference of about 2%, and the accuracy of the measurement instrument is determined to be too high (for example, 0·5. Grade) is unreasonable and uneconomical. As far as the flow meter itself is concerned, the accuracy between the detection element (or sensor) and the conversion display instrument should also be properly determined, such as the real-time calibration of differential pressure devices such as constant-velocity, wedge, and elbow, etc. Between 5% and 5%, it does not make sense to use a high-precision differential pressure gauge to match (at present, the error of the ROSEMOUNT3051 used by our factory is 0.075%).
The accuracy class shown in the flow meter specification is within a relatively wide flow range. If the use condition is within a certain flow or a very narrow flow range, the measurement accuracy used at this time can be higher than the specified value; if it can be The measuring points are specifically calibrated to increase the accuracy, for example from 0·5 to 0·25 or higher. When used for higher accuracy of business accounting, storage, and material balance requirements, it is also necessary to consider the durability of accuracy, whether it is easy to re-check and other key factors, and whether there is the possibility of online verification.
4. Repeatability Repeatability is an important indicator in process control applications. It is determined by the principle of the instrument itself and the quality of manufacture. Accuracy, in addition to repeatability, is related to the calibration system. Strictly speaking, the repeatability refers to the consistency of multiple measurements in the same direction over a period of time for a certain flow value under the condition of constant environmental conditions, medium parameters, and the like. However, in practical applications, excellent reproducibility of the meter is affected by many factors including fluid viscosity, density, and the like, and is often mistaken for poor repeatability of the meter. Therefore, there are places where the parameters change, and do not choose instruments sensitive to this parameter change. For example, float meters are susceptible to fluid density, and turbine meters are susceptible to fluid viscosity when used for high viscosity measurements. If the output characteristics of the instrument are non-linear, the effect is more prominent.
5, linearity flow meter output mainly linear and square root non-linear two. Most of the non-linearity errors of flowmeters are not listed in the individual indicators but are included in the basic error. However, linearity is an important indicator for a meter with a wide flow range pulse output as a totalizer. Using the same meter constant within the flow range, the linearity difference may reduce the meter accuracy. With the development of microprocessor technology, signal adaptation technology can be used to modify the instrument system nonlinearity, thereby increasing the accuracy of the instrument and extending the flow range.
6, the upper limit flow and flow range upper limit flow is also called full flow. The caliber of the selected flow meter should be selected according to the flow range used by the pipeline under test and the upper and lower limit flows of the selected instrument, instead of simply using the path of the pipeline. The maximum flow rate of the pipe fluid is usually designed according to the economic flow rate. Because the flow rate selection is too low, the diameter of the tube is large, and the investment is large. If the flow velocity is too high, the power is transmitted and the operating cost is increased. For example, low viscosity liquids such as water have an economic flow rate of 1.5 to 3 m/s, and high viscosity liquids have an economic flow rate of 0.2 to 1 m/s. The flow rate of most of the flow meter's upper limit flow rate is close to or slightly higher than the economic flow rate of the pipeline. Therefore, there are many opportunities for the instrument to select the same diameter and diameter, and the installation is more convenient. If they are not the same, they will not differ too much. Generally, there is a difference between the specifications and the use of different-diameter pipes. The different types of meter upper limit traffic of the same caliber are restricted by their respective working principles and structures, which are quite different. Some meter flow upper limit values ​​can not be changed after ordering, such as positive displacement meters and float-type meters; differential pressure meter orifices, etc., are designed to determine the lower limit flow rate can not be changed, but the flow rate upper limit can be adjusted to adjust the differential pressure transmitter range. Adaptation; some meters do not pass the real flow calibration user can reset the flow upper limit, such as certain types of electromagnetic flow meters and ultrasonic flow meters.
7. The extent of the range is the ratio of the upper limit flow to the lower limit flow. The larger the value is, the wider the flow range is. Linear instruments have a larger range, generally 10:1; non-linear instruments are smaller, usually only 3:1, to meet the general process control flow measurement and total business accounting measurement. However, some commercial accounting instruments require a wide range of degrees, for example, the daytime and nighttime of utility water quantity measurement and the winter and summer seasons are very different, requiring a wide range of degrees. The widening of the range of differential pressure meters has seen some breakthroughs in recent years, mainly in the differential pressure transmitter and microcomputer technology applications, but also up to 10:1, but the price of the instrument is more than doubled. Some models of electromagnetic flowmeter users can adjust the flow upper limit, the upper adjustable ratio (the ratio of the maximum upper limit value and the minimum upper limit value) can reach 10, and then multiplied by the set upper limit of 20:1 range. The range of significance of an instrument's expansion (ie, considering the upper limit adjustable ratio) can reach (50~200):1, and some models have the function of automatically switching the upper limit flow value. Some manufacturers have indicated that their range is wide. The maximum flow rate of the upper limit flow rate is very high. The liquid is 7~10m/s and the gas is 50~75m/s. In fact, such a high flow rate is generally not available. The key is Whether the lower limit flow rate adapts to the measurement requirements generally requires a lower range of the lower limit flow rate to be lower.
8, pressure loss In addition to unimpeded flow sensor (electromagnetic, ultrasonic, etc.), most of the flow sensor or to change the direction of flow, or in the flow channel set static or moving detection elements, resulting in changes with the flow rate Unrecoverable pressure loss, sometimes up to several 10 kPa. The pumping energy consumption is directly proportional to pressure loss, fluid density, and flow rate, and excessive pressure loss due to improper selection often affects process efficiency. For flowmeters with a pipe diameter greater than 500mm, the energy loss caused by pressure loss should be taken into consideration, and the pumping cost should not be increased if the pressure loss is too great. The pump with a large pressure loss often pays more for pumping in a few years. The cost of low pressure loss is more expensive than the purchase of the expensive instrument.
9. The response time applied to the pulsating flow field should pay attention to the instrument's response to the flow step change. Some use sites require the meter output to follow the flow changes, while others require only a slower response output in order to obtain a composite average. The transient response is often expressed as a time constant or response frequency. The former range from a few milliseconds to several seconds. The latter is below several hundred hertz. The use of a display instrument may considerably increase the response time, and the meter's flow rate rise and fall may not respond dynamically. Symmetry can drastically increase the measurement error.
In summary, the selection of the flow meter is a systematic project. There may be several options for the meter that can be used for a specific application site. When making a selection, it is only necessary to make decisions based on past experience and simply considering the initial installation cost. Lose the chance to choose the best instrument. Therefore, in the flow measurement design, the instrument performance that is selected according to the site conditions can exactly meet the actual operating parameter requirements in order to make it fully play its role and achieve the purpose of accurate measurement.
1. First, determine whether it is necessary to install a flow meter. If you only want to know whether the fluid in the pipeline is flowing or not, and observe its general flow, then using a flow window or flow indicator can achieve this goal at a lower cost. Because of its simple structure, the domestic flow meter manufacturing industry pays insufficient attention to the window viewing and flow indicators, and only a few companies provide products. On the other hand, from the introduction of foreign petrochemical, steam turbine, and other complete sets of equipment, more flow is installed on the station. Peeper or indicator.
If the measurement requirements are high, you need to install a flow meter. Site conditions should be fully utilized. Flow sensors do not have to be specially installed. The bends in the pipe system can be used, as can the pressure drop of other turbulent pipe sections (eg T-tubes). The same purpose can usually be achieved. The relationship between flow rate and differential pressure can be checked online by making some simplifying assumptions (such as bends) or using an external clamped portable ultrasonic flowmeter to check online at an appropriate location upstream (or downstream) of the resistance section.
2. Analyze factors Collect all types of instrument samples, technical data, and selection manuals to fully understand the specification performance of various types of instruments. Then, according to performance requirements and instrument specifications, fluid characteristics, installation sites, environmental conditions, and economical application of five factors, According to the questions raised later, one by one analysis, list comparison. Different measurement objects have their own measurement purpose and have different focuses on the performance of the instrument. For example, business accounting and storage and transportation require high accuracy; continuous measurement process control usually requires good reliability and repeatability, sometimes requires a wider measurement range, and places measurement accuracy requirements on a secondary level; Better than production, we hope to have good accuracy. The order of consideration generally begins with "performance requirements and instrument specifications", with due consideration of other factors. If it is considered that "economic factors" are the main factors (such as low pumping costs for large pipe transmission requirements and small measurement errors for trade accounting requirements), other factors are secondary factors.
3. Accuracy In fluid flow measurement, if it is not only the total amount of measurement but it is used in a flow control system, the accuracy of the detection instrument must be determined under the overall system control accuracy requirement because the entire system not only has the error of flow detection. It also contains errors and various influencing factors such as signal transmission, control adjustment, and operation execution. For example, the operation execution link often has a return difference of about 2%, and the accuracy of the measurement instrument is determined to be too high (for example, 0·5. Grade) is unreasonable and uneconomical. As far as the flow meter itself is concerned, the accuracy between the detection element (or sensor) and the conversion display instrument should also be properly determined, such as the real-time calibration of differential pressure devices such as constant-velocity, wedge, and elbow, etc. Between 5% and 5%, it does not make sense to use a high-precision differential pressure gauge to match (at present, the error of the ROSEMOUNT3051 used by our factory is 0.075%).
The accuracy class shown in the flow meter specification is within a relatively wide flow range. If the use condition is within a certain flow or a very narrow flow range, the measurement accuracy used at this time can be higher than the specified value; if it can be The measuring points are specifically calibrated to increase the accuracy, for example from 0·5 to 0·25 or higher. When used for higher accuracy of business accounting, storage, and material balance requirements, it is also necessary to consider the durability of accuracy, whether it is easy to re-check and other key factors, and whether there is the possibility of online verification.
4. Repeatability Repeatability is an important indicator in process control applications. It is determined by the principle of the instrument itself and the quality of manufacture. Accuracy, in addition to repeatability, is related to the calibration system. Strictly speaking, the repeatability refers to the consistency of multiple measurements in the same direction over a period of time for a certain flow value under the condition of constant environmental conditions, medium parameters, and the like. However, in practical applications, excellent reproducibility of the meter is affected by many factors including fluid viscosity, density, and the like, and is often mistaken for poor repeatability of the meter. Therefore, there are places where the parameters change, and do not choose instruments sensitive to this parameter change. For example, float meters are susceptible to fluid density, and turbine meters are susceptible to fluid viscosity when used for high viscosity measurements. If the output characteristics of the instrument are non-linear, the effect is more prominent.
5, linearity flow meter output mainly linear and square root non-linear two. Most of the non-linearity errors of flowmeters are not listed in the individual indicators but are included in the basic error. However, linearity is an important indicator for a meter with a wide flow range pulse output as a totalizer. Using the same meter constant within the flow range, the linearity difference may reduce the meter accuracy. With the development of microprocessor technology, signal adaptation technology can be used to modify the instrument system nonlinearity, thereby increasing the accuracy of the instrument and extending the flow range.
6, the upper limit flow and flow range upper limit flow is also called full flow. The caliber of the selected flow meter should be selected according to the flow range used by the pipeline under test and the upper and lower limit flows of the selected instrument, instead of simply using the path of the pipeline. The maximum flow rate of the pipe fluid is usually designed according to the economic flow rate. Because the flow rate selection is too low, the diameter of the tube is large, and the investment is large. If the flow velocity is too high, the power is transmitted and the operating cost is increased. For example, low viscosity liquids such as water have an economic flow rate of 1.5 to 3 m/s, and high viscosity liquids have an economic flow rate of 0.2 to 1 m/s. The flow rate of most of the flow meter's upper limit flow rate is close to or slightly higher than the economic flow rate of the pipeline. Therefore, there are many opportunities for the instrument to select the same diameter and diameter, and the installation is more convenient. If they are not the same, they will not differ too much. Generally, there is a difference between the specifications and the use of different-diameter pipes. The different types of meter upper limit traffic of the same caliber are restricted by their respective working principles and structures, which are quite different. Some meter flow upper limit values ​​can not be changed after ordering, such as positive displacement meters and float-type meters; differential pressure meter orifices, etc., are designed to determine the lower limit flow rate can not be changed, but the flow rate upper limit can be adjusted to adjust the differential pressure transmitter range. Adaptation; some meters do not pass the real flow calibration user can reset the flow upper limit, such as certain types of electromagnetic flow meters and ultrasonic flow meters.
7. The extent of the range is the ratio of the upper limit flow to the lower limit flow. The larger the value is, the wider the flow range is. Linear instruments have a larger range, generally 10:1; non-linear instruments are smaller, usually only 3:1, to meet the general process control flow measurement and total business accounting measurement. However, some commercial accounting instruments require a wide range of degrees, for example, the daytime and nighttime of utility water quantity measurement and the winter and summer seasons are very different, requiring a wide range of degrees. The widening of the range of differential pressure meters has seen some breakthroughs in recent years, mainly in the differential pressure transmitter and microcomputer technology applications, but also up to 10:1, but the price of the instrument is more than doubled. Some models of electromagnetic flowmeter users can adjust the flow upper limit, the upper adjustable ratio (the ratio of the maximum upper limit value and the minimum upper limit value) can reach 10, and then multiplied by the set upper limit of 20:1 range. The range of significance of an instrument's expansion (ie, considering the upper limit adjustable ratio) can reach (50~200):1, and some models have the function of automatically switching the upper limit flow value. Some manufacturers have indicated that their range is wide. The maximum flow rate of the upper limit flow rate is very high. The liquid is 7~10m/s and the gas is 50~75m/s. In fact, such a high flow rate is generally not available. The key is Whether the lower limit flow rate adapts to the measurement requirements generally requires a lower range of the lower limit flow rate to be lower.
8, pressure loss In addition to unimpeded flow sensor (electromagnetic, ultrasonic, etc.), most of the flow sensor or to change the direction of flow, or in the flow channel set static or moving detection elements, resulting in changes with the flow rate Unrecoverable pressure loss, sometimes up to several 10 kPa. The pumping energy consumption is directly proportional to pressure loss, fluid density, and flow rate, and excessive pressure loss due to improper selection often affects process efficiency. For flowmeters with a pipe diameter greater than 500mm, the energy loss caused by pressure loss should be taken into consideration, and the pumping cost should not be increased if the pressure loss is too great. The pump with a large pressure loss often pays more for pumping in a few years. The cost of low pressure loss is more expensive than the purchase of the expensive instrument.
9. The response time applied to the pulsating flow field should pay attention to the instrument's response to the flow step change. Some use sites require the meter output to follow the flow changes, while others require only a slower response output in order to obtain a composite average. The transient response is often expressed as a time constant or response frequency. The former range from a few milliseconds to several seconds. The latter is below several hundred hertz. The use of a display instrument may considerably increase the response time, and the meter's flow rate rise and fall may not respond dynamically. Symmetry can drastically increase the measurement error.
In summary, the selection of the flow meter is a systematic project. There may be several options for the meter that can be used for a specific application site. When making a selection, it is only necessary to make decisions based on past experience and simply considering the initial installation cost. Lose the chance to choose the best instrument. Therefore, in the flow measurement design, the instrument performance that is selected according to the site conditions can exactly meet the actual operating parameter requirements in order to make it fully play its role and achieve the purpose of accurate measurement.
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