1 Foreword
With the continuous advancement of science and technology, thermal measurement technology has also been greatly developed. The introduction of new thermal automation instruments frequently has played a positive role in solving the problems in the field of thermal measurement. However, various new types of instruments.
Due to its different measurement principles and structures, the occasions for adaptation are also different. To fully understand the measurement principle and structural characteristics of various measuring instruments, according to the different measured media and use occasions, the rational selection of measuring instruments, to improve the measurement accuracy, reliability and reduce equipment investment, improve the design quality is very necessary. This article talks about the types of flow measurement instruments and the problems that should be paid attention to in the selection of models for your reference. 
2 Types of flow measuring instruments
Flow measurement instruments mainly have the following types of measurement principles:
2.1 The throttling differential pressure flow device measures flow by measuring the final differential pressure of the throttling device installed in the pipe. Commonly used throttling devices include standard orifice plates, circular orifice plates, standard nozzles, long-diameter nozzles, Venturi tubes, and wedge-type flow replacement. 
2.2 Dynamic Pressure Velocimetry Flow device, which uses the velocity measuring element set in the pipeline to measure the flow rate of the measured medium to measure the flow. The commonly used measuring devices are dynamic pressure measuring tubes (also known as pitot tubes), constant velocity tubes (also known as annubars) and wing-type wind measuring devices. 
2.3 Float flowmeter (also known as rotameter), which uses the relative position of the float when measuring the force of the fluid on the float and the gravity balance of the float to measure the flow. Commonly used glass rotor flow meter and metal rotor flowmeter. 
2.4 impeller speed flow meter, which is the use of measurement under the action of the fluid, the impeller rotation speed to measure the flow, commonly used turbine flowmeter, water meter and so on. 
2.5 Vortex flowmeter, also known as vortex flowmeter, uses the Karman principle to measure the vortex frequency after the non-streamlined columnar body is placed in the pipe to measure the flow. 
2.6 volumetric flow meter, which is similar to a standard container that can be periodically rotated. When the fluid passes through the device, due to the effect of differential pressure at the inlet and outlet, the rotating parts in the flow meter are periodically rotated, and the flow is measured according to the number of rotations. Commonly used positive displacement flowmeters include oval gear flowmeters, waistwheel flowmeters (also known as Roots flowmeters), dual-rotor flowmeters, and the like. 
2.7 electromagnetic flowmeter, which is a flow meter designed using electromagnetic induction law. The flow rate is measured by measuring the electromotive force of the conductive fluid that is proportional to the speed of cutting the magnetic flux when passing through the magnetic field. 
2.8 Ultrasonic flowmeter, which uses the principle of measuring the ultrasonic time difference to measure the flow. According to the different installation methods, it can be divided into fixed installation and clamping installation. The former has higher accuracy than the latter. 
2.9 mass flow meter, which measures the mass flow of a fluid according to the Coriolis force principle. That is, when a fluid moves linearly in a rotating system, the force received is proportional to the mass and velocity of the fluid, ie, mass flow. 
2.10 Thermal type (also called thermal diffusion method) gas mass flow meter, which uses the fluid when passing through the pipe to take away the heat from the heating element in the pipe, and the heat taken away is proportional to the flow velocity and density of the fluid. Measuring the output power of the circuit (ie, the power consumed by the heating element), the corresponding mass flow of the fluid can be known. According to the different structure of the instrument, there are two kinds of pipeline and plug-in. 
2.11 Weighing flowmeter, which uses the momentum principle in mechanics to measure the weight of solid particles. Commonly used weighing flowmeters have electronic belt scales. Track scale, computer fixed flowmeter and so on. 
In addition, there are target flowmeters that measure the principle of force balance: elbow flowmeters that use elbow measurements, and steam flow rates into turbines that measure the pressure after the turbine's governor stage. This is not repeated here.
3 flow measurement instrument selection. 
The various flow meters introduced above, because of their different measuring principles and different structures, therefore, their accuracy, price, and adaptable media and occasions are also different. Therefore, when selecting flow meters, we should select the most appropriate ones according to their respective conditions. Flow measuring instrument. 
Below, we will give a brief introduction on the applicable occasions of various types of flowmeters for reference when selecting a type. 
3.1 throttling differential pressure method, which is a kind of most traditional flow measurement method, it has a simple structure, mature technology does not require calibration, there is no strict requirements on the pressure, temperature of the medium, a wide range of adaptation, the price is cheaper, but it The installation of the straight pipe section requires a long, throttling loss is also large, the orifice (or nozzle) inlet edge is easy to wear, wear and tear on the measurement accuracy. Therefore, for the clean gas and water flow measurement, when there are enough straight pipe sections installed and there is no strict requirement for pressure loss, the measuring devices such as orifice plates and nozzles should be used. For plug-in venturis. The double venturi tube is mainly used to measure the air volume of the air duct, and the wedge flow device is mainly used to measure the high viscosity fuel oil. 
3.2 Annubar flowmeter in dynamic pressure measurement method, it has a small throttling loss and high precision, can be used in various shapes (round, square, rectangular) and caliber of the pipeline, but it also needs to have The longer straight section, while it is also not suitable for measuring turbid medium, and the price is relatively high, so it is suitable for measuring large-diameter clean water pipelines or air flow. The wing-type air flow measuring equipment requires low installation of straight sections. The pressure loss is small, and the differential pressure value is stable and moderately suitable. It is a common device used for measuring the air volume of various air ducts. 
3.3Floater flowmeter, low requirements for installation of straight pipe sections, relatively large range, local instructions, when using special materials, can measure corrosive media, but it is suitable for medium pressure, low temperature, with remote metal Rotor flowmeters are also expensive. Therefore, it is more suitable for a water treatment system that does not have a sufficient diameter to install and has a low pressure and temperature. 
3.4 Vortex flowmeter, the range is relatively large, the accuracy is also high, but its rotating parts are easy to wear, it should not be measured unclean media, when used to measure water. 
3.5 vortex flowmeter, which is a flow meter with a wide range of applications, it has a large range ratio, low pressure loss, its flow characteristics are not affected by the pressure, temperature, viscosity, density, etc. of the fluid, so it is particularly suitable for measuring Water, low pressure steam and gas flow. 
3.6 volumetric flowmeter, high precision, large range, installation without straight pipe requirements, but requires the installation of filters. Pressure losses are also large and are often used in power plants to measure the flow of fuel. 
3.7 electromagnetic flowmeter, this is a new type of flow meter, it has high precision, large range ratio, low pressure loss, installation of straight pipe section requirements are short, but the price is higher, can not measure non-conductive media, can not adapt to high temperature, high pressure medium. Therefore, it is generally used for measuring the mixture of water or water with high precision. 
3.8 Ultrasonic flowmeter, which is also a new type of flow measurement instrument. The flowmeter has strong applicability and can be adapted to almost all industrial environments with high precision and low pressure loss. However, it also needs to have enough straight sections and the price is relatively high. Therefore, it is generally used in power plants to measure large-diameter water flow (measurement of circulating water flow tubes), fuel flow, and gas flow. 
3.9 Mass flow meter, this is a high-precision mass flow meter, it has high precision, large range ratio, can measure high-viscosity medium, and the requirement for straight pipe section is also short. Imported mass flow meters are expensive. Therefore, this type of flowmeter is generally used where precise metering is required. Power plants are often used to measure fuel flow and gas flow. 
3.10 Thermal diffusion flowmeter, which is a new type of gas mass flow meter. The range is very large. Can be used to measure gases other than acetylene. It is mainly used for air flow measurement and flue gas flow measurement in power plants. 
3.11 Weighing flowmeters are generally used for the measurement of coal and pulverized coal in power plants. 
In selecting the flow meter, in addition to considering the pressure and temperature of the media that the flow meter can adapt to, the flow state of the medium, that is, the Reynolds number, should be considered for the flow meter using the throttling differential pressure method and dynamic pressure measurement method. When it is too low (less than 5X103), it is not appropriate to use this type of measuring instrument. 
For the flow meter for measuring heavy oil, in addition to considering the pressure and temperature of the medium, the pressure and temperature of the purge medium should also be satisfied. Here are some of the flow meter performance comparison and usage occasions commonly used in the project, the list is as follows:
Note: Some of the technical data in the attached table are based on the same kind of instruments compiled by some manufacturers' product samples. Due to the difference in manufacturing plants, the performance difference is also relatively large. In the specific selection, the sample of each manufacturer shall prevail. The table is only for reference when selecting peers.
With the continuous advancement of science and technology, thermal measurement technology has also been greatly developed. The introduction of new thermal automation instruments frequently has played a positive role in solving the problems in the field of thermal measurement. However, various new types of instruments.
Due to its different measurement principles and structures, the occasions for adaptation are also different. To fully understand the measurement principle and structural characteristics of various measuring instruments, according to the different measured media and use occasions, the rational selection of measuring instruments, to improve the measurement accuracy, reliability and reduce equipment investment, improve the design quality is very necessary. This article talks about the types of flow measurement instruments and the problems that should be paid attention to in the selection of models for your reference. 
2 Types of flow measuring instruments
Flow measurement instruments mainly have the following types of measurement principles:
2.1 The throttling differential pressure flow device measures flow by measuring the final differential pressure of the throttling device installed in the pipe. Commonly used throttling devices include standard orifice plates, circular orifice plates, standard nozzles, long-diameter nozzles, Venturi tubes, and wedge-type flow replacement. 
2.2 Dynamic Pressure Velocimetry Flow device, which uses the velocity measuring element set in the pipeline to measure the flow rate of the measured medium to measure the flow. The commonly used measuring devices are dynamic pressure measuring tubes (also known as pitot tubes), constant velocity tubes (also known as annubars) and wing-type wind measuring devices. 
2.3 Float flowmeter (also known as rotameter), which uses the relative position of the float when measuring the force of the fluid on the float and the gravity balance of the float to measure the flow. Commonly used glass rotor flow meter and metal rotor flowmeter. 
2.4 impeller speed flow meter, which is the use of measurement under the action of the fluid, the impeller rotation speed to measure the flow, commonly used turbine flowmeter, water meter and so on. 
2.5 Vortex flowmeter, also known as vortex flowmeter, uses the Karman principle to measure the vortex frequency after the non-streamlined columnar body is placed in the pipe to measure the flow. 
2.6 volumetric flow meter, which is similar to a standard container that can be periodically rotated. When the fluid passes through the device, due to the effect of differential pressure at the inlet and outlet, the rotating parts in the flow meter are periodically rotated, and the flow is measured according to the number of rotations. Commonly used positive displacement flowmeters include oval gear flowmeters, waistwheel flowmeters (also known as Roots flowmeters), dual-rotor flowmeters, and the like. 
2.7 electromagnetic flowmeter, which is a flow meter designed using electromagnetic induction law. The flow rate is measured by measuring the electromotive force of the conductive fluid that is proportional to the speed of cutting the magnetic flux when passing through the magnetic field. 
2.8 Ultrasonic flowmeter, which uses the principle of measuring the ultrasonic time difference to measure the flow. According to the different installation methods, it can be divided into fixed installation and clamping installation. The former has higher accuracy than the latter. 
2.9 mass flow meter, which measures the mass flow of a fluid according to the Coriolis force principle. That is, when a fluid moves linearly in a rotating system, the force received is proportional to the mass and velocity of the fluid, ie, mass flow. 
2.10 Thermal type (also called thermal diffusion method) gas mass flow meter, which uses the fluid when passing through the pipe to take away the heat from the heating element in the pipe, and the heat taken away is proportional to the flow velocity and density of the fluid. Measuring the output power of the circuit (ie, the power consumed by the heating element), the corresponding mass flow of the fluid can be known. According to the different structure of the instrument, there are two kinds of pipeline and plug-in. 
2.11 Weighing flowmeter, which uses the momentum principle in mechanics to measure the weight of solid particles. Commonly used weighing flowmeters have electronic belt scales. Track scale, computer fixed flowmeter and so on. 
In addition, there are target flowmeters that measure the principle of force balance: elbow flowmeters that use elbow measurements, and steam flow rates into turbines that measure the pressure after the turbine's governor stage. This is not repeated here.
3 flow measurement instrument selection. 
The various flow meters introduced above, because of their different measuring principles and different structures, therefore, their accuracy, price, and adaptable media and occasions are also different. Therefore, when selecting flow meters, we should select the most appropriate ones according to their respective conditions. Flow measuring instrument. 
Below, we will give a brief introduction on the applicable occasions of various types of flowmeters for reference when selecting a type. 
3.1 throttling differential pressure method, which is a kind of most traditional flow measurement method, it has a simple structure, mature technology does not require calibration, there is no strict requirements on the pressure, temperature of the medium, a wide range of adaptation, the price is cheaper, but it The installation of the straight pipe section requires a long, throttling loss is also large, the orifice (or nozzle) inlet edge is easy to wear, wear and tear on the measurement accuracy. Therefore, for the clean gas and water flow measurement, when there are enough straight pipe sections installed and there is no strict requirement for pressure loss, the measuring devices such as orifice plates and nozzles should be used. For plug-in venturis. The double venturi tube is mainly used to measure the air volume of the air duct, and the wedge flow device is mainly used to measure the high viscosity fuel oil. 
3.2 Annubar flowmeter in dynamic pressure measurement method, it has a small throttling loss and high precision, can be used in various shapes (round, square, rectangular) and caliber of the pipeline, but it also needs to have The longer straight section, while it is also not suitable for measuring turbid medium, and the price is relatively high, so it is suitable for measuring large-diameter clean water pipelines or air flow. The wing-type air flow measuring equipment requires low installation of straight sections. The pressure loss is small, and the differential pressure value is stable and moderately suitable. It is a common device used for measuring the air volume of various air ducts. 
3.3Floater flowmeter, low requirements for installation of straight pipe sections, relatively large range, local instructions, when using special materials, can measure corrosive media, but it is suitable for medium pressure, low temperature, with remote metal Rotor flowmeters are also expensive. Therefore, it is more suitable for a water treatment system that does not have a sufficient diameter to install and has a low pressure and temperature. 
3.4 Vortex flowmeter, the range is relatively large, the accuracy is also high, but its rotating parts are easy to wear, it should not be measured unclean media, when used to measure water. 
3.5 vortex flowmeter, which is a flow meter with a wide range of applications, it has a large range ratio, low pressure loss, its flow characteristics are not affected by the pressure, temperature, viscosity, density, etc. of the fluid, so it is particularly suitable for measuring Water, low pressure steam and gas flow. 
3.6 volumetric flowmeter, high precision, large range, installation without straight pipe requirements, but requires the installation of filters. Pressure losses are also large and are often used in power plants to measure the flow of fuel. 
3.7 electromagnetic flowmeter, this is a new type of flow meter, it has high precision, large range ratio, low pressure loss, installation of straight pipe section requirements are short, but the price is higher, can not measure non-conductive media, can not adapt to high temperature, high pressure medium. Therefore, it is generally used for measuring the mixture of water or water with high precision. 
3.8 Ultrasonic flowmeter, which is also a new type of flow measurement instrument. The flowmeter has strong applicability and can be adapted to almost all industrial environments with high precision and low pressure loss. However, it also needs to have enough straight sections and the price is relatively high. Therefore, it is generally used in power plants to measure large-diameter water flow (measurement of circulating water flow tubes), fuel flow, and gas flow. 
3.9 Mass flow meter, this is a high-precision mass flow meter, it has high precision, large range ratio, can measure high-viscosity medium, and the requirement for straight pipe section is also short. Imported mass flow meters are expensive. Therefore, this type of flowmeter is generally used where precise metering is required. Power plants are often used to measure fuel flow and gas flow. 
3.10 Thermal diffusion flowmeter, which is a new type of gas mass flow meter. The range is very large. Can be used to measure gases other than acetylene. It is mainly used for air flow measurement and flue gas flow measurement in power plants. 
3.11 Weighing flowmeters are generally used for the measurement of coal and pulverized coal in power plants. 
In selecting the flow meter, in addition to considering the pressure and temperature of the media that the flow meter can adapt to, the flow state of the medium, that is, the Reynolds number, should be considered for the flow meter using the throttling differential pressure method and dynamic pressure measurement method. When it is too low (less than 5X103), it is not appropriate to use this type of measuring instrument. 
For the flow meter for measuring heavy oil, in addition to considering the pressure and temperature of the medium, the pressure and temperature of the purge medium should also be satisfied. Here are some of the flow meter performance comparison and usage occasions commonly used in the project, the list is as follows:
Note: Some of the technical data in the attached table are based on the same kind of instruments compiled by some manufacturers' product samples. Due to the difference in manufacturing plants, the performance difference is also relatively large. In the specific selection, the sample of each manufacturer shall prevail. The table is only for reference when selecting peers.
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