Views: 3 Author: Site Editor Publish Time: 2022-01-03 Origin: Site
Flow rate is one of the main measurement parameters of the process industry production process, and a variety of factors need to be considered when selecting a suitable and highly reliable flow measurement instrument. Follow the selection principle, introduce the classification of flow meters and the characteristics and applications of commonly used flow meters.Flow rate is one of the main measurement parameters of the process industry production process, and a variety of factors need to be considered when selecting a suitable and highly reliable flow measurement instrument. Follow the selection principle, introduce the classification of flow meters and the characteristics and applications of commonly used flow meters.
Flow meters can be divided into two categories: volume measurement and mass measurement according to the measurement method.
Volume measurement can be divided into speed type, variable area type, volume type and differential pressure type.
Velocity flow meters include ultrasonic flow meters, vortex flow meters, electromagnetic flow meters, target flow meters, and turbine flow meters.
Variable area type: float type flowmeter, piston type flowmeter.
Positive displacement: oval gear flowmeter, scraper flowmeter, Roots flowmeter and dual rotor flowmeter.
Differential pressure type: Venturi flowmeter, orifice flowmeter, pitot tube, averaging tube, annubar flowmeter, V-cone flowmeter, etc. Mass measurement is divided into Coriolis mass flowmeter and thermal mass flowmeter.
The Coriolis mass flow meter is a direct mass flow meter made based on the Coriolis force that is proportional to the mass flow of the fluid when the fluid flows in the vibrating tube. Mainly used for trade settlement, high-precision process measurement control, performance evaluation and measurement, etc. For example, the measurement and measurement of crude oil and various oil products, liquid raw materials, chemical products, catalysts, polymerization inhibitors, etc., can also be used to measure bidirectional fluids and small flow rates.
Coriolis mass flowmeters should not be used in strong vibration, strong magnetic field, water hammer effect in the fluid, strong pulsating flow, entrained air flow, etc.
Thermal mass flow meter is a meter that uses the relationship of heat exchange between fluid and heat source to measure flow. Divided into two kinds of heat distribution and heat dissipation. Thermal mass flow meters have slow response and are not suitable for flow measurement of pulsating fluids, large changes in composition, viscous or dirty fluids.
Thermally distributed mass flow meters are used for flow measurement of ultra-clean gases, such as H2, NH3, etc.; thermally dissipative mass flow meters are used for HVAC, water supply and drainage, water treatment, fan flow, air emissions, etc.
Thermal mass flow meters are used to measure small flow rates at low flow rates, diatomic gases such as N2, O2, H2, etc., without moving parts, high reliability, and wide measurement range. When measuring the small gas flow, make sure the piping is clean, purge and add a double filter in front of the sensor.
Electromagnetic flowmeter is a kind of flowmeter for measuring conductive liquid manufactured according to the principle of Faraday's law of electromagnetic induction. Its limitations can be seen from its measurement principle: it cannot measure steam and gas, the measured medium must be conductive, and the conductivity cannot Less than 5μs/cm. Its advantage is that it can measure dirty, viscous and corrosive fluids, such as pulp, mud, sewage, etc. There is no pressure loss caused by flow monitoring. The flow range is large and the caliber range is wide. For large-caliber pipelines, plug-in electromagnetic flowmeters can be used.
The electrode and lining of the electromagnetic flowmeter are in direct contact with the measured medium, so choose the appropriate electrode and lining material for the corrosiveness and operating temperature of the measured medium.
The vortex flowmeter is researched and produced according to the Karman vortex principle. It is mainly used for the flow measurement of industrial pipeline fluids. It is suitable for many types of fluids, such as gas, liquid, vapor and other media. It is characterized by small pressure loss, large measuring range and high accuracy. The disadvantage is that it is not suitable for the measurement of low Reynolds number fluids, the installation requires a long straight pipe section, and the vortex flowmeter is not suitable for the occasions where the pipeline vibration is large.
Turbine flowmeter is the main type of impeller flowmeter. The sensor uses a multi-blade rotor to pass the average flow rate of the fluid to obtain the flow rate.
It is suitable for flow measurement of low-viscosity media such as tap water, gasoline, diesel, light refined oil, air, natural gas, etc. The highest accuracy can reach ±0.5% and ±0.2%.
Limitations: It cannot maintain its verification characteristics for a long time, and it needs to be verified regularly;
Ordinary turbine flowmeters cannot measure high-viscosity media;
The sensor requires long upstream and downstream straight pipe sections, and a rectifier can be installed to reduce the length of the straight pipe section;
The cleanliness of the medium is relatively high. Although a filter can be installed to remove impurities in the medium, the installation of a filter will increase the pressure loss and increase the amount of maintenance.
There are many types of differential pressure flow meters, such as orifice flow meters, balanced flow meters, pitot tubes, averaging tubes, annubars, wedge flow meters, rectangular flow meters, and V cone flow meters mentioned above. The differential pressure flow meter is based on the differential pressure generated when the fluid flows through the throttling device to achieve flow measurement.
Standard throttling devices include orifice, nozzle, venturi nozzle and venturi tube.
Non-standard throttling devices include: restrictive orifice, eccentric orifice, balanced flowmeter, wedge flowmeter, averaging velocity tube, etc.
There are many types of differential pressure flowmeters. Now, a brief introduction will be made on the application of non-standard throttling devices and balanced flowmeters.
The measurement principle of the balanced flowmeter is based on the Bernoulli law of conservation of energy to achieve equal Reynolds number and momentum balance. Through the optimized design of the sensor of the porous disc throttling device (balanced flow sensor), when the fluid passes through the porous disc throttling device, the fluid is balanced and adjusted by the sensor, and the eddy current is minimized to form an ideal flow field through the throttling device Take the pressure to obtain the differential pressure to calculate the fluid flow.
The reasonable design and selection of the flow meter should be a solution with high reliability, convenient maintenance and cost saving under the premise of meeting the requirements of use.
The factors that need to be considered in the selection of flow meters mainly include: fluid characteristics, flowmeter characteristics requirements, installation conditions, environmental conditions and costs.
In addition, the selection of flow meters cannot fully pursue high performance and high precision, which will not only increase the purchase cost, but also the cost of spare parts.
Accurate measurement and adjustment of fluid flow is the basis for ensuring the safety of petrochemical production processes, improving product quality, and reducing energy consumption. Due to the complexity and diversity of the media for flow measurement, there is no universal flow meter, and no flow meter can be applied to various media under any different conditions. Each type of flowmeter has its advantages and limitations. Flowmeters with different principles, different structures, and different functions play a role in their respective applicable ranges. Therefore, the selection of flow meters needs to follow standard specifications and selection principles while comprehensively considering various factors that affect flow measurement.