Flow detection and instrumentation is a type of instrument designed to measure the flow or total amount of fluid passing through a pipe or equipment per unit time. These instruments are widely used in various industrial production process control, energy management, environmental protection, transportation, scientific research and other fields, and are essential for monitoring and regulating the flow of fluids such as liquids, gases or steam.
I. Classification
Flow detection instruments can be divided into many types according to different working principles, such as:
1.1 Differential pressure flowmeters (such as orifice flowmeters, venturi flowmeters) : use the relationship between the pressure difference generated by the fluid flowing through the throttle element and the flow rate to measure the flow rate.
1.2 Positive displacement flowmeters (such as elliptical gear flowmeters, scraper flowmeters) : by measuring the number of times in a unit time through a known fixed volume of space to determine the flow rate.
1.3 Velocity flowmeters (such as turbine flowmeters, electromagnetic flowmeters, ultrasonic flowmeters) : directly or indirectly measure the flow rate of fluid in the pipeline, so as to calculate the flow rate.
1.4 Mass flowmeters (such as thermal mass flowmeters, Coriolis mass flowmeters) : can directly measure the mass flow of the fluid without being affected by changes in the physical properties of the fluid.
In addition, the flow meter can also distinguish between instantaneous flow measurement and cumulative flow measurement according to its function, the former reflects the flow value at a certain time, the latter statistics the total amount of fluid through a period of time.
Second, the working principle of common flow meters
2.1 Working principle of rotameter
The working principle of the rotameter is based on the balance of Archimedes' buoyancy and force. It is mainly composed of a conical tube and a rotor that can float up and down in the conical tube. When the fluid flows in from the bottom of the conical tube and flows upward, the rotor is affected by the fluid and rises. According to the different fluid flow, the buoyancy force of the rotor also changes. When the buoyancy is balanced with the rotor's own gravity and the damping force due to fluid resistance, the rotor will stabilize in a certain position.
This position is directly proportional to the flow through the pipe, that is, the greater the flow, the higher the rotor rises. The rotor is usually equipped with a permanent magnet, whose height and position changes can be sensed by external indicators (such as Pointers) and transmitted through magnetic coupling, thus displaying the current flow value.
The smart rotammeter also uses microprocessor control technology that utilizes dual hall sensors to detect magnetic fields that change with the position of the rotor and measure its vertical and horizontal components. This data is processed and compared with a preset magnetic field calibration table, and then the instantaneous flow rate is converted to the actual flow value by interpolation calculation, which is displayed on the display screen and may output a standard signal (e.g. 4-20mA) for remote monitoring or control use.
2.2 Working principle of electromagnetic flowmeter
The Electromagnetic Flowmeter (EMF) operates on the basis of Faraday's law of electromagnetic induction. Specifically: Inside the electromagnetic flowmeter, there is a measuring tube made of non-permeable magnetic material, and the inner wall of the tube is covered with a layer of insulating material as a liner to ensure good electrical insulation between the fluid and the measuring tube. An electromagnetic coil is arranged on both sides of the measuring tube, and a stable magnetic field is generated around the coil after being energized, and the magnetic field is perpendicular to the axis of the measuring tube. When a conducting fluid (such as water, acid and alkali solution, slurry, etc., whose conductivity must be greater than 5μs/cm) passes through this magnetic field, the electromotive force (E) will be induced because the charged particles in the fluid cut the magnetic field lines, and its size is proportional to the fluid speed. Inside the tube are two electrodes in contact with the fluid, which are used to detect this induced electromotive force. This electromotive force is transmitted to the converter part of the meter. The converter will amplify and filter the induced weak signal, and convert the induced electromotive force into the corresponding flow value through the pre-set calculation formula. Finally, the flow meter displays the volume flow or mass flow (after density compensation) of the fluid flowing through the pipe.
In summary, the electromagnetic flowmeter is a kind of instrument that determines the flow of fluid by measuring the induced electromotive force generated by the movement of conductive fluid in the magnetic field.
2.3 Working principle of vortex flowmeter
Vortex flowmeters are flow measuring instruments that operate on the Karman Vortex Street principle. The working principle is as follows: Inside the vortex flowmeter, there is usually a specific shape of the resistance fluid (such as a triangular column or cylinder), the resistance fluid is called the vortex generator, which is vertically installed on the center line of the fluid pipeline. When the fluid flows through the vortex generating body, due to the separation effect caused by the fluid bypassing the non-streamlined object, a series of regular vortices will be alternately generated on both sides of the vortex generating body, and these vortices are like a vortex street, so it is named vortex flowmeter.
The generated vortex has a certain frequency, which is directly related to the velocity of the fluid flowing through the vortex generator, and under certain conditions, the frequency of the vortex is linearly proportional to the volume flow of the fluid. By detecting elements (such as piezoelectric crystals or other types of sensors), vortex flowmeters can capture and record the frequency of the vortex, and then calculate the actual flow value through a preset conversion formula.
In summary, vortex flowmeter is a device that indirectly determines fluid flow by measuring the vortex frequency caused by the vortex generator, and is suitable for measuring the flow of various gases, liquids and steam.
Installation of differential pressure flowmeter
3. Selection of flow measuring instrument
The selection of flow measuring instruments is a comprehensive consideration of many factors in the process, the following are some basic selection principles and steps:
3.1 Fluid characteristics: temperature, pressure, density, viscosity, chemical corrosion, wear, scaling tendency, whether there is miscible or phase change, electrical conductivity (important for electromagnetic flowmeters), sound velocity (related to ultrasonic flowmeters), thermal conductivity and specific heat capacity (heat flowmeters may be involved), etc.
3.2 Measurement requirements: Determine whether you need to measure instantaneous flow or cumulative flow, or both. Instruments are selected according to the desired level of accuracy to ensure accuracy requirements for process control or trade handover are met. Determine the flow range and select a meter with the right range (range ratio) to ensure accurate measurement throughout the operating range.
3.3 Instrument performance: Consider the accuracy, repeatability, linearity, response time and pressure loss of the instrument. Select the appropriate instrument type according to the characteristics of the output signal, such as analog signal (4-20mA), pulse signal or digital communication interface (HART, MODBUS, PROFIBUS, etc.).
3.4 Installation conditions: the layout direction of the pipeline, the flow direction and the length of the upstream and downstream straight pipe section have a great impact on the performance of the flowmeter, especially for differential pressure flowmeters and vortex flowmeters. Make sure there is enough space for installation, and consider the diameter of the pipe, ease of maintenance, and auxiliary equipment that may be needed, such as filters, sterilizers, etc.
3.5 Environmental conditions: ambient temperature, humidity, electromagnetic compatibility, safety, explosion-proof grade, anti-pipeline vibration ability, etc. are all factors that need to be considered when selecting.
3.6 Specific application: Select the appropriate flowmeter type according to different occasions, such as: for easy crystallization or viscous media, electromagnetic flowmeter or positive displacement flowmeter may be suitable. For clean gases or liquids, a turbine flowmeter, vortex flowmeter, or ultrasonic flowmeter may be a good choice. For steam, vortex flowmeters, V-cone flowmeters, or orifice flowmeters with differential pressure transmitters may be selected. For large diameter pipes with limited installation space, Anoba flowmeters or venturi tubes may be ideal solutions.
To sum up, the selection of flow meters is a systematic project, which needs to be carefully analyzed and evaluated according to various factors such as specific process parameters, field conditions and economic benefits. In practical applications, it is also necessary to consult the detailed product specifications and technical manuals provided by the manufacturer to ensure that the selected flowmeter can adapt and meet the project requirements.
Four, commonly used flow measurement instrument selection
There are many kinds of commonly used flow measuring instruments, and each instrument has its own applicable occasions and characteristics. The following is a summary of the different types of flowmeters and their typical application scenarios and selection reference points:
4.1 Electromagnetic Flowmeter
Application scenario: Applicable to the measurement of various conductive liquids, including water, wastewater, corrosive fluids, etc., not applicable to the measurement of non-conductive liquids (such as petroleum products) and gases.
Selection reference: According to the conductivity of the measured flow, maximum flow, minimum flow, fluid pressure, temperature and pipe material and other factors to determine.
4.2 Float flowmeter (Rotameter)
Application scenario: Suitable for small diameter, low flow rate of small flow liquid, gas and steam measurement.
Selection reference: Consider the physical properties of the fluid (density, viscosity, etc.), working pressure, temperature range, as well as flow range and accuracy requirements.
4.3 Vortex flowmeter (vortex flowmeter)
Application scenario: Widely used in industrial pipeline gas, liquid and steam flow measurement, especially suitable for small pressure loss and do not need frequent maintenance occasions.
Selection reference: Pay attention to fluid properties, flow range, pressure level, pipeline vibration, and pay attention to avoid use in strong vibration environment.
4.4 Turbine Flowmeter
Application scenario: Ideal for fast and accurate measurement of clean, low-viscosity liquids and gases, especially in trade settlement and process control.
Selection reference: Consider fluid cleanliness, minimum measurable flow, maximum allowable pressure and temperature, and whether bidirectional measurement is required.
4.5 Ultrasonic Flowmeter
Application scenario: Suitable for all kinds of pure or containing a small amount of impurities of the liquid and gas, especially large diameter pipelines and do not need to contact the fluid occasions.
Selection reference: you need to know the sound speed of the fluid, the material and size of the pipe, and the sediment condition in the pipe.
4.6 Coriolis force mass flowmeter
Application scenario: It is suitable for accurate measurement of mass flow of various fluids in chemical, petroleum, food and beverage industries, regardless of fluid temperature and pressure changes.
Selection reference: Focus on the physical state of the fluid, changes in fluid composition, and whether real-time density and concentration measurements are required.
4.7 Differential pressure flowmeter (such as orifice flowmeter, Venturi flowmeter) :
Application scenario: It is widely used in flow measurement of various fluids, especially under high pressure, high temperature and large flow conditions.
Selection reference: According to the fluid characteristics of the calculation of the required hole plate or venturi throat diameter, considering the range and accuracy requirements of the pressure transmitter.
In the specific selection, it is also necessary to consider the cost, maintenance convenience, reliability, life expectancy and other factors. In addition, each flowmeter has specific technical specifications and performance indicators, such as range ratio, accuracy level, repeatability, zero stability, etc., which should be carefully reviewed and compared in the selection.