Application of common flowmeter

The instruments for measuring fluid flow are collectively referred to as flowmeter or flowmeter. Flowmeter is one of the important instruments in industrial measurement. It is widely used in metallurgy, electric power, coal, chemical industry, petroleum, transportation, construction, light textile, food, medicine, agriculture, environmental protection, people’s daily life and other fields of national economy. It is to develop industrial and agricultural production and save energy, It is an important tool to improve product quality, economic efficiency and management level, and plays an important role in the national economy. In order to adapt to various purposes, various types of flow meters have been invented one after another. At present, more than 60 kinds of flow meters have been put into use. According to the most popular and extensive classification at present, it is divided into: differential pressure flowmeter, vortex street flowmeter, turbine flowmeter, float flowmeter, digital target flowmeter, electromagnetic flowmeter and ultrasonic flowmeter.

1 differential pressure flowmeter
1.1 differential pressure flowmeter
Differential pressure flowmeter is an instrument that calculates the flow according to the differential pressure generated by the flow detector installed in the pipeline, known fluid conditions and the geometric dimensions of the detector and the pipeline. Differential pressure flowmeter is one of the most widely used flowmeters in industry. Its measurement accuracy is determined by its measurement principle, structure, manufacturing process level, the nature and service conditions of the measured fluid. Differential pressure flowmeter is composed of primary device (detection part) and secondary device (differential pressure conversion and flow display instrument). Differential pressure flowmeter is usually classified in the form of detection parts, such as orifice flowmeter, V-cone Flowmeter, etc.
1.1.1 orifice flowmeter
Working principle of orifice flowmeter: a throttling device is installed on the fluid flow pipe, which is equipped with an orifice plate, and a circular hole is opened in the center. Its hole diameter is smaller than the inner diameter of the pipe. The fluid flows stably forward in front of the orifice plate. When the fluid flows through the orifice plate, the stable flow state is disturbed due to the smaller hole diameter and the contraction of cross-sectional area, so the flow rate will change and the speed will be accelerated, The static pressure of the gas decreases, so there is a pressure drop before and after the orifice, that is, the differential pressure (the pressure is large where the section in front of the orifice is large, and the pressure is small where the section through the orifice is small). There is a definite numerical relationship between the magnitude of differential pressure and fluid flow, that is, when the flow is large, the differential pressure will be large, and when the flow is small, the differential pressure will be small. The flow is proportional to the square root of the differential pressure

1.1.2 V-cone Flowmeter
V-cone Flowmeter, originated from American mcrometer, is a new type of differential pressure flow instrument with great advantages. Since its birth more than 20 years ago, it has been widely used and praised in the field of flow measurement because of its many advantages that cannot be compared with conventional differential pressure instruments. V-cone Flowmeter is a new differential pressure flow metering device. It changes almost all the shortcomings of the traditional throttling device with a unique side wall gradually shrinking throttling mode. It is a revolutionary achievement of differential pressure flowmeter. Its principle is the same as other differential pressure flow meters. It is the classic principle of energy conservation and flow continuity in closed pipeline, and has the functions of self rectification, self-cleaning and self-protection; The straight pipe section is required to be very short, free of dirt and blockage, and can maintain long-term stability.
The working principle of V-cone Flowmeter is the same as that of orifice flowmeter. When the medium passes through the V-cone, due to the existence of the V-cone, the flow area of the fluid changes and the flow rate changes. According to the Bernoulli equation, the change of the flow rate causes the change of the pressure, which has a certain relationship with the flow rate. The purpose of measuring flow is achieved by measuring the pressure difference. Although the principle is the same as that of the orifice plate, the most essential difference is that the orifice plate is the central contraction throttling device, while the V-cone is the side wall contraction throttling device.
The signal-to-noise ratio of V-cone is much smaller than that of orifice plate. Due to the low signal-to-noise ratio, the V-cone can be measured accurately even under small working conditions when measuring small flow.
V-cone Flowmeter also has some disadvantages, such as calibration and high price.

2 Vortex flowmeter
2.1 characteristics of vortex flowmeter
Vortex flowmeter is mainly used for flow measurement of medium fluid in industrial pipeline, such as gas, liquid, steam and other media. It is characterized by small pressure loss, large range, high precision and good repeatability. When measuring the volume flow under working conditions, it is almost not affected by the parameters such as fluid density, pressure, temperature and viscosity. There are no movable mechanical parts, so it has high reliability and small maintenance. The instrument parameters can be stable for a long time.
The vortex flowmeter measures the flow by applying the fluid oscillation principle. When the fluid passes through the vortex flow transmitter in the pipeline, two rows of vortices proportional to the flow rate are alternately generated up and down behind the vortex generator of the triangular column. The release frequency of the vortex is related to the average velocity of the fluid flowing through the vortex generator and the characteristic width of the vortex generator, which can be expressed by the following formula

f=Stv/d

Where: F is the release frequency of vortex, Hz; V is the average velocity of the fluid flowing through the vortex generator, M / S; D is the characteristic width of vortex generator, m; ST is Strouhal number, dimensionless, and its numerical range is 0.14-0.27.
ST is a function of Reynolds number, St = f (L / re). When the Reynolds number Re is in the range of 102 ~ 105, the st value is about 0.2. Therefore, in the measurement, try to meet the Reynolds number of the fluid in the range of 102 ~ 105 and the vortex frequency f = 0.2v/d.
Vortex flowmeter is a new type of flowmeter for measuring fluid flow in closed pipeline based on Carmen vortex principle. Because of its good medium adaptability, it can directly measure the working condition volume flow of steam, air, gas, water and liquid without temperature and pressure compensation. It is equipped with temperature and pressure sensors to measure the standard condition volume flow and mass flow. The disadvantage is that the installation pipeline is generally required to be straight, and the front and rear straight pipe sections are required to meet the requirements of vortex flowmeter. The inner diameter of the equipped pipeline must also be consistent with the inner diameter of vortex flow transmitter, and be far away from vibration sources and places with strong electromagnetic interference.

3 turbine flowmeter
Turbine flowmeter is composed of turbine, bearing, preamplifier and display instrument. The measured fluid impacts the turbine blade to make the turbine rotate, and the speed of the turbine changes with the change of flow, that is, the flow is large, and the speed of the turbine is also large. Then, the speed of the turbine is converted into electrical pulse of corresponding frequency by the magnetoelectric conversion device, amplified by the preamplifier, and sent to the display instrument for counting and display, The instantaneous flow and cumulative flow can be calculated according to the number of pulses and cumulative pulses per unit time [4]. The working principle of turbine transmitter is that when the fluid flows along the axis of the pipe and strikes the turbine blade, there is

Q=fk

Where: Q – flow through the transmitter, L / S;
F – electric pulse frequency;
K – instrument coefficient, times / L.

The force of the fluid in the pipe acts on the blades and drives the turbine to rotate. While the turbine rotates, the blade periodically cuts the magnetic force lines generated by the electromagnet and changes the magnetic flux of the coil. According to the principle of electromagnetic induction, a pulsating potential signal will be induced in the coil. The frequency of this pulsating signal is directly proportional to the flow of the measured fluid. K is an important characteristic parameter of the turbine transmitter. It represents several pulses per cubic meter of flow or several pulses per liter of flow. Different instruments have different K and change with the wear of the instrument in long-term use. Although the design dimensions of the vorticity meter are the same, the geometric parameters of the actually processed turbine are not exactly the same, so the instrument constant k of each turbine transmitter is not exactly the same. It is usually calibrated by the manufacturer with clean water at room temperature. The pulse signal output by the turbine transmitter is amplified by the preamplifier and sent to the display instrument to realize the flow measurement [5]. Turbine flowmeter has the advantages of high precision, good repeatability, no zero drift, high range ratio and so on. Turbine flowmeter has high-quality bearings and specially designed guide vanes, so it greatly reduces wear and is not sensitive to peak value. Even under bad conditions, it can give reliable measurement variables. Disadvantages: (1) measuring gas-liquid mixture or fluid with high viscosity will produce great errors; (2) The measured fluid containing particles needs to be filtered in advance to prevent the turbine from getting stuck.

4 float flowmeter
Rotameter, also known as rotameter, is a volume flow meter that measures the flow by changing the flow annular gap area formed between the floats in the vertical conical tube with the change of flow [6].
According to different manufacturing materials, the float flowmeter can be divided into three types: glass tube float flowmeter, plastic tube float flowmeter and metal tube float flowmeter. Glass tube rotameter and plastic tube rotameter have simple structure, clear and readable float position and low cost. They are commonly used to measure normal temperature, atmospheric pressure, transparent and corrosive media. Such as air, gas, ammonia, etc. It is convenient for on-site visual inspection and is mostly used for proportioning measurement of industrial raw materials. Although glass tube rotameter has many advantages, it is only suitable for local indication, and the signal cannot be transmitted remotely. The strength of glass tube is not enough to measure high temperature, high pressure and opaque fluid. Therefore, metal tube rotameter is widely used in industrial production. The flow detection element of the float flowmeter is composed of a vertical conical tube expanding from bottom to top and a float group moving up and down along the conical tube axis. The working principle is that when the measured fluid passes through the annular gap formed by the cone tube and the float from bottom to top, the differential pressure is generated at the upper and lower ends of the float to form the rising force of the float. When the rising force of the float is greater than the weight of the float immersed in the fluid, the float rises, the annular gap area increases, the flow velocity at the annular gap immediately decreases, and the differential pressure between the upper and lower ends of the float decreases, The lifting force acting on the float will also decrease until the lifting force is equal to the weight of the float immersed in the fluid, and the float will stabilize at a certain height. There is a corresponding relationship between the height of the float in the cone tube and the flow through it.
The rotameter has many advantages, such as simple structure, reliable operation, small and stable pressure loss, and can measure low flow medium. The rotameter has a wide flow range, generally 10 ∶ 1, the lowest 5 ∶ 1 and the highest 25 ∶ 1. The output of the flow detection element is close to linear. Low pressure loss. However, it is generally not applicable to measure particles in liquid or droplets in solid gas. The flow indication shall be corrected before use.

5 digital target flowmeter
Target flowmeter began to be used in industrial flow measurement in the 1960s. It is mainly used to solve the flow measurement of high viscosity and low Reynolds number fluids. It has experienced two development stages: pneumatic meter and electric meter.
Structure composition and working principle of digital target flowmeter: it is mainly composed of measuring tube, stress element (target), sensing element (capacitive force sensor, pressure sensor, temperature sensor), transmission part, integrator and its display and output part. When the medium flows in the measuring tube, due to its own kinetic energy and the pressure difference caused by the choke, a force on the choke is generated, and its action is directly proportional to the square of the medium flow rate. The advantages are: the whole instrument has solid structure, no movable parts, plug-in structure and convenient disassembly; The instrument is equipped with self inspection program, which has high precision, good repeatability, high temperature resistance, etc; Disadvantages: high requirements for fluid, such as: the measured fluid must be Newtonian fluid, the fluid must fill the measuring tube of the flowmeter, etc.

6 electromagnetic flowmeter
The electromagnetic flowmeter has two combined forms: one type and split type. The output stage adopts electrical isolation, which can be easily matched with the rear instrument to realize the functions of flow recording, control and regulation. At the same time, the flowmeter is equipped with RS-485 communication interface, which can be interconnected with the computer. The working principle of electromagnetic flowmeter is Faraday’s law of electromagnetic induction. Conductive liquid flows in a magnetic field, cutting magnetic lines of force and generating induced potential. Expression: e = kblv, where B is magnetic induction intensity; L is the distance between measuring electrodes; V is the average velocity of the measured fluid moving in the magnetic field; K is the proportional constant. Electromagnetic flowmeter is mainly composed of flow sensor and signal converter. The sensor converts the flow of the measured liquid into the corresponding induced potential. The function of the signal converter is to amplify and convert the millivolt voltage signal output by the electromagnetic flow sensor proportional to the flow into the standard DC current, voltage or pulse signal output that can be received by the industrial instrument, so as to cooperate with the instrument and regulator to realize the indication, recording and operation of flow. The advantages of electromagnetic flowmeter are:
(1) It can measure the volume flow of liquid containing solid particles, suspended solids or acid, alkali, salt solution with certain conductivity; Bidirectional measurement can also be carried out. (2) There are no movable parts and blocking fluid in the measuring catheter, so there is no pressure loss and mechanical inertia, and the response is very sensitive. (3) Large measurement range. (4) In fact, the volume flow measured by electromagnetic flowmeter is not significantly affected by the changes of fluid density, viscosity, temperature, pressure and conductivity (as long as it is above a certain threshold).
The disadvantages are: (1) the electromagnetic flowmeter can not measure the liquid with very low conductivity. (2) Gases, vapors and liquids containing large bubbles cannot be measured. (3) Cannot be used for higher temperature liquids.

7 ultrasonic flowmeter
Ultrasonic flowmeter consists of ultrasonic transducer, electronic circuit and flow display and accumulation system. It is a velocity flow instrument that uses ultrasonic pulse to measure fluid flow. The ultrasonic transmitting transducer converts electric energy into ultrasonic energy and transmits it into the measured fluid. The ultrasonic signal received by the receiver is amplified by the electronic circuit and converted into an electrical signal representing the flow, which is supplied to the display and integration instrument for display and integration, so as to realize the detection and display of flow [7]. Ultrasonic flowmeter is generally divided into: plug-in ultrasonic flowmeter, pipe section ultrasonic flowmeter, external clip ultrasonic flowmeter, etc. Advantages of ultrasonic flowmeter: (1) ultrasonic flowmeter is non-contact measurement, suitable for large pipe diameter and large flow measurement, and is not affected by fluid temperature, viscosity, density and other parameters. (2) Ultrasonic flowmeter can measure various media such as water, gas or oil. (3) The operation of ultrasonic flowmeter consumes very little energy and can easily realize long-term battery power supply. In addition, the advanced intelligent host can easily carry out network wireless communication, so its application prospect is broader. Disadvantages: at present, the disadvantages of ultrasonic flowmeter are mainly that the temperature range of measurable fluid is limited by the temperature resistance of ultrasonic transducer and coupling material between transducer and pipeline, and the original data of sound transmission velocity of measured fluid at high temperature is incomplete.


Post time: 17-02-22