Heng Aode's new ultrasonic flowmeter is a non-contact type of medium that can measure both large pipe diameters and media that are difficult to contact and observe.
The external clamp type or pipe type ultrasonic flow meter is a meter for measuring the liquid flow in a circular tube based on the principle of "speed difference method". It adopts advanced multi-pulse technology, signal digital processing technology and error correction technology, which makes the flow meter more adaptable to the industrial scene environment, and the measurement is more convenient, economical and accurate. The products have reached the advanced level at home and abroad and can be widely used in petroleum, chemical, metallurgy, electric power, water supply and drainage and other fields.
Ultrasonic flow meters are meters that measure the flow by detecting the effect of fluid flow on the ultrasound beam (or ultrasound pulse).
principle
According to the principle of signal detection, ultrasonic flowmeter can be divided into propagation velocity difference method (direct time difference method, time difference method, phase difference method and frequency difference method), beam offset method, Doppler method, cross correlation method, spatial filtering method. And noise law, etc.
Ultrasonic flowmeter is the same as ultrasonic flowmeter. It is an unobstructed flowmeter because it does not have any obstruction in the flow passage of the instrument. It is a kind of flowmeter suitable for solving the problem of difficult flow measurement, especially in the measurement of large-diameter flow. The advantage is that it is one of the fastest growing types of flow meters.
advantage
Ultrasonic flowmeters are non-contact meters that measure both medium flow in large diameters and media that are difficult to access and observe. Its measurement accuracy is very high, and it is almost undisturbed by various parameters of the measured medium. In particular, it can solve the problem of flow measurement of strong corrosive, non-conductive, radioactive and flammable and explosive materials that other instruments cannot.
Disadvantage
The shortcomings of today are mainly that the temperature range of the measurable fluid is limited by the temperature resistance of the ultrasonically-transformed aluminum and the coupling material between the transducer and the pipe, and the original data of the sound velocity of the fluid to be measured at a high temperature is incomplete. At present, China can only be used to measure fluids below 200 °C. In addition, the measurement line of the ultrasonic flowmeter is more complicated than the general flowmeter. This is because, in general industrial metering, the flow rate of liquid is often several meters per second, and the propagation speed of sound waves in liquid is about 1500 m/s. The change in flow velocity (flow rate) of the measured fluid brings the maximum change in sound velocity. 3 orders of magnitude. If the accuracy of measuring the flow rate is required to be 1%, the accuracy of measuring the speed of sound needs to be of the order of 10-5~10-6, so it is necessary to have a perfect measuring circuit to achieve, which is the only ultrasonic flowmeter. In the premise of the rapid development of integrated circuit technology, the reason for practical application can be obtained.
Ultrasonic flowmeter consists of three parts: ultrasonic transducer, electronic circuit and flow display and accumulation system. The ultrasonic transmitting transducer converts electrical energy into ultrasonic energy and emits it into the fluid to be measured. The ultrasonic signal received by the receiver is amplified by the electronic circuit and converted into an electrical signal representing the flow to be supplied to the display and integrated meter for display. And the total calculation. This enables the detection and display of traffic.
The piezoelectric elements of the ultrasonic flowmeter transducer are often made into a circular sheet that vibrates along the thickness. The diameter of the sheet exceeds 10 times the thickness to ensure the directionality of the vibration. The piezoelectric element material is mostly lead zirconate titanate. In order to fix the piezoelectric element, the ultrasonic wave is injected into the fluid at a suitable angle, and the component is placed in the acoustic wedge to form a whole transducer (also called a probe). The material of the acoustic wedge not only requires high strength and aging resistance, but also requires that the ultrasonic energy after the wedge is small, that is, the transmission coefficient is close to 1. The commonly used acoustic wedge material is plexiglass, because it is transparent, and the assembly of piezoelectric elements in the wedge can be observed. In addition, some rubber, plastic and bakelite can also be used as the acoustic wedge material.
Application overview
The propagation time method is applied to clean, single-phase liquids and gases. Typical applications include factory effluent, blame, liquefied natural gas, etc.
Good experience in the application of high pressure natural gas for gas applications;
surroundings
The Doppler method is suitable for two-phase fluids with a low heterogeneous content, such as untreated sewage, factory effluent, dirty process fluids; usually not suitable for very clean liquids.
Precautions
The correct selection of ultrasonic flowmeters can ensure better use of ultrasonic flowmeters. The type of ultrasonic flowmeter to be selected should be determined according to the physical properties and chemical properties of the fluid medium to be tested, so that the diameter, flow range, lining material, electrode material and output current of the ultrasonic flowmeter can be adapted to the properties of the fluid to be measured. And flow measurement requirements.
1, precision function check
Accuracy class and function According to the measurement requirements and use occasions, the instrument accuracy level is selected to achieve economical efficiency. For example, in the case of trade settlement, product handover and energy measurement, the accuracy level should be higher, such as 1.0, 0.5, or higher; for process control, select different accuracy levels according to control requirements; It is to detect the process flow, no need to do precise control and measurement, you can choose a lower accuracy level, such as 1.5, 2.5, or even 4.0, then you can use a low-cost plug-in ultrasonic flowmeter.
2, measurable medium
When measuring the medium flow rate, meter range and caliber to measure the general medium, the full flow of the ultrasonic flow meter can be selected within the range of 0.5-12 m/s of the measured medium flow rate, and the range is relatively wide. The selection of the meter specification (caliber) is not necessarily the same as the process piping. It should be determined according to whether the measured flow range is within the flow rate range. That is, when the pipeline flow rate is too low to meet the requirements of the flow meter or the measurement accuracy is not guaranteed at this flow rate. It is necessary to reduce the gauge diameter, thereby increasing the flow rate inside the tube and obtaining satisfactory measurement results.
Measuring principle
When the ultrasonic beam propagates in the liquid, the flow of the liquid will cause a small change in the propagation time, and the change in the propagation time is proportional to the flow velocity of the liquid, and its relationship conforms to the following expression.
among them
θ is the angle between the sound beam and the direction of flow of the liquid
M is the number of linear travels of the sound beam in the liquid
D is the inner diameter of the pipe
Tup is the propagation time of the sound beam in the positive direction
Tdown is the propagation time of the sound beam in the reverse direction
ΔT=Tup –Tdown
Let the speed of sound in the stationary fluid be c, the velocity of the fluid flow be u, and the propagation distance be L. When the sound wave is in the same direction as the fluid flow direction (ie, the downstream direction), the propagation velocity is c+u; otherwise, the propagation velocity is cu. Two sets of ultrasonic generators and receivers (T1, R1) and (T2, R2) are placed at two places separated by L. When T1 is in the forward direction and T2 transmits ultrasonic waves in the reverse direction, the time required for the ultrasonic waves to reach the receivers R1 and R2 respectively is t1 and t2, then
T1=L/(c+u); t2=L/(cu)
Since the flow velocity of the fluid in the industrial pipeline is much smaller than the sound velocity, that is, c>>u, the time difference between the two is â–½t=t2-t1=2Lu/cc. Thus, the propagation velocity of the acoustic wave in the fluid is known. When it is known, the flow rate u can be obtained by measuring the time difference â–½t, and the flow rate Q can be obtained. The method of measuring the flow using this principle is called the time difference method. In addition, a phase difference method, a frequency difference method, or the like can be used.
Phase difference method
If the ultrasonic transmitter emits a continuous ultrasonic pulse or a long pulse train, a phase difference â–½O is generated between the received signals in the forward and reverse current transmissions, ie â–½O=wâ–½t=2wLu/cc
Where w is the ultrasonic angular frequency. When â–½O is measured, u can be obtained, and the flow rate Q can be obtained. This method replaces the measurement of the small time difference â–½t by measuring the phase difference â–½O, which is beneficial to improve the measurement accuracy. However, the influence of the speed of sound c on the measurement results exists.
Frequency difference method
In order to eliminate the influence of the speed of sound c, the frequency difference method is often used. As can be seen from the past, the difference between the frequencies of the ultrasonic waves received by the upstream and downstream receivers is â–½f, which can be expressed by the following equation: =f=[(c+u)/L]-[(cu)/L]=2u/L
It can be seen that the flow rate Q can be obtained as long as the â–½f is measured, and this method is independent of the speed of sound. Ultrasonic technology and its application I. No measurement of water level
Hydropower stations often use float level gauges or input level gauges for water level measurement. The shortcomings are: low measurement accuracy, unreliable, frequent floating of the float and sensor clogging, resulting in inaccurate maintenance; large maintenance workload, inconvenient installation and debugging, only the analog alarm signal is collected, can not directly enter the power plant computer surveillance system. Not practical for unattended power plants.
Through the repeated comparison of the water level measuring system of the trash rack, the final scheme design is optimized. The ultrasonic level gauge is used to accurately monitor the water level in front of the grid and the water level in the grid. The ultrasonic level gauge uses PLC to process the collected amount. And the real-time water level and differential pressure data are sent to the central control room, and the ultrasonic level gauge displays and exceeds the limit alarm. The ultrasonic level gauge adopts the RS422/RS232 interface at the same time, and sends the real-time data to the industrial control unit of the dam centralized control room, processes it into a computer communication message, and finally sends the collected quantity to the upper computer of the power plant computer monitoring system.
After the implementation of the project, it not only satisfies the multi-point real-time monitoring of the front and rear gates of the grid, but also the alarm function, and ends the situation that the trash rack measurement system works independently and cannot communicate with the computer monitoring system of the power plant. Integration with the monitoring functions of the gate system, control functions, and ON-CALL paging system functions at the time of failure. Meet the needs of unattended power stations. This technology is still the first in the power system of Yunnan Province.
The principle of ultrasonic level gauge for measuring water level and installation requirements
When the ultrasonic level gauge is working, the high-frequency pulse sound wave is emitted by the transducer (probe), and the surface of the measured object (water surface) is reflected, and the reflected echo returned by the same transducer is received by the same transducer (probe) and converted into an electrical signal. . The time between the transmission and reception of the pulse (the motion time of the sound wave) is proportional to the distance from the transducer to the surface of the object. The relationship between the distance S of the sound wave transmission and the speed of sound C and the transmission time T can be expressed as: S = CXT /2
For example, the sound velocity C=344m/s and the transmission time is 50ms, the distance of transmission can be calculated to be 17.2m, and the measurement distance is 8.6m.
3. The effect of the programmable ultrasonic anti-staining grid water level measuring system applied in Tianba Power Station
Measuring the dam water level with an ultrasonic level gauge is not common in China today, and there is no experience in technology. Under such circumstances, we make full use of the advanced technology in the field of PLC and ultrasonic level gauge, and design the project according to the overall plan, long-term consideration, one-time, avoiding repeated transformation and repeated investment. Completed this issue in a comprehensive and smooth manner. More valuable experience has been gained in this area. It provides an example for the implementation of the dam water level monitoring system in China's domestic hydropower stations.
principle
Ultrasonic waves carry information about the fluid flow rate as they propagate through the flowing fluid. Therefore, the flow rate of the fluid can be detected by the received ultrasonic wave, and converted into a flow rate. According to the detection method, it can be divided into different types of ultrasonic flowmeters such as propagation velocity difference method, Doppler method, beam offset method, noise method and correlation method. Ultrasonic flowmeter is a kind of application that has been applied since the rapid development of integrated circuit technology in the past decade.
Non-contact instrument for measuring fluids that are difficult to access and observe, as well as large pipe runoff. It is linked to a water level gauge for flow measurement of open water flow. The use of ultrasonic flow rate does not change the flow state of the fluid without installing the measuring element in the fluid, and does not generate additional resistance. The installation and maintenance of the instrument can not affect the operation of the production pipeline and is an ideal energy-saving flowmeter.
As we all know, industrial flow measurement generally has the problem of large diameter and large flow measurement difficulty. This is because the general flowmeter will bring difficulties in manufacturing and transportation with the increase of the measuring pipe diameter, and the cost will increase and the energy loss will increase. Installation is not only a disadvantage, but ultrasonic flowmeters can be avoided. Because all kinds of ultrasonic flowmeters can be installed outside the pipe, non-contact flow measurement, the cost of the instrument is basically independent of the size of the pipe to be tested, while other types of flowmeters increase with the increase in caliber, so the cost is increased. The flowmeter is superior to the other functions of the same type of flowmeter. It is considered to be a good large-diameter flow measuring instrument. The Doppler ultrasonic flowmeter can measure the flow of two-phase medium, so it can be used for the measurement of dirty sewage such as sewers and sewage. In power plants, the use of portable ultrasonic flowmeters to measure large pipe diameters such as turbine water inflow and turbine circulating water is much more convenient than in the past. Ultrasonic flow juice can also be used for gas measurement. Pipe diameters range from 2cm to 5m, from a few meters wide open channels, culverts to 500m wide rivers.
In addition, the accuracy of the flow measurement of the ultrasonic measuring instrument is almost independent of the temperature, pressure, viscosity, density and other parameters of the measured fluid, and can be made into non-contact and portable measuring instruments, so it can solve the problem that other types of instruments are difficult to measure. Flow measurement problems for corrosive, non-conductive, radioactive, and flammable and explosive media. In addition, in view of the non-contact measurement characteristics, coupled with reasonable electronic circuits, one instrument can adapt to a variety of pipe diameter measurements and a variety of flow range measurements. The adaptability of ultrasonic flowmeters is also unmatched by other instruments. Ultrasonic flowmeters have some of the above advantages, so it has received more and more attention and has been developed into a series of products and generalization. It has been made into standard, high-temperature, explosion-proof and wet instruments of different channels to adapt to different media. Flow measurement for occasions and different pipeline conditions.
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