According to the principle of signal detection, the current ultrasonic flowmeter can be roughly divided into propagation velocity difference method (including: direct time difference method, time difference method, phase difference method, frequency difference method) beam shift method, Doppler method, correlation method, spatial filtering and noise method and other types of methods. Among them, the noise method principle and structure of the most simple, easy to measure and carry, cheap but less accurate, suitable for use in the flow measurement accuracy requirements are not high occasions.
Because of the direct time difference method, time difference method, frequency difference method and phase difference method of the basic principle is to measure the ultrasonic pulse downstream and countercurrent transmission speed difference to reflect the fluid flow rate, so collectively referred to as the propagation velocity difference method. Among them, the frequency difference method and time difference method overcomes the error brought about by the change of the speed of sound with the fluid temperature, the accuracy is higher, so it is widely used. According to the transducer configuration method is different, the propagation velocity difference dialing is divided into: Z method (through the method), V method (reflection method), X method (cross method) and so on.
Beam offset method is the use of ultrasonic beam propagation direction in the fluid with the fluid flow rate changes and produce offset to reflect the fluid flow rate, low flow rate, the sensitivity is very low applicability.
Doppler method is the use of acoustic Doppler principle, through the measurement of inhomogeneous fluid scatterer scattered ultrasonic Doppler shift to determine the fluid flow rate, suitable for containing suspended particles, bubbles and other fluid flow measurement.
Correlation method is the use of correlation technology to measure the flow rate, in principle, the accuracy of the measurement of this method has nothing to do with the speed of sound in the fluid, and therefore has nothing to do with the fluid temperature, concentration, and so on, and therefore the measurement of the accuracy of the measurement is high, the scope of application. However, the correlator is expensive and the line is complicated. After the popularization of microprocessors, this disadvantage can be overcome.
Noise method (listening method) is the use of fluid flow in the pipeline when the noise generated by the fluid flow rate and the principle of flow rate, through the detection of noise indicates the flow rate or flow value. Its method is simple, the equipment is cheap, but the accuracy is low.
The above methods have their own characteristics, should be measured according to the nature of the fluid. Flow rate distribution, pipeline installation location and the measurement of accuracy requirements and other factors to choose. Generally speaking, due to industrial production, the temperature of the fluid often can not maintain a constant, so more frequency difference method and time difference method. Only in the pipe diameter is very large when the direct time difference method. The principle of choice for the transducer installation method is generally: when the fluid flows parallel to the axis of the tube, the choice of Z method; when the flow direction and the pipe uranium is not parallel or pipeline installation locations so that the transducer installation interval is limited, the use of V method or X method. When the flow field distribution is not uniform and the table before the straight pipe section is short, can also be used to overcome the flow rate of multi-channel (eg, two-channel or four-channel) flow measurement error caused by flow velocity perturbation. Doppler method is suitable for measuring two-phase flow, can avoid the conventional meter by suspended particles or bubbles caused by the clogging, wear and tear, adherence and can not run the disadvantages, and therefore the rapid development. With the development of industry and energy saving work, kerosene mixing (COM), coal-cement combined (CWM) fuel delivery and application and fuel oil with water to fuel the development of energy-saving methods, are for the Doppler ultrasonic flowmeter applications to open up broad prospects.
Two, composition: ultrasonic flowmeter by ultrasonic transducer, electronic circuit and flow rate display and accumulation system consists of three parts. Ultrasonic flowmeter electronic circuit including transmission, reception, signal processing and display circuit. Measured instantaneous flow rate and cumulative flow rate value with digital or analog display. Ultrasonic transmitter transducer will be converted into ultrasonic energy, and will be launched into the measured fluid, the receiver received ultrasonic signals, amplified by the electronic circuit and converted to represent the flow of electrical signals supplied to the display and accumulation instrument for display and accumulation. This will realize the flow detection and display. Ultrasonic flow meter commonly used piezoelectric transducer. It utilizes the piezoelectric effect of piezoelectric materials, the use of suitable launch circuit to add electrical energy to the launch transducer piezoelectric element, so that it produces ultrasonic vibration persuasion. The ultrasonic wave propagates into the fluid at a certain angle, and is then received by the receiving transducer and changed into electrical energy by the piezoelectric element for detection. The transmitting transducer utilizes the inverse piezoelectric effect of the piezoelectric element, while the receiving transducer utilizes the piezoelectric effect.
Three, the advantages: ultrasonic flowmeter non-contact instrumentation, suitable for measuring the fluid is not easy to contact and observation, as well as large diameter flow. It can be linked with the water level meter open flow measurement. The use of ultrasonic flowmeter, do not have to install measuring elements in the fluid, so it will not change the flow state of the fluid, does not produce additional resistance, instrumentation, installation and maintenance can not affect the production pipeline operation is therefore an ideal energy-saving flowmeter. Doppler method of ultrasonic flowmeter can measure the flow of two-phase media, so it can be used for sewerage and sewage water and other dirty flow measurement. In the power plant, with portable ultrasonic flowmeter to measure the turbine water intake, turbine circulating water and other large pipe diameter flow, much more convenient than the past Pi off the tube flow meter. Ultrasonic was flow juice can also be used for gas measurement. The applicable range of pipe diameter from 2cm to 5m, from a few meters wide nullahs, culverts to 500m wide rivers can be applied.
Four, disadvantages: mainly measurable fluid temperature range by the ultrasonic transducer aluminum and transducer and the coupling material between the pipeline temperature degree of restriction, as well as high temperatures in the measured fluid sound transmission velocity of the original data is incomplete. At present, our country can only be used to measure the fluid below 200 ℃. In addition, the ultrasonic flowmeter measurement line is more complex than the general flowmeter. This is because, in general, industrial measurement of liquid flow rate is often a few meters per second, and sound waves in the liquid propagation speed of about 1500m / s or so, the measured fluid flow rate (flow rate) changes to bring the speed of sound is the maximum amount of change is also 10-3 orders of magnitude. If the accuracy of the required measurement of flow rate of 1%, the accuracy of the measurement of the speed of sound needs to be 10-5 ~ 10-6 orders of magnitude, so there must be a perfect measurement line in order to achieve, which is precisely the ultrasonic flowmeter only in the rapid development of integrated circuit technology under the premise of the reason for the practical application of the flowmeter.
Five, ultrasonic flowmeter installation steps
Installation of ultra-flow can be operated in accordance with the following steps:
One: Observe the installation site pipeline to meet the installation of straight section of the first 10D after 5D as well as the distance from the pump 30D. (D for the inner diameter of the pipe)
Second: Confirm the fluid medium in the pipe and whether the pipe is full.
Three: confirm the pipeline material and wall thickness (taking into account the thickness of the scaling of the inner wall of the pipeline)
Four: confirm the service life of the pipeline, in the use of 10 or so of the pipeline, even if it is made of carbon steel, it is best to use the insertion type installation.
Fifth: After the completion of the first four steps can confirm the use of what kind of sensor installation
Sixth: Begin to enter parameters to the meter to determine the installation distance.
7: Very important: measure the installation distance accurately.
(1) external clamp optional installation sensor approximate distance, and then constantly debugging activities sensor in order to achieve the signal and transmission ratio
the best match
(2) Insert the use of special tools to measure the distance from the installation point on the pipeline, the distance is very important, which directly affects the table
the actual measurement of the accuracy, so it is best to carry out a number of measurements in order to achieve a higher degree of accuracy. high accuracy.
Eight: Installation of the sensor - debugging signal - waterproofing - return the signal cable - clean up the site wire and other waste. -Clean up the site wire and other waste - the end of the installation - acceptance signature
Six, ultrasonic flowmeter in the use of common problems:
1, ultrasonic flowmeter probe for some time, there will be occasional alarms. There will be irregular alarm. Especially when more impurities in the delivery medium, this problem will be more common. Solution: clean up the probe regularly (recommended once a year).
2, ultrasonic flowmeter conveying media containing water and other liquid impurities, flowmeter pressure pipe is prone to produce liquid, the lower temperature will be the pressure pipe freeze phenomenon, especially in the northern region in winter is more common. Solution: the pilot tube for blowing or adding electric heat
Ultrasound in the propagation process, due to the medium and the impurities in the medium of the obstacles or absorption, its intensity will produce attenuation. Whether it is ultrasonic flowmeter or ultrasonic level meter, the strength of the accepted sound wave have certain requirements, so they have to inhibit a variety of attenuation.