According to the principle of signal detection, ultrasonic flow meters can be roughly divided into propagation velocity difference method (including direct time difference method, time difference method, phase difference method and frequency difference method), beam migration method, Doppler method, correlation method, spatial filtering method and noise method. Among them, the noise method is the simplest in principle and structure, easy to measure and carry, cheap but low in accuracy, and suitable for use in occasions where the accuracy of flow measurement is not high.
because the basic principles of direct time difference method, time difference method, frequency difference method and phase difference method all reflect the fluid velocity by measuring the difference between the forward and backward propagation velocities of ultrasonic pulses, they are also collectively called propagation velocity difference method. Among them, frequency difference method and time difference method overcome the error caused by the change of sound velocity with fluid temperature, and have high accuracy, so they are widely used. According to the different configuration methods of transducers, the difference of propagation speed can be divided into Z method (transmission method), V method (reflection method) and X method (cross method).
The beam migration method reflects the fluid velocity by using the shift of the propagation direction of ultrasonic beam in fluid with the change of fluid velocity. At low velocity, the sensitivity is very low and the applicability is not great.
The Doppler method uses the acoustic Doppler principle to determine the fluid flow by measuring the ultrasonic Doppler frequency shift scattered by scatterers in inhomogeneous fluid, which is suitable for the flow measurement of fluids containing suspended particles and bubbles.
correlation method is to measure the flow rate by using related technologies. In principle, the measurement accuracy of this method has nothing to do with the sound speed in the fluid, so it has nothing to do with the temperature and concentration of the fluid, so it has high measurement accuracy and wide application range. But the correlator is expensive and the circuit is complicated. This shortcoming can be overcome after the popularization and application of microprocessors.
the noise method (listening method) is based on the principle that the noise generated when the fluid flows in the pipeline is related to the flow velocity of the fluid, and indicates the flow velocity or flow value by detecting the noise. The method is simple and the equipment is cheap, but the accuracy is low.
The above methods have their own characteristics, and should be selected according to the measured fluid properties, velocity distribution, pipeline installation location and requirements for measurement accuracy. Generally speaking, frequency difference method and time difference method are often used because the temperature of working substance can not be kept constant in industrial production. The direct time difference method is only used when the pipe diameter is very large. The selection principle of transducer installation method is generally: when the fluid flows parallel to the tube axis, Z method is selected; When the flow direction is not parallel to the tube uranium or the installation location of the pipeline limits the installation interval of the transducer, V method or X method is adopted. When the flow field distribution is uneven and the straight pipe section in front of the table is short, multi-channel (such as two-channel or four-channel) can also be used to overcome the flow measurement error caused by velocity disturbance. Doppler method is suitable for measuring two-phase flow, which can avoid the disadvantages that conventional instruments are blocked, worn and attached by suspended particles or bubbles, so it has developed rapidly. With the development of industry and the development of energy-saving work, the transportation and application of kerosene (COM) and coal cement (CWM) fuels and the development of energy-saving methods such as adding water to fuel oil for combustion have opened up broad prospects for the application of Doppler ultrasonic flowmeter.
ii. composition: the ultrasonic flowmeter consists of an ultrasonic transducer, an electronic circuit and a flow display and accumulation system. The electronic circuit of ultrasonic flowmeter includes transmitting, receiving, signal processing and display circuits. The measured instantaneous and cumulative flow values are displayed by digital or analog quantities. The ultrasonic transmitting transducer converts electric energy into ultrasonic energy, and emits it into the measured fluid. The ultrasonic signal received by the receiver is amplified by electronic circuit and converted into an electrical signal representing the flow, which is supplied to the display and integration instrument for display and integration. In this way, the detection and display of traffic are realized. Piezoelectric transducer is commonly used in ultrasonic flowmeter. It makes use of the piezoelectric effect of piezoelectric materials, and uses an appropriate transmitting circuit to add electric energy to the piezoelectric element of the transmitting transducer, so that it can generate ultrasonic vibration. Ultrasonic waves are injected into the fluid at a certain angle for propagation, and then received by the receiving transducer, and converted into electric energy by the piezoelectric element for detection. The transmitting transducer uses the inverse piezoelectric effect of piezoelectric elements, while the receiving transducer uses the piezoelectric effect.
3. Advantages: Ultrasonic flowmeter is a non-contact instrument, which is suitable for measuring fluids that are not easy to contact and observe and large pipe runoff. It can be linked with the water level gauge to measure the flow of open water. Ultrasonic flowmeter is an ideal energy-saving flowmeter because it does not need to install measuring elements in the fluid, so it will not change the flow state of the fluid and produce additional resistance, and the installation and maintenance of the instrument will not affect the operation of the production pipeline. Doppler ultrasonic flowmeter can measure the flow of two-phase medium, so it can be used to measure dirty flows such as sewers and sewage. In power plants, it is much more convenient to use portable ultrasonic flowmeter to measure large pipe runoff such as water inflow of turbine and circulating water of steam turbine than the previous pitot-tube velocimeter. Ultrasonic flow juice can also be used for gas measurement. The application range of pipe diameter is from 2cm to 5m, and it can be applied from open channels and culverts with a width of several meters to rivers with a width of 5m m.
4. Disadvantages: The temperature range of the measurable fluid is mainly limited by the temperature resistance of the ultrasonic transducer aluminum and the coupling material between the transducer and the pipeline, and the original data of the sound transmission speed of the measured fluid at high temperature are incomplete. At present, China can only be used to measure fluids below 2℃. In addition, the measuring circuit of ultrasonic flowmeter is more complicated than general flowmeter. This is because, in general industrial measurement, the velocity of liquid is often several meters per second, while the propagation velocity of sound wave in liquid is about 15 m/s, and the maximum change of the velocity (flow) of the measured fluid is 1-3 orders of magnitude. If the accuracy of measuring the velocity is required to be 1%, the accuracy of measuring the sound velocity needs to be 1-5 ~ 1-6 orders of magnitude, so it must have a perfect measuring circuit to realize it.
V. Installation steps of ultrasonic flowmeter
The superfluid can be installed according to the following steps:
1. Observe whether the pipeline at the installation site meets the requirements of the first 1 days and the last 5 days of the straight pipe section and the distance from the pump for 3 days. (d is the inner diameter of the pipeline)
2: Confirm the fluid medium in the pipeline and whether the pipeline is full.
third, confirm the material and wall thickness of the pipeline (fully considering the scaling thickness on the inner wall of the pipeline)
fourth, confirm the service life of the pipeline. Even if the pipeline is made of carbon steel, it is best to use plug-in installation.
five: after the first four steps are completed, you can confirm which sensor to install
six: start inputting parameters into the table body to determine the installation distance.
seven: very important: accurately measure the installation distance.
(1) the approximate distance of the external clamp type optional installation sensor, and then continuously debug the movable sensor to achieve the best matching of signal and transmission ratio
(2) insert a special tool to measure the distance of the installation point on the pipeline, which is very important and directly affects the actual measurement accuracy of the meter
, so it is best to make multiple measurements for higher accuracy.
VIII. Installing sensors-debugging signals-waterproofing-putting signal cables in order-cleaning up waste such as thread ends on site-completion of installation-acceptance signature
VI. Common problems in the use of ultrasonic flowmeter:
1. When the probe of ultrasonic flowmeter is used for a period of time, there will be irregular alarm. Especially when there are many impurities in the conveying medium, this problem will be more common. Solution: Clean the probe regularly (once a year is recommended).
2. When the conveying medium of ultrasonic flowmeter contains liquid impurities such as water, the pressure-inducing tube of flowmeter is prone to accumulate liquid, and the pressure-inducing tube will freeze up when the temperature is low, especially in winter in northern areas. Solution: Purge the pressure pipe or add electricity for heat tracing
During the propagation of ultrasonic wave, its intensity will be attenuated due to the obstruction or absorption of medium and impurities in the medium. Both ultrasonic flowmeter and ultrasonic level meter have certain requirements on the sound wave intensity, so all kinds of attenuation should be suppressed.