Ultrasonic flowmeters are instruments that measure volume by detecting the effect of ultrasonic pulses on fluid flow. Closed pipes are classified according to the measurement principle using ultrasonic flowmeters: (1) propagation time method; (2) Doppler effect method; (3) wave to offset method; (4) correlation method; (5) noise method. Since we actually use ultrasound flowmeters to measure more clean media, it is better to use the propagation time method. Acoustic waves propagate in the fluid, the propagation speed of acoustic waves increases in the downstream direction, and the direction of countercurrent decreases. The same propagation distance has different propagation time. Use the difference between the speed of propagation and the flow rate of the fluid under test. Therefore, the measured flow rate is called the propagation time method. The following describes the working principle of the ultrasonic flowmeter of the propagation time method and its application in actual production.
1 How it works
The propagation speed C of ultrasonic waves sent from the sensor 1 to the sensor 2 is accelerated by the fluid flow velocity Vm, which is
The propagation speed C of ultrasonic waves sent from the sensor 2 to the sensor 1 is slowed down by the fluid flow velocity Vm.
(1)
Formula (1) minus (2), to:
(2)
Where: L - the length of the ultrasonic wave propagation path between sensors, (m);
X—the axial component of the propagation path, (m);
T12, t21 - propagation time from sensor 1 to sensor 2 and sensor 2 to sensor 1, (S);
C—speed of propagation of ultrasound in a stationary fluid, (m/s);
Vm—The average flow rate over the channel between the sensors 1 and 2, m/s according to the flow equation:
Volume flow is: 
K - flow distribution correction factor. π
DN—Inner diameter of the pipe.
From the above formula, we can know that when K is a certain value, π, DN is a fixed value, so as long as we know the value of Vm, we can get the volume flow of the fluid. Therefore, the difference in the ultrasonic wave propagation time between two sensors can be measured to obtain the flow rate of the fluid.
2 Comparison of Installation Methods of Ultrasonic Flowmeters in Practical Applications
Ultrasonic flowmeter consists of two parts: sensor and converter. The sensor and converter are connected by a dedicated signal cable. The installation of the converter can be divided into: on-site installation and installation of cabinets. For on-site installation of the converter, attention should be paid to the explosion-proof requirements in the dangerous locations.
The installation of the sensor is divided into Z method installation, V method installation, W method installation, as shown in Figure 2.
In actual use, we often use the Z method and the V method to install. These two methods have their advantages and disadvantages. When used, they are used according to actual conditions.
(1) Z method installation. The sound wave propagation time is short and the effect is good, but the Z-method installation requires that the two sensors should be on the same section of the pipe, so the installation is required to be accurate.
(2) Installation of the V method. It is more suitable for measurement installation on a straight pipe. If it is on a horizontal pipe, it is not easy to measure the signal when the fluid is not full.
(3) The method of transmission and reception of channel arrangement is to use Z method and V method. The principle of selection is to have enough sound path and medium absorption sound wave program, but also to consider the roughness of the inner wall of the pipeline.
3 Comparison of sensor type selection
(1) Ultrasonic flowmeters are installed at the portable site and are easy to use. They are most suitable for evaluating the flow conditions of pipelines or for checking the operating conditions of other flow meters installed on pipelines.
(2) The clamped sensor does not have the problem of direct corrosion, but the corrosion and crystallization of the inner surface of the pipe can also cause inaccurate measurements.
(3) Clamp-type sensors Because the coupling agent is prone to dryness after a period of use, it affects the detection effect and should not be used on larger diameter pipes.
(4) Clip-on sensors can be used for short-term measurements, and can also be used with petroleum jelly and other lipids. Long-term measurements can be made with silicone grease or silicone.
(5) The pressure of the fluid has no influence on the clip-on sensor, and generally there is no problem with the insert-type sensor.
(6) Clip-on sensors are not suitable for measuring gases.
(7) The plug-in sensor is more suitable for pipes with DN>100mm, but there must be enough installation space outside the pipe.
4 Issues to be aware of during installation and commissioning
(1) When installing the plug-in sensor, pay attention to the input parameter setting to obtain the installation distance. The wall thickness of the pipe should be actually measured. The resulting sensor installation distance can be accurate and the measurement accuracy can be improved.
(2) To ensure that there is a certain length of straight pipe, the minimum is 1OD upstream and 5D downstream.
(3) The outlet of the sensor should be sealed and waterproof.
(4) When installing the cartridge type sensor, the outer wall of the pipeline should be polished to avoid sag, and the inner wall scale layer of the pipeline should not be too thick.
(5) There is a sufficient coupling agent between the clamped sensor working face and the pipe wall, and air and solid particles cannot exist.
(6) If the receiving signal is not good during debugging, check whether the parameters are set correctly. For the installation of the plug-in sensor, whether the working surfaces of the two sensors are correct or not will also affect the signal reception.
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