Measurement principle and development status
0 Preface
Ultrasonic flowmeter has the advantages of non-contact measurement, low pressure loss, wide measurement range, etc. It has been widely used in the flow measurement of petrochemical industry, and is especially suitable for large-diameter pipelines and non-conductive flow measurement.
1 Basic Measurement Principles of Ultrasonic Flowmeters
Ultrasonic flowmeters use ultrasonic waves to measure different propagation speeds in the forward and reverse flow of a fluid. The measurement principle is shown in Figure 1. The most common time difference method, speed difference method and frequency difference method.
Fig. 1 Schematic diagram of ultrasonic flow meter measurement principle
Time difference method:
Differential speed method:
Frequency difference method:
In the formula, L is the linear distance between A and B transducers and θ is the angle between the fluid flow and the ultrasonic path.
Because the speed of sound c is not easily neglected due to temperature, while the frequency difference in equation (5) is proportional to the flow velocity v and does not contain the speed of sound c, the measurement result is not affected by temperature, so the frequency difference method is more simple and practical. There is also an ultrasonic flowmeter developed specifically to exploit the characteristics that the temperature distribution of the fluid in the pipe section changes with the fluid flow, and the temperature distribution causes the change in the sound speed.
Because of the general flow rate, the time difference between the forward flow and the reverse flow depends on the size of the pipe diameter and is only a few hundred nanoseconds to a few microseconds. Therefore, the measurement method of high-precision time difference and frequency difference is closely related to the measurement accuracy of ultrasonic flowmeters, such as the Netherlands. Instroment uses a high-speed digital processing circuit technology that can detect microsecond time differences and achieve nanosecond timing accuracy.
2 Type selection and installation of external clamp ultrasonic flowmeter
According to different installation methods of transducers, ultrasonic flowmeters can be divided into: external clamp type, pipe segment type and insert type. External clamp type ultrasonic flow meter means to attach the transducer 
Ultrasonic flowmeter outside the pipe. This installation method is the most convenient one for installation and maintenance of the three transducer installation methods of ultrasonic flowmeters, but the transmission and reception signal channels of the transducer signals are the most complex and must pass through pipes, (liners) and fluids. Medium, measurement accuracy is also relatively low.
2.1 Selection Considerations
Instrument selection is a very important task in the application of instruments. Ultrasonic flowmeter selection should pay special attention to collecting the following information:
2.1.1 Basic Information
Name of the fluid to be measured; maximum flow, minimum flow; maximum temperature, minimum temperature; pipe material; pipe lining material; pipe outside diameter D (or pipe wall thickness), pipe inside diameter.
2.1.2 Protection Level Selection
There are two types of IP65 and IP68. IP65 is a spray-proof type water jet that can withstand a certain pressure and amount of water in use. IP68 is a submersible type and can work in water for long periods of time.
2.1.3 Selection of measuring points
1) The measurement position should be selected at the position where the probe is more than 10D upstream and downstream is greater than 5D, full of liquid straight pipe;
2) Selection of measuring points should be as far away as possible from pumps, valves, etc. to avoid interference. The reason is the same as above, because it is necessary to measure the flat DC body as much as possible to avoid measurement errors caused by turbulence;
3) The measurement point selection should be as far away as possible from high-power stations, strong magnetic field interference sources, etc.
4) Pipeline pipe should be even and dense, easy to transmit ultrasonic waves;
5) Fully consider the condition of fouling in the pipe, and choose the pipe section with no scale to measure as much as possible.
2.2 Installation Precautions
1) The area of ​​the metal pipe to be installed with the transducer must be polished to 3 times the area of ​​the probe, remove the rust paint, smooth the surface of the pipe wall, expose the original surface of the pipe metal and maintain the original arc;
2) The signal shield of the transducer can be left unconnected, but it cannot be short-circuited with the positive and negative terminals;
3) The contact part of the transducer and pipe should be sealed with a coupling agent to prevent air, dust and rust from entering;
4) The fixture should be fixed in the center part of the transducer, making it difficult to slide.
3 Development Status of External Clamp Ultrasonic Flowmeter
One of the characteristics of ultrasound is that the attenuation of propagation is large. The gas has the strongest ability to absorb ultrasonic energy, followed by liquid and solid. Therefore, in a single-fluid ultrasonic flowmeter, the external clamp type gas ultrasonic flowmeter generally has low measurement accuracy in practical applications. In order to improve the measurement accuracy and signal strength, some ultrasonic flow meters adopt a multi-channel measurement method and use the multi-channel measurement data to calculate the average flow velocity according to the weights. Some ultrasonic flow meters use wide beam technology. The technique is that the transducer emits a sound beam with a wide frequency range to scan and search the pipe wall, so that the pipe becomes part of the transducer, so that the acoustic beam can cover the cross section of the measured medium with a larger area. There are data showing that wide beam technology can achieve very high stability and repeatability with fewer channels, with a measurable minimum tube diameter of 6.35mm.
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