Factors affecting the measurement accuracy of clamp-on ultrasonic flowmeter mainly include the installation and location of the transducer, pipe diameter and wall thickness, media factors, external interference.
Installation and location
Clamping ultrasonic flowmeter transducer installation usually have Z method, V method, N method, W method and so on.
V method of installation is very common and standard installation of ultrasonic flowmeter, installation should pay attention to the upstream and downstream of the transducer level alignment, so that its center line and pipe axis level.
Z method of installation is usually applied to wider diameter or more impurities in the medium of the pipeline, scale wall is too thick will make the V method signal distortion, equipment measurement effect is not good.
N-method installation is characterized by extending the ultrasonic transmission distance and improving the measurement accuracy. When installed, the ultrasonic beam can be reflected twice in the pipe and pass through the fluid three times, so it is more suitable for measuring small diameter pipes.
W-mode installation, like N-mode, improves measurement accuracy in small diameter pipes by extending the ultrasonic transmission distance. The difference is that the ultrasonic beam is reflected three times in the pipe and passes through the fluid four times in W-mode installation, which is suitable for measuring small pipes under 50 mm. Installation should be strictly in accordance with the calculated distance of the flowmeter to install two transducers, press the probe on the pipe wall to apply a constant pressure, and always use the same method of applying the coupling agent to avoid the effect of fluctuations caused by different thicknesses.
Installation position of external clamp-on ultrasonic flowmeter
Pipeline installed in many pressure reducers, elbows, valves, pumps, etc. will become a source of interference, the chosen installation location to maintain a sufficient distance from these installation points. Try to choose a longer straight pipe section as the measurement pipeline, in order to meet the requirements of different sources of interference in the straight pipe section, priority should be given to the installation of the upstream of the regulating valve. If it is a horizontal pipeline, the transducer should be installed on one side of the pipeline so that the emitted sound wave can propagate horizontally in the pipeline. If it is a vertical pipeline, the position where the liquid flow is downward should be selected as the measuring point, and the pipeline must be completely filled with medium. Care should be taken to avoid selecting the measurement point near the deformed, defective and welded position of the pipe.
Pipe factors
The installation distance between the transducer is clamp-on ultrasonic flowmeter according to the pipe diameter and wall thickness automatically calculated. The outer diameter of the pipe circumference measurement by a steel tape measure to obtain the calculation, the wall thickness is measured directly by the thickness gauge. In order to reduce the uncertainty introduced in the measurement process should be measured several times to take the average value. Clamp-on ultrasonic flowmeter in the data entry of the pipe diameter, material, lining to meet the actual, pipe material and lining on the measurement of the impact reflected in the sound wave propagation speed of the difference in various media, if the material set the speed of sound is greater than the actual speed of sound, then the measurement results are large, and vice versa, is small. If the pipeline is old, the thickness of the scale is also taken into account. Errors in the input parameters can lead to inaccurate calculation of the transducer mounting distance and affect the measurement results. Measurement must ensure the smoothness of the pipe surface, the measured surface roughness is easy to affect the coupling effect and thus cause the measurement data deviation, such as rust and other debris will also affect the ultrasonic signal, so the pipe surface should be cleaned before placing the transducer.
Medium factors
Pipeline media containing more impurities will also affect the measurement accuracy of ultrasonic flowmeter. For example, the accumulation of impurities in the pipeline will lead to the measurement of the effective diameter of the pipeline is reduced, the flowmeter reading is high. Flowmeter transducer installation section of the inner wall with dirt, resulting in ultrasonic waves in the propagation process, the signal attenuation, while the inner wall surface is not smooth will also affect the flow distribution. In addition to calibration, but also consider whether the medium flow rate to meet the normal use of the instrument being measured.
External interference factors
Fluid temperature in the pipeline will affect the clamp-on ultrasonic flowmeter signal speed of sound, which in turn affects the distance between the ultrasonic flowmeter, so in the work should be considered in the selected clamp-on ultrasonic flowmeter’s normal operating temperature range, and strictly in accordance with the instructions required to operate.
Many companies or manufacturers of external clamp-on ultrasonic flowmeter installed in the pump room, a lot of high-power equipment and power supply lines, control lines, etc. and the measured pipeline distance is very close. High-power electrical equipment will produce a strong magnetic field, affecting the normal operation of the ultrasonic flowmeter transducer. Correct installation location and avoid electromagnetic interference can effectively improve the measurement accuracy.
