Straight pipe requirement for clamp-on ultrasonic flowmeter

The straight pipe requirement for clamp-on ultrasonic flowmeter is strict, and the following are detailed instructions:
Theoretical basis
Ultrasonic flowmeter is to calculate the flow rate of the fluid by measuring the propagation time of the ultrasonic wave in the fluid, and then obtain the flow rate. A stable and uniform flow pattern is essential for accurate measurement of ultrasonic propagation time. If the length of the straight pipe section is insufficient, the flow pattern of the fluid will become chaotic after passing through the elbow, valve and other pipe fittings, such as vortices and uneven flow velocity distribution, which will change the propagation path and time of the ultrasonic wave, resulting in an increase in measurement error.
Specific requirements
Upstream straight pipe section
In general: under ideal installation conditions, that is, when there is no or only a few pipe fittings in the upstream pipe system that will interfere with the flow pattern, the external clamp ultrasonic flowmeter requires that the upstream straight pipe section length is not less than 10 times the pipe diameter. For example, for a pipe with a diameter of 200mm, the length of the upstream straight pipe section should not be less than 2000mm. This allows the fluid to have enough distance before entering the measurement area to stabilize the flow state, so that the flow velocity distribution tends to be uniform, and provides good flow field conditions for accurate measurement.
When there is interference pipe fitting: if there are elbows, tees, valves, reducing pipes and other pipe fittings causing flow disorder in the upstream, in order to eliminate the influence of these pipe fittings on the flow state, the length of the upstream straight pipe section needs to be further increased. Under normal circumstances, the length of the straight pipe section should be increased to 15 or even 20 times the pipe diameter. For example, when the pipe diameter is 300mm and there are elbows upstream, the length of the upstream straight pipe segment may need to reach 4500mm-6000mm. This is because these pipe fittings will cause the fluid to produce whirlpool, drift and other irregular flow phenomenon, only through a long enough straight pipe section, so that the fluid has been fully developed and adjusted, in order to return to a stable and uniform flow state to meet the measurement accuracy requirements.
Downstream straight pipe section: Generally, the length of the downstream straight pipe section is not less than 5 times the pipe diameter. For a pipe with a diameter of 400mm, the length of the downstream straight pipe section should not be less than 2000mm. Its purpose is to ensure that the fluid has enough straight pipe sections to maintain a stable flow state after measurement, and to prevent downstream pipe fittings from interfering with the flow field in the measurement area. If the downstream straight pipe section is too short, the flow pattern at the measurement point may change due to the influence of the downstream pipe fitting, thus affecting the accuracy of the measurement.
Special situations and countermeasures
Limited field space: In some practical application scenarios, due to the limited field space, it may not be possible to meet the standard requirements of the straight pipe length. At this time, some compensatory measures can be taken to minimize the measurement error. For example, a flow regulator can be installed, which can adjust the disturbed flow pattern so that the fluid returns to as uniform and stable a state as possible before entering the measurement area. However, the installation of the flow regulator also requires a certain length of the straight pipe section, which is generally required to have a straight pipe section of at least 3 times the diameter of the upstream and a straight pipe section of 2 times the diameter of the downstream.
Evaluation of non-standard installation conditions: If all of the above conditions cannot be met on site, a detailed evaluation by a professional is required based on the specific pipe layout, fluid characteristics, pipe fitting type and other factors. It may be necessary to determine the measurement error under the actual installation conditions through numerical simulation, field testing and other methods, and take appropriate corrective measures, such as adjusting the parameter Settings of the flowmeter or adopting more advanced measurement algorithms, to improve the measurement accuracy as much as possible. However, under such non-standard installation conditions, it is more difficult to achieve the same measurement accuracy as the standard installation, and it is necessary to pay close attention to the accuracy of the measurement data in practical applications, and carry out calibration and maintenance regularly.