Knowledge||Precautions for Installing Ultrasonic Flow Meters

Thread Content

1.1 Selection of Transducer Types (1) Travel Time Method: A variety of transducer types can be used in this type of instrument. Different manufacturers produce transducers with different structures, and these transducers are suitable for varying fluid conditions (temperature, pressure, etc.), pipeline conditions (material, shape, diameter, straight section length, etc.), as well as installation requirements. It is also related to the method of setting channels, and the method of setting channels is closely connected to factors such as measurement accuracy and repeatability. For gases, USF is used because the efficiency of ultrasonic propagation at the solid-gas interface is very low; only direct-transmission transducers can be employed. Therefore, external USF is generally not used for gas flow measurement. (2) Refractive transducers for Doppler-type instruments of this kind. At present, most domestic products use clamped types, but their technical specifications such as the transmission frequency of the clamped transducers used in the propagation time method are different, so they cannot be used interchangeably. However, the pipeline conditions applicable to both are essentially the same. 1.2 Considerations regarding installation and layout 1) Installation location and flow direction: The flow sensing part of the USF (ultrasonic flow sensor or ultrasonic transducer) can generally be installed in horizontal, inclined, or vertical pipes. For vertical pipes, it is preferable to choose locations where flow occurs from bottom to top; if flow is from top to bottom, there should be sufficient backpressure downstream, such as subsequent pipes above the measurement point, to prevent partial flow at that measurement point. 2) Unidirectional or bidirectional flow: It is usually unidirectional, but it can also be designed for bidirectional flow using more complex electronic circuits; in such cases, the lengths of the straight sections on both sides of the flow measurement point should be arranged in accordance with the requirements for the upstream straight sections. 3) Pipeline conditions: The deposition layer on the inner surface of externally clamped USF pipelines can cause poor transmission of sound waves as well as deviations from the intended path and length of the sound channel, which should be avoided ； The outer surface is less affected due to its ease of handling. The contact surface between the clamped transducer and the pipe should be coated with a coupling agent. Attention should be paid to pipes made of granular structural materials (such as cast iron and concrete), as sound waves are likely to be scattered, and most of the sound waves fail to reach the fluid, thereby reducing performance. There must be no gaps between the pipeline lining or rust layer at the transducer installation site and the pipe wall. In the reverse assumption using the V-method, welds and joints must be avoided (see Figure 11). 4) Upstream flow disturbances: Like most other flow meters, the USF is sensitive to the velocity profile of the flow passing through it; therefore, it also requires a considerable length of straight pipe upstream. The requirements for straight pipe sections were discussed earlier. 5) Preventing acoustic interference: Attention should be paid to the acoustic interference caused by high pressure drops in control valves, etc., which is particularly important when measuring gas flow rates; efforts should be made to avoid it. For example, the USF display units from Instromet company provide real-time measurement alerts for acoustic interference ； The measure of using a elbow as shown in Figure 9 to block acoustic interference is adopted in the pipeline measurement. 2. Installation of flow sensors or transducers (1) Installation of flow sensors (i.e., plug-in transducer assemblies with measurement tubes) 1) When installing such flow sensors, the flow in the pipeline system must be stopped, and the pipe at the measurement point must be disconnected before it is connected to the flow sensor. 2) The inner diameter of the pipe connecting the flow sensor must be the same as that of the flow sensor, with a difference within ±1%. 3) The sensor on the flow sensor should, as much as possible, be positioned within an angle of 45 degrees to the horizontal diameter as shown in Figure 10, avoiding installation near the vertical diameter. Otherwise, when measuring liquids, the sound wave surface of the transducer is prone to being affected by gases or particles, while when measuring gases, it is affected by liquid droplets or particles. 4) The installation position must be filled with liquid when measuring it. 5) There should be sufficient straight sections upstream and downstream. (2) Installation of externally clamped transducers. In addition to paying equal attention to items 2), 3), 4), and 5) above, the following points should also be taken into consideration. 1) Remove the insulation and protective layers from the installation section completely, and polish the wall surface at the location where the transducer will be installed. Avoid local depressions, smooth out protrusions, and remove the paint rust layer. 2) For vertically installed pipes, in the case of instruments using the single-channel travel time method, the transducer should be installed as close as possible within the plane of the bend axis of the upstream elbow (see Figure 11), in order to obtain an average value that is closer to the actual value after flow field distortion at the elbow. 3) The transducer installation location and the pipe wall reflection points must be kept away from joints and welds, as illustrated in Figure 12 with an example using the V-method. 4) The pipe lining and scale layer at the transducer installation location must not be too thick. There should be no gaps between the lining, the rust layer, and the pipe wall. For pipes severely corroded, a hammer can be used to strike the pipe wall in order to knock off the rust layer on it, ensuring normal propagation of sound waves. But care must be taken to prevent dents from forming. 5) Sufficient coupling agent should be maintained between the working surface of the transducer and the tube wall; there should be no air or solid particles to ensure good coupling. 6) The Doppler clamped transducer can be installed in two ways: symmetrically or on the same side, as shown in Figure 13. Symmetrical installation is suitable for pipes with medium and small diameters (usually less than 600 mm) and liquids containing few suspended particles or bubbles ； In-situ installation is suitable for pipes of various diameters and liquids containing many suspended particles or bubbles.