What is a multipath ultrasonic flow meter?
A multipath ultrasonic flow meter is a high-precision instrument that measures fluid flow using the principle of acoustic wave propagation. Compared with single-path flow meters, multipath designs improve measurement accuracy and response speed by employing multiple acoustic paths. The structural design of the measurement pipe is a key factor in ensuring the performance of the flow meter.
Structural design of multipath ultrasonic flowmeters
When designing the measurement pipe of a multipath ultrasonic flow meter, the following three basic principles must first be considered:
Flow uniformity
The pipe design must ensure uniform fluid flow and avoid measurement errors caused by local turbulence or flow non-uniformity.
Acoustic wave propagation characteristics
The pipe design must ensure that acoustic waves can propagate uniformly throughout the pipe and minimize attenuation or interference caused by pipe walls, fluid properties, or other factors affecting acoustic propagation.
Sensor arrangement
The positions and layout of acoustic transmitters and receivers must be reasonably arranged to ensure accurate measurement data.
It is worth noting that geometric shape and dimensions of the pipe have a direct impact on acoustic wave propagation. The selection of pipe diameter must be based on nominal pressure, design pressure of the medium, medium temperature, flow range, and the optimal performance range of the flow meter. A larger pipe diameter can reduce disturbance to fluid flow, but it can lead to greater acoustic attenuation. In general, the pipe diameter should be reasonably selected according to the fluid flow rate and velocity.
Notice
The length of the pipe has an important influence on flow uniformity. Generally, the pipe length should be sufficiently long to allow the fluid to develop into a fully stabilized flow, reducing the influence of inlet and outlet disturbances on measurement.
Dimensions of the multipath ultrasonic flow meter
The dimensional requirements include nominal diameter DN (mm), length L (mm), height H (mm), width D (mm), internal flow passage diameter φd (mm), and bolt hole diameter φn. The flow meter adopts flange connections, and the flange dimensions comply with GB/T 9124.1-2019 Steel Pipe Flanges, Part 1: PN Series.
This ultrasonic flow meter working principle adopts high anti-noise design technology and is equipped with a built-in three-stage plate-type flow conditioner or other structural built-in flow conditioners. The required upstream and downstream straight pipe lengths of the flow meter ultrasonic shall meet the requirements shown in Figures.
Straight pipe length requirements
For a unidirectional ultrasonic gas flow meter (with built-in flow conditioner), along the flow direction, the upstream straight pipe length shall be ≥ 5D, and the downstream straight pipe length shall be ≥ 3D.
For a unidirectional ultrasonic gas flow meter (without built-in flow conditioner), along the flow direction, the upstream straight pipe length shall be ≥ 10D, and the downstream straight pipe length shall be ≥ 5D.
For a bidirectional ultrasonic flow meter (bidirectional type without built-in flow conditioner), along the flow direction, the recommended upstream straight pipe length shall be ≥ 10D, and the downstream straight pipe length shall be ≥ 10D.
For a bidirectional ultrasonic flow meter (bidirectional type with built-in flow conditioner), along the flow direction, the recommended upstream straight pipe length shall be ≥ 5D, and the downstream straight pipe length shall be ≥ 5D.
In addition to considerations of external structural dimensions, the selection of measurement pipe materials is also important for acoustic wave propagation and fluid flow. Pipe materials must have low acoustic absorption characteristics to reduce signal attenuation caused by material absorption during wave propagation. Common materials include stainless steel, special plastics, and composite materials. Material selection must be compatible with the fluid medium to prevent corrosion or chemical reactions during long-term operation. For highly corrosive fluids, stainless steel or corrosion-resistant alloys are usually selected. Environmental factors such as temperature, humidity, high pressure, and low pressure should also be considered when selecting materials.
Multipath of ultrasonic flow meter applications
A multipath ultrasonic gas flow meter mainly consists of ultrasonic sensors for transmitting and receiving acoustic signals, symmetric, crossed, or annular acoustic path configurations, pipe adapters that fix the ultrasonic sensors to the pipe, and a signal processing unit responsible for processing the acoustic signals received from the sensors. The signal processing unit includes amplifiers, filters, analog-to-digital converters, and other components, which convert analog signals into digital signals and calculate the flow velocity.
Ultrasonic transducer Pipe layout method
There are many ways to arrange the acoustic paths of ultrasonic pipe flow meter in the measurement pipe, mainly including parallel arrangement, parallel/cross arrangement, cross arrangement, and network arrangement, as shown in Figure.
The parallel arrangement can fully reflect the velocity distribution in the pipe. The cross arrangement can reduce system errors by comparing the flow velocities measured by two crossed acoustic paths. Therefore, these two arrangements are widely used in practice. The network arrangement increases the acoustic propagation distance through reflections and can fully reflect the velocity distribution in the pipe; however, signal attenuation is extremely severe, so it is mostly used in laboratory experimental research. At present, the parallel and cross arrangements are the most commonly used internationally.
Regardless of the arrangement selected, the ultimate goal is to improve the performance of the flow meter. The positions of the acoustic paths and the weighting coefficients of each path have a direct impact on the accuracy of fluid flow measurement in the pipe. To investigate optimal acoustic path layouts and weight distributions, many researchers have conducted studies and proposed a series of data fusion methods, such as neural network algorithms and genetic algorithms. However, these new methods mostly remain at the theoretical research stage. Currently, commonly used acoustic path layout schemes include Gaussian–Legendre, Chebyshev, Tailored, and OWICS schemes. In each scheme, the relative positions of the acoustic paths and their weighting coefficients are fixed and can be obtained from lookup tables. These layout schemes have demonstrated good performance in multipath gas flow measurement.
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