Ultrasonic Flow Meters – Operating principle ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Ultrasonic Flow Meters – Operating principle

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Ultrasonic Meters have undergone a lot of improvement and development over the years and have transitioned from the engineering lab to wide commercial use. It is fast becoming the primary device of choice to measure gas volume for fiscal metering.

Types of Ultrasonic Meters
Inline Systems
Ultrasonic flow meters are available in two variants. There are inline systems and clamp-on systems. In the inline design the ultrasonic transducers are mounted rigidly in the pipe wall and are directly or indirectly in contact with the measuring medium. These measuring
systems can be calibrated and achieve a measuring accuracy of ± 0.5 % of rate and better.

Clamp-on Systems
The clamp-on technology is different. The ultrasonic transducers are mounted on the outside of the piping.. The sound pulse must traverse the pipe wall and any coatings which may be present with differing sound velocities twice. During installation the laws of refraction and reflection must be considered. They are preferred when measuring large flow in large diameter installations. These measuring systems can only be dry calibrated in a flow laboratory in the manufacturer’s works and achieve an accuracy better than ± 2.0 % of rate. If an on-site calibration can be conducted then accuracies up to ± 0.5 % of rate are possible.

Typical Specifications of Ultrasonic Flow Meters
Nominal Diameter DN 25 - 3000mm. Above DN 600, clamp-on systems are preferred
Flow Velocity 1 – 10m/s
Measuring Accuracy ±0.5% of rate for inline systems.
±2.0% of rate for clamp-on systems
Maximum Measuring Medium Temperature -40 °C - 260 °C . With special designs up to 500 °C and higher

Operating Principle of Clamp-on Multipath Ultrasonic Meters
Multipath Ultrasonic meters typically used for gas custody transfer applications calculate volume of gas from velocity measurement’s made over a pipe’s cross-section:
As shown below, pairs of Ultrasonic transducers are installed in a meter body (pipe’s cross-section) and are used to make transit time measurements of Ultrasonic pulses which each transducer both transmits and receives. Pulses shot in the downstream direction are accelerated while those shot upstream are decelerated by the flowing medium (gas). At no flow condition, transit times in the upstream and downstream directions are equal.
After measurement of the transit times (tu & td), velocities are calculated for each transducer pair or path from the measured time difference between pulses shot in the upstream and downstream directions. The multiple path velocities are averaged into the bulk velocity using a weighting scheme that depends on the path’s location in the pipe’s cross-section for which the velocity is measured.

Finally, flow rate is determined by multiplying the bulk velocity (v) by the meter body’s cross-sectional area. As can be seen, velocity measurements are made along multiple paths using transducer pairs arrayed in known position in the meter body. Fluctuations in pressure, temperature and fluid composition (in this case gas) don’t affect the velocity measurements of Ultrasonic flow meters.

Basic Equations of Ultrasonic Flow Measurement
Advantages of Ultrasonic Flow Meters
1. Reliability
2. Accuracy
3. Repeatability
4. High Rangeability (Measuring large flow variation)

1. Flow measurement dependent on flow profile
2. Long meter runs – 15 pipe diameters upstream and 10 pipe diameters downstream         
    are required for good flow profile.