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  Ultrasonic flow measurement in non-stationary conditions

Recap of ultrasonic flow measurement techniques.

Flow rate measurements are crucial for the monitoring of any hydro equipment. Currently, different techniques exist: electromagnetic, laser/optical, X-ray, electromechanical, ultrasonic, … Among all these techniques, the ultrasonic flow measurement systems are the most suitable in practical application, since they can be non intrusive (by the use of clamp-on sensors) , easy to install and flexible.

The main technique of ultrasonic flow metering is the time-of-flight measurement, described by the following picture:

Time of flight technique for ultrasonic flow measurement

This technique allows the flow rate measurement only if the flow speed profile is completely known in the whole measurement section. It is generally true if the flow is stationary in both time and space. See condition stated by the IEC 60041 standard: Field acceptance tests to determine the hydraulic performance of hydraulic turbines, storage pumps and pump-turbines.

Ultrasonic velocity profile determination may be impossible…

In practice, several limitations restrict the use of this ultrasonic technique. In particular, the non-respect of these above standard’s constraints which can be interpreted as spatial and time non-stationary context. 

Performing in the spatial configuration out of the conditions defined in the standard  cause an inaccurate result.

So, the non stationary flow conditions, whether from water system configuration or water speed variation, is a blocking point. Making it impossible to determine the velocity profile  by means of ultrasonic measurement. For example:

Ultrasonic flow measurement close to a pipe bent
The distance between elbow and measurement section is much lower than 20 diameters – out of standard conditions
Ultrasonic flow measurement in disrupted flow caused by a diaphragm
The presence of diaphragm makes the flow rate profile asymmetrical

Flow blow effect caused by fast variation of flow rate
Water speed: Fast variation of flow rate (high dynamic fluids), the acoustic sensing can loss its property (flow blow effect, illustrated in the figure)

ABIUS broadens the scope of clamp-on ultrasonic velocity profile determination

By using ABIUS software for  » Adaptive Beamforming for non-Instrusive Ultrasonic Sensors”, MOTRHYS provides the all level processing of acoustic signals of a multi-sensor configuration for the estimation of flow speed profile in the in stationary conditions – see next figure:

  • Single sensor level: the wide band signal transmitted by the EM sensor will contain, at the receiver sensors, information of time of flight as well as Doppler deformation. The processing of the received signal will allow estimating these parameters
  • Multi-element sensor: as suggested by the next figure, each sensor will have four elements. By digitally controlling the phase delay between these elements, ABIUS allows the electronically beam forming, helping to focus the acoustic path to the direction of maximum of the energy. In this way, it is possible to have a much wider range of flow rate
  • Multi-sensor configuration: by using many acoustic path, it is possible to get the 3D flow rate profile, independently of the spatial configuration.

ABIUS  test bench proofs-of-concept

Flow profile estimation by means of non intrusive acoustic measurements
Estimation of an asymmetric flow profile [2], [3]
Blue line – real vertical flow profile, measured with a Pitot tube;
Red line – Estimated vertical profile with clamp-on
ultrasonic sensors and ABIUS processing

Double function : flow rate metering and transient pressure measurement…

Thanks to ABIUS processing, clamp-on ultrasonic sensors allow two functions in one: flow rate metering and transient pressure measuring

Water hammer wave detection by means of non intrusive acoustic sensors
With ABIUS technology, it becomes possible to use a standard acoustic flow metering system to measure the speed of the transient pressure wave created by an opening/close of a valve, for instance. [5], [6]

Water hammer wave detection by means of non intrusive acoustic sensors