VTT MIKES developed a clamp-on ultrasonic flowmeter that facilitates the installation of sensors

Clamp-on flowmeters based on ultrasonic transit time measurement are widely used in industry because they can be installed externally on the pipe without process interruptions. However, the measurement accuracy of clamp-on meters depends strongly on the installation of the transducers. The measurement solution developed at VTT MIKES aims to address this key challenge.

The clamp-on flowmeter developed by VTT MIKES is based on fixed measurement geometry. It opens new possibilities, particularly in applications where meters need to be relocated, replaced or used temporarily. With further development, the method can be adapted to different pipe sizes and potentially to applications requiring measurements at multiple positions along a pipeline.

In conventional clamp-on ultrasonic flowmeters, two separate transducers are mounted on the pipe surface at a certain distance from each other, where the spacing is determined case by case based on parameters such as pipe diameter and material. This makes the installation prone to errors and reduces accuracy, for example when the meter is removed and reinstalled or relocated to another measurement site.

The solution developed at VTT MIKES uses two commercial piezoelectric ultrasonic transducers whose spacing is fixed by a mechanical mounting structure. This eliminates one of the most significant uncertainty factors related to installation. The transducers are mounted externally on a steel pipe, and measurements are carried out in a full, pressurized pipe without contact with the flowing medium.

The measurement setup has been tested in VTT MIKES liquid flow calibration laboratory in Kajaani. The calibration laboratory provides an ideal environment for instrument development due to well-characterised and highly accurate reference systems. The measurement uncertainty of the applied reference system is 0.3%, and the uncertainty evaluation of the ultrasonic system has naturally been included from the beginning of the development process. So far, measurements have been carried out in a DN150 pipe. The results show a strong linear correlation between the phase difference measured by the ultrasonic system and the reference volumetric flow rate in the range from 20 l/s to127 l/s. Based on a preliminary uncertainty analysis, the expanded uncertainty of the method was 1.7% (k = 2), which is comparable to commercial clamp-on solutions. The applicability of the method will be studied in more detail in various industrial installation environments.

Technically, the solution is based on the difference in ultrasonic transit times upstream and downstream of the flow. Instead of direct transit time measurement, a phase shift measurement approach is used. The flowing fluid causes a phase shift between ultrasonic signals propagating in opposite directions, and this phase shift can be used to determine the flow velocity and, consequently, the volume flow rate. A lock-in amplifier technique was used in the measurements, enabling reliable detection of even very small phase differences.
 

From an industrial perspective, the developed measurement solution offers several practical advantages: faster and simpler installation of the ultrasonic flowmeter, relatively good measurement accuracy, and improved repeatability in reinstallation scenarios. The solution is suitable for various industrial applications, such as the energy sector, water utilities, and process industries. Collaboration with industrial partners is ongoing to bring the method into practical use.

The method has been developed in the ERDF-funded projects CEMIS-UURA and CEMIS-MESI, funded by the Regional Council of Kainuu. The latter project is currently ongoing and investigates the applicability of the technique for pulp flow measurements.