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Flow metering system

No components inside the measuring tube

The Flowave from Buerkert is a flow metering system without sensor elements in the measuring tube. It communicates via CANopen.

The Flowave doesn’t need sensor elements in the measuring tube (Photo: Buerkert)

WHEN IT COMES TO FLOW METERING, the market offers a multitude of solutions. Today, magnetic inductive flow metering is well established due to its broad range of applications. Devices are available in many variations. Products based on ultrasound and the coriolis effect experience high growth rates. Of course every technology has its weaknesses and limitations so that selecting the most suitable one for a specific application is a challenge. The metering task must be fulfilled, reliably, throughout the entire life cycle of the device, and the operative hassle it involves should remain within reasonable limits. Downtimes should be minimized. The German company Buerkert set out to find a technology that would take account of all that and offer more value to users. The Surface Acoustic Wave Technology (SAW) fits the bill. The system relies on the propagation of waves for measurements, a little bit like the waves in seismic activities.

Distinguishing laminar and turbulent flows

One of the features of SAW for inline flow metering of liquids is that it can do without any components or constrictions inside the measuring tube. The interior surface can be manufactured with the same quality applied to the rest of the pipeline. In terms of hygiene, cleaning, and flow conditions, there are no differences compared to any other straight piece of pipe. No pressure drops, no effects of the fluid on sensor elements. No selection process to come up with the best sensor variant. Transmitter, sensor, and measuring tube stand up to hygienic requirements. Besides the volume flow, temperature and density can be measured, too.

Based on this data, the mass flow rate can be calculated. The metering principle works even with stagnant liquids so that results are available even for smallest flow volumes. The technology recognizes flow rate changes and is suitable for fast filling processes. The excitation frequency of 1,5 MHz avoids disturbances due to inherent vibrations in the plant. Magnetic and electrical effects have no influence on measurements. This technology does not depend on the conductivity of the fluid. Certain effects, which might occur in liquids due to gas bubbles or solid particles, are less pronounced with SAW, or can be detected with better reliability. Last but not least: SAW is the only metering technology that can distinguish between laminar and turbulent flows.

Putting the technology into the device

Implementing this technology leads to a compact and lightweight flow meter, says the company. The flow meter can be installed in any position. The SAW sensor is an integral part of the measuring tube. It is linked up to a transmitter, which comprises the user interface and generates the required output signals. The transmitter is based on Buerkert's electronic platform, which will be used for all future devices. The design is modular, which allows for a diverse use of individual components. The link-up of the electric sub-assemblies is fully digital. The company uses CANopen with extensions, which allows for the design of network topologies. Volker Erbe, product manager for sensors at Buerkert, recommends the SAW technology especially in those cases where, so far, low or non-conductive fluids have excluded magnetic inductive sensors and forced users to opt for more expensive Coriolis devices.

The Flowave doesn’t need sensor elements in the measuring tube (Photo: Buerkert)

Surface Acoustic Wave technology - how it works

SAW relies on the propagation of a wave front, a little bit like with seismic activities. The waves are generated at the excitation center and spread out along the surface of a solid material. In the case of Flowave, this would be the surface of the pipeline. The system works with so-called inter-digital converters. An electric pulse induces them to generate SAWs that propagate like the shock waves of an earthquake. And that is the big difference to ultrasound, where a directional signal travels from A to B in a straight line. Flowave works with four to five inter-digital converters, each consisting of a piezo element and a special electrode.

Each inter-digital converter is a sender and a receiver simultaneously. When one is in sender mode, the two most distant ones act as receivers. The SAWs travel along the surface of the measuring tube, but a portion will uncouple and travel through the liquid. The decoupling angle depends on the liquid and/or the propagation velocity of the wave, which is specific to the liquid. On the other side of the measuring tube, portions of the wave recouple and continue their way along the pipe surface to the next inter-digital converter. Another portion is uncoupled again and travels back to the other side of the measuring tube where the effect repeats itself and the transducer on this side detects the wave.

Excitation of any one transducer leads to a sequence of input signals on two other transducers in the distance. Two of the transducers send their signals in the direction of flow, two in the other direction. The absolute time the waves need to travel from sender to receiver depends on the nominal width of the pipe and the type of liquid. And the difference in travel times in both directions is proportional to the flow rate. A final analysis of all signals, and comparisons with a view to frequency, amplitude, travel times etc. facilitate an evaluation of the quality of the measurement and the type of liquid. Another determining factor can be a comparison of signals and measurements taken with a view to the number of trips the wave takes through the cross-section of the pipe.

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