Flow Measurement & Control


Revolutionary flow measurement idea

May 2000 Flow Measurement & Control

Many instrumentation engineers are extremely weary when ultrasonic flow measurement is mentioned. However with the new helical sound guidance system, (patented by Siemens), engineers can now certainly rethink their opinions about ultrasonic flow measurement.

Operation principle

The propagation rate of ultrasonic waves in a medium depends upon the speed of sound in the medium and its flow velocity. Ultrasound reflectors in the flow tube produce a helical ultrasonic signal path along the flow tube – this is unlike the traditional method, where the signal is transmitted across the diameter of the pipe.

The two ultrasonic transducers alternatively transmit and receive ultrasonic signals. Propagation delays of the two ultrasonic signals will be different when a flow is present, with the delay being shorter when the signal travels with the flow, and longer when against the flow. The resulting difference is used to determine the flow velocity. This result is independent of the speed of sound in the medium, and therefore independent of the medium. It is also independent of the flow profile. This means that flow can be measured from turbulent flow through the transition region, and into laminar flow. Changes in temperature, density, viscosity and conductivity have no influence on the result. This means that almost all liquids, conductive and non-conductive, organic and inorganic and also almost all hydrocarbons can successfully be measured with this ultrasonic flow measurement principle. Since the measurement in the difference of propagation delay, the flow can be measured in both directions. The input of fluid density values would then allow masflow to be calculated.

There is no pressure drop across the flow tube, thanks to a design that makes use of a construction having no obstructions to the flow. Even the flow rates of slurries can be measured with success. A high degree of accuracy is obtainable, namely: 0,5% with a turndown of 25:1, and 1% with a turndown of 100:1. The alternate signal transmission permits bi-directional flow to be measured with the same degree of accuracy in both directions.



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