Flow Measurement & Control

Revolutionary new flow measurement idea

April 2003 Flow Measurement & Control

Many instrumentation engineers become weary when ultrasonic flow measurement is mentioned. Siemens believes that its new helical sound guidance system will cause engineers to 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. A helical ultrasonic flow path is set up along the flow tube by means of ultrasonic reflectors. (Unlike the traditional method, where the measurement is made straight, or diagonally across the diameter of the pipe).

Two ultrasonic transducers alternatively transmit ultrasonic signals in opposite directions in the pipe. The propagation time for the signal over the two helical paths will be different if a flow is present. Propagation time with the flow is shorter than that against the flow. The resulting difference is then an indication of the flow velocity.

This result is independent of the speed of sound in the medium, and therefore independent of the medium. This result is also independent of the flow profile. This means that flow can be measured anywhere from turbulent flow, through the transition region and into the laminar flow region.

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 is derived from the difference of propagation times, the flow can be measured in both directions. A calculated massflow measurement is also obtainable once the density of the fluid is known.

The manner of construction of the flow tube results in no obstructions to the flow, which means that there is no pressure loss. Even 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.

For more information contact Keshin Govender, Siemens Southern Africa, +27 (0)11 652 2412, keshin.govender@siemens.com, www.siemens.co.za


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