Permanent Multipath Clamp-On Transit Time Flow Meter


For many years now, ultrasonic flow measurements with 'wetted' sensors have been a well established procedure to measure discharge in big pipes in the field of hydro power plants. A lot of research studies show that very precise measuring results can be achieved, under the provision of correct application and consideration of diverse ancillary conditions. Depending on the flow conditions and the definition of the task, systems working in one or several paths are used. Either the sensors are installed inside the pipe wall (internal) or installed from outside (insertion) by drilling holes into the pipe. By doing so, the acoustic path can be aligned in specific planes, which cover the measurement of the velocity profile throughout the cross section of the pipe. Using modern integration procedures (OWICS), the single velocities are weighted and the flow is calculated with reasonably high accuracy. Typically an accuracy of +/-0.83% can be achieved when a straight length of at least 5 pipe diameters is between the measurement section and any bend or change in pipe diameter upstream and 3 pipe diameters downstream of any important irregularity. It could be better if more straight lengths are available.

For a new penstock, a 'wetted' sensor system can always be considered because it gives the chance to work on the dewatered pipe. But for operational power plants, especially old and very old power plants, installing a 'wetted' sensor would be highly challenging and most of the time, the idea to measure flow accurately is dropped. Even in the new power plants, many times the designers are reluctant to suggest a 'wetted' sensors system, because of possible maintenance requirements, abrasion of sensors due to high silt conditions etc, and hence, the flow measurement in the penstock is compromised. This paper will discuss the multipath clamp-on technology and explain why it is superior in many applications where drilling and internal mounting of the sensors is not allowed by the operator and where are excessive silt/abrasive materials in the flow that will damage the sensors.

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