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Tuesday, February 19, 2019

Turbine flowmeters

These flowmeters refer to velocity measurement devices, since the action (rotation) of their measuring element (turbine) is proportional to stream velocity, which, in its turn, is proportional to the flow of fluid in the pipe. Turbine flowmeters provide accurate measurements in the wide flow range. However, their application is limited to clean liquids. The name of this device comes from the operational principle of this flowmeter (see Fig. 6.5). 

Fig . 6.5A Basic Parts of the turbine flow meter

The housing of this device 1 is connected to pipes 2 and 3. A turbine 4, sometimes called a rotor, is placed co-axial in this housing in the path of the flowing liquid. This liquid imparts the force to the blades 5 of the rotor and causes the rotor to rotate on the shaft 6, which is connected with the housing by a support 7 with bearings. In order to straighten the stream of the passing fluid, several radial-straightening vanes 8 are placed on the shaft before the rotor in upstream direction. The rotational speed of the rotor is proportional to the fluid velocity only when a steady rotational speed of the rotor has been reached. If we measure the number of turbine wheel revolutions per unit time, then this will be a measure of flowrate. Therefore, we need to measure the number of rotor revolutions. Several methods are used to transmit rotor revolutions through the meter housing to the readout device, which is placed outside the housing. The first method employs a mechanical device, which by use of selected gear trains 9 transmits the rotation of the turbine directly to the register 10. Another, electrical method, employs a permanent magnet with several coils mounted close to the rotor but external to the fluid channel. When one blade of the rotor passes the coil, the total flux through the coil changes and a pulse of voltage is generated (one cycle of voltage). The frequency of voltage pulses is proportional to the fluid flowrate, and the total number of pulses is an indicator of the total flow.

Figure 6.5. Turbine flowmeter.

Article Source:: Dr. Alexander Badalyan, University of South Australia


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