Ultrasound flowmeters

This method is based on the relationship between the flowrate of the stream and the velocity of ultrasound introduced in this stream. There are several modifications of this method, such as Doppler-effect method and transit-time method. The first one is based on the Doppler effect, saying that frequencies of received waves are dependent on the motion of the source or receiver (observer) relative to the propagating medium. We will describe the second method, which is shown schematically in Fig 6.6.

Figure 6.6. Transit-time flowmeter.

A source of ultrasound 1 is attached outside to the pipe 2 with a flowing fluid 3 inside it. A sonic beam is propagating the flowing fluid at a specific velocity, proportional to the properties of the fluid (temperature, pressure, and density). An ultrasound beam 4 will travel faster in the direction of flow, and slower in the opposite direction. This beam arrives in to the receiver 5 faster than an ultrasound beam 6 from the transmitter 7 to the receiver 8.


Transit time of ultrasound beam from transducer the 1 to the receiver 5 can be evaluated as follows (from Bentley J. P. Principles of Measurement Systems, Longman, 1995, p. 411-412):

(6.46)

Transit time of ultrasound beam from the transducer 7 to the receiver 8 can be evaluated as follows:(6.47)

Let’s evaluate the time difference:

(6.48)

The ratio , therefore,(6.49)


Using (6.49) we can reduce (6.48) to the following form:

(6.50)

where, 

These devices can not be used for flow measurements of fluids with air bubbles or solid particles, since they will interfere with the transmission and receipt of ultrasound radiation. These particles serve as reflectors of ultrasound radiation.

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

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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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Venturi flow nozzle

There is another modification of the variable differential pressure technique for flowrate measurements. This technique employs a Venturi flow nozzle, which is shown schematically in Fig. 6.4. 


The Venturi flow nozzle is installed in pipes with internal diameter varying from 65 to 500 mm. It produces a large differential pressure with a minimum loss of static pressure. This nozzle is able to measure flowrates of fluids with suspended solids. However, Venturi flow nozzles are very expensive. It consists of three parts: a profiled inlet 1, a cylindrical throat 2, and a conical outlet 3. A pipes 4 and 5 are connected to the inlet and outlet of the Venturi flow nozzle. The nozzle may be long and short. In the first case the biggest diameter of the outlet cone D_c^max is equal to the internal diameter of the pipe  D_p, in the second case it is less than D_p. The restriction diameter of Venturi flow nozzles D_n ³ 15 mm. The differential pressure is measured as in the case for orifice flowmeters with the only difference, that the downstream pressure in the cylindrical throat is sensed through radially drilled holes 6.

Figure 6.4. Venturi flow nozzle.

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

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