Turbine flowmeters are flow measurement instruments which use axial fluid momentum to turn a multibladed rotor, the angular velocity of which is approximately proportional to the volume flowrate. With appropriate calibration and operation, turbine meters offer precise and highly repeatable measurement of throughput and flowrate. Furness (1982) provides an excellent description of turbine meter design, the theory of operation and a review of the important aspects of performance.
The majority of turbine meters use a design very similar to that shown in Figure 1 below.
The free-running rotor is supported between upstream and downstream supports and the blades sweep out the full bore of the meter except for a small tip clearance. As the blade tips move past a pick-up mounted externally, a train of pulses is produced. Under calibration, the frequency of the pulses is used to determine a calibration coefficient, normally referred to as the meter factor (pulses per unit volume throughput). In service the reciprocal of the meter factor is multiplied by the measured pulse frequency to provide the flowrate measurement.
Because of the free-running nature of the rotor, the performance of this type of flowmeter is influenced by fluid property effects, and there is a sensitivity to upstream flow disturbance effects. Most notable are the effects of viscosity and swirl.
For liquid metering both free gas entrainment, or cavitation on the meter blades must be avoided. Both are likely to cause significant deviations in the meter factor. Kinghorn (1981) reported changes in meter factor of up to 50 per cent with only trace levels of free gas entrainment.
Furness, R. A. (1982) Turbine flowmeters, Development in Flow Measurement, Vol. 1., R. W. W. Scott edn., Applied Science Publishers.
Kinghorn, F. C. and McHugh, A. (1981) The performance of turbine meters in two-component gas/liquid flow, Flow, Its Measurement and Control in Science and Industry, Vol 2. Instrument Society of America, St. Louis, Mo.