Turbine Flowmeter

This flow meter is named so as it measures flow by counting the rotation of a turbine that is positioned in the flow line. The fluid flowing through the pipe makes the blades of turbine rotate at a speed that is proportional to the flow velocity of the fluid. The number of revolutions is monitored by either a gear train or by a magnetic or optical pick-up.

The reader or a “pick-up” is installed perpendicular to the rotor. Two types of pick-up assemblies are commonly used: magnetic pick-up and no drag pick-up.

The magnetic pick-up consists of a permanent magnet with a coil wrapped around the magnet. When the turbine blades cut through the magnetic field, an alternating current, the frequency of which is proportional to the flow, is induced in the coil.

The no drag pick-up consists of an oscillator that transmits a high-frequency carrier wave to the coil of the pick-up. The rotation of the turbine modulates the carrier wave depending on the velocity of the rotor. In both cases pulses proportional to the flow are produced.

Every turbine flow meter is characterized by the K-factor, a coefficient that for a specific flow of a particular fluid shows how many pulses per liter are generated by the flow meter.

The K-factor can be calculated from:

K = 60*f/Qv

where

Q_V à the flow [liter/minute]

f à frequency [pulse/s]

Kà K-factor [pulse/liter].

Ideally, the value of K-factor should be constant i.e. relationship between the meter output and the flow rate should be linear. In reality, however, the driving torque of the fluid on the blade is
balanced by the influence of viscous, frictional and magnetic drag effects.

Since these vary with the flow rate, the shape of the K-factor curve depends on
viscosity, flow rate, bearing design, blade edge sharpness, blade roughness and the nature of
the flow profile at the turbine leading edge. In practice, all these influences have differing
effects on the meter linearity and thus all turbine meters, even from the same manufacturing
batch, should be individually calibrated.

The linear relationship of the K-factor is confined to a flow range of about 10:1 – sometimes
extending up to 20:1.

At low flows, the poor response of the meter is due to turbine rotor bearing friction, the effect of fluid viscosity and magnetic drag on the rotor due to the use of a magnetic pick-up. The humping section of the curve flattens as the viscosity decreases – with resultant increase in accuracy.

Practical installation

-The turbine flow meter requires equalization pipes at upstream and downstream sides of the instrument. The length of the horizontal pipe segment on both sides depends on the flow conditions. As a rule of thumb equalization pipe of 10D upstream and one of 5D downstream is recommended.

– The accuracy of the instrument is strongly influenced by the quality of the blades and the friction of the rotor against its axis.

– Inertia of the rotor can greatly influence the response time, especially when dealing with gases.
– To perform the digital-to-analog conversion, a frequency-to-voltage converter can be used, which transforms the pulses into a standard electrical signal.

– To stop any contamination that might block the turbine, a filter can be placed in front of the turbine flow meter if desired.

– Providing a bypass is efficient for continuous processes as it allows the replacement and cleaning of the flow meter (and of the filter) without interrupting the process.

Characteristics

Advantages

-          An extensive selection of ranges is available, for gases as well as for liquids.

-          A high level of accuracy, (0.2-0.3%), is attainable under specific circumstances because of the digital output.

-          Excellent repeatability (± 0.05 %).

-          Wide rangeability up to 20:1

-          Wide range of temperature applications from -220 to 600 °C

-          Measurement of non-conductive liquids.

-          Suitable for very low flow rates.

Disadvantages

-          Very sensitive to wear, especially with highly contaminated fluids and at high speeds.

-          Linear only in a limited area, which reduces the measuring range.

-          Not suitable for high viscous fluids.

-          Viscosity must be known.

-          10 diameter upstream and 5 diameters downstream of straight pipe is required.

-          Not effective with swirling fluids.

-          Only suitable for clean liquids and gases.

-          Relatively expensive.