TURBINE METER THEORY

The basic theory behind Daniel liquid turbine meters is relatively simple. Fluid flow through the meter impinges upon the turbine blades which are free to rotate about an axis along the center line of the turbine housing. The angular (rotational) velocity of the turbine rotor is directly proportional to the fluid velocity
through the turbine. These features make the turbine meter an ideal device for measuring flow rate.

The output of the meter is taken by an electrical pickoff(s) mounted on the meter body. The pickoff’s output frequency is proportional to the flow rate. In addition to its excellent rangeability, a major advantage of the turbine meter is that each electrical pulse is also proportional to a small incremental volume of flow. This incremental output is digital in form, and as such, can be totalized with a maximum error of one pulse regardless of the volume measured.

The turbine meter and associated digital electronics form the basis of any liquid metering system. An expanding blade hanger assembly holds the turbine rotor in alignment with the fluid flow. The angle of the turbine blades to the stream governs the angular velocity and the output frequency of the meter. A sharper blade angle provides a higher frequency output. In general, the blade angle is held between 20º and 40º to the flow. Lower angles cause too low of an angular velocity and loss of repeatability, while larger angles cause excessive end thrust.

MAGNETIC PICKOFF OF ROTOR VELOCITY

The angular velocity of the turbine rotor is taken through the turbine meter wall by means of a magnetic pickoff. The stainless steel meter body is non-magnetic and offers negligible effect on a magnetic field set up by a permanent magnet in the pickoff coil.

Turbine blades, made of a paramagnetic material (which properties cause it to be attracted by a magnet), rotate past the pickoff coil, generating irregular shaped voltage pulses. The frequency of these pulses is linearly proportional to the angular velocity of the rotor and thus to the flow rate. Additionally, each pulse is incrementally proportional to a small unit of volume. The amplitude of the pulses will vary in proportion to blade velocity but is not considered in the measurement process. Flow rate and total flow information is transmitted by frequency and by counting (totalizing) the pulses.

The permanent magnet produces a magnetic field which passes through the coil and is concentrated to a small point at the pickoffs. In Figures 5 and 6 below, as a turbine blade (A) moves into close proximity to the pickoff point, its magnetic properties cause the magnetic field to deflect to accommodate its presence. This deflection causes a voltage to be generated in the coil. As the blade passes under the pickoff point (B), this voltage decays, only to build back in the opposite polarity as the leaving blade – now in position (C). This causes the magnetic field to deflect in the opposite direction. So as each blade passes the pickoff, it produces a separate and distinct voltage pulse. Since the fluid surrounding each blade represents a discrete unit of volume, each electrical pulse also represents a discrete unit of volume. Turbine meter output is rated in pulses per gallon, pulses per liter, or other standard engineering units.

References:

EMERSON (2015) Daniel Meter – Liquid Turbine Flow Meter

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• 2017-05-07 at 00:48