What is Viscosity? How
does Viscosity affect the performance of the Precision Turbine Flowmeter?
Viscosity is the measure of a liquids resistance to flow.
Kinematic viscosity is the ratio of the absolute viscosity to the specific gravity,
usually expressed in centistokes (cs), where the resistance to flow is measured in square
millimeters per second (mm2/s).
Viscosity has two distinct effects on the PTF rotor. First, the profile
causes boundary layer thickness to increase as viscosity increases for a fixed volume.
This means that rotor-blade shape and length will be important in determining the K-factor
as the flow around the blade tip region changes with respect to viscosity. This boundary
layer thickness causes the turbine flowmeter to be non-linear. Formation of a shroud
around the turbine rotor, with the shroud outer diameter slightly smaller than the inside
diameter of the flow tube, increases the viscosity and creates a drag (resistance to
rotation). This drag offsets the non-linear effect of the boundary layer.
The second effect of viscosity is one of viscous shear-force change on
the rotor and increased viscous drag within the bearing.
These effects act to slow the rotor while the profile effect acts to
speed the rotor. The relative magnitude of all these forces changes the Reynolds number.
As previously indicated, some turbine flowmeter designs introduce a device or shroud that
introduces viscous drag, eliminating the hump that normally occurs in the transition
While linearity is affected by viscosity, repeatability is not.
Linearity can be achieved when using the EFR design meter with viscous fluids if the meter
is electronically compensated for temperature.
The minimum flow rate of a PRECISION TURBINE FLOWMETER becomes a factor
of viscosity versus the degree of accuracy. As product viscosity increases, the minimum
flow rate required to maintain a specific degree of accuracy increases. The maximum rate
of flow allowable becomes a factor of viscosity versus the pressure drop across the PTF.
As the product viscosity increases, the maximum flow rate decreases in accordance with the
maximum allowable pressure drop across the PTF. In order to arrive at the minimum and
maximum rate of flow limits for a particular PTF size and application, these factors must
first be determined:
The viscosity of the product to be metered (or maximum value of
viscosity for products with varying viscositys at 37.8B
The degree of accuracy required.
The maximum amount of pressure drop allowed across the flowmeter.
Using an area-of-operation diagram for a particular PTF size and
charting the factors for viscosity accuracy and pressure drop will determine the minimum
and maximum flow rates. Operating the PTF within this flow range will meet the operating
requirements unique to that application.