Viscosity is the measure of a liquid’s 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 Precision Turbine Flowmeter (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 region.
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 (PFT) 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 viscosity’s at 37.8B (100B F).
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.