VISCOSITY
Viscosity if the measure of the liquid products resistance to flow. Kinematics 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 EFFECTS ON RANGEABILITY
Viscosity has two different effects on the turbine flowmeter rotor. First of all, 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 since the flow around the blade tip region changes with respect to viscosity. This boundary layer thickness causes the turbine flowmeter to be non-linear. Supplying 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, which eliminates the hump that normally, occurs in the transition region.
While linearity is affected by viscosity, repeatability is not.

FLOW RANGE
The minimum flow rate of a 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 flowmeter. As the product viscosity increases, the maximum flow rate decreases in accordance with the maximum allowable pressure drop across the flowmeter. In order to arrive at the minimum and maximum rate of flow limits for a particular turbine flowmeter 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 (100BF).
· The degree of accuracy required.
· The maximum amount of pressure drop allowed across the flowmeter.

Using an area-of-operation diagram for a particular turbine flowmeter size and charting the factors for viscosity accuracy and pressure drop will determine the minimum and maximum flow rates.

Operating the flowmeter within this flow range will meet the operating requirements unique to that application. Technical bulletins providing area of operation for turbine flowmeter sizes with varying viscosity fluids can be obtained from the various meter manufacturers.

CAVITATION
Cavitation in a turbine flowmeter will take place when the local pressures fall close to or below the vapor pressure of the liquid product. The formation of bubbles and their collapse or local vaporization of product as it passes over the rotor blade surface can cause erratic behavior in the turbine flowmeter and excessive wear due to over speeding. Maintaining a system backpressure of 2 times the flowmeter pressure drop plus 25 times the product vapor pressure is sufficient to prevent cavitation as shown by the following formula:

BP= (P x 2) + (VP x 1.25)

Where,
BP= Required back pressure
P= Pressure drop at maximum flow.
VP= Absolute vapor pressure at maximum temperature.

Cavitation usually causes the rotor to speed up at the high flow rate due to the increased flow volume and causes the accuracy curve of the turbine flowmeter to be adversely affected.

INSTALLATION
The term swirl is used to describe the rotational velocity or tangential velocity component of fluid flow in a pipe or tube. Depending on its degree and direction, swirl will change the angle of attack between the fluid and the turbine rotor blades, causing a different rotor speed at a constant flow rate to non-swirling conditions at the same flow rate. Liquid swirl and non-uniform velocity profiles may be introduced upstream of the turbine flowmeter by variations in piping configurations or projections and protrusions within the piping. Swirl may be effectively reduced or eliminated through the use of sufficient lengths of straight pipe or a combination of straight pipe and straightening vanes installed upstream of the turbine flowmeter.

APPLICATIONS
Turbine flowmeters, when first introduced, were used mainly by the aircraft industry in small sizes. Turbine flowmeters are now used on many applications (figure 3). Reasons for this increased used are sizes up to 12", weight and size versus flow rate, extended flow ranges, operating pressures up to 10,000 pounds per square inch, temperature range of -450° to 1000°F and a wide variety of construction materials including stainless steels.
In recent years, turbine flowmeters have been competing successfully with positive displacement flowmeters in many applications due to the economy of installation, low maintenance costs, weight, size and high flow rates per comparable connection size. You must exercise caution when making this comparison, especially on viscous products. Following the parameters outlined previously will prevent most misapplications of the turbine flowmeter.
When products are used in which viscosity changes with seasonal temperature, a proving run should be done at a time when the product temperature would be changing. For instance, fuel oil may change 50°F in ambient temperature between summer and winter. A change of this magnitude would affect the flowmeter curve and directly affect the flow range.
Increased expertise with electronics such as linearization is permitting turbine flowmeters to be used more widely (figure 4).


PROVING
Proving is a method of checking a measuring device against an accepted standard to determine the accuracy and repeatability of that measuring device. Turbine flowmeters should be proven immediately after installation, after repair, following removal from service (for any reason) when changing products, when product viscosity changes, or to chart the flow patterns of the flowmeter during a period of time.
In general, provings should be quite frequent in the early history of an installation. When sufficient results have been gathered to establish meter factor versus flow rate curves for each product, frequently proving can taper off unless one of the aforementioned reasons for proving occurs.

METHODS
There are several different methods of proving. Volumetric proving consists of a measured volume of fluid being compared to a known standard, such as a seraphin can or piston prover.
Gravimetric proving entails measuring weight of a fluid by scale or load cell, then converting it by a known formula.
Master-meter proving is the comparison of a test flowmeter to another flowmeter previously calibrated in one of the above methods.

CONCLUSION
Turbine flowmeters are becoming more prominent in the field of liquid flow measurement. Turbine flowmeter manufacturers continue to respond to industry interest with improvements.