Turbine type flowmetering devices are applied worldwide to the measurement and control of liquid products in the industrial, chemical and petroleum marketplaces. Significant advantages associated with the use of turbine flowmeters, in lieu of other metering principles, make increased future use inevitable. Newcomers to the field of flow measurement should become familiar with fundamental characteristics and conditions surrounding the turbine flowmeter in order to better understand its usage. Consequently, this article is provided as a brief guide to the operation and application of turbine flowmeters for liquid product measurement.

Barry Ellison

The basic construction of the turbine flowmeter incorporates a bladed turbine rotor installed in a flow tube. The rotor is suspended axially in the direction of flow through the tube. The turbine flowmeter is a transducer, which senses the momentum of the flowing stream. The bladed rotor rotates on its axis in proportion to the rate of the liquid flow through the tube.

TURBINE ROTATION
As the liquid product strikes the front edge of the rotor blades, a low-pressure area is produced between the upstream cone and the rotor hub.
The blades of the turbine rotor will tend to travel toward this low-pressure area as a result of this pressure differential across the blades. The pressure differential (or pressure drop) constitutes the energy expended to produce movement of the rotor. The initial tendency of the rotor is to travel downstream in the form of axial thrust. But since the rotor is restrained from excessive downstream movement, the only resulting movement is rotation.
Fluid flowing through the meter impacts an angular velocity to the turbine rotor blades, which is directly proportional to the linear velocity of the liquid. The degree of the angular velocity or number of revolutions per minute of the turbine rotor is determined by the angle of the rotor blades to the flowing stream of the approach velocity.

ROTOR BALANCE
With axial thrust forcing the turbine rotor downstream, the friction resulting from contact between the turbine rotor and the downstream cone would cause excessive wear if there were not some means of balancing the turbine rotor on its axis between the upstream and the downstream cone.
Bernoulli's Principle states that when flow velocity decreases, the static pressure increases. Therefore, a high-pressure area exists at the downstream side of the turbine rotor exerting an upstream force on the rotor. As a result, the turbine rotor is hydraulically balanced on its axis.

SIGNAL OUTPUT
Electrical output is generated using the principle of reluctance. A pickup coil, wrapped around a permanent magnet, is installed on the exterior of the flow tube or the meter body immediately adjacent to the perimeter of the rotor (Figure 1). The magnet is the source of the magnetic flux field that cuts through the coil. Each blade of the turbine rotor passing in close proximity to the pickup coil causes a deflection in the existing magnetic field. This change in the reluctance of the magnetic circuit generates a voltage pulse within the pickup coil.
Each pulse generated represents a discrete amount of volumetric throughput. Dividing the total number of pulses generated by the specific amount of liquid product that passed through the turbine flowmeter determines the K-Factor. The K-Factor, expressed in pulses per unit volume, may be used with a factoring totalizer to provide an indication of volumetric throughput directly in engineering units. The totalizer continuously divides the incoming pulses by the K-Factor (or multiplies them with the inverse of the K-Factor) to provide factored totalization. The frequency of the pulse output, or number of pulses per unit time, is directly proportional to the rotational rate of the turbine rotor. Therefore, this frequency of the pulse output is proportional to the rate of the flow.

By dividing the pulse rate by the K-Factor, the volumetric throughput per unit time of the rate of flow can be determined. Frequency counters or converters are commonly used to provide instantaneous flow rate indication. Plotting the electrical signal output versus flow rate provides the characteristics profile or calibration curves for the turbine flowmeter.
Electrical output is also generated using the principle of inductance. A pickup coil is installed on the exterior of the flow tube immediately adjacent to the perimeter of the turbine rotor. The magnetic source of the flux field in this type of output is either the rotor itself or small magnets installed in the rotor. In the case of the rotor, the material of construction would be nickel or some other easily magnetized flux field. The results are identical to that of the reluctance principal.

Figure 1

ACCURACY
The accuracy of a turbine flowmeter is derived from its output (electrical or mechanical) and is the measure of the deviation of an indicated measurement from the referenced standard. Turbine meter accuracy is dependent upon several items.
The accuracy must include the error associated with the calibration standard. In the USA, the National Institute of Standards and Technology represents the flow standard.
Linearity is the variation of the flowmeter K-factor from a nominal value of a point on a curve. Normally during calibration, a value is chosen which makes linearity fall in line with accuracy. Linearity may remain constant during meter life although the absolute accuracy level has changed.
Repeatability is the ability of a turbine flowmeter to reproduce its output indefinitely under constant operating conditions at any point over its specified operating range.

Figure 2 (click on the image to enlarge picture)

SPECIFIC GRAVITY
The specific gravity of a liquid is the ratio of its density to that of water at 4BC (39.2BF) and is dimensionless. While changes in specific gravity do not affect the average turbine meter K-factor value, the overall linear range of the flowmeter is changed (Figure 2). The linear range represents the minimum to the maximum flow rate within which the linearity of the flowmeter is specified.
As stated previously, the rotor rotates due to pressure differential across the rotor blades. Specific gravity is one of the factors affecting this pressure differential. As the specific gravity decreases, the pressure differential decreases. On a fluid with a low specific gravity and a low flow rate, the pressure differential across the blades is very low. This leaves almost no energy for turning the rotor. Consequently, the rotor cannot turn in proportion to the liquid throughput and the K-factor drops off.

Therefore, the angle of the rotor blades is changed to help compensate for the change to a lower specific gravity. This allows products with lower specific gravity's to be measured accurately by the turbine flowmeter.

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.

Figure 4: The turbine flowmeter position should not adversely affect velocity or the smooth rotation of the rotor. The rotor should decelerate and stop in a smooth uninterrupted fashion. An abrupt sticking motion indicates bearing failure.

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.