Principle of Operation
Hoffer inline turbine flowmeters are velocity measuring devices, which is to say they measure the average velocity of a fluid flowing through the body of the meter.



Mounted within the body of a Hoffer liquid turbine flowmeter is a vaned rotor. The rotor is centered on a shaft and allowed to rotate by means of bearings. The shaft is supported in the housing by tube bundles that also provide a measure of flow conditioning for the fluid stream. The rotor is made from a ferromagnetic material or contains a magnet within the hub of the rotor.
Liquid flowing through the meter body engages the rotor forcing it to rotate.The rotational velocity of the rotor is proportional to the average linear velocity of the liquid flow stream. This rotational velocity is transformed into an electrical frequency signal by means of a non-intrusive sensor or coil threaded partially into the body of the meter aligned with the rotational circumference of the rotor. Being in close proximity to the ferromagnetic or magnetic rotor creates an electromagnetic coupling with the coil. The output frequency of the coil then is directly proportional to the rotational velocity of the rotor.
This frequency can then be converted to a flow rate indication or scaled signal by dividing the frequency by the meter’s scaling or K-factor (e.g. pulses per gallon or pulses/liter). The K-factor is established by factory calibration of the flowmeter at the time of manufacture.

Performance Characteristics
The general performance characteristics of a Hoffer turbine flowmeter are illustrated below.



For liquids with a viscosity in the range of approximately 0.8 to 2 centistokes (cstks.), the turbine meter will produce an output signal that is typically linear to within +/-0.5% of reading or better over a turndown range of 10:1. The meter will produce an output signal that is typically repeatable within +/-0.1% of reading over a turndown range of 20:1 to as wide as 100:1 depending on the coil used and the type bearing used. Accuracy and flow range specifications are given for each size and coil type combination are given in the individual meter series datasheets.

Installation
 Turbine flowmeters measure the average velocity of the flowing stream and this requires that the velocity profile of the fluid stream be symmetrical across the internal diameter of the flowmeter body when it engages the rotor. In order to assure that the flowing stream’s velocity profile is symmetrical, it is necessary to provide straight runs of pipe into and out of the flowmeter. For most applications, a length equal to 10 diameters of the nominal turbine meter inlet size upstream and 5 diameters of the nominal turbine meter inlet size downstream of the meter are adequate to assure this condition. For flowmeters with nominal inlet piping sizes less than 1", 10" of upstream and 5" of downstream straight piping are recommended. For applications requiring the best accuracy, such as custody transfer or legal-for-trade, a flow straightening tube bundle in the upstream section is normally required to eliminate any rotation of the flow stream about its axis of flow.



Hoffer offers a complete line of installation kits and flanged flow straightening sections that provide the required straight runs of piping in and out of the flowmeters as an option and meet API and AGA design criteria for custody transfer applications as well. Consult factory with your specific requirements.
A strainer is generally recommended for installation upstream of the straight run section in order to prevent debris from entering into or damaging the flowmeter. Specific maximum particle sizes and recommended strainer sizes for each size of turbine meter are given in Table 2. A typical installation schematic for a uni-directional turbine flowmeter is shown in Figure 2. The bypass flow loop shown in Figure 2 is optional but provides a convenient method for removing the meter from the line for servicing without the necessity of shutting down the flow. For liquid service, it is recommended that the turbine meter and straight run sections be installed in a horizontal plane relative to ground level. They may also be installed in a vertical or slanted line with the flow direction being up through the flow meter. Vertical or slanted downflow installations are not recommended due to the difficulty in maintaining the line full of liquid and the probability of accelerating bearing wear. Turbine flowmeters should always be installed in positive pressure lines and will not perform well under vacuum flow conditions. Adequate back pressure is required downstream of the flowmeter in order to assure that the flowmeter will remain completely full of liquid during measurement operations and to prevent fluid cavitation. Normally a minimum back pressure of 5 to 10 PSIG is sufficient to prevent cavitation. For more volatile, low density liquids such as hydrocarbon based liquids, the minimum required back pressure can be calculated as follows:

P(b) = (2 x ?P) + (1.25 x P(e))

Where: P(b) = minimum required back pressure in PSIG ?P = pressure drop through the turbine flow meter at the maximum flow rate for the liquid being measured in PSIG P(e) = equilibrium vapor pressure of the liquid at the operating temperature in PSIA

Pressure Drop Characteristics


Viscosity Effects
 Hoffer inline turbine flowmeters may be used for measuring relatively viscous fluids. The exception is the Teflon® Series which is only recommended for use on viscosities of 3 cSt. and below. The maximum viscosity that can be successfully addressed will depend on the available operating pressure and pressure drop across the meter or allowable pressure drop. The usual maximum recommended viscosity for the inline turbine meters is illustrated below.



Ball bearings are usually preferred for viscous applications when a choice is available. It is also usually recommended that the meter be calibrated on a viscosity equal to or similar to the operating viscosity. Finally, signal linearization is often required for such applications.
It is also possible that the viscosity of a particular fluid may vary with operating temperature. This will also create a non-linear output from the turbine flowmeter. A technique known as Universal Viscosity Curve (UVC) correction may be employed to compensate for this effect over the linear range of the turbine meter. Consult Hoffer Technical Note TN-21 for additional information on this option as well as Technical Note TN-47 for an expanded discussion on viscosity effects on turbine flowmeters.