In-line Turbine & Paddle Wheel Flow Meters
Commercially introduced in the 1940s, turbine flow metering has become one of the most popular volumetric flow measurement technologies offered for monitoring the flow of most liquids and gases, in pipes of virtually all sizes.
This flow monitoring technology offers a variety of advantages, including:
- Good accuracy at low cost
- Good repeatability, especially in turbulent flow regime
- Wide flow rangeability
- Simple, rugged and field repairable
- Variety of construction materials available for greater chemical compatibility
- Wide range of temperature and pressure ratings available
- Variety of process connections available
- Flexibility to interface with a variety of flow control/readout devices
In-line Turbine Meter
The turbine flow meter is offered in several different designs. We will focus on the LB45 Series In-line Turbine in this introduction.
Turbine Meter Operating Principle
The in-line turbine body contains a turbine rotor, mounted on a shaft with bearings and rotor support assembly.
Fluid flowing through the meter is channeled through the inlet flow straightener section (upstream rotor support assembly). This reduces the turbulent flow pattern* to a more stable, laminar flow, prior to coming in contact with the multi-bladed turbine rotor. Flow through the rotor's angular blades cause the turbine rotor to spin at a speed proportional to the velocity of the flowing media.
As the turbine wheel rotates, an electrical (AC) voltage is generated in a magnetic pick up coil, mounted on the meter housing (body), located immediately adjacent to the flow path. As each rotor blade passes through the magnetic field, a pulse is generated. The waveform produced is a modified sine wave that approaches a pure sine wave as the rotor speed increases. The frequency of the AC signal is proportional to the flow rate through the turbine meter.
In turbine flow meters, the housing (body) is constructed of non-magnetic materials, such as aluminum, 316 stainless steel or a variety of rugged plastics (for chemical compatibility). The turbine rotor is constructed of magnetic or magnetized materials, such as 400 series (martensitic) stainless steel. The rotor may also be "slugged" with mini magnets when an inductive-type magnetic pickup or Hall Effect pickup is used.
Three types of bearings are used in turbine flow meters: the ball bearing, the sleeve (journal) bearing and the pivot bearing.
The ball bearing is the most widely used, usually in 440 stainless steel, offering relatively sturdy, "low drag" operation.
The journal or sleeve bearing is most frequently offered in tungsten carbide for its extreme durability. It is most suitable for fluids containing abrasive particles.
The pivot bearing employs hard jewel (sapphire) materials at point of contact (shaft and support). This provides less friction than ball bearings. It is also relatively impervious to chemical attack. However, the load-carrying capacity and allowable speeds of this type bearing are also lower than the ball bearing.
Turbine flow meters use a variety of pickup transducers to convert the rotational energy (speed) of the turbine wheel to a measurable electrical signal. These transducers then transmit a proportional output signal to external readout displays or other interfacing electronic data acquisition equipment.
The Mechanical Gear System uses a shaft which is mechanically driven by a gear mounted on the rotor shaft. This shaft, in turn, drives a mechanical readout which may display flow rate, total batch, or both. (Similar to an automotive speedometer and odometer).
The Magnetic Reluctance type is the most common method. In this design, the turbine rotor is constructed of magnetic 400 series stainless steel. The pickup contains a bar magnet wrapped in a sensing coil. As the rotating turbine blades pass the base of the transducer, an (AC) signal is generated. The frequency of the signal is proportional to the velocity of the flowing media.
The Magnetic Inductive sensing method features magnetic pins inserted in the turbine rotor blades. The transducer contains a simple sensor coil and core. An electrical pulse is induced in the coil as each blade passes the base of the transducer coil. This method offers less magnetic drag than the Magnetic Reluctance sensing method.
The Modulating Carrier RF pickup uses a high frequency signal that is amplitude-modulated by each passing turbine rotor blade. This method imposes no significant magnetic drag on the rotor, thus providing superior performance at low flow rates or with low density media.
The Hall Effect sensing method uses micromagnetic pins imbedded in each of the turbine blades. The Hall Effect Sensor interacts with each of the high flux magnets located in each of the passing blades, inducing a separate (Hall Effect) voltage across the sensor outlets. This induced voltage (VH) is proportional to the current flowing through the sensor and the density of the magnetic field in the rotor blades. In other words, the voltage (amplitude) will increase and/or decrease as each passing blade travels toward and/or away from the sensor position.
This transducer operates without any "magnetic drag" and it transmits a square wave signal up to 2000 ft. without a transmitter using unshielded cable for direct interface with most electronic flow controls and/or PLC's, counters, computers, etc.
Turbine Meter K-Factor
The K-factor represents the number of output pulses transmitted per volumetric unit of fluid passing through the meter's turbine (pulses per gal.) However, turbine meters are not consistent (linear) throughout the full flow range of the meter.
There are several forms of "friction" that retard the rotational movement of the turbine rotor, at the lower end of the flow range. They are "magnetic drag" created by electromagnetic force of the magnetic pickup attracting the rotor, "mechanical drag" due to bearing friction and "viscous drag" produced by processed fluid.
As flow rate increases, the retarding forces (friction) are overcome and the free spinning turbine rotor becomes more linear (consistently proportional to flow). The K-factor maintains linearity throughout the balance of linear flow range. This is approximately a 10:1 "turndown" ratio from maximum flow rate down to minimum flow rate. Above the maximum rated capacity of the flow range, the meter becomes functionally less reliable, due to cavitation, excessive pressure drop and/or bearing overspeeding.
To define the K-factor mathematically in "Gallons per Minute", we can use; K = 60f/Q
Where; f = pulses/second (frequency)
Q = flow rate (GPM)
K = pulses per gallon
To select an appropriate readout device, such as:
Frequency Sensing Devices
Amplifiers for Mag Pick ups
Frequency-to-Analog Devices to convert frequency to 4-20 mA output.
the following can be used:
f = KQ/60
Where: Q = Minimum or Maximum linear flow rate
Paddle Wheel Flow Meters
The paddle wheel flow meter is frequently considered as a low cost alternative to the turbine-type flow meter in applications with less demanding accuracy requirements.
Paddle Wheel Operating Principle
In paddle wheel flow meters, the paddle wheel (rotor with rotary vaned blades) is perpendicular to the flow path, not parallel as in the traditional turbine-type flow meter. The rotor's axis is positioned to limit contact between the paddles and the flowing media to less than 50% of the rotational cycle. This imbalance causes the paddle to rotate at a speed proportional to the velocity of the flowing media.
In paddle wheel flow meters, a sensor is used to detect the proximity of micromagnets imbedded in each of the passing paddle wheel blades. As in a turbine meter, the frequency of the output signal is proportional to the fluid velocity and can be transmitted directly to external remote readout/data acquisition equipment.