Turbine flow meter use a free-spinning turbine wheel to measure fluid velocity, much like a miniature windmill installed in the flow stream.
Turbine Flow Meter Working Principle
Turbine Flow Meter is a volumetric measuring turbine type. The flowing fluid engages the rotor causing it to rotate at an angular velocity proportional to the fluid flow rate.
The angular velocity of the rotor results in the generation of an electrical signal (AC sine wave type) in the pickup. The summation of the pulsing electrical signal is related directly to total flow.
The frequency of the signal relates directly to flow rate. The vaned rotor is the only moving part of the flow meter.
The Turbine flow meter (axial turbine) was invented by Reinhard Woltman and is an accurate and reliable flow meter for liquids and gases.
It consists of a flow tube with end connections and a magnetic multi bladed free spinning rotor (impeller) mounted inside; in line with the flow.
The rotor is supported by a shaft that rests on internally mounted supports.
The Supports in Process Automatics Turbine Flow Meters are designed to also act as flow straighteners, stabilizing the flow and minimizing negative effects of turbulence.
The Supports also house the unique open bearings; allowing for the measured media to lubricate the bushes – prolonging the flow meters life span.
The Supports are fastened by locking rings (circlips) on each end.
The rotor sits on a shaft ,which in turn is suspended in the flow by the two supports. As the media flows, a force is applied on the rotor wings.
The angle and shape of the wings transform the horizontal force to a perpendicular force, creating rotation. Therefore, the rotation of the rotor is proportional to the applied force of the flow.
Because of this, the rotor will immediately rotate as soon as the media induces a forward force. As the rotor cannot turn thru the media on its own, it will stop as soon as the media stops.
This ensures an extremely fast response time, making the Turbine Flow Meter ideal for batching applications.
A pick-up sensor is mounted above the rotor. When the magnetic blades pass by the pickup sensor, a signal is generated for each passing blade.
This provides a pulsed signal proportional to the speed of the rotor and represents pulses per volumetric unit.; and as such the flow rate too.
The fundamental design goal of a turbine flow meter is to make the turbine element as free-spinning as possible, so no torque will be required to sustain the turbine’s rotation.
If this goal is achieved, the turbine blades will achieve a rotating (tip) velocity directly proportional to the linear velocity of the fluid, whether that fluid is a gas or a liquid:
Turbine Flow Meter Construction
A cut-away demonstration model of a turbine flow meter is shown in the following photograph. The blade sensor may be seen protruding from the top of the flow tube, just above the turbine wheel:
Note the sets of “flow conditioner” vanes immediately before and after the turbine wheel in the photograph. As one might expect, turbine flow meters are very sensitive to swirl in the process fluid flow stream.
In order to achieve high accuracy, the flow profile must not be swirling in the vicinity of the turbine, lest the turbine wheel spin faster or slower than it should to represent the velocity of a straight-flowing fluid.
A minimum straight-pipe length of 20 pipe diameters upstream and 5 pipe diameters downstream is typical for turbine flow meters in order to dissipate swirl from piping disturbances.
Mechanical gears and rotating cables have also been historically used to link a turbine flow meter’s turbine wheel to indicators.
These designs suffer from greater friction than electronic (“pickup coil”) designs, potentially resulting in more measurement error (less flow indicated than there actually is, because the turbine wheel is slowed by friction).
One advantage of mechanical turbine flow meters, though, is the ability to maintain a running total of gas usage by turning a simple odometer-style totalizer.
This design is often used when the purpose of the flow meter is to track total fuel gas consumption (e.g. natural gas used by a commercial or industrial facility) for billing.