Insertion-style Turbine and Paddle Wheel Flow Metering
In spite of the introduction of many new flow metering technologies in the past five decades, the turbine flow meter continues to be one of the most widely accepted flow measurement technologies available for measuring volumetric flow with superior accuracy and repeatability, especially in the turbulent flow regime.
- Good accuracy at low cost
- Flexibility to fit a wide range of pipe sizes
- Wide flow rangeability
- Variety of construction materials available for greater chemical compatibility
- Quick, easy installation/serviceability
- Simple, rugged and field repairable
- Flexibility to interface with a variety of flow control/readout devices
The insertion-style turbine flow meter is frequently considered as a lower cost alternative to in-line style turbine flow measurement, especially in larger diameter pipe sizes.
The turbine flow meter is offered in many different designs. We will focus on the ES45-D Series Insertion-style Turbine Flow Meter in the introduction to this section.
Turbine Meter Operating Principle
The Insertion-style Turbine Flow Meter features a helical turbine rotor, mounted on a shaft and bearings to operate (rotate) in an axial orientation to the flow stream. The turbine rotor assembly is enclosed within a rotor housing which is affixed to the tip of a tubular insertion probe/flow meter body.
This insertion-style tubular body contains an adjustable adapter assembly that allows the turbine rotor to be inserted into pipes of various diameters and set at the proper depth to measure the average velocity of the flow stream passing through the pipe. Flow through the helical blades cause the turbine rotor to spin at a speed proportional to the velocity of the flowing media.
This unique rotor design features a polished nickel-bonded tungsten carbide shaft with precisely machined sapphire jewel bearings for minimum "mechanical drag" and to assure longer abrasion resistant life.
In this design series, magnetic pins are embedded in each of the turbine rotor blades. A Hall Effect sensor interacts with each of the high flux magnets in each passing blade of the rotating turbine. This interaction generates a voltage across the sensor output. 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 or decrease as each passing blade travels toward or away from the sensor position.
This transducer operates without any "magnetic drag". It transmits a square wave signal up to 2000 feet without a transmitter, using unshielded cable for direct interface with most electronic flow controls and/or PLC's, counters, computers, etc.
Insertion-style Paddle Wheel Flow Meter
Similar to their "in-line" counterparts, the insertion-style paddle wheel flow meter is frequently considered as a lower cost substitute for the insertion-style turbine in flow monitoring application that do not require the higher degree of accuracy.
Paddle Wheel Meter Operating Principle
The Insertion-style Paddle Wheel Flow Meter is structurally similar to the Turbine Flow Meter. However, the paddle wheel axis is mounted perpendicular to the direction of flow, causing the flowing stream to contact the radially vaned paddle blades on the flat broad side surface.
The paddle wheel axis is positioned within the sensor housing to limit contact between the paddle blades and the flowing media to less than 50% of the rotational cycle. This imbalance causes the paddle wheel to rotate at a speed proportional to the velocity of the flowing media.
Fluid Flow Profiles
During the late 19th century, Osborne Reynolds developed a method of describing the nature of fluid flow in enclosed conduits (pipes).
In the laminar flow regime
- Viscous forces (internal friction of fluid) dominate fluid behavior
- Local velocity (across flow path) creates a parabolic profile (see drawing below)
- Velocity at the center line of the flow path is twice the mean velocity
In the turbulent flow regime
- Inertial forces (dynamic forces) dominate fluid behavior
- Lateral and transverse forces cause immediate dispersion of media (intimate mixing of fluid)
- Resulting in a flow profile with relatively small changes in local velocity across the flow path (cross sectional area of pipe)
Reynolds also created a formula with a dimensionless numerical value, which is called a Reynolds number.
This is defined as:
ReD = vpD / = 3160 Qgpm S.G. / cp Din
Where: Re = Reynolds Number
v = Average fluid velocity
p = Fluid density
D = Pipe diameter
Q = Absolute viscosity
S.G. = Fluid specific gravity
Reynolds Numbers under 2000 indicate flow is in a laminar flow regime: i.e., fluid reaction and velocity at any point in the flow profile does not vary with time. Reynolds Numbers over 4000 indicate flow is in a turbulent flow regime: i.e., the fluid direction at any point in the flow cross section varies with time. Reynolds Numbers between 2000 and 4000 indicate flow in a transitional flow regime: i.e., flow may intermittently change between laminar and turbulent regimes. Most flow meter applications are in turbulent flow regime.
|Turbulent Flow Velocity Profile||Laminar Flow Velocity Profile|