Water Conditioning & Purification Magazine

Instrumentation: Flow Measurement with Impeller Flow Sensor

Summary: This article describes flow measurement terminology and definitions pertaining to the operation of impeller flow sensors. A comparison between impeller flow sensors from different manufacturers is included along with useful equations to aid in the decision-making process.

There are a number of flow measurement techniques. One category of flow meters is the direct measurement type. These are positive displacement meters that sample discrete, specifically-sized batches of liquid and count the batches. A rate of flow or total volume is then calculated. Full flow isn’t normally required for this measuring technique. It’s used for low flow applications or when high measurement accuracy is required. Direct measuring flow meters include piston, rotary vane and oval gear types.

Another category of flow meters uses an indirect measurement technique where the flow is determined by measuring the liquid’s velocity or change in kinetic energy and by applying an appropriate formula. Types of indirect measuring flow meters include differential pressure devices, turbines, impellers, vortex shedding, magnetic and ultrasonic. The flow rate can be calculated by multiplying the pipe’s cross-sectional area (assuming it to be constant) by the average velocity:

Equation 1: Flow = Area × Velocity ×
    Scaling Factor

Other factors that influence the flow of the liquid include viscosity, density, temperature, and the friction of the liquid in contact with the wall of the pipe. A dimensionless unit called the Reynolds Number has been created to quantify these factors:

Equation 2: Reynolds Number =     
         Velocity × I.D.    
     Kinematic Viscosity

Where: I.D. = interior diameter

At very low velocities (flow rates) or high viscosities (thick liquids), the Reynolds Number is low—and the liquid flows in smooth layers with the highest velocity at the center of the pipe. The lowest velocity is at the pipe wall, where the adhesion to the wall (friction) tends to restrain it. This is called laminar flow and is associated with a Reynolds Number below 2,000. A characteristic of laminar flow is the parabolic shape of its velocity profile, shown at the top of Figure 1.

Most applications involving turbulent flow have Reynolds Numbers above 5,000. Turbulent flow occurs at higher velocities and/or lower viscosities. The liquid breaks up into turbulent eddies that flow through the pipe with the same average velocity. The velocity profile for turbulent flow is more uniform than laminar flow. Transitional flow occurs between laminar and turbulent flow, depending on pipe roughness and a combination of other factors such as velocity, viscosity, temperature and solids content.

Viscosity effects on flow
Viscosity is one of the most critical factors affecting liquid flow. Conceptually, viscosity is the “thickness” of the fluid. Viscosity is greatly influenced by temperature—thicker viscosity at lower temperatures and thinner viscosity at higher temperatures. Two important terms relating to viscosity are “relative” viscosity and “kinematic” viscosity.

Relative viscosity is the ratio of the liquid’s absolute viscosity with respect to the viscosity of water. The unit of measurement for relative viscosity is centipoise (cP).

Kinematic viscosity is defined as the ratio of the liquid’s relative viscosity to its relative density. It’s measured in units of centistokes (cS). Table 1 lists kinematic viscosities for specific concentrations of common acids, bases and solvents. (Water has a value of 1.000 centistokes at 20°C.)

Understanding impeller flow sensors
Impeller flow sensor blades are perpendicular to the flow. A turbine flow meter’s shaft and blades, which have a helical twist, are in line with the flow. An impeller flow sensor is relatively inexpensive, with a large turndown ratio of about 30 to 1. It maintains about 99 percent accuracy over its full range. Figure 2 shows the practical lower and upper limits.

At flow rates below 1 foot per second (ft./sec.), the inertia needed to overcome the bearing friction, impeller mass and fluid drag is greater than the fluid can generate. Cavitation can occur at rates above 30 ft/sec, which causes an increase in reading (and may cause excessive parts wear). As the velocity increases, the reading will eventually decrease with respect to true velocity.

Comparing impeller flow sensors
Impeller flow sensors may appear nearly identical in principle; however, there are significant differences that make a sensor preferable. In addition to superior construction, a flow sensor may use a more efficient sensing method to convert flow velocity into an electronic signal.

Other sensors may use a multi-bladed impeller with a magnet imbedded in each blade. A pickup coil located inside the sensor housing generates an electrical pulse each time a blade passes it. This same principle is used in electrical generators; a changing magnetic field in a coil of wire produces a changing voltage measured at the ends of the coil. Figure 3 illustrates this magnetic generator principle.

This principle is simple, but it has drawbacks. A sensor using the generator principle emits a weak signal that cannot be transmitted over great distances and is easily disrupted by other changing magnetic fields in the vicinity of the coil. Also, ferrous contamination is common in industrial installations, causing the magnet in each blade to attract iron particles in the stream. This not only affects sensor accuracy but can impede or stop the rotating impeller. At low flows, the magnetic attraction between each rotating blade and the pick-up coil increases the force required to turn the impeller. This results in poor linearity. The force of this magnetic attraction can be demonstrated by spinning the impeller blade on the magnetic sensor. The impeller always stops with a blade directly in line with the sensor body.

The impeller of a more efficient flow sensor has six, forward-swept blades to provide more uniform force distribution with smoother rotation. A non-ferrous, conductive encoder is sealed inside the impeller of the sensor. Since the encoder is non-magnetic, there’s no magnetic drag to impede the impeller as it rotates.

This sensor has encapsulated electronics that sense, condition and output a pulse train proportional to impeller rotational velocity. An RF oscillator and sensing coil, integral to the electronic circuit, induce current into the impeller encoder as it passes within close proximity to the coil. The induced current causes the amplitude of the RF oscillator to decrease as the encoder passes. This amplitude modulation is conditioned and transmitted as a low impedance square-ware signal, suitable for transmission over great distances without further amplification (see Figure 4).

The advantages of this pulse signal method are significant. The sensor electronics produce a strong, clean signal that can be easily transmitted over long distances on inexpensive cable. Since magnets aren’t used, ferrous particles aren’t attracted from the fluid; and impeller movement isn’t impeded at low flows, resulting in improved linearity throughout the sensor flow range.

There are many different types of flow sensors available. No one style is able to handle all applications. It’s important to consider the system requirements and choose the most appropriate technology to attain optimum measurement results.

About the author
The above article appears as Technical Bulletin No. TB-F1, Rev. 3-201, for Milwaukee’s GLI International, which became a subsidiary of Danaher Corp. as part of its acquisition of the Viridor Instrumentation Group in February. It’s reprinted with permission here. GLI can be contacted at (800) 454-0263, (414) 355-3601, (414) 355-8346 (fax), email: info@gliint.com or website: www.gliint.com

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