Vortex Meter

Pic: Vortex Meter Calculation of flow velocity

Theodor von Karman discovered that, when a non-streamlined object (also called a bluff body) is placed in the path of a fast-flowing stream, the fluid will alternately separate from the object on its two downstream sides, and, as the boundary layer becomes detached and curls back on itself, the fluid forms vortices (also called whirlpools or eddies). He also noted that the distance between the vortices was constant and depended solely on the size of the rock that formed it. 

On the side of the bluff body where the vortex is being formed, the fluid velocity is higher and the pressure is lower. As the vortex a flagpole (which acts as a bluff body); this is what causes the regular rippling one sees in a flag. Vortices are also shed from bridge piers, pilings, offshore drilling platform supports, and tall buildings. The forces caused by the vortex-shedding phenomenon must be taken into account when designing these structures. In a closed piping system, the vortex effect is dissipated within a few pipe diameters downstream of the bluff body.

Today, the majority of vortex meters use piezoelectric or capacitance- type sensors to detect the pressure oscillation around the bluff body. These detectors respond to the pressure oscillation with a low voltage output signal which has the same frequency as the oscillation. Such sensors are modular, inexpensive, easily replaced, and can operate over a wide range of temperature ranges—from cryogenic liquids to superheated steam. Sensors can be located inside the meter body or outside. Wetted sensors are stressed directly by the vortex pressure fluctuations and are enclosed in hardened cases to withstand corrosion and erosion effects. External sensors, typically piezoelectric strain gages, sense the vortex shedding indirectly through the force exerted on the shedder bar. External sensors are preferred on highly erosive/ corrosive applications to reduce maintenance costs, while internal sensors provide better rangeability (better low flow sensitivity). They are also less sensitive to pipe vibrations. The electronics housing usually is rated explosion- and weatherproof, and contains the electronic transmitter module, termination connections, and optionally a flow-rate indicator and/or totalizer.

 St = f(d/V)

Where St is the Strouhal number, f is the vortex shedding frequency, d is the width of the bluff body, and V is the average fluid velocity. The value of the Strouhal number is determined experimentally, and is generally found to be constant over a wide range of Reynolds numbers. The Strouhal number represents the ratio of the interval between vortex shedding where B is the blockage factor, defined as the open area left by the

bluff body divided by the full bore area of the pipe. This equation, in turn, can be rewritten as:

Q = f K

where K is the meter coefficient, equal to the product (A f d B). As with turbine and other frequency-producing flowmeters, the K factor can be defined as pulses per unit volume (pulses per gallon, pulses per cubic foot, etc.). Therefore, one can determine flowrate by counting the pulses per unit time. Vortex frequencies range from one to thousands of pulses per second, depending upon the flow velocity, the character of the process fluid, and the size of the meter.

Pic: Vortex Detecting Sensor

Vortex flow meters are flow sensors that detect the frequency of vortices shed by a bluff body placed in a flow stream. The frequency of the vortices is proportional to the flow velocity. Vortex flow meters are used to measure the flow of liquids and/or gases. Some devices can accommodate mixed-phase materials such as steam. Others can measure liquids with suspended solids (slurries). Operating pressure range and media temperature range are important media specifications to consider. Operating pressure range is the maximum head pressure of the process media that devices can withstand. Maximum temperature range is the maximum temperature of the media that can be monitored. Additional parameters include velocity flow rate, which is usually measured in distance/time. Vortex flow meters that can also measure temperature, density or media levels are commonly available.

Pipe size and mounting style are important consideration when selecting vortex flow meters. Many products are calibrated in terms of pipe diameter, a measurement that is expressed in English units such as inches (in) or metric units such as centimeters (cm). There are several mounting styles for vortex flow meters. Insertion-type devices are inserted perpendicular to the flow path. They usually require a threaded hole in the process pipe, or another means of access. In-line flanged devices are inserted parallel to the flow path, typically between two pieces of flanged process piping. In-line threaded products are inserted parallel to the flow path and threaded into two existing process pipes. National pipe thread (NPT) is the most common thread type. In-line clamp vortex flow meters are also available. These devices are inserted parallel to the flow path and clamped between two existing process pipes.

Vortex flow meters differ in terms of outputs and interfaces. Flow rate information can be output as an analog voltage signal, an analog current signal, a frequency or pulse, or a switch. Analog voltage signals such as 0 – 10 mV are a simple, usually linear function of the measurement. They are continuous, rather than pulsed or discrete. Analog current signals such as 4 – 20 mA use feedback to provide the appropriate current regardless of line noise and impedance. These devices, which are often called transmitters, are useful when sending signals over long distances. Flow rate information can also be output as an encoded signal via amplitude modulation (AM) or frequency modulation. Vortex flow meters with switch outputs use media flow to trigger a switch based on preset flow rates. Both serial and parallel computer interfaces are commonly available. Serial interfaces include RS232, RS422, and RS485. Parallel interfaces use the general-purpose interface bus (GPIB), a Centronics port, or a printer port.

Vortex flow meters provide a variety of features. Multi-insertion devices determine the flow rate by taking flow rate measurements across several points in the flow path. Programmable flow meters typically include a built-in microprocessor and can be adjusted electronically for different materials, ranges, and outputs. Devices with recorder or totalizer functions can plot or chart flow history for a given unit of time. Some vortex flow meters include built in audible or visual alarms and not just alarm contacts. Others provide controller functions. Products that are rated for sanitary applications are suitable for medical or food processing applications.

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