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 and are best suited for turbulent flow with a Reynolds number greater than 10,000. Some devices can accommodate mixed-phase materials such as steam. Others can measure liquids with suspended solids (slurries). The advantages of this device are low initial set up costs, and low maintenance requirements when used in clean flow conditions. Vortex flow meters are also insensitive to temperature, pressure, and viscosity. The disadvantage of a vortex flow meter is that there is a low to medium pressure drop due to the obstruction in the flow path.

The basic principle for a vortex flow meter is that a barrier is placed in a moving stream. As the flow goes around the object it alternates creating vortices (swirls of media) from the top or bottom of the object. The swirls are created from the increase in pressure and decrease in velocity on one side of the object and a decrease in pressure and increase in velocity on the other side. This process creates a swirl on one side of the object and as the process alternates, a swirl is created on the other side.

Vortex shedding. Image Credit: Cesareo de La Rosa Siqueira

The frequency of this alternating process is proportional to the velocity of the flowing stream as it passes the point of contact. The series or pattern created by the vortex swirls is known as "a von Karman vortex street." The volume flow rate (assuming steady-state upstream flow) is

Where:

is the frequency of vortex shedding

is the diameter of the pipe

is the width of bluff body over diameter of pipe

is the a constant for all fluids given design of meter. A k factor is introduced to compensate for the non-uniform profile of pipe flow.

is the Strouhal number, determined experimentally.

Here is a table of some common Strouhal numbers.

In order to measure flow, an ultrasonic, electronic, or fiber optic sensor monitors the change in the vortex pattern downstream from the object and tramsits a pulsating output signal to external readouts.

Video explaining Vortex Flow Measuring Principle. Video Credit: EndressHauserAG / CC BY 3.0

Vortex Specifications

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.
• Velocity flow rate is usually measured in distance/time. The maximum flow rate of a flow meter is given by the manufacturer in addition to the turndown ratio which can be divided into the maximum flow rate to determine the minimum flow rate. Additional formulas on calculating flow rate can be found here.
• Pipe size is important since 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). Turbulence in the pipe can affect the accuracy of most flow meters; therefore vortex flow meters should be placed at the recommended distance from turbulence creating devices like pumps and valves.

System Media

Different media solutions, such as liquid, or gas, can affect the capability of the vortex flow meter.

Liquid - Liquids with heavy particle suspensions will wear down or clog internal parts, such as the bluff body, causing flow meter errors. Vortex flow meters struggle with liquids that have a high viscosity since they tend to dampen the formation of vortices and reduce the effective range. Particles and internal bubbles do not usually affect vortex meters. Most manufactures are calibrated for liquids of 1 CST viscosity. Anything above that will raise the minimum readable flow rate, reducing rangeability.

Gas- Gas is compressible and changes volume when placed under pressure, heated, or cooled. For systems where gas is the primary media, the vortex flow media should have sensors which monitor changes in temperature and pressure in the system.

Performance Specifications

Material Selection

Pipe material- The material of the pipe should be selected after consideration of the fluid transported by the system. It is important to know the concentration, operating temperature range, and operating pressure range so the pipe can withstand the system requirements.

Flow meter material- Choosing the material for a vortex flow meter requires additional considerations to the ones mentioned above for pip material. These specifications include fluid viscosity, suspended particles, density of the fluid, and expected flow range. If you are planning to install a flow meter into a metal pipe system temperature, media, and pressure should be considered to ensure compatibility.

Mounting Style

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.

For more information on end fittings, please read How to Select Flow Switches.

Outputs and Interfaces

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.

Output Options

• 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.

Interface Options

• Serial interfaces include RS232, RS422, and RS485.
• Parallel interfaces use the general-purpose interface bus (GPIB), a Centronics port, or a printer port.

Features

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.
• Audible or visual alarms can be built in to alert the user if the system goes above and/or below a preset specification.
• Controller functions have or receive sensor input, provide control, (limits, PID, logic, etc.) and output a control signal.
• Sterilization is important for sanitary applications such medical or food processing applications.
• Vortex flow meters that can also measure temperature, density or media levels are commonly available

Resources

efunda - Vortex Flowmeters Overview

Instrumart - Vortex Shedding Flow Meters

Image credits:

Sierra Instruments, Inc. | Badger Meter