An Overview of Vortex-Shedding Flow Meters

Key Takeaways

• Vortex-shedding flow meters are widely used in process industries to measure volume and mass flow. 

• There is a direct relationship between the frequency of the vortex shedding and the velocity of the liquid passing through the meters, which is used for volume measurement. 

• The repetitive pattern of alternating vortices formed on each side of a bluff body is called the Karman Vortex street and the effect of vortex shedding is referred to as the Von Karman effect.

The one instrument commonly used in most of these industries to measure the volume flow of fluids is the vortex-shedding flow meter

The volumetric measurements of gas, liquid, and steam are essential in a variety of industries, including the chemical, petrochemical, fertilizer, and steel industries as well as in thermal power plants. A commonly-used instrument to measure the volume flow of fluids is a vortex-shedding flow meter.

A vortex-shedding flow meter can measure differing fluids such as compressed air, liquified gas, superheated steam, solvents, heat transfer oils, saturated steam, flue gases, fully demineralized water, and much more.

Let’s take a closer look at what vortex-shedding flow meters are all about.

Vortex-Shedding Flow Meters: The Basics

Vortex-shedding flow meters are widely used in process industries to measure volume and mass flow. These meters offer a variety of benefits: 

• Simple installation.
• Wide measurement range.
• High sensitivity to small flow variations.
• Excellent performance due to the compensation scheme for pressure and temperature variations.
• Highly accurate measurements.
• No moving parts, requiring less maintenance and repair.
• Lower probability of malfunction.

The vortex-shedding flow meter makes use of the natural phenomenon that occurs when a liquid flows around a bluff object. The principle under which a vortex-shedding flow meter works is called “vortex shedding”. Vortex shedding is when eddies or vortices shed alternately downstream of an object. There is a direct relationship between the frequency of the vortex shedding and the velocity of the liquid passing through the meters, which is used for volume measurement.

Next, we will dive into the vortex-shedding process.

What Happens During Vortex Shedding?

Vortex shedding is a naturally occurring phenomenon discovered by Theodor Von Karman. When a bluff body or non-streamlined object is placed in the path of a fast-flowing stream, the fluid gets separated from the object into two downstream sides. It also forms vortices (otherwise called eddies or whirlpools) as the boundary layer detaches and curls back onto itself. The separation between the vortices is constant and depends on the size of the bluff body.

On the two sides of the bluff body, vortices are generated and experience higher fluid velocity and lower fluid pressure. The length and size of the vortex increase as it moves downstream. Eventually, it detaches and sheds from the bluff object. Subsequently, vortices are formed on the other side of the bluff body. The repetitive pattern of the alternating vortices formed on each side of the bluff body is called the Karman Vortex street and the effect of vortex shedding is referred to as the Von Karman effect.

Vortex-Shedding Flow Meter Design

The primary parts of a vortex-shedding flow meter are:

1. Bluff Body

The bluff body is a non-streamlined object placed perpendicular to the axis of the pipeline. The pipeline is typically made of stainless steel. Through the pipeline, the fluid flows around the bluff body. As mentioned earlier, the frequency of the vortices is directly proportional to the velocity of the medium. This vortex-shedding frequency is utilized to determine the volume mass flow.

St is the Strouhal number, f is the frequency of vortex shedding, v is the flow velocity, and d is the width of the body.

2. Sensor Assembly

The pressure and velocity oscillations generated on either side of the bluff body by the vortices are recorded using the sensor assembly. The sensor generates the linear digital output signal. 

3. Transmitter

The transmitter is used to convert the sensor output into a current signal between 4-20mA. The transmitter translates the sensor’s zero and full scale reading into 4 and 20mA, respectively.

Types of Vortex-Shedding Flow Meters

Vortex-shedding flow meters can be classified based on their bluff body shape. Characteristics such as linearity, Reynolds number limitations, and sensitivity vary slightly depending on the bluff body shape. The typical bluff body shapes include T, rectangular, square, and trapezoidal.

Based on the type of sensors, vortex-shedding flow meters can be classified into:

1. Piezoelectric type sensor-based flow meters
2. Capacitance type sensor-based flow meters 

Internal as well as external sensors are incorporated in a vortex-shedding flow meter to detect the pressure oscillations and velocity variations.

Obtain Smart Measurements With Vortex-Shedding Flow Meters

Vortex-shedding flow meters are developed to cope with the smart industrial environment. The present-day vortex-shedding flow meters measure vortex frequency as well as vortex pulse strength to detect the volume and mass flow. Cadence’s CFD tools can capture vortex shedding precisely and can help engineers perform analysis to gain more information.

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