Vortex Flowmeters: Measurement Principle and Measured Fluids

Contents

 

1. Introduction

Yokogawa launched the world’s first vortex flowmeter in 1969, and in 1979, we launched general-purpose vortex flowmeters. For over 50 years, Yokogawa’s vortex flowmeters have sold more than 500,000 units worldwide. Over the years our vortex flowmeters contributed to improving our customers' productivity, and in 2022 we launched the latest Vortex Flowmeter VY Series, continuing that tradition of improved productivity. 
There are many methods for measuring flow. Why have vortex flowmeters been a trusted method for such a long period? 

We updated this article to introduce—through its measurement principle—the reliability of vortex flowmeters, and the objects they measure. 

 

2. Measurement Principle

Fig.1 shows the measurement principle behind the vortex flowmeter.
When the vortex shedder bar is placed in a flow, it generates alternating vortices on either side. The vortex flowmeter calculates flow rate by counting the number of generated vortices and multiplying that number by a certain factor unique to each vortex flowmeter. Although the measuring method is very simple, it is based on fundamental physical phenomena and meticulously designed engineering.
This section explains the details of the measurement principle. 

Fig.1: Vortex Flowmeter’s Measurement Principle

 

2.1. Number of Vortices Generated is Proportional to Velocity

In the early 20th century, the Hungarian-born mathematician and physicist Theodore von Karman discovered that when a liquid or gas flows perpendicular to an obstacle, it generates alternating vortices behind that obstacle. The pattern of vortices is called a “Von Karman vortex street.” He further found that the number of vortices generated is proportional to the velocity of the fluid that generates them.
The relationship between the number of vortices generated and flow velocity is determined for each vortex flowmeter. Therefore, if this relationship is known in advance, each vortex flowmeter can calculate flow rate by counting the number of vortices generated. 

Fig.2: Fluid Simulation of Karman Vortices 

 

2.2. Force to Vortex Shedder Bar

The Karman vortices generate low-pressure and high-pressure areas on both sides of the vortex shedder bar. As a wave-pattern, this pressure differential applies a force to the vortex shedder bar, and it is important to measure this force accurately. 

Fig.3: Force on the Vortex Shedder Bar 

 

2.3. Sensor Converts Force to Electrical Signals

When a force is applied to an object, it slightly changes the object’s form and generates stress within the object. However, this stress is very small, and the vortices are generated at very short time intervals. A piezo electric sensor makes it possible to generate a charge from this stress (force) from Karman vortices and convert it into sinusoidal electrical signals.

Fig.4: Sensor Changes Force into Electrical Signals

 

2.4. Number of Karman Vortices

The vortex flowmeter counts the number of Karman Vortices. When the amplitude of Karman vortex-generated electrical signal is over a reference value, one pulse is generated. The number of pulses is counted by the pulse counter. With this mechanism, the vortex flowmeter can count the number of Karman vortices accurately and measure the flow rate with high accuracy. 

Fig.5: Generates Pulses and Counts the Number of Karman Vortex 

 

2.5 Flow Rate Calculation

The volumetric flow rate can be calculated using the number of generated vortices and a factor of each vortex flowmeter. The factor means the volumetric flow rate for each pulse, the total flow volume can be determined by multiplying the number of vortices and the factor. Furthermore, the volumetric flow rate can be calculated by dividing the number of Karman vortices by the time interval Δt during which they were measured. The factor is the relationship between the number of Karman vortices and the volumetric flow rate, and it is called the flow coefficient, which is determined through flow calibration before shipment.
Thus, the principle based on simple natural laws provides the foundation for stable measurements.   

Fig.6: Calculate Flow Rate by Pulse 

 

3. Measured Fluids

As mentioned above, vortex flowmeters can perform stable measurements through very simple principles. Additionally, the wide range of measurable fluids is also characteristic of vortex flowmeters since Karman vortices proportional to the flow velocity are generated regardless of the type of fluid. In this section, we will show the various fluids that vortex flowmeters can support.

 

3.1. Types of Fluid

Up to now we have been using the term fluids.
Fluids can be broadly divided into liquids and gases. Liquids are further categorized into water, pure water, oil, liquefied petroleum gas (LPG), liquefied natural gas (LNG), liquid nitrogen, and liquid helium. Gases are categorized into air, compressed air, steam (saturated steam, superheated steam), ammonia gas, ethylene gas, chlorine gas, and fuel gas, and other gasses. Vortex flowmeters can measure these fluids because Karman vortices are generated in them. 

Fig.7: Measurable Fluids: Water, Gas, and Steam (Excerpt from the Video, “VY Series Product Overview”) 

 

3.2. Wide Measurable Range

The measurable flow velocity range of vortex flowmeters is approximately 0.3 m/s to 10 m/s (1 ft/s to 32 ft/s) for liquid, and approximately 5 m/s to 80 m/s (16 ft/s to 260 ft/s) for gas—this wide measurable range is characteristic of vortex flowmeters. For water, they can measure a range from what can be felt by touch to the velocity of a tall fountain. For air, from a velocity weaker than a gentle wind to the velocity exceeding that of a storm. The image below depicts this wide measurement range. 
The VY series lineup offers connection sizes from 15 mm to 400 mm (1/2 inch to 16 inch) and can measure volumetric flow rates from approximately 0.3 m³/h to 28,000 m³/h (10 CFH to 1,000,000 CFH). 

Fig.8: Wide Measurable Range 

 

3.3. Measures in Horizontal, Vertical, and Inclined Piping

As a principle of vortex flowmeters, the phenomenon of vortices generated downstream of the vortex shedder bar is not affected whether in horizontal, vertical, or inclined piping. Therefore, vortex flowmeters can be used in various locations and orientations if the pipe is always filled with uniform fluid. 

Fig.9: Vortices are generated in any piping orientation. 

 

3.4. Wide Temperature Range

The vortex flowmeter can measure fluids of a wide temperature range because of their measurement principle and structure. They can even measure liquefied natural gas (LNG) at -162°C (-260℉), liquefied nitrogen at -196°C (-321℉), and superheated steam in excess of 400°C (752℉).
Yokogawa’s Vortex Flowmeter VY Series can measure in a range from -196°C to 450°C (-321℉ to 842℉).

 

3.5. Wide Pressure Range

High temperature superheated steam necessarily becomes highly pressurized. Vortex flowmeters are robust, with high-pressure-resistant structures. Therefore, Yokogawa’s Vortex Flowmeter VY Series includes models to measure fluid pressures of up to 40 MPa (5,800 psi).

 

3.6. Selection of Sizing and Specifications

Vortex flowmeters can stably measure flow rate under conditions where Karman vortices are stably generated. Which vortex flowmeter you need (including sizing) depends on complex process conditions (flow rate, temperature, pressure, and other fluid properties).
Detailed sizing information is provided, so please refer to the Vortex Flowmeter VY Series product website.

 

4. Conclusion

In this article we introduced the measurement principle behind the vortex flowmeter, and the fluids it can measure. In addition to this principle based on physical phenomena, and its reliability backed by of sales over 500,000 units, the latest Vortex Flowmeter VY Series offers a wide range of models to meet various customer needs. The VY Series includes calculation functions for standard volumetric flow, mass flow, and energy flow, signal processing functions for stable measurement, and diagnostic functions to support stable operations.
Please visit the product page for the Vortex Flowmeter VY Series (see Related Products & Solutions, below) and consider adding our vortex flowmeters with their diverse capabilities to your system.

We hope this article helps you understand vortex flowmeters, and how they contribute to improving productivity.

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