5 Must-Have Features in a Vortex Flow Meter

This measuring principle is based on the fact that vortices are formed downstream of an obstacle in a fluid flow, either in a closed pipe or in an open channel. This phenomenon can be observed by looking at the eddies (“vortex street”) formed downstream of a bridge pillar, for example (Fig. 1). The frequency of vortex shedding down each side of the pillar (bluff body) is proportional to the mean flow velocity and therefore to the volume flow. As early as , Leonardo da Vinci had sketched stationary vortices downstream of obstacles shedding flow.

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In , Strouhal was attempting to describe in scientific form the eddies that form behind bluff bodies. His studies revealed that a wire stretched tight in a jet of air will oscillate. He found that the frequency of this oscillation is proportional to the velocity of the air jet. This phenomenon can be observed in a car or house: the whistling tone of the wind is caused by vortex shedding and rises or falls as velocity changes. This is called the “aeolian tones”.

The physicist Theodore von Kármán laid down more of the theoretical groundwork for flow measurement with vortex meters in , when he described what has become known as the “vortex street”. His analysis of the double row of vortices behind a bluff body in a fluid flow revealed a fixed ratio between their transverse spacing (d) and longitudinal spacing (L). If the bluff body is cylindrical, this ratio is 0.281, for example. With a uniform pipe diameter, the volume of the individual vortices is therefore constant. Presuming that the vortices are of the same size despite differences in operating conditions, flow can therefore be derived directly from the number of vortices per unit of time.

The flow reaches its maximum velocity at the widest part of the bluff body and subsequently loses some of this speed. Figure 3 shows that the flow tries to break away (a) from the contour of the bluff body, instead of continuing to follow it. This causes localized low pressure, producing backflows and, ultimately, vortices (b). These vortices shed alternately down each side of the bluff body and are carried away by the fluid.

Bluff bodies vary in shape from manufacturer to manufacturer. They can be rectangular, triangular, round, delta-shaped or one of several proprietary and patented designs. The design must be such that the Strouhal number remains constant over the entire measuring range, in other words, the vortex frequency is independent of pressure, temperature and density. It is this constant range (Re > 10.000) that is utilized for measuring volume flow with vortex meters (see Fig. 4). Delta-shaped bluff bodies exhibit almost ideal linearity and have proved particularly reliable. NASA engineers have subjected these bluff-body designs to exhaustive studies. Measuring accuracy can be ±0.75% o.r., and reproducibility is around 0.1%.

It is usual to define the characteristics of vortex flowmeters in terms of the “K factor”. This factor represents the number of vortices in unit time (pulses per unit of volume). The manufacturer obtains this K factor by calibration and includes this information on the instrument name plate. It is dependent on bluff body shape and pipe size.

Vortex flowmeters are used in numerous branches of industry to measure the volume flow of steam, liquids and gas. Vortex meters are becoming more and more common in applications that were formerly the preserve of differential pressure flowmeters such as orifice plates. This trend is ongoing, for the simple reasons that vortex meters are easier to install and have a wider range of turndown. Figure 5 shows an example of such a case.

The focus of end customers has evolved from purely volumetric measurement to compensated mass measurement. This development makes it possible to draw up precise balance sheets. By taking pressure and temperature into account, accurate mass measurements can be achieved, which is essential for accurate balancing, process control and optimization.

Furthermore, special wet steam measurement (dryness fraction/steam quality) can help operators to understand the quality of their steam and detect potential accumulation of wetness online. This way, safety and efficiency can be improved reliably. Best accuracy results become possible in saturated/wet steam environments enabling customers to close potential gaps in their mass balances.

Once you’ve decided that a vortex flow meter vs other meters is the right solution for your application, you must determine what vortex meter will meet your specifications. This comprehensive guide to choosing a flow meter covers:

• Parameters and standards
• Vortex flow meter types
• How to size a flow meter
• What to check after installation

ifm specializes in vortex flow meters for water-based media. This general article helps you choose a vortex flow meter for a wider variety of conditions and media types.

Established in , ifm is an industry leader in sensors and controls for today’s demanding Industry 4.0 applications. With 23,000 customers nationwide and $260 million in annual sales, ifm USA  produces 800,000 temperature sensors, flow sensors, and connectors yearly.

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Vortex flow meter parameters

Important parameters to consider when selecting a vortex flow meter are:

• Reynolds number of the process fluid
• Viscosity and fluid density of the process fluid
• Rangeability and conductivity of the process fluid
• Process pressure and temperature requirements
• Pipe diameter
• Minimum flow rate and fluid velocity

Manufacturers offer different models that serve various specifications. Therefore, it’s essential to know the characteristics of your media type (e.g., liquid, gas, steam, etc.) and your facility’s characteristics (e.g., flow rate, straight pipe runs, etc.). Match all these factors to the specs of the models you’re considering.

Vortex flow meter standards

Vortex meters have a few standard specifications to which most meters adhere. The typical pipeline size diameter range is 0.6” to 12” (0.015m to 0.3m.). These meters often require an inside pipe diameter smaller than the process pipe. But, the process connections of the meter will match the nominal pipe size.

There are also standards for the meter installation parameters:

• Meter accuracy increases with longer straight-line pipe
• The usual recommendation for the upstream straight-line pipe is 35 times the diameter of the pipe
• The typical downstream straight line pipe is five pipe diameters

Vortex meters require a straight run pipe upstream and downstream from the installation location. Vertical installation is possible if the media is flowing upwards. Since downward flow is typically inconsistent across the meter, it’s not recommended.

Finding a suitable location to accommodate proper installation requirements can be difficult. In some cases, a vortex swirl meter (see below) is the better option. These allow for smaller upstream and downstream straight pipe lengths.

Vortex meter low flow cutoff

The vortex meter low flow cutoff is the point where the output on the meter automatically clamps to zero. The low flow cut off exists because, at low flow rate, eddies (or vortices) don't form consistently. Therefore, the meter cuts off measurement at certain low flow point. The fluid’s viscosity determines the cutoff. It varies depending on the fluid’s temperature and composition.

Vortex meters also have minimum flow rate limitations specified by the manufacturer. Make sure your flow range exceeds the stated low flow cutoff.

Vortex flow meter density correction

Vortex flow meters don’t output the mass flow rate. A density correction converts the volumetric flow measurement found by the vortex meter into mass flow rate by multiplying the volume flow rate by the density of the fluid. However, this correction only applies if the density is constant.

Pressure compensation of a vortex meter

Extra friction caused by the vortex meter can result in a pressure drop. You can calculate the pressure loss using this equation:

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DP=1.2r × V2(Pa)

Where: 

• DP: pressure loss (Pa)
• r: density of the medium (Kg/m3)
• V: Average flow velocity in the pipe (m/s)

When dealing with liquid flow, ensure the sensor meets the following formula to prevent vaporization:

P≥2.6DP + 1.25P1 (Pa absolute pressure)

Where:

• DP: pressure loss value (Pa)
• P1: The vapor pressure of the fluid (Pa absolute pressure)

You can achieve pressure compensation with a vortex meter by decreasing the diameter of the meter to increase pressure. Typically, vortex meters are 1 to 2 sizes smaller than the pipeline.

We have more details below about determining what size vortex meter you’ll need to measure your medium accurately.

Types of vortex flow meters and other options

Vortex flow meters have various advantages and disadvantages, available in different types and with distinct features for different applications. These options include:

Vortex shedding design

This is the traditional vortex meter design. It consists of a bluff body that sheds vortices downstream. The vortices cause differential pressure across a sensor that transmits a signal proportional to the flow rate. This design requires a long, straight line of pipe upstream from the meter.

Inline type meter

An inline meter replaces a portion of the pipeline. This option is ideal for continuous measurement. This installation type comes in flanged, threaded, or clamped options.

Insertion type meter

These meters are inserted into the existing pipeline and can be removed and used at various locations along the pipeline. They’re often cheaper than inline meters for larger pipe diameters with lower installation and removal costs.

These are suitable for intermediate readings but not continuous measurement. They often have a less accurate flow velocity profile than an inline meter.

Vortex precession swirl meter

A swirl meter is similar to a traditional vortex meter. It still has no moving parts, but the bluff body includes a swirling element that increases the velocity through the meter. It uses a deswirling element after the sensor to return the fluid to its previous tangential velocity. It tends to be more expensive than the traditional vortex meter.

Analog or remote display

Analog meters will display the measurement on the screen of the meter. Remote display models transmit readings to a PLC or higher-level system.

Output signal options

Vortex flow meters can provide various output signals, including analog voltage, analog current, and frequency modulation.

Integrated temperature sensor

In addition to measuring the flow rate, some flow meters can also provide temperature readings. These can be stationary or removable. These sensors are usually built into the design of the bluff body to avoid disrupting the vortices.

How to size a vortex flow meter

Vortex flow meter sizing depends on process parameters. Each meter will have a range of pipe diameters it can suitably measure.

What size is a vortex flowmeter?

When sizing an inline meter, the meter should be 1 to 2 sizes smaller than the process pipe. For example, if you have a 5-inch pipe, you can choose a meter with a 3-inch diameter. The meter will connect to the pipe with 5-inch flanges.

When sizing an inline vortex precession or swirl meter, the meter can be the same diameter as the process pipe.

The swirling mechanism increases the velocity profile across the sensor. This way, the diameter doesn’t need to be decreased to achieve that effect.

An insertion meter should have the detection probe sitting at roughly the center of the process pipe’s diameter.

How to check a vortex flow meter

Once the meter is installed, perform the necessary checks for proper installation:

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• Ensure the meter is installed with alignment to the flow path
• Check for signs of physical damage or leaks
• Confirm that the electrical components are connected and grounded properly
• Ensure the k-factor is accurately configured and the unit is calibrated.