Vortex Flow Meter

A Vortex Flow Meter utilizes a bluff body or cylinder mounted in a pipe spool that creates alternating vortices behind it. The frequency of these vortices is directly proportional to the fluid velocity. These flow meters have no moving parts, minimal maintenance, and are highly reliable for clean fluids. The vortex signal is electronically processed to determine flow rate. Vortex meters work well with most clean fluids and have similar application ranges to DP flow meters.

⚙️ Working Principle

As fluid passes around a bluff body, vortices are alternately formed on either side. The rate of vortex formation is proportional to the fluid’s velocity. Sensors detect these pressure fluctuations and convert them into electrical pulses proportional to flow rate. This principle is known as the vortex shedding principle.

Vortex flow meters utilize a bluff body or cylinder mounted in a pipe spool that creates alternating vortices behind the cylinder. The frequency of the alternating vortex is proportional to the fluid velocity. Vortex flow meters have no moving parts to maintain or repair, and the signal is read electronically and simply converted to a flow rate. Vortex meters work well with most clean fluids and have similar application ranges to DP flow meters. Vortex flow meters are also referred to as vortex shedding flow meters or oscillatory flow meters. These types of flow meters are used to measure the vibrations of the downstream vortexes caused by an obstruction in the flowing stream. Each obstruction has a vital liquid flow speed at which vortex shedding takes place. This vortex shedding occurs at the instant when alternating low pressure zones gets created in the downstream. These sporadic pressure zones enable the barrier to move towards the low pressure zone. By means of sensors gauging the vortices the flow rate can be easily detected.

🧩 Major Components

Bluff body (vortex generator)
Vortex sensor (piezoelectric or capacitance type)
Transmitter for signal amplification and conditioning

🌟 Main Features

Measures both flow rate and total flow
High accuracy (0.5–1%) with wide rangeability
Unaffected by fluid temperature, pressure, or viscosity changes
Compact, reliable, and low maintenance
Suitable for gas, liquid, and steam measurement

Vortex meters are equally appropriate for flow rate or flow totalization measurements.

Use of vortex meters is usually not preferred for slurries or high viscosity liquids. Also their usage is not suggested for batching or other intermittent flow applications.

Since there is rise in viscosity with the drop in Reynolds number, vortex flow meter range ability degrades as and when the viscosity increases. The maximum viscosity limit, as a function of permissible accuracy and rangeability, is found to be somewhere between 8 and 30 centipoises.

In case of gas and steam services, one can gain rangeability better than 20:1 whereas in low-viscosity liquid applications, rangeability offered by a properly sized vortex meter is over 10:1.

With Reynolds numbers more than 30,000, inaccuracy of majority of vortex flowmeters is 0.5-1% of rate.

Vortex metering error increases with the decreasing Reynolds number.

Vortex flowmeters are available in typical flange sizes ranging from 1/2 in. to 12 in.

Wafer body vortex flowmeters i.e. flange less flowmeters are inexpensive as compared to flanged meters. However, flanged meters are considered ideal for applications where the process fluid is perilous or is at a high temperature.

Nowadays, nearly all vortex meters make use of piezoelectric or capacitance-type sensors to determine the pressure oscillation around the bluff body

📈 Vortex Shedding Frequency

The shedding frequency (f) of vortices is proportional to flow velocity (V) and is expressed as:

St = f × (d / V)

Where:

St – Strouhal Number (dimensionless)
f – Frequency of vortex shedding
d – Width of bluff body
V – Fluid velocity

🛠️ Installation Guidelines

For accurate results, install the vortex flow meter on straight pipe sections with minimal turbulence. Follow these guidelines:

Install vertically for gas/steam (upward flow preferred).
Maintain straight lengths: 10D upstream and 5D downstream.
Avoid installation near pumps, valves, or elbows.
Electronics should be mounted sideways for steam applications.

📐 Technical Specifications

Size Range: 0.5" to 12" (13–300 mm)
Pressure Rating: up to 2000 PSIG (138 bar)
Temperature Range: -330°F to 750°F (-200°C to 400°C)
Material: Stainless Steel / Plastic
Accuracy: ±0.75% (liquids), ±1% (gas/steam)
Output: 4-20mA, Pulse, HART/Fieldbus options

✅ Advantages

No moving parts — long service life
Low maintenance, simple structure
Wide measuring range (turndown ratio up to 10:1)
Minimal pressure loss
Applicable for liquids, gases, and steam

⚠️ Limitations

Not suitable for low flow or pulsating flow
Requires clean media (not for dirty or viscous fluids)
Vibration may cause signal interference
Needs sufficient straight pipe length

Why Are Flow Meter Straight Lengths Important?

Like most flow devices, a Vortex flow meter requires a well-developed and symmetrical flow velocity profile, free from any distortions or swirls, to achieve good accuracy and repeatability. To ensure accurate measurement, it is standard practice to install the meter at a certain distance from sources of turbulence. Common sources of turbulence include pumps, valves, bends, reducers, or changes in pipe size or direction.

How Are Flow Meter Straight Lengths Defined?

Most manufacturers specify minimum straight length distances for their products, typically expressed in pipe diameters (D). For example, 10D means that the flow meter should be placed ten times the pipe’s internal diameter away from a turbulence source.

Because turbulence both upstream and downstream can affect measurement accuracy, these straight length requirements are given for both sides of the flow meter.

Manufacturer Variations

Different manufacturers specify different straight length requirements. Some promote meters that require fewer straight lengths as an advantage for compact installations. Ideally, the flow transmitter should be installed with as many straight lengths as possible — preferably more than the minimum recommended, but never less.

Installation Considerations

In compact plants, obtaining the necessary straight lengths can be challenging. Fortunately, Vortex flow meters can be installed vertically, horizontally, or at any angle, as long as the line remains flooded. When installed vertically, the flow should preferably move upward to ensure consistent measurement.

Pressure and Temperature Compensation

When compensating for pressure and temperature:

Allow 3–4 pipe diameters (D) between the flow meter and any downstream pressure taps.
Ensure thermowells are small and located 5–6 D downstream of the meter.

General Rule of Thumb

As a rule of thumb, the required straight lengths should be roughly equivalent to those for an orifice installation with a beta ratio of 0.7. These distances help stabilize the flow profile and ensure the meter delivers accurate and repeatable readings.

💡 Tip: Always follow the manufacturer’s installation guidelines for straight lengths to achieve the best accuracy and minimize flow disturbances.

Recommended Straight Lengths for Flow Meter Installation

The table below shows typical upstream and downstream straight length requirements for common pipeline configurations. Values are expressed in terms of pipe diameters (D).

Pipe Configuration	Upstream Straight Length (D)	Downstream Straight Length (D)
Single 90° Bend	10 D	5 D
Two 90° Bends (Same Plane)	15 D	5 D
Two 90° Bends (Out of Plane)	25 D	5 D
Fully Open Valve Upstream	5 D	3 D
Partially Closed Valve Upstream	20 D	5 D
Reducer or Expander	10 D	5 D
Pump Discharge Line	30 D	5 D

🏭 Top Manufacturers

Yokogawa
ABB
Emerson
Bopp-Reuther
Krohne

📋 Applications

Custody transfer of natural gas
Steam and condensate measurement
Water and chemical flow monitoring
HVAC and power generation
Pharmaceutical and process industries