Vortex flow meters are versatile instruments widely used for measuring steam, gas, and clean liquid flows. They offer a good balance of accuracy, reliability, and cost.
The Principle: Von Kármán Vortex Street
When fluid flows past a bluff body (a non-streamlined obstruction), alternating vortices are shed from each side. This phenomenon is called a von Kármán vortex street. The frequency of vortex shedding is directly proportional to the fluid velocity.
The relationship is described by the Strouhal equation: f = St × V / d
Where f is the vortex shedding frequency, St is the Strouhal number (a dimensionless constant, typically around 0.26), V is the fluid velocity, and d is the width of the bluff body.
Key Components
Bluff Body (Shedder Bar)
The shedder bar is positioned across the flow path to generate vortices. Its shape is carefully designed to produce strong, stable vortices across a wide flow range. Common shapes include T-bar, delta, and rectangular profiles.
Sensor
A sensor detects the pressure fluctuations caused by the vortices. Modern vortex meters use piezoelectric sensors that are highly sensitive and reliable. The sensor is typically located just downstream of the bluff body.
Transmitter
The transmitter processes the vortex frequency signal, calculates flow rate, and provides output signals. Advanced transmitters can also compensate for temperature and pressure variations.
Advantages of Vortex Flow Meters
- No moving parts, resulting in low maintenance
- Works with liquids, gases, and steam
- Good accuracy, typically ±1% of rate
- Wide temperature range capability (up to 400°C or higher)
- Relatively low cost compared to Coriolis meters
- Long-term stability with minimal drift
Limitations
- Requires adequate straight pipe runs (15-20D upstream, 5D downstream)
- Minimum flow velocity required for vortex generation (Reynolds number above ~20,000)
- Not suitable for high-viscosity fluids (above 30 cP)
- Not suitable for dirty or abrasive fluids
- Vibration can interfere with measurement
- Limited turndown ratio compared to mag or Coriolis meters (typically 20:1 to 40:1)
Steam Measurement
Vortex meters are particularly popular for steam measurement because they can handle the high temperatures involved, they measure volumetric flow which can be converted to mass flow with temperature and pressure compensation, and they have no moving parts that could fail in steam service.
For saturated steam, a temperature or pressure measurement allows accurate mass flow calculation. For superheated steam, both temperature and pressure measurements are needed.
Sizing Vortex Meters
Proper sizing is critical for vortex meters. The fluid velocity must be within the meter's operating range. Too low a velocity and vortices will not form reliably. Too high a velocity and excessive pressure drop or vibration can occur.
The ideal velocity range is typically 0.3 to 30 m/s for liquids and 3 to 80 m/s for gases and steam.
Common Applications
Vortex flow meters are commonly used for saturated and superheated steam measurement, natural gas and compressed air, clean liquids in chemical and petrochemical plants, HVAC hot and chilled water systems, and general utility metering in industrial facilities.