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How Does a Vortex Flow Meter Work? Von Karman Shedding Frequency Math Explained
Quick Answer: A vortex flow meter uses a bluff body to create alternating vortices in a moving fluid. The vortex shedding frequency is proportional to flow velocity. The formula f = St x v / d links frequency, Strouhal number, velocity, and shedder bar width. Silver Automation Instruments supplies vortex flow meters from DN15 to DN300 for steam, gas, and low viscosity liquids.
This article explains the von Karman shedding frequency math in plain industrial terms. It covers the sensor, the calculation, sizing limits, and applications. If you need a quote, send us your fluid, pressure in bar, temperature in °C, pipe size DN, and flow range. You can reach Silver Instruments by Tel: +86-25-68650347, Whatsapp: +86-25-52155837, or WeChat: +86 15365082610.
What Happens Inside the Pipe
A vortex flow meter has a shedder bar installed across the pipe diameter. When fluid passes this bluff body, the flow separates. Vortices form alternately on each side of the shedder bar. Each vortex creates a local low pressure zone. A piezoelectric sensor detects these pressure pulses. The pulse frequency equals the vortex shedding frequency. The meter body has no moving parts. That is a real advantage for saturated steam and dirty liquids.
Here is the thing. The sensor does not measure flow velocity directly. It counts pressure pulses. The transmitter converts pulse frequency into flow velocity. Then it converts velocity into volumetric flow. This conversion depends on the von Karman equation.
Von Karman Shedding Frequency Math
The standard formula is f = St x v / d. In this formula, f is the shedding frequency in Hertz. St is the Strouhal number, a dimensionless value. v is average flow velocity in meters per second. d is the shedder bar width in meters.
Example: a DN50 vortex meter has a shedder bar width of 0.014 m. Water flows at 3 m/s. The Strouhal number is 0.25. Frequency = 0.25 x 3 / 0.014 = 53.6 Hz. The meter electronics count 53.6 pulses per second. That frequency maps to a volumetric flow rate.
The Strouhal number is not fixed for all conditions. It stays nearly constant over a wide Reynolds number range. Many industrial vortex meters work from Re 20,000 to Re 7,000,000. Below that range, the vortex street is weak or unstable. The meter output may be cut off. This is why vortex meters have a minimum flow velocity. In practice, most engineers check the minimum Reynolds number at low flow. We have seen this on customer sites many times. A paint manufacturer in Southeast Asia had a vortex meter sized for normal batches but not for low flow rinse cycles. The meter did not read below 0.5 m/s.
From Frequency to Flow Rate
The flow rate Q equals velocity v multiplied by pipe cross-sectional area A. A DN80 pipe has an area of about 0.00503 m2. At 4 m/s, the volumetric flow is 0.0201 m3/s or 72.4 m3/h. The meter measures f, calculates v, and then multiplies by A. The output updates several times per second. Users get volumetric flow or mass flow if pressure and temperature compensation is available.
For saturated steam mass flow, the transmitter may use an external PT100 temperature sensor and a pressure transmitter. Silver Instruments can supply integrated pressure and temperature compensation in the vortex meter electronics. This works well for steam and compressed air. Typical accuracy is about 0.75 percent of reading for liquids and 1 percent for gas and steam.
Why the Strouhal Number Matters
Not all bluff body shapes have the same Strouhal number. A trapezoidal shedder bar is common. Its St value is often near 0.25. Some designs use a T-shaped bar or dual bluff bodies. Calibration fixes the St value for a given line size. Any change in the shedder bar shape chang
Common Industrial Applications
A vortex flow meter fits saturated steam, superheated steam, compressed air, natural gas, and clean or slightly dirty liquids. A vortex meter is not ideal for very viscous fluids. Viscosity above 10 cP can suppress vortex formation. A wastewater plant in Vietnam used a vortex meter for aeration air flow. It worked well because the air was dry and pressure was stable at 3 bar. A chemical plant in Chile measured 12 bar steam with a DN150 vortex meter. The local display showed kg/h after pressure compensation.
For custody transfer or very low flow, other meter types may work better. But for steam and gas, a vortex flow meter is a practical choice. It has no moving parts and can handle standard temperatures up to 250 °C. Silver Instruments can supply versions up to 350 °C with extended electronics.
Installation and Sizing
Vortex meters need straight run upstream and downstream. Typical values are 15D upstream and 5D downstream. If there is a control valve or elbow, more straight run is needed. A flow conditioner can help in tight pipe racks. Vibration is a known issue. Avoid installing near large compressors or pumps. The sensor can interpret pipe vibration as flow pulses. Some Silver Instruments meters include vibration filtering in the electronics. That helps on offshore platforms and ships.
The meter is available in flanged, wafer, and sanitary connections. Line sizes range from DN15 to DN300. Pressure ratings include PN16, PN40, ANSI 150, and ANSI 300. For aggressive fluids, the wetted parts can be 316L stainless steel. For steam, the meter body may be 304 or 316 stainless steel.
Model Pairing and Outputs
Silver Instruments recommends the SIVF series vortex flow meter for steam and gas. For liquid applications, the same series works if viscosity is below 10 cP and flow is turbulent. The transmitter supports 4-20 mA HART, pulse output, and optional RS485 Modbus. For hazardous areas, Silver Instruments can supply ATEX Zone 1 and IECEx versions. The local LCD shows total flow, actual flow, and diagnostic codes. In a beverage plant in the Philippines, the pulse output fed a PLC batch controller for CIP water. The setup was simple and stable.
If you need a quote, send us the fluid name, operating pressure in bar, temperature in °C, pipe size DN, and flow range in m3/h or kg/h. Our engineers will select the meter and transmitter options. You can reach us by Tel: +86-25-68650347, Whatsapp: +86-25-52155837, or WeChat: +86 15365082610. More product details are on flow-meter.com.au.
FAQ
How does a vortex flow meter calculate flow rate?
The meter counts vortex shedding frequency. It uses the equation f = St x v / d to find flow velocity. Then it multiplies velocity by pipe cross-sectional area to get volumetric flow rate.
What is the Strouhal number in a vortex flow meter?
It is a dimensionless number that links vortex frequency, flow velocity, and shedder bar width. Most industrial meters use a value near 0.22 to 0.27.
Can a vortex flow meter measure low flow?
No. Below a minimum Reynolds number around 20,000, the vortex street is weak and the output may drop to zero. Check your minimum flow before sizing.
What fluids work with a vortex flow meter?
Steam, gas, and clean low viscosity liquids work well. Avoid very thick liquids above 10 cP and slurries with hard solids.
Which vortex meter does Silver Instruments recommend for steam?
The SIVF series with pressure and temperature compensation. It gives mass flow for saturated steam and works with a 4-20 mA HART signal.
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