The versatility of vortex flow meters makes them useful for measuring liquids, gases, and steam. They are to be given priority status, with the caveat that their suitability for a given application must be confirmed. Since vortex meters count how often a “bluff body” or “shedder bar” produces a vortex, they are, in essence, frequency meters.
Vortices are only generated at a particular velocity threshold (Re-number), vortex meters will have a higher zero value (the “cut-off” point) after that velocity threshold is reached. The meter’s output will be shut off before the velocity reaches zero.
Some vortex meters may generate an output signal at a specific back-flow (above cut-off point), which could be misinterpreted.
Similar to an orifice meter, a vortex meter measures the volumetric flow rate of a fluid and accurate steam measurement. Intrusive flow meters, such as orifice meters, will permanently reduce pressure as flow rates are increased. liquids close to their boiling point may cause cavitation if the pressure across the meter falls below the vapor pressure of the liquid.
If the pressure rises over the vapour pressure, the bubbles will pop. Avoiding cavitation is crucial since it causes the vortex steam flow meter to malfunction.
What is a Vortex Flow Meter?
A vortex meter to measure the volume of a liquid as it flows past an immovable object. The vortex shedding principle, by which vortices (or eddies) are shed sporadically downstream of an object, is the basis for the operation of vortex flow meter. The vortex shedding happens at a rate proportional to the liquid’s speed as it passes through the meter.
When measuring flows when the insertion of moving elements is problematic, vortex steam meters excel. They come in a variety of materials, including plastic, brass, and industrial grade. Since there are no moving parts, the meter has less wear and less sensitivity to process changes.
Working Principle of Vortex Flow Meter
Vortices are created when a fluid moves at a specific speed and passes through a stationary obstacle. Karman’s vortices will be formed, and their apex will be located around 1.2D downstream of the bluff body.
Once a stretched wire begins to vibrate in an airflow, as Strouhal discovered, the frequency will be precisely proportional to the velocity of the air.
St = f*d/V0
Strouhal’s number = St
wire frequency = f
wire diameter = d
Velocity = V0
“Vortex shedding” describes this process, while “Karman’s Vortex street” refers to the line of vortices.
Vortex shedding frequency is proportional to the fluid velocity and is affected by the bluff body’s geometry and face width. Since the pipe’s inner diameter and the obstruction’s breadth will remain relatively constant, the frequency can be calculated as follows:
V=Fluid Velocity at the Sheddar Bar (in meters per second)
f=Vortex Frequency (in Hertz)
St=Dimension minus Strouhal’s Number
D=Pipe’s Inside Diameter (in millimeters)
ratio d/D = constant (c).
d= Sheddar bar face width in millimeters
The vortex meter’s pressure loss gradient will look like an orifice meter’s. Sheddar bars (analogous to an orifice meter’s vena contracta) have pressure that is at its lowest. The pressure downstream of this location will gradually recover, leading to a net loss of pressure in the long run. Cavitation can be avoided by paying attention to the pressure drop at the vena-contracta.
In order to prevent cavitation, the following minimum back pressure should be maintained:
Pmin=3.2*Pdel + 1.25*Pv
Five pipe diameters down stream from the flow meter, Pmin = minimum needed pressure in bars.
Pdel = predicted long-term pressure drop in bars
Pv = Vapor Pressure (in Bars) at the Desired Operating Temperature
Frequency Sensing Principle
Piezoelectric sensors—the sheddar bar incorporates two piezoelectric crystals. The shedding frequency will subject the sheddar bar to opposing forces, and the piezo-crystals will experience the same thing.
A pair of capacitance sensors with adjustable ranges are included into the sheddar bar. The capacitors’ capacitance will vary as the sheddar bar undergoes alternate micro motions due to stresses induced by the shedding frequency.
Uses for Vortex-Based Flow Meters
Although vortex flow meters have a wide range of potential uses, they perform best with clean, low-viscosity, medium-to-high speed fluids in a number of different sectors.
The most common applications include:
- Gas metering custody transfer
- The Evaluation of Steam
- Suspension fluids in motion
- The Use of Water in Everyday Life
- Fluid chemicals and medicines
Range and Accuracy
The rangeability of vortex flowmeters degrades with increasing viscosity because the Reynolds number decreases. Tolerable precision and variation place a cap on maximum viscosity in the 8–30 centipoise range. Rangeability of a suitably sized vortex steam meter should be greater than 20:1 for gas and steam service and greater than 10:1 for low-viscosity liquid applications.
For Reynolds values above 30,000, the typical error range for vortex meters is 0.5 to 1% of the rate. The metering inaccuracy rises as the Reynolds number decreases. Under a Reynolds number of 10,000, the margin of error can be as high as 10% of the true flow rate.
While other flowmeters keep giving readings even when the flow rate is close to zero, the vortex meter has a cut-off point. The meter’s output is fixed below this threshold at 0 mA (0 mA for analog transmitters). This limit applies to systems with a Reynolds number of 10,000 or below. This is not an issue if the minimum flow that needs to be measured is greater than twice the cut-off flow. However, this can still be an issue in other scenarios, such as when start-up, shutdown, or other upset conditions call for information about low flow rates.
Benefits of Using a Vortex Meter
As opposed to other flow meters, such as turbine meters, vortex steam flow meters don’t have any moving components and hence never need to have their bearings oiled or replaced.
They are frequently used in confined manufacturing facilities due to their versatility in mounting orientation. To learn more about how to install a vortex meter properly, check out our installation manual.
Vortex meters have a permanent pressure loss of just a few psi, which is about half that of an orifice plate. However, in order to attain a sufficiently high Reynolds number, the meter size is typically chosen to be “one size down” from that of the pipe, such as a 4″ meter in a 6″ line. Changing the flow meter’s line size can result in a permanent pressure drop of about 10 psi.
The availability of SIL-rated instruments, like vortex meters, for usage in potentially hazardous regions is another factor to consider.
Problems With Measuring Vortex Flow
Fluids with a low flow rate are a nightmare for vortex meters. It’s possible that the Reynolds number is too low for vortices to form in these conditions. This is why you will only sometimes hear them advised for uses involving batching or other forms of intermittent flow.
They want fluids that are fairly sanitary. Therefore, shredder bars are not suggested for use where sludge or slurry might coat them and prevent vortices from forming.
Straight sections of pipe upstream and downstream of the flowmeter are necessary for accurate steam measurement. ICS Vortex meters have shorter straight-length requirements than many other flow instruments, however this may still be an issue in some settings.
The Summing Up
The pressure pulse produced by a vortex is proportional to the density of the fluid times the square of the velocity of the fluid. The meter’s sensitivity is set by the amount of turbulence in the flow and the force needed to activate the sensor. In order to be distinguished from random fluctuations, this force must be rather strong. For instance, a standard 2-inch vortex meter may measure water flows anywhere from 12 to 230 gallons per minute. The meter’s range will shift if the fluid’s density or viscosity is not the same as water’s.
Selecting a vortex steam flow meter that can withstand the minimal and maximum process flows being measured is crucial for keeping accurate steam measurement noise to a minimum. At least twice the meter’s minimum detectable flow rate should be the minimum flow rate to be monitored. At least five times the highest possible flow rate should be built into the meter’s maximum capacity.