Working Principle of Vortex Flow Meter

Vortex Flow Meters

• Vortex flow meters determine the vortex’s frequency generated through a “bluff body” or “shedder bar,” they are normally known as frequency meters.
• Vortex meters are precise volume flow meters.
• They are restrictive meters, equivalent to orifice meters.
• The pressure drop within the flow meter exists with an increase in flow and tends to permanent loss.
• Vortices occur after reaching a certain velocity called Re-number, so vortex meters will have a cut-off point known as an elevated zero.
• In some vortex flow meters output signal is obtained above the cut-off point resulting in a false interpretation.
• The output of the meter reads to zero before the velocity of the meter drops to a minimum or zero value.
• The bubbles will impede as soon as the pressure rises above the vapor pressure.
• Cavitation damages to the meter should be avoided at all costs.
• A vortex flow meter can determine the flow of a wide range of fluids, like liquids, gasses     , and steam.
• They are to be considered as first preference, about the confirmation to make sure they satisfy the specifications of the particular assessment.

Principle of Vortex Flow Meters

• Vortices are formed when a liquid passes at a constant rate and passes through a fixed obstruction.
• The formation of vortices is known as Karman’s Vortices, and the vortices’ pinnacle point will be roughly 1.2D downstream of the bluff body.
• Strouhal discovered that when a stretched wire starts to oscillate in an airflow, the frequency is directly proportional to the air velocity.

St= F*D/V₀

Where

• St= Strouhal’s number, 
• F=frequency of wire,
• D=diameter of wire, &
• V₀= Velocity

• This is known as “vortex shedding,” and the route of vortices is known as “Karman’s Vortex street.”
• The frequency of vortex shedding depends on the shape and facewidth of the bluff body and is a straightforward function of fluid velocity.
• Because the width of the constriction and the inner diameter of the pipe will be relatively constant.
• The frequency is given by the expression

                  F= (St*V)/C*D

• Where
• F= vortex frequency, Hz
• St=strouhal’s number, dimension less
• V=Fluid velocity at the shedder bar, m/s
• D=Inner diameter of the pipe, m
• C=constant (ratio d/D)
• d= Face width of shedder bar, m

• The pressure loss curve across the vortex meter represents a similar sketch compared to an orifice meter.
• The shedder bar will have the minimum pressure compared to orifice meters vena contracta.
• The pressure downstream at this point gets restored progressively which leads to permanent pressure loss.
• This pressure loss at the vena-contracta is essential to prevent cavitation.
• The minimum back pressure required to prevent cavitation is:

P_min=3.2*P_loss + 1.25*P_v

Where

• P_min= minimum pressure required at 5D downstream of the flow meter.
• P_loss= permanent pressure loss
• P_v= vapor pressure

• The value of d/D for most vortex flow meters ranges between 0.22 to 0.26.
• The frequency of vortices is based on the size of the meter, i.e. frequency of vortices is inversely proportional to the size of the meter means the larger the meter lowers the frequency of the vortices and vice versa.
• The control vortex meter having maximum diameter is restricted since the resolution is a major issue.
• To address this issue, onboard digital multipliers are often used, which multiply the vortex frequency without incorporating additional error.

The performance of Vortex meters is influenced by

1. Erosion has been caused due to change in the geometry of the shedder bar.
2. Modification in shedder bar geometry owning to deposits, i.e. Wax corrosion of upstream piping.
3. Modification in shedder bar position if it is not protected fully.
4. Hydraulic noise.

Parts of Vortex Flow Meter

Generally, a vortex flow meter includes

1. Pick-up elements,
2. AC-pre amplifiers,
3. AC-amplifier with filters,
4. Noise abatement features,
5. Schmitt trigger,
6. Microprocessor

Features of Vortex Flow Meters

• The vortex shedding meter generates a linear digital or analog output signal that does not require additional transmitters or converters, attempting to make equipment installation easier.
• Meter accuracy is good across a potentially broad flow range, though this range is reliant on system parameters.
• The shedding frequency is defined by the measurements of the bluff body and, as a physical occurrence, to assure great long calibration consistency and accuracy rate of less than + 0.15%.
• Since this is a fixed frequency system, it doesn’t have the drift.
• Since there aren’t any moving or wearing parts in the meter, it is more reliable and requires less or no maintenance.
• The fact that there are no valves or manifolds to cause leakage problems further reduces maintenance.
• The dearth of valves or manifolds results in a particularly safe installation, which is extremely crucial for hazardous or toxic process fluids.
• If the sensor is quite sensitive, the same vortex-shedding meter can be used on both gas and liquid applications.
• Moreover, if the meter is used for gas or liquid application, the calibration is practically independent of the system parameters such as viscosity, density, pressure, temperature, and so on.
• The vortex shedding meter also has a low installed cost, especially in pipe sizes smaller than 6 in. (152 mm), which compares favorably to the mounted price of an orifice plate and differential pressure transmitter.
• The meter size range is one of the limitations.
• Meters with diameters less than 0.5 inches are unrealistic.
• The meters with diameters greater than 12 inches have limited application due to their high expense compared to an orifice system and their limited output pulse resolution.
• With increasing pipe diameter, the number of pulses generated per unit volume decreases on a cube law.
• As a result, at 10 ft/s (3 m/s) fluid velocity, a 24-inch diameter vortex shedding meter has a general blockage ratio of 0.3 has a full-scale frequency output of about 5 Hz.

Selection and Sizing of Vortex Flow Meters

• The specification of the vortex flow meter must be compared with operating conditions such as
  • The temperature of the process fluid,
  • Ambient temperature,
  • Line pressure
• The meter bonding substances and sensors must be further evaluated for the chemical attack and safety functionality with the process fluid.

• Nonferrous materials, for instance, should always be avoided or contacted with great caution when functioning with O₂.
• The minimum and maximum flow rates of the meter for the given application should then be determined.
• The minimum flow rate of the meter is determined by a
• Reynolds number of 10,000 to 10,500,
• The fluid density
• Minimum acceptable shedding frequency by the electronic component.
• The maximum flow rate is determined by
  • Meter pressure loss with two velocity heads     ,
  • Cavitation onset with liquids, and
  • Sonic velocity flows with gasses     .
• As a result, the flow range for any application is determined solely by the operating fluid viscosity,
  • Fluid density,
  • Vapor pressure
  • The maximum flow rate
  • Line pressure.
• For low-viscosity products like water, petrol, and liquid ammonia, vortex shedding meters can have a rangeability of about 20:1, with an application maximum velocity of 15 ft/s (4.6 m/s) and a pressure loss of about 4 PSIG (27.4 kPa).
• Because of the meter’s high (“of rate”) accuracy and digital linear output signal, it can be used over a wide range of flow rates.
• The rangeability of the process decreases proportionally as its viscosity, density, or maximum flow velocity increases.
• Consequently, these vortex flow meters are not suitable for high-viscosity liquids.

Vortex Meter Advantages

1. Low wear
2. The cost of installation is less
3. Low or no maintenance cost
4. Low sensitivity
5. Highly Stable
6. Long term accuracy
7. Long term repeatability
8. Applicable for a wide temperature range
9. Applicable for a wide variety of pipe sizes
10. Vortex meters can also be employed for liquids, gasses and steam applications.

Vortex Flow Meter Limitations

1. Not suitable for very low flow rates
2. A small length of straight pipe is required upstream and downstream of the vortex meter

Vortex Flow Meter Applications

1. Natural gas metering      
2. The measurement of steam
3. Suspension liquid flow
4. Water applications in general
5. Pharmaceuticals and liquid chemicals

Frequently Asked Questions:

What is the principle of the Vortex Flow Meter?

The Von Karman effect, which says that when fluid flows by a bluff body, a recurring pattern of swirling vortices is generated, is used by vortex flow meters to quantify fluid velocity.

What is the Vortex Shedding Principle?

Vortices are shed conversely from one side to the other when the wind is blowing across a structural component.

How is vortex flow measured?

Several factors affect the formation of vortices frequency, & shedding,

1. The velocity of the fluid (υ),
2. Width of the shedder (d),
3. Reynolds number (re).

• Vortex flow is measured by the relationship of velocity with flow and frequency
  • f = S × υ/d,
• where S is the Strouhal number.

What are the different types of vortex shedding?

Vortex shedding depends on forcing frequency and amplitude, the types are

1. Anti-symmetric,
2. Symmetric, or
3. Chaotic.