Vortex Flow Meter

Working Principle of Vortex Flow Meter

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. Hence,

Major components of a vortex flow meter include

A bluff body strut-installed across the flow meter bore
A sensor to indicate the presence of the vortex and to produce an electrical impulse
A signal amplification and conditioning transmitter which gives an output proportional to the flow rate

Vortex flow meter Main Features

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 vortex shedding frequency is the vibrating frequency of the vortex shedding. It is the frequency which is directly proportional to the velocity of the liquid flowing in the pipe, and hence to volumetric flow rate. It is independent of fluid properties such as density, viscosity, conductivity, etc., except that the flow must be turbulent for vortex shedding to occur. The basic relationship between vortex shedding frequency and fluid velocity is given below:

St = f(d/V)

In the above equation, St represents the Strouhal number which is a typical dimensionless calibration factor used to differentiate a variety of bluff bodies.

Other parameters are

F = vortex shedding frequency

d = Width of the bluff body

V = Average fluid velocity.

The Strouhal number is generally defined as the ratio of the interval between vortex shedding (l) and width of the bluff body (d). If the Strouhal number of two different bluff bodies is equal, then they will work in the same manner.

Installation

If the vortex flowmeter will be installed in a vertical orientation

Install upward or downward flow for gas or steam. Install upward flow for liquids. For steam and fluids with small solids content, it is recommended to have the flowmeter installed with the electronics to the side of the pipe.

Size Ranges Available :- 0.5 to 12 in. (13 to 300 mm).

Design Pressure :-2000 PSIG (138 bars)

Design Temperature :- − 330 to 750 ° F ( − 201 to 400 ° C)

Materials of Construction:- Mostly stainless steel, some in plastic.

Inaccuracy :- 0.5 to 1% of rate for liquids, 1 to 1.5% of rate for gases and steam with pulse outputs; for analog outputs, add 0.1% of full scale.

Price :- Plastic and probe units cost between $250 and $1500; stainless steel units in small sizes cost about $2500; insertion types cost about $3000

Advantage

The vortex flowmeter has no moving parts, and the measuring component has a simple structure, reliable performance and long service life.
The vortex flowmeter has a wide measuring range. The turndown ratio can generally reach 1:10.
The volumetric flow rate of the vortex flowmeter is not affected by thermal parameters such as temperature, pressure, density or viscosity of the fluid being measured.
It measures the flow of liquids, gases or vapors, has very wide applications
It causes little pressure loss.
High accuracy, and low maintenance.

Disadvantages

Low to medium pressure drop due to the obstruction in the flow path
Not suitable for very low flow rates

Limitations

It has poor anti-vibration performance. External vibrations can cause measurement errors in the vortex flow meter and may not even work properly.
The high flow velocity shock of the fluid causes vibrations in the vortex body, which reduces the measurement accuracy.
Cannot measure dirty media
Straight pipe requirements are high when mounting the vortex flow meter
Not suitable for low Reynolds number fluids measurements;
Low meter factor (compared to turbine flow meter)
It is not suitable for the pulsating flow.

Vortex Flow meter technical specification

Measurement Fluid :- Liquid, gas, saturated steam, superheated steam (avoid multi-phase flow and adhesive fluid)

Process Temperature :- -29 to 250 °C

Process Pressure :- -0.1 MPa according to flange rating

Diameter

Wafer :- 15 to 100 mm
Flange :- 15 to 400 mm

Accuracy

Liquid :- ± 0.75 % of reading (depends on Reynolds number)
Gas, Steam :- ± 1.0 % of reading (depends on the flow velocity)

Output Signal :- Analog output, transistor contact output (simultaneous output possible)

Communication Type :- HART 7 / HART 5, BRAIN, FOUNDATION fieldbus

Explosion Protected Type :- TIIS Ex d, FM Ex d / Ex ia, ATEX Ex db / Ex ia, CSA Ex d / Ex ia, IECEx Ex db / Ex ia

Vortex Flow meter Installation Guide

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 if it is to give good accuracy and repeatability. To achieve good accuracy and repeatability it is standard practice to place the meter some distance from sources of turbulence. Common sources of turbulence include pumps, valves, changes in line direction (i.e. bends), changes in line size etc.

How are flow meter straight lengths defined?

Most manufacturers provide the user with minimum distances for their particular products. These distances, referred to as straight lengths, are indicated in Pipe Diameters (D). For example, 10 D means place the flow meter ten times the pipe’s internal diameter away from the source of turbulence. Because turbulence both upstream and downstream can reduce accuracy, manufacturers provide straight length requirements for both upstream and downstream of the meter.

Different manufacturers claim differing requirements, with fewer straight lengths being marketed as an advantage for the end user. Ideally the flow transmitter should be sited with as many upstream and downstream straight pipe lengths as possible, preferably more than recommended by the manufacturer, but definitely not less.

Obtaining the necessary straight lengths can be difficult, especially in compact plants. Therefore it is worth remembering that Vortex meters can be installed vertically, horizontally, or at any angle, as long as they are kept flooded. So if installing vertically try to have upward flow.

If you are compensating for pressure and temperature then allow 3 to 4 pipe diameters between the meter and downstream pressure taps, and thermowells should be small and located 5 to 6 D downstream of the meter. As a general rule of thumb straight lengths should be about the same as that required for an orifice installation with a beta ratio of 0.7 – see the table below

Vortex Flow meter Manufacturer

Yokogawa
ABB
Emerson
Bopp-Reuther
Krohne

Applications

Custody transfer of natural gas metering
Steam measurement
Flow of liquid suspensions
General water applications
Liquid chemicals & pharmaceuticals

Vortex Flow Meter working principle and applications