Vortex flow meter

Vortex flow measuring system

The vortex flow meter (VDM), also known as vortex flow meter, is a flow meter for determining volume or mass flows based on the Kármán vortex street .

Vortex flow meters are among the standard measuring devices for determining the volume flow of liquids, gases and vapors today. Typical applications are the use in saturated steam, superheated steam, liquid and gaseous hydrocarbons, demineralized water, liquid and gaseous ammonia and in gases such as nitrogen, oxygen, chlorine, air etc. Vortex flow meters can also be used in non-conductive media and provide a Addition to the electromagnetic flow meter (MID).

Using vortex flow measurement (also known as vortex measurement), volume flows of gases, vapors and liquids can be determined. To determine mass flows, a temperature and pressure-dependent correction is necessary, with many device types having an integrated temperature measurement.

history

Story Vortex

Leonardo da Vinci (* 1452; † 1519), especially famous as a painter, was not only concerned with art in his time. As the engineer responsible for the waterways of the Po Valley, he was interested in erosion in the river bed and how to avoid it. He observed the flow around various obstacles, such as bridge piers and the various types of vortex systems that formed. So he tried, as in the Codex Leicester , to analyze the behavior of water and researched the currents in rivers as well as the formation of eddies. In 1513 he recorded the vortex behavior for the first time. His aim was to describe and systematize the various flow forms.

His note bears the note in the margin: (Quote) "Observe the movement on the surface of water, which resembles that of hair, which has two types of movement; one depends on the weight of the hair, the other on the direction of the curls; so forms swirling eddies in the water, part of which is caused by the main flow and the other by the secondary flow and reflux. " (At that time Leonardo used reflux to refer to the flow against the main flow)

Through his observations , detailed studies and drawings, Leonardo da Vinci found a way to vividly describe turbulent currents in his manuscripts and to record them for posterity.

The so-called vortex streets were then calculated by Theodore von Kármán for the first time in 1912 and formed the basis for today's measurement technology.

In 1970 the first industrially usable vortex meters came onto the market as measuring sensors with an inlet section of 5 to 10 times the length of the pipe, which was necessary in order to achieve a useful accuracy.

Inlet sections above from 1970 to below 2008

Due to the piezo ceramic sensors used from 1983 onwards, the vortex vibrating body could not be exposed to any impacts.

In 1986 Endress + Hauser launched the first vortex device with a capacitive sensor and vibration compensation.

Measuring principle

The measuring principle is based on the Kármán vortex street, with opposite vortices occurring behind a body in the flow. This fact is exploited in the vortex flow measurement by in a media-carrying pipeline, usually in a special measuring tube, a bluff body brings behind which the named vortex street is formed. Since the eddies run in opposite directions and offset from one another, local pressure differences are formed which can be recorded by a corresponding sensor . The sensor determines the so-called eddy frequency by counting the pressure pulses occurring per unit of time,

${\displaystyle f={\frac {v}{d}}\ Sr}$

where is the flow rate of the medium through the pipeline, the characteristic dimensions of the bluff body and the Strouhal number . The Strouhal number depends, among other things, on the geometry of the bluff body and the Reynolds number of the flowing medium. About the flow velocity ${\displaystyle v}$ ${\displaystyle d}$ ${\displaystyle Sr}$

${\displaystyle v={\frac {f\cdot d}{Sr}}\ }$

can be adjusted to the volume flow

${\displaystyle {\dot {V}}={\frac {\pi \cdot D^{2}\cdot f\cdot d}{4\cdot Sr}}}$

or temperature and pressure dependent on the mass flow

${\displaystyle {\dot {m}}=\rho (T;p){\frac {\pi \cdot D^{2}\cdot f\cdot d}{4\cdot Sr}}}$

conclude. If all constants are combined into a proportionality constant , the linear relationship between volume flow and vortex frequency becomes apparent. ${\displaystyle K}$

${\displaystyle {\dot {V}}=K\cdot f}$

Sensors

Sensor function

The vortex flowmeters available on the market differ greatly in terms of the sensor that records the frequency of the vortex breaks. Essentially, a distinction must be made here between the use of pressure sensors that detect the frequency directly on the basis of the pressure fluctuations (implemented e.g. with capacitive sensors, membranes or piezo elements), and strain gauges that are set into oscillation by the vortices, which corresponds to the eddy frequency or thermistors, which are periodically cooled to different degrees by the eddy (the evaluation is then usually carried out in a bridge circuit ).

Applications

Steam measurement

Vortex flow measurement is used in many industries, including the petrochemicals , energy technology , heat supply , pharmaceuticals, paint production, agrochemicals and cosmetics / health care as well as the food industry. One of the main uses of the Vortex flow meter is steam (mass flow) measurement.

In addition, this type of sensor is used in the field of tank combat. The US-American "M1 Abrams" battle tank uses a vortex flow measurement to determine the wind transverse to the direction of action of its projectiles, which is used in the ballistics computer to correct the angle of the tube.

advantages

There are advantages over measuring devices such as turbines, diaphragms or pitot tubes:

• lower installation costs,
• low pressure loss,
• large measuring range dynamics up to 45: 1,
• fast display of measured values ​​(after less than half a second)
• small errors (0.75% of the measured value for liquids and 1.00% of the reading for gases)
• Medium properties such as density and viscosity have no influence on the measurement accuracy with Reynolds number: Re> 20000.
• Can be used in a wide temperature range from −200 ° C to +400 ° C.
• Ex versions

Application limits

In the case of vortex flowmeters, the following problem areas must essentially be named, which limit the possible uses of the vortex devices. The first problem point concerns the relatively high susceptibility of the devices to soiling. Depending on the functional principle of the sensor, dirt-laden media or media that tend to crystallize influence the measurement result in different ways. Especially in the case of measurement setups with bores in the bluff body, blockages in thermoplastic media such as B. bitumen come. When the process is stopped, residues can harden and prevent movement of the sensor elements. Furthermore, impacts of the particles against the bluff body can falsify the measurement result.

The second problem point relates to the sensitivity of the vortex flowmeters to vibrations in the system, which result in unstable measurement results. Vibrations can be caused, for example, by pumps that are installed before or after the flow meter.

Another application limit results from the minimum flow velocity required to form measurable eddies. This results in a measuring range that is always greater than zero. This means that in order to be able to detect a stagnant or very slowly flowing medium, further separate measures are necessary.

Another limitation, which applies to the majority of all flow measurements, is that the measurement is restricted to one flow direction. In order to be able to measure the opposite flow direction, another device is required.

Individual evidence

1. ↑ Uni-Frankfurt Retrieved May 22, 2019
2. ↑ Operating instructions from the early days of the vortex counter ( Memento from August 27, 2008 in the Internet Archive )
3. ↑ Messages from the Max Planck Institute for Flow Research and the Aerodynamic Research Institute ISSN  0374-1257
4. ↑ Annual reports of the Bavarian Research Foundation. Retrieved March 23, 2018 . 
5. ↑ Crosswind Sensors for Military use by J-TEC Associates | J-TEC Associates. Retrieved February 23, 2019 . 

swell

• Herbert Oertel, Martin Böhle, Ulrich Dohrmann; Fluid mechanics; Basics, basic equations + solution methods; 4th edition from 2006, Vieweg + Teubner Verlag | ISBN 3-8348-0206-9

Web links

• 3D animation of the vortex meter flow measurement principle