Vortex flowmeters consist of several components, including the measuring tube, the solid body, the sensor, the preamplifier and electronic counter (see Figure below). In most counters, the sensors have no moving parts, so do not wear out or require any maintenance.

The two most common Vortex flow meters are flanged and non-flange. These latter are known as accountants for installation between flanges (type "sandwich" or "wafer") and are designed to be installed between two pipe flanges. Some models for installation between flanges have a standardized total length of 65 mm (2.5 "), which allows them to directly replace disk groups full stops.

The field measured values ​​on the market standard covers nominal sizes from DN 15 to 300 (from 1 / 2 to 12 "), and some versions reach up to DN 400 (16"). Pressure ranges can reach up to PN 250 (ANSI Class 2500). The vortex shedding frequencies for nominal sizes above DN 300 (12 ") are very low and require some signal processing to achieve a stable signal. For applications with large diameters, Vortex flow meters are relatively expensive compared with the disc diaphragm. Many manufacturers also offer models for very high temperatures or very low (-200 to +400 ° C / -330 to +750 ° F).

Devices with two independent sensors and electronics are a special case (Fig. below). This model is mainly used in industries where redundant measurements are considered important.

Ill.: "vortex flowmeters Prowirl" E + H (two-wire device). Left: Flange. Center: Version "wafer" for installation between flanges. Right: bidirectional version with two sensors and electronic

Sensors for measuring

To measure the local fluctuations in the flow pressure caused by vortices generated in the solid body electric signals and convert several types of sensors. Each manufacturer recommends his favorite type of sensor from the wide range of existing, including capacitive sensors, piezo-resistive, ultrasonic, thermistors, pressure and mechanical stress. In most cases, the sensor is integrated on the same solid body or is located immediately behind. Today, most of today's sensors measure the vortex shedding capacitive sensors or piezo-electric.

DSC sensor (differential switched capacitor):

The DSC sensor (differential capacitance switches) using E + H consists of a shovel-shaped sensor that penetrates inside the solid body (Fig. below). This shovel (a) transmits pressure fluctuations due to the vortices to an electrode sleeve-shaped central (c) together with the external electrode (d) make the capacitors C1 and C2 semilaminares. A variation in the extent of separation of the two plates causes a change in capacitance proportional to the pressure differential caused by the vortex, which changes regularly and is processed by the electronic counter. These measurement systems are highly insensitive to vibration of the pipe by the fact that the sensor be in mechanical equilibrium.

Figure 57: Design of a DSC sensor E + H. a = Sensor Pala, b = focal point of the sensor system, c = central electrode, d = external electrode

Here are the main advantages of the DSC sensors:

- They are resistant to extreme temperature variations, for example, in cryogenic applications or vapor treatment systems. The stainless steel DSC sensors have no moving parts or components overly sensitive, making them extremely robust.

- They are resistant to the pulsating flow bursts, for example in steam systems.

- Insensitive to vibration of the pipe. The vibration accelerations have no effect on the distance between the central electrode and outer electrodes. The sensor blade and the center electrode are in relative balance and acceleration forces due to vibrations act upon the center of gravity of the sensor system, so that vibrations do not generate additional signals.

- Are quite insensitive to the presence of foreign bodies, because the DSC sensor is mounted freely in the measuring tube. In the worst case, depositions on the blade itself sensor could cause a slight reduction (turndown) of field measured values, but would not affect the accuracy of the measurement.

Advantages and disadvantages

Advantages:

- Are universally applicable for measuring volumetric flow rates of steam, liquids and gases.

- They are practically insensitive to changes in pressure, temperature and viscosity.

- Installation is simple.

- Offer a wide range of nominal diameters from DN15 (1 / 2 ") to 300 (12") and up to DN450 (18 ") on request.

- They have a large reduced field, typically from 1:10 to 1:30 for gas / vapor or liquid 1:40.

- The losses are low (typically 30 mbar).

- No moving parts.

- Wide temperature range: -200 to +400 ° C (-330 to +750 ° F).

- The linear behavior of the frequency is independent of process conditions and fluid.

- They have a high long-term stability (the parameter K is maintained throughout life), no deviations from zero experience.

- The measuring accuracy can reach values ​​as good as ± 0.75% fluid vl, y + l ° / o vi gases (Re> 20,000).

- The reproducibility is between 0.2 and 0.3%.

Disadvantages:

- Pulsating flows and the vortices negatively affect measurement accuracy.

- Depending on the type of accessory that may be upstream sections are needed for entry and release times.

- Not used for highly viscous fluids.

- They can not measure too low fluid velocities (Re <4,000).

Common Applications

Vortex flowmeters are used in many areas of industry to measure volumetric flow rates of steam, liquids and gases. These counters are increasingly common in applications that previously had differential pressure flowmeters, such as disk diaphragm. This trend holds even for two reasons: Vortex flow meters are easier to install and also have a reduced field (turndown) wider.

Steam Flow Measurement:

Since the early eighties of the twentieth century, Vortex flowmeters have become very popular, particularly in all industrial sectors of steam flow measurement. Vortex flowmeters measure volumetric flow only, but steam systems also often carry a certain amount of mass and energy content, so that these counters are often used in combination with a pressure sensor and / or temperature and energy flow meter .

Figure: Application of steam. Photo: Vortex flow in pipe insulation.

Liquid Flow Measurement:

In contrast to magnetic meters, Vortex flow meters for determining the non-conductive fluid flow or only slightly drivers as oil or demineralized water, condensate or feedwater of a boiler. Can also be used under high pressures and temperatures much higher than magnetic meters.

Gas flow measurement:

In applications of this nature, Vortex flowmeters are widely used in flow measurement of compressed air, natural gas or individual components of air such as nitrogen, oxygen, carbon dioxide, hydrocarbons. .