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Application of Vortex Flowmeter in Trade Measurement

2022-12-22 14:25:24

Due to the rising energy prices, the cost of energy consumption has become one of the main operating costs of many industrial enterprises, and the measurement of energy has increasingly become the focus of attention of society. The reasonable allocation of energy-measuring instruments and the accuracy of measurement are closely related to the vital interests of enterprises. As the original signal of the vortex flowmeter is a digital signal, it has the advantages of high measurement accuracy, simple compensation calculation, and easy analysis in case of abnormal conditions. It has a place in the trade measurement of energy. This paper introduces the measurement principle of the vortex flowmeter, analyzes the advantages of vortex flowmeter in trade measurement, and points to attention when vortex flowmeter is used in trade measurement.

A vortex flowmeter is a flowmeter based on the Karman Vortex Street principle, which is used to measure liquid, gas, and steam, as well as turbid liquid containing tiny particles and impurities. It is widely used in petroleum, chemical, pharmaceutical, paper making, metallurgy, power, environmental protection, food, and other industries.

Working principle of vortex flowmeter

The vortex flowmeter is based on the Karman vortex street theory: when the fluid flows through a non-linear triangular cylinder retainer, regular vortex signals will be generated alternately on both sides of the retainer, and the separation frequency of the vortex is proportional to the flow velocity of the fluid.
According to the principle of Karman vortex street, there is the following relationship:

M - the ratio of an arch area on both sides of the vortex generator to the cross-section area of the pipeline (dimensionless);
D - drift diameter of meter body (unit: m);
D - frontal width of vortex generator (unit: m);
F - Frequency of vortex occurrence (unit: Hz);
U1 - average velocity at both sides of vortex generator (unit: m/s);
Sr - Strouhal number (dimensionless);
U - average velocity of the incoming flow of measured medium (unit: m/s);
Qv - instantaneous volume flow (unit: m3/s);
K-instrument coefficient of flowmeter (unit: m-3)

It can be seen from the formula that the size of the average instrument coefficient is only related to the geometric size of the vortex flow sensor, that is, the vortex diameter, the width of the barrier, and the Strouhal number Sr. When Reynolds number Re is at 2 × 104~7 × 106, Sr can be regarded as a constant. That is to say, for a vortex flowmeter whose diameter and the geometric dimension of the barrier have been determined, its instrument coefficient is constant within a certain flow velocity range.

Characteristics of vortex flowmeter

1) It can measure the flow of liquid, gas, and steam;
2) The output is pulse frequency, which is proportional to the actual volume flow of the fluid to be measured, and is not affected by the fluid composition, density, pressure, and temperature;
3) Simple structure, no mechanical moving parts, stable and reliable long-term operation;
4) The measuring range ratio is moderate, the measuring range ratio of ordinary sensors can reach more than 10 ∶ 1, and the measuring range ratio of intelligent vortex flow can reach 40 ∶ 1;
5) The pressure loss is small, about 1/4~1/2 of the orifice flowmeter.

Advantages of vortex flowmeter in trade measurement

1) Vortex flow signal is in digital frequency mode, with high measurement accuracy

The vortex separation frequency of the vortex street flowmeter is proportional to the volume flow, and the separation frequency is not affected by the fluid composition, density, pressure, and temperature. As long as the number of vortices is measured, the volume flow can be accurately calculated. The measurement of the number of vortices is a purely digital concept, and there is no error in the measured values. The vortex street flowmeter has the characteristics of accurate flow signals within the full range, which is incomparable to other analog flowmeters.

2) Simple compensation operation, high calculation accuracy, and few unknown parameters

The volume flow of the vortex street flowmeter can be obtained by measuring the vortex separation frequency. The steam density, gas density, and compressibility coefficient can be compensated and calculated by measuring the temperature and pressure. The calculation formulas are all national standards or international formulas with high accuracy. Compared with the differential pressure flowmeter, the calculation results of the differential pressure flowmeter are affected by the changes in density, viscosity, isentropic index, compressibility coefficient, gas expandability coefficient, and structure size from thermal expansion to cold contraction. The compensation calculation is more complex, and many differential pressure flowmeters do not have the compensation calculation formula for the gas expandability coefficient. Therefore, the measurement results will have a certain deviation.

3) When the instrument measurement is abnormal, it can quickly analyze and judge

Since the Karman vortex street flow signal is reflected by the vortex frequency value, under normal conditions, the vortex separation frequency is stable, and only when the sensor is abnormal or the working condition changes, the vortex separation frequency will have a great change. Therefore, by observing the frequency change range of the vortex street flow sensor, it is very convenient to analyze whether the sensor has faults, There will be no measurement error that is difficult to find due to zero shift or analog conversion error when analog sensors are used.

Observe the waveform of the vortex flowmeter after amplification and shaping. If it is a uniform square wave signal, it is a normal flow change; If there is obviously an uneven square wave signal, it may be caused by external interference, out of measurement range, or sensor failure.

Precautions for Vortex Flowmeter in Trade Measurement

In addition to the conventional type selection design, the following points should also be noted when a vortex flowmeter is used for trade measurement:

1) Influence of Steam State Change on Flow Measurement

When measuring steam, after the steam is transported through valves or long pipes, the steam state may change:

(1) Superheated steam becomes saturated steam

In the process of pipeline transportation, superheated steam will produce heat loss, that is, temperature reduction. When the temperature drops to a certain extent, it will enter the critical dry saturation state or supersaturation state from the superheated steam state.

If it enters the dry saturation state, there will be no metering error theoretically. Because the dry saturated steam still belongs to the category of steam, the temperature and pressure double compensation method is generally used for measuring superheated steam, which is also applicable to the density compensation of superheated steam and dry saturated steam.

If it enters the supersaturated state (i.e. wet saturated state), the humidity will exist in the form of water droplets and mist, which will inevitably affect the density of the medium. However, the steam density compensation algorithm is only applicable to the measurement of dry steam, so the greater the humidity (the smaller the dryness), the greater the error. Generally, when the dryness is ≥ 95%, the error is small; When the dryness is less than 95%, the error is too large.

① Shorten the distance of the transmission pipeline;
② Thermal insulation of pipeline shall be well done;
③ When the transmission pipeline is too long, the instrument installation pipeline shall be raised, or the liquid discharge device shall be added.

(2) Saturated steam becomes superheated steam

The ideal saturated steam state is that the temperature, pressure, and density are in one-to-one correspondence. Knowing one of the parameters, the other two can be calculated. Therefore, the measurement of saturated steam generally adopts a single compensation method, that is, a single temperature compensation or a single pressure compensation. However, when the saturated steam is greatly depressurized through the pressurizing valve, it may change from a saturated state to superheated steam. At this time, if the density error calculated by single temperature compensation is large, the density error calculated by single pressure compensation is relatively small; If temperature and pressure compensation modes can be selected, no error will occur.

The saturated steam (1 MPa, 184.1 ℃) at the boiler outlet of the chemical plant changes into the superheated steam state (0.42 MPa, 162.4 ℃, density 2.7048 kg/m3) after passing through the pressure-reducing valve. The temperature compensation error is as high as 27.61%, while the pressure compensation error is only 2.60%.

① The flowmeter is installed upstream of the pressure relief device;
② If it is installed at downstream of the pressure relief device, try to use the temperature and pressure double compensation mode or the single pressure compensation mode.

2) Select a vortex flowmeter with a built-in high-precision standard algorithm

The software algorithm will greatly affect the measurement accuracy of the vortex flowmeter. For example, steam density and gas compressibility are key parameters that affect the calculation accuracy.
There are two calculation methods for steam density: the look-up table method and the formula method. The formula method in the two methods has high relative accuracy and can judge the saturated and overheated areas of steam; The coverage area of the table lookup method is limited, and there is no accurate limit for the steam saturation and overheating area.

The compressibility coefficient of gas also varies with the working conditions. If the default value is 1, there will be some deviation in the calculation results. Therefore, when measuring gas, especially natural gas, a vortex flowmeter with a compression coefficient compensation function should be selected.


A vortex flowmeter is one of the most suitable flowmeters for trade measurement at present because the stability of vortex frequency output by vortex street can directly judge whether the flowmeter is abnormal, the pressure loss is small, and the installation is convenient. In long-term use, the meter body structure deviation produced by the vortex flowmeter has little impact on the instrument coefficient, so it can be used as a measuring instrument for a long time.

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