Working principle and advantages of intelligent vortex flowmeter

Working principle of intelligent vortex flowmeter:

Intelligent vortex flowmeter uses the principle of fluid oscillation to measure flow. When the fluid passes through the vortex flow transmitter in the pipeline, two vortices proportional to the flow velocity are generated alternately up and down behind the vortex generator of the triangular column. The release frequency of the vortex is related to the average velocity of the fluid flowing through the vortex generator and the characteristic width of the vortex generator, which can be expressed by the following formula:

f=Stv/d

Where: f is the release frequency of the vortex, Hz; v is the average velocity of the fluid flowing through the vortex generator, m/s; d is the characteristic width of the vortex generator, m; St is the Strouhal number, dimensionless, and its value range is 0.14-0.27. St is a function of the Reynolds number, St=f(l/Re).

When the Reynolds number Re is in the range of 102~105, the St value is about 0.2. Therefore, in the measurement, the Reynolds number of the fluid should be between 102~105 as much as possible, and the vortex frequency f=0.2v/d.

It can be seen that by measuring the vortex frequency, the average velocity v of the fluid flowing through the vortex generator can be calculated, and then the flow rate q can be calculated by the formula q=vA, where A is the cross-sectional area of ​​the fluid flowing through the vortex generator.

What are the advantages of intelligent vortex flowmeter:

1. Easy to install and maintain, unlike ordinary differential pressure flowmeters, the longer pressure pipeline is easy to leak, block, and freeze;
2. Output a pulse signal proportional to the flow, and then use digital technology to process it without losing accuracy;
3. There is no zero drift problem;
4. Reasonably determine the caliber to obtain a wider measurement range.
5. It can measure high-temperature, high-pressure liquid and gas media.
6. It can measure corrosive media.

The minimum caliber of the vortex flowmeter currently available on the market is DN15. The maximum caliber, domestic products have DN600, but after the caliber is larger, the resolution is lower, so foreign instrument companies generally only provide products of DN300 and below. Vortex flowmeters are still difficult to use. Pipes or industrial sites where flowmeters are installed will inevitably vibrate, and some vibrations are quite strong. For example, in some compressor and blower plants, not only the pipes vibrate violently, but even the entire plant and the roads around the plant vibrate violently.

Intelligent vortex flowmeter is a fluid oscillation instrument. Its working principle is that after the fluid flows through the vortex generator, vortices proportional to the flow rate are generated. The number of vortices is detected by the sensor, and pulses equal to the number of vortices are output. When the pipeline or environment vibrates, this vibration also acts on the sensor to generate a corresponding electrical signal. When the amplitude of this electrical signal is large enough and higher than the resolution threshold, it is mixed in the electrical signal generated by the vortex, and is sent to the output end after amplification and shaping, making the output frequency higher. If the actual flow is zero, it appears to be "creating something out of nothing".

The vibration resistance of the vortex flowmeter is related to its own quality. The vortex flowmeters manufactured according to different principles have different vibration resistance. Generally speaking, the vortex flowmeters made with capacitive sensors are superior to those made with piezoelectric sensors. The vibration resistance of vortex flowmeters of different brands, or even the same brand but with different calibers and models, is also different. The vibration resistance

of the intelligent vortex flowmeter is also related to its installation position. Although the instrument manual says that the vortex flowmeter can be installed vertically or horizontally, it is found in practice that the vibration resistance of the instrument installed horizontally (that is, the generator is in a horizontal position) is significantly reduced. The vibration resistance of the

intelligent vortex flowmeter is poor when the flow rate of the measured fluid is very low and zero flow. This is because the thrust of the vortex on the sensor is proportional to the square of the flow rate of the fluid in the pipeline. When the flow rate of the measured fluid is very low and zero flow, the thrust of the vortex on the sensor is small, the signal amplitude generated is low, so the signal-to-noise ratio is also small, and the signal generated by vibration is more significant.

The vibration resistance of the intelligent vortex flowmeter is also related to the density of the medium it measures. Because when the pipeline vibrates, the sensor vibrates with the pipeline. When the measured medium is a liquid, the medium surrounding the sensor is a liquid with a high density, which has a good damping effect on the sensor itself. Therefore, the amplitude of the electrical signal generated by the sensor is small. If the medium surrounding the sensor is a gas, the amplitude of the electrical signal generated by the vibration is large due to poor damping.

In order to improve the vibration resistance of the vortex flowmeter, people have conducted long-term research and taken some measures, such as the use of dual sensor method, spectrum signal processing (SSP) technology, etc., which have achieved certain results. However, in places with strong vibration, the application of vortex flowmeters is still difficult.

When the intelligent vortex flowmeter is used to measure gas and steam flow, the minimum measurable flow rate is subject to the density of the fluid. This is because the vortex generated by the vortex generator of the measured fluid acts on the sensor probe, and accordingly generates the same number of electrical pulses. The amplitude of this electrical pulse is related to the thrust generated by the vortex on the probe. The greater the thrust, the greater the pulse amplitude, and this thrust is proportional to the square of the flow velocity and the density of the fluid. When the density is small to a certain extent and the flow velocity is low, the thrust is too small, so that the pulse amplitude is correspondingly too small, resulting in being submerged by noise, and finally, the pulse cannot be reliably detected. However, if the flow velocity is high, the thrust can still be increased to a large enough level, so that the pulse amplitude increases to a large enough level, and finally it can still be reliably detected.

Some measured fluids, such as hydrogen and semi-water gas, have low densities. Some low static pressure and low flow velocity measurement objects, such as city gas, have low flow velocities in the pipeline, and the flow velocity cannot be increased by local reduction. Be very careful when designing and selecting. When the density of the measured fluid is known, the minimum measurable flow rate can be obtained by calculation. Some manufacturers have tested their own products and provided useful data on products of different calibers. (end)