What are the performance of acetylene flowmeters in the measurement of different flow rates and what causes the flow limitation problem?

Do you know why there are different performance situations in different flow rate measurements and why the flow rate is limited when using acetylene flow meters? The editor has provided an analysis and details on the following points for everyone. Let's take a look together and see what it really is?

At present, there are many types of flow meters in industrial production, which work based on different measurement principles, such as mechanical principles, thermal principles, electrical principles, and optical principles. Acetylene flow meter is a volume flow meter that uses the Karman vortex street principle to measure the volume flow rate, standard volume flow rate, or mass flow rate of gas, steam, or liquid. Mainly used for flow measurement of industrial pipeline medium fluids, such as gases, liquids, steam, and other media. The characteristics are small pressure loss, large range, high accuracy, and almost unaffected by parameters such as fluid density, pressure, temperature, and viscosity when measuring the volumetric flow rate under working conditions. No movable mechanical parts, therefore high reliability and low maintenance. The instrument parameters can be stable for a long time. The acetylene flow meter adopts a piezoelectric stress sensor with high reliability and can operate within the working temperature range of -20 ℃ to+250 ℃. There are analog standard signals and digital pulse signal outputs, which are easy to use in conjunction with digital systems such as computers. It is a relatively advanced and ideal measuring instrument. Acetylene flow meters have significant advantages in inherent principles, structure, installation and maintenance, operating costs, and energy consumption, making them one of the best choices for gases and low viscosity liquids at present. However, acetylene flow meters have technical difficulties that need to be overcome in certain aspects. Next, we will mainly explore some limitations in the detection of acetylene flow meters under the conditions of low flow rate and high flow rate, and propose some solutions and implementation effects.

1. Advantages and disadvantages of acetylene flow meters

Acetylene flow meters are superior to traditional orifice flow meters in many applications. For example, there are about 20 static sealing points in one measuring circuit of an orifice flowmeter. In comparison, the static sealing points of acetylene flow meters are only three, making them less prone to leakage. They are not affected by fluid temperature, pressure, density, etc., and the flow coefficient remains unchanged for a long time. However, there are also some issues with the use of acetylene flow meters.

1) Due to the fact that the original signal of the acetylene flowmeter is a frequency signal, the acetylene flowmeter is actually a digital instrument. As long as it can work properly, its accuracy must be guaranteed. But once it cannot work properly, the measurement error generated will be very large, and even the trend of flow rate changes cannot be indicated, completely unable to work.

2) The lift of a vortex is directly proportional to the square of the flow rate and directly proportional to the density of the fluid. Therefore, when the flow rate decreases, the vortex signal sharply weakens in a second-order relationship, while the vortex signal of gas is much lower than that of liquid. When used for gas flow detection, the vortex signal is weak due to low density and flow rate, which can easily be submerged in interference. The flow meter cannot correctly identify the vortex, resulting in measurement failure.

3) Due to the sensitivity of acetylene flow meter sensors to detect small vortex lift forces at low flow rates, the structure of the sensor is directly limited. In response to the above issues, some discussions will be conducted below.

2. Working Principle and Structure of Acetylene Flowmeter

2.1 The working principle of acetylene flow meters is often seen in daily life, such as the phenomenon of vortex street. For example, a flag in the wind swings due to the vortex street generated by the flagpole. The stronger the wind, the faster the flag swings - the oscillation frequency is proportional to the wind speed. The design of bridge piers, chimneys, and tall buildings also needs to consider the destructive force of vortex streets. Acetylene flow meter refers to the principle of vortex street phenomenon in daily life, by inserting a column of appropriate size and shape (i.e. vortex street generator) into the pipeline. When the fluid flows through, alternating vortices are generated on both sides of the vortex street after the body occurs, and this type of vortex is called the Carmen vortex. The frequency of the vortex is directly proportional to the flow rate. It can be represented by the following equation: F=stv/d In equation (1), f is the frequency of the vortex; V is the average velocity of the fluid flowing through the vortex street generator; D is the width of the flow surface of the vortex street generator; St is the Strouhal number, with a range of values ranging from 0.14 to 0.27. When measuring, it is generally assumed that St=0.2. From this, by measuring the frequency of the vortex, the average velocity v of the fluid flowing through the vortex street generator can be calculated, and then the flow rate q can be calculated from the following equation: Q=vA. Among them, A is the cross-sectional area of the fluid flowing through the vortex generating body.

2.2 Structure of acetylene flow meter

The basic structure of acetylene flow meters consists of two parts: sensors and converters. The sensor includes vortex generator, detection components, etc; The converter includes an amplification circuit, a filtering and shaping circuit, and a D/A conversion circuit; The common types of vortex street generators include cylindrical, T-shaped, quadrangular, and triangular columns. Currently, it is widely used, and the feedback is good with the triangular column type vortex generator. The detection components include piezoelectric wafers, thermistors, ultrasonic waves, and strain gauge differential capacitors. The converter part is basically intelligent, and all microprocessor chips are installed in it. Vortex flow can be directly installed on pipelines, with strong interchangeability, small volume, and high long-term operation accuracy, making it suitable for measuring most liquids, vapors, and gases.

3. The limitation of small flow and low flow rate measurement is based on the principle of acetylene flow meters. The strength of the flow signal is proportional to the square of the flow rate, that is, when the flow rate decreases, the vortex signal will sharply decrease in a square relationship.

Figure 2 shows the waveform recording of the vortex street signal when the flow rate increases from zero. Under the same conditions, the vortex force generated by a gas flow rate of 1m/s is only 1/25 of that at a flow rate of 5m/s. To ensure the detection of small flow rates, it is necessary to have extremely high eddy vibration detection sensitivity, amplifying the flow signal thousands of times, which makes the acetylene flow meter extremely sensitive to the vibration of steam pipelines. When there is no flow rate, the actual indication is a vibration interference signal, which is a major problem in the practical application of acetylene flow meters. The detection component of the acetylene flow meter uses piezoelectric chips to detect the frequency f of the vortex, thereby obtaining a voltage signal. This voltage signal needs to go through an amplification circuit and trigger device to convert the vortex frequency * * * into a pulse signal that the instrument can display. This pulse signal is sent back to the conversion instrument device to convert it into the measured flow rate that can be displayed. Among them, the amplification factor A of the amplifier and the threshold voltage of the trigger can both be adjusted.

As shown in Figure 3. In Figure 3, the input signal voltage is E, the noise signal is converted to the voltage input terminal as V, the threshold voltage U is output as u through the amplifier, and the amplification factor of the amplifier is A. Since u=AU, changing A or U has the same effect. As shown in Figure 4. To make the output signal of the trigger a valid signal, it is necessary to make the input valid signal u of the trigger much greater than the noise signal. Therefore, the necessary conditions for the normal operation of acetylene flow meters are: E>u>V. When measuring low flow rate and small flow rate fluids using acetylene flow meters, based on the above analysis, it is necessary to increase the signal-to-noise ratio, try to increase the effective signal of the input flow rate, and reduce the amplitude of the interference signal generated by mechanical vibration. Therefore, the structural shape of the resistive fluid can be modified to enable the sensor to better receive the pulsation frequency of the vortex, which can significantly increase the amplitude of the effective signal. Another more practical and effective method is to install a pair of symmetrical piezoelectric crystals at both ends of the vortex generator, use a differential piezoelectric sensor to sense the signal, and use a differential amplification circuit to amplify the signal, as shown in Figure 4. Due to the fact that the interference generated by mechanical vibration in the circuit exerts the same force on the two piezoelectric crystals, and the fluid vortices are alternately generated on both sides of the blocking fluid, the signal generated by the interference is amplified through differential amplification, and the mechanical vibration signals cancel out and reduce each other due to the same effect. However, the flow signals of the opposite piezoelectric crystals are added up and enhanced. As a result, the interference of mechanical vibration signals is greatly reduced.

4. The limitation of high flow rate and high flow rate measurement is usually believed to be that the steam flow rate in the pipeline does not exceed 60m/s. When selecting a flow meter, a range of 60m/s is sufficient. However, when using online real-time spectrum analysis, it has been found that pipelines below 80 often have high flow rates above 80m/s. Nearly half of them have high flow rates exceeding 100m/s, and even more so, the flow rate can reach as high as 180m/s. When the flow rate of general acetylene is too high, it is difficult to estimate the magnitude of the ultra-high flow rate due to the severe leakage wave phenomenon. As shown in Figure 5, the leakage wave phenomenon reduces the flow rate by 44.3%. In response to this phenomenon, spectral analysis+dynamic filtering is adopted to improve signal fluctuations and eliminate the phenomenon of "leakage waves". The signal can be analyzed from both the time domain and frequency domain. The signal image in the time domain is based on the time axis as the horizontal axis; The frequency domain signal image is based on the frequency value as the horizontal axis. The time-domain analysis of signals mainly focuses on the intuitive impression of the signal, such as the period of the signal, the amplitude of the signal at a certain point in time, etc. The frequency domain analysis of signals uses Fourier transform to transform X (t) into X (f). The specific transformation method will not be repeated here. The frequency spectrum of the signal indicates the size of its components at different frequencies, providing more specific and rich frequency domain images than time-domain images. In the Pico Scope oscilloscope, its spectrum analysis function can be used to observe the spectrum of the signal. Signal filtering is usually a commonly used method in signal processing. The main purpose of signal filtering is to obtain the desired signal and filter out signals that do not meet the experimental requirements. There are usually several methods, including low-pass, high pass, bandpass, and bandstop. In practical applications, it is usually to design a filtering circuit to filter the circuit. In testing and measurement, it is often necessary to filter out the clutter in the signal. Try to eliminate the influencing factors as much as possible, and directly perform real-time spectrum analysis on the raw signal of the acetylene * * * sensor to obtain the flow rate value at ultra-high flow rates. As shown in Figure 6.

Due to its easy compatibility with digital electronic devices, acetylene flow meters are a relatively advanced and ideal measuring instrument. The lift of a vortex is directly proportional to the square of the flow rate and directly proportional to the density of the fluid. Therefore, when there is a small flow rate, low flow rate, or a large flow rate, high flow rate, higher requirements are put forward for acetylene flow meters. This article explores this issue accordingly. In order to enable acetylene flow meters to measure low flow rates and small flow rates as much as possible, it is necessary to improve the signal-to-noise ratio by using differential piezoelectric sensors and differential amplification circuits to maximize the amplitude of the effective flow signal and reduce the amplitude of mechanical vibration interference signals. To address the issue of leakage waves caused by high flow rates and high flow rates, spectral analysis and dynamic filtering methods are adopted to minimize leakage waves as much as possible. Figure 7 shows the sensor signal and amplifier output signal of the flowmeter when not processed. The upper part of Figure 7 (a) shows the original signal output by the sensor, and the lower part shows the amplifier output signal; Figure 7 (b) shows the expanded view. Figure 8 shows the sensor signal and amplifier output signal output by the processed flowmeter. The upper part of Figure 8 (a) shows the original signal output by the sensor, and the lower part shows the amplifier output signal; Figure 8 (b) shows the expanded view.

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