Anti-interference measures of steam flow meter

Abstract: The information on anti-interference measures of steam flowmeters is provided by excellent flowmeter and flowmeter manufacturers and quotation manufacturers. Anti-interference measures for steam flowmeters Introduction Among the many flow detection solutions, steam flowmeters have the characteristics of high measurement accuracy, small pressure loss, easy installation, not affected by the physical properties of the measured medium, and easy remote transmission of signals. technology. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following is the article details of anti-interference measures for steam flowmeters. Anti-interference measures for steam flowmeters Introduction Among the many flow detection solutions, steam flowmeters have the characteristics of high measurement accuracy, small pressure loss, easy installation, not affected by the physical properties of the measured medium, and easy remote transmission of signals. The technology is becoming more and more perfect, especially in the flow measurement of large pipe diameter and liquid media such as water and oil, which is more widely used. The steam flowmeter is based on the Karman vortex principle in fluid dynamics. Within a certain range of Reynolds number, the flow velocity or volume flow of the fluid is proportional to the vortex frequency and has nothing to do with the physical properties of the fluid (pressure, temperature, density, etc.), namely: Q=k f where: Q is the volume flow; k is the instrument constant; f is the vortex frequency. According to the above measurement principle, one of the characteristics of the steam flowmeter is that it is susceptible to electromagnetic and mechanical vibration interference, which limits the normal use of the steam flowmeter in some occasions, which is also the working condition of the steam flowmeter. Solving the problem of anti-interference is an effective way to expand the lower limit of the range and improve the steam flowmeter. 2 Working conditions The bluff body of the steam flowmeter uses piezoelectric crystals to detect the vortex frequency, and the piezoelectric signal is amplified and triggered by AC to convert the vortex frequency into a pulse signal. The pulse signal is sent to the secondary instrument to display the measured flow after conversion. Among them, the amplification factor K of the AC amplifier and the threshold voltage of the trigger can be adjusted, as shown in Figure 1. In Figure 1, the signal voltage is E, the interference signal is converted to the input end as V, the threshold voltage U is converted to the input end as u, and the AC amplification factor is K. Since u=UK, the effect of adjusting K or U is the same. In order to make the threshold voltage prevent the interference signal and ensure that the trigger can output an effective signal, the interference signal V must be less than the threshold voltage u, and the effective signal voltage E is greater than the threshold voltage u, that is, the working conditions of the steam flowmeter are: E> u> The magnitude of the V interference signal V determines the lower limit of the range of the steam flowmeter. Therefore, expanding the lower limit of the steam flowmeter must start with reducing the interference signal. Adjusting the AC magnification K can only strengthen the output signal, and the lower limit of the range cannot be expanded. 3 Anti-interference measures The interference signals of the steam flowmeter mainly include electromagnetic interference and mechanical vibration interference. How to solve these two kinds of anti-interference problems becomes the key to improving the steam flowmeter. The steam flowmeter usually adopts a metal casing, and the shielding effect of the casing can prevent electric field and radio frequency interference; for the magnetic field interference, it can be solved by selecting non-magnetic components and rational wiring of printed circuit boards in the internal circuit design. Development and refinement of the manufacturing process are also not a problem. Therefore, the anti-electromagnetic interference is mainly anti-ground current interference. The piezoelectric crystal of the steam flowmeter is mounted on the bluff body structure, and one end of the piezoelectric crystal is connected to the casing, so the signal preamplifier must be grounded. The output signal of the steam flowmeter is sent to the secondary instrument, and the DC power supply required for signal amplification is provided by the secondary instrument. There may be a step voltage between the ground wire of the piezoelectric crystal and the ground wire of the secondary instrument to form a current. When this current flows in the ground wire of the signal amplifier, there will be a voltage drop. This voltage drop is superimposed with the effective signal and cannot be separated, which is the ground wire current interference. The solution to the ground wire current interference is to reduce or eliminate the ground wire current. The most thorough solution is to isolate the DC power supply from the secondary instrument. That is, the DC power supply is isolated by a transformer and then rectified into a DC supply to the steam flowmeter, so that there is no electrical connection between the ground wire of the secondary instrument and the ground wire of the piezoelectric crystal. At the same time, the effective measurement signal is converted into a pulse signal after pre-amplification, and is output to the secondary instrument through the pulse transformer, which fundamentally eliminates the influence of the ground current and is an extremely effective anti-interference measure. However, the method of transformer isolation is relatively expensive, bulky, and difficult to implement in the manufacturing process, which greatly reduces the practicability. Optical isolation current limiting and anti-interference measures can effectively reduce the interference of ground current. The principle is shown in Figure 2. In the figure, a is the grounding point of the piezoelectric crystal, and b is the grounding point of the secondary instrument. A resistor r is connected to the ground loop, so the ground current between points a and b is limited by the resistor r, and the voltage drop between the two points a and b is across the resistor r. The voltage drop across the resistor r reflected on the positive line of the power supply is blocked by the three-terminal voltage regulator R. The resistance of the preamplifier ground loop is much smaller than that of the resistor r. There is only a small ground current in the preamp ground. After the effective signal of the piezoelectric crystal is amplified, it is isolated and output by the optical isolation device. In this way, the interference of the ground current can be reduced by at least an order of magnitude.

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