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Industry Information
How to detect vortex frequency is an important issue in the development of vortex flowmeters
2012-3-30 11:32:02

的流量信号是由旋涡的频率反映的,所以涡频如何检出,是涡街流量计研制的一个重要课题. The flow signal of the vortex flowmeter is reflected by the frequency of the vortex, so how to detect the vortex frequency is an important subject for the development of the vortex flowmeter. According to the current vortex frequency signal detection principle, there are roughly two types: ① detecting the frequency of flow changes near the vortex generator after the vortex is generated, which is mainly completed by the thermal element; ② detecting the force on the vortex generator after the vortex is generated; Change frequency, this is mainly done by piezoelectric sensor

In the following, we will discuss specific examples of these two detection methods respectively. The first detection method uses the hot-wire type of a cylindrical vortex generator as an example. The surface of the cylinder is provided with pressure guiding holes, which communicate with the internal cavity of the cylinder. The cavity is divided into two parts by the partition wall, and there is a small hole in the central part of the partition wall, and a platinum resistance wire for detecting fluid flow is installed in the small hole. When the vortex is generated on the downstream side of the cylinder, the pressure under the cylinder is higher than the pressure due to the lifting force. The fluid under the cylinder enters the hollow glue from the pressure guiding hole below the cylinder under the effect of the pressure difference between the cylinder and the platinum resistance wire through the small hole in the central part of the partition wall and flows out from the pressure guiding hole above. If the platinum resistance wire is heated to a temperature value higher than the temperature of the fluid, when the fluid flows through the platinum resistance wire, it will take away heat and change its temperature, that is, change its resistance value. When a vortex is generated above the cylinder, the fluid enters from the upper pressure guiding hole, flows out from the lower pressure guiding hole, passes through the platinum resistance wire again, and changes its resistance value again. It can be seen that the change in resistance value corresponds to the change in flow, and therefore also to the frequency of the vortex. Therefore, the eddy frequency can be obtained by detecting the resistance change frequency of the platinum resistance wire, and then the flow value can be obtained. A circuit that converts the change in the resistance value of a platinum resistance wire into an electrical signal. The output of the platinum resistance wire on one arm of the bridge is differentially amplified, and the output current of the power amplifier A2 is fed back to the bridge, so that the temperature of the platinum resistance wire is lower than that of the fluid. The temperature is higher than a constant value. The second detection method uses a triangular cylinder vortex generator as an example. Its structure can be seen in Figure 3-10. Two elastic metal films are installed on the two sides of the triangular column, which are also the poles of a capacitor, and an electrode plate is installed inside. The electrode plate and the metal membrane are filled with oil to transmit pressure. In this way, when a vortex is generated below the triangular ridge, and the pressure below is higher than the upper pressure, the metal film below the triangular ridge is pushed in, and the upper metal film is ejected outward, which changes the respective capacitances of the two capacitors. . In this way, the capacitance of the two sets of capacitors changes differentially in response to the lift generated alternately. Therefore, the change in capacitance corresponds to the change in lift, and therefore to the frequency of vorticity. In this way, the vortex frequency can be obtained from the capacitance change frequency, and then the flow value can be obtained. The circuit frame eddy detection sensor that converts the capacitance change into an electrical signal is composed of two sets of metal films and electrode plates. A differential change capacitor is placed on the static capacitance detection bridge and is excited by the RF oscillation circuit. When the eddy is generated, the electrostatic capacity changes, resulting in an unbalanced electric bridge. The unbalanced potential is amplified by an RF amplifier circuit. After detection, a signal corresponding to the eddy frequency is obtained. The signal is amplified and shaped into a rectangular wave, and the constant current loop is output as a given current pulse. The feedback loop is used to compensate for bridge imbalance caused by temperature changes.

测焦炉煤气不稳定解决办法:1、在仪表安装、连接过程中,应确保每一个环节的准确无误,其中包括安装前对现场的考察、安装过程中仪表接线、系统接地线等方面,从而确保检测到真实数据并能够准确输出。 Vortex flowmeter measuring coke oven gas instability solution: 1. In the process of instrument installation and connection, ensure that each link is accurate, including site inspection before installation, instrument wiring during installation, and system ground wire And other aspects to ensure that real data is detected and accurately output. 2. For the measurement system in operation, the method of “dual-track measurement, comparison and confirmation”, and the “replacement method” can be used to confirm and eliminate the measurement instrument failure in operation. 3. Regularly drain the pipeline, especially the water before the straight pipe section, and set a special person to discharge it regularly according to the specific conditions, reduce the water in the metering pipe section as much as possible, and eliminate the pulsation in the fluid to the maximum extent. 4. Strengthen the management of the measurement system data, set the timing printing function, and analyze the operation of the instrument based on the printed data combined with the production status. 5. Regularly clean the instrument as a whole. If necessary, purge the sensor head to avoid condensation of impurities at the sensor head. In the cold season, adding a heat tracing device to the metering straight pipe section and the meter part is also beneficial to alleviate the condensation of impurities at the metering meter. Compared with the two detection methods, the first method has high sensitivity and a wide applicable frequency range, so the corresponding flow range is also wide. However, its fine structure is easily affected by impurities in the fluid. The second method has a rugged and reliable structure and is widely used in industrial measurement. However, since the lift is proportional to the square of the flow velocity, that is, a 10-fold increase in the flow velocity, the lift will change 100-fold. Therefore, if the flow meter is required to be applicable to a large flow range, and it can be guaranteed to have a certain degree of flexibility, this method is difficult. When the coke oven gas is unstable, the output signal of the vortex flowmeter is unstable. The reasons are as follows: 1. The vortex flowmeter is not suitable for installation in strong vibration. When changing, the vortex flow sensor will detect a signal output higher than normal. Practice has proved that in a scene where no gas flows, when the vortex flow sensor is in a changing magnetic field, the vortex flow sensor will sense an error signal and output it when the magnetic field changes. When the change is over, the meter is in a stable magnetic field. The meter will output a normal signal. 2. Due to the high temperature and high humidity of the coke oven gas when it leaves the factory, there will be moisture in the gas transportation process. The gas flow drives the water back and forth to form a pulsating flow. When the vortex flow sensor is in this fluid state, the output data is suddenly large or small, and it cannot reflect the production status at all. 3. The crimping wire is not solid during the instrument wiring process, which causes the signal to be intermittent during transmission. 4. The ground wire of the instrument does not meet the requirements of the specification, so that 50Hz interference in strong electricity enters. When the normal signal is higher than 50Hz, a normal signal is output, otherwise an error signal is output. 5. Because the coke oven gas has many impurities, it is easy to crystallize, and the impurities are condensed on the sensor head, which causes measurement inaccuracy. When the temperature increases, the impurities evaporate, the sensitivity increases, and the signal increases; on the contrary, it decreases. As a result, data is unstable.