The measured fluid impacts the turbine blades and causes the turbine to rotate. The speed of the turbine changes with the change of the flow. That is, when the flow is large, the speed of the turbine is also large. The magnetoelectric conversion device converts the speed of the turbine into electrical pulses of the corresponding frequency. After the preamplifier is amplified, it is sent to the display instrument for counting and display. According to the number of pulses per unit time and the cumulative number of pulses, the instantaneous flow of the measured fluid can be found to impact the turbine blades, causing the turbine to rotate, and the speed of the turbine changes with the flow rate. The change, that is, the flow rate is large, the turbine speed is also large, and then the turbine speed is converted into the electric pulse of the corresponding frequency by the magnetoelectric conversion device. After being amplified by the preamplifier, it is sent to the display instrument for counting and displaying, according to the unit time The instantaneous flow rate and the accumulated flow rate can be obtained by the number of pulses and the cumulative pulse number in. The working principle of the turbine transmitter is that when the fluid flows along the axis of the pipeline and impacts the turbine blades, a force proportional to the product of the flow rate qv , the flow velocity V and the fluid density ρ acts on the blades to push the turbine to rotate. While the turbine is rotating, the blades periodically cut the lines of magnetic force generated by the electromagnet, changing the magnetic flux of the coil. According to the principle of electromagnetic induction, a pulsating electric potential signal will be induced in the coil. The frequency of this pulsating signal is proportional to the flow rate of the fluid being measured, namely: where qv is the total volume of the fluid, and N is the pulsation generated by the transmitter Total; ξ is the flow coefficient. ξ is an important characteristic parameter of turbine transmitters. Different meters have different ξ and change with the wear of the meter for a long time. Its meaning is the number of pulses output by the transmitter when the unit volume flow passes through the transmitter. . The pulse signal output by the turbine transmitter is amplified by the front amplifier and sent to the display instrument to realize the flow measurement. 2 Turbine flowmeter selection (1) It is better to use 316 stainless steel in the area of u200bu200bthe flowmeter body to prevent corrosion. If it is an explosion-proof area, it must be explosion-proof. (2) Bearings generally have three specifications: tungsten carbide, polytetrafluoroethylene, and carbon graphite: tungsten carbide has the highest precision, which is used as a standard part of industrial control; PTFE and carbon graphite can be anti-corrosion, generally preferred in chemical sites . The life velocity of the bearing is proportional to the square of the flow velocity, so the best velocity is 1/3 of the maximum velocity. (3) The induction probe detects the movement of the rotating body and converts it into a pulse digital electrical signal. The voltage output value of its electromagnetic coil is close to a sinusoidal curve. The frequency range of the pulse signal changes linearly with the measured flow rate. The typical range is 10 :1, 25:1 and 100:1 three specifications. The resistance of the electromagnetic coil is generally less than 2000Ω, and it may be damaged if it is greater than this value. 3 The installation of turbine flowmeter (1) The power cord of the transmitter adopts metal shielded wire, and the grounding should be good and reliable. The power supply is DC 24V, 650Ω impedance. (2) The transmitter should be installed horizontally, avoid vertical installation, and ensure that there are suitable straight pipe sections before and after it, generally 10D at the front and 5D at the back. (3) Ensure that the flow direction of the fluid is consistent with the direction of the arrow on the instrument housing, and it must not be installed in reverse. (4) The measured medium must not corrode the turbine, especially the bearing, otherwise measures should be taken. (5) Be careful not to collide with the magnetic induction part. 4 The calibration method for the configuration and calibration standard of the turbine flowmeter is the ten-point water calibration method, but the value of the calibration is different for different viscosity, so the viscosity calibration curve is usually required. 5 Turbine flowmeter display instrument The task of the display instrument is to convert the number of output pulses per unit time and the total number of output pulses into instantaneous flow and total flow, and display them. The pulse signal output by the pre-amplifier has irregular amplitude and waveform. After entering the display instrument, it needs to be shaped into a regular rectangular electric pulse signal with a certain amplitude after entering the display instrument. The current conversion circuit converts the frequency signal into the corresponding current signal (4～20mA) and then converts the instantaneous flow value. The total amount is obtained by the conversion and integration circuit. Some display instruments display on-site, and some send DCS display. 6 Notes (1) Before installing the turbine flowmeter, the pipeline should be cleaned. When the measured medium is not clean, add a filter. Otherwise, the turbine and bearing are easy to be stuck, and the flow cannot be measured. (2) When disassembling and installing the flowmeter, do not collide with the magnetic induction part. (3) Before putting into operation, set the instrument coefficient. Check carefully to make sure that the wiring of the meter is correct and the grounding is good, then power can be sent. (4) When installing the turbine flowmeter, the front and rear pipe flanges should be level, otherwise the pipe stress will have a great influence on the flowmeter. 7 Development prospects With the continuous development of science, today's turbine transmitter has developed into a miniaturized, highly integrated module, designed with powerful functional software, and equipped with RS232 standard computer communication interface, which provides convenience for maintenance and repair . It can be connected to DCS for communication. DCS replaces the display instrument, such as HOFFER flowmeter, which is more convenient and practical in the near-production process of industry. In short, the turbine flowmeter is a speed type flow meter. Because of its high measurement accuracy, fast response, wide measurement range, low price, and easy installation, it is widely used in chemical production.
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