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technical article
Introduction of liquid turbine flowmeter
2011-7-2 17:36:53

1 The task of the display meter head of the turbine flow meter is to convert the number of output pulses and the total number of output pulses per unit time into instantaneous flow and total flow, and display it. The amplitude and waveform of the pulse signal output by the front amplifier are irregular. After entering the display meter, it needs to be shaped into a regular rectangular electrical pulse signal with a certain amplitude by a shaping circuit, and then passed through the frequency / 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 accumulation circuit.
2 Turbine transmitter working principle: Turbine flowmeter consists of turbine, bearing, preamplifier and display meter. The measured fluid impacts the turbine blades, causing the turbine to rotate, and the rotation speed of the turbine changes with the change in flow rate, that is, the flow rate is large, and the rotation speed of the turbine is also large. After the preamplifier is amplified, it is sent to the display instrument for counting and display. The instantaneous flow rate and accumulated flow rate can be obtained according to the number of pulses and the accumulated pulse number in a unit time. 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 velocity V and the fluid density ρ acts on the blades, pushing the turbine to rotate. While the turbine is rotating, the blade periodically cuts the magnetic lines of force generated by the electromagnet, changing the magnetic flux of the coil. According to the principle of electromagnetic induction, a pulsating potential signal will be induced in the coil, and the frequency of this pulsating signal is proportional to the flow rate of the measured fluid. The pulse signal output by the turbine transmitter is amplified by the pre-amplifier and sent to the display instrument to realize the flow measurement.
3 The inductive probe detects the motion of the rotating body and converts it into a pulsed digital electrical signal. Its electromagnetic coil voltage output value is close to a sine curve. The frequency range of the pulsed signal changes linearly with the measured flow rate. The typical range is 10: 1. , 25: 1 and 100: 1 specifications. The resistance of the electromagnetic coil is generally less than 2000Ω, and greater than this value may be damaged.
4 Turbine flowmeter installation (1) The power line of the transmitter uses metal shielded wire, and the grounding must be good and reliable. The power source is DC 24V, 650Ω impedance.
(2) The transmitter should be installed horizontally, avoid vertical installation, and ensure that there are adaptable straight pipe sections at the front and back, generally the first 10D and the rear 5D.
(3) Ensure that the flow direction of the fluid is consistent with the direction of the arrow on the instrument case, and it must not be installed in the opposite direction.
(4) The measured medium must not corrode the turbine, especially the bearings, otherwise measures should be taken.
(5) Be careful not to collide with the magnetic induction part.
5 The calibration method of the configuration and calibration standard of the turbine flowmeter is a ten-point water calibration method, but the calibration value is different for different viscosity, so the viscosity calibration curve is usually made.
6 Bearings generally have three specifications of tungsten carbide, polytetrafluoroethylene, and carbon graphite: tungsten carbide has the highest accuracy, which is used as a standard component for industrial control; polytetrafluoroethylene and carbon graphite can be anticorrosive, and it is generally preferred in chemical sites. The life of the bearing is proportional to the inverse square of the flow rate, so the best flow rate is better than 1/3 of the maximum flow rate.
7 Precautions (1) Before installing the turbine flowmeter , clean the pipeline. When the measured medium is not clean, add a filter. Otherwise, the turbine and bearing will be easily jammed and the flow cannot be measured.
(2) When disassembling or disassembling the flowmeter, do not collide with the magnetic induction part.
(3) Set the meter coefficients before commissioning. Carefully check to make sure that the wiring of the meter is correct and the ground is good before transmitting power.
(4) When installing the turbine flowmeter, the front and rear pipeline flanges must be level, otherwise the pipeline stress will have a great impact on the flowmeter.
8 Development Prospects With the continuous development of science, today's turbine transmitters have developed into miniaturized, highly integrated modules, designed with powerful functional software, and equipped with RS485 standard computer communication interfaces, which facilitates maintenance and repair. . Can communicate with DCS, DCS replaces the display instrument.
In short, the turbine flow meter is a speed-type flow meter . It has been widely used in chemical production due to its advantages such as high measurement accuracy, fast response speed, wide measurement range, low price, and easy installation.