On the working principle of turbine flowmeter and turbine transmitter
1 How Turbine Transmitters WorkTurbine flowmeters consist of turbines, bearings, preamplifiers, and display instruments.
The measured fluid impinges on the turbine blades and causes the turbine to rotate. The rotational speed of the turbine changes with the flow rate, that is, the flow rate is large, and the rotational speed of the turbine is also large. Then, the rotational speed of the turbine is converted into an electrical pulse of a corresponding frequency by the magnetoelectric conversion device. After the preamplifier is amplified, it is sent to the display meter for counting and display. The instantaneous flow rate and the accumulated flow rate can be calculated based on the pulse number and the accumulated pulse number in the unit time.
The working principle of the turbine transmitter is that when the fluid flows along the axis of the pipeline and strikes the turbine blades, a force proportional to the product of the flow qv, the flow velocity V, and the fluid density Ï acts on the blades to rotate the turbine. While the turbine is rotating, the blades periodically cut the magnetic lines of force generated by the electromagnets and change the magnetic flux of the coils. According to the principle of electromagnetic induction, a pulsed potential signal will be induced in the coil, and the frequency of the pulse signal is proportional to the flow of the fluid to be measured.
The pulse signal output by the turbine transmitter is amplified by the amplifier and sent to the display meter to measure the flow rate.
2 Turbine flow meter selection
(1) The flow meter body is preferably treated with 316 stainless steel for corrosion protection in the selected area. If it is an explosion-proof area, it must also be an explosion-proof result.
(2) Bearings usually have carbonized tungsten, polytetrafluoroethylene, carbon graphite three kinds of specifications: the highest accuracy of tungsten carbide, which is used as a standard part of industrial control; PTFE, carbon graphite can be anti-corrosion, generally preferred in chemical industry places . The square of the bearing life flow rate is positive and negative, so the best flow rate is 1/3 of the maximum flow rate.
(3) The sensing probe detects the movement of the rotating body and converts it into a pulsed digital electrical signal. Its output voltage of the electromagnetic coil is close to a sinusoidal curve. The frequency range of the pulse signal varies linearly with the measured flow rate. The typical range is 10 :1,25:1
And 100:1 three specifications. The resistance of the solenoid coil is generally less than 2000Ω, which may be damaged if larger than this value.
3 Turbine Flowmeter Installation
(1) The power cable of the transmitter adopts metal shield wire, and the grounding should be good and reliable. The power supply is DC 24V, 650Ω impedance.
(2) The transmitter should be installed horizontally to avoid vertical installation, and to ensure that it has a straight section adapted before and after, generally the first 10D, after the 5D.
(3) Ensure that the flow direction of the fluid is the same as the direction of the arrow of the instrument housing and must not be reversed.
(4) The measured medium cannot corrode the turbine, especially at the bearing, otherwise measures shall be taken.
(5) Note that the magnetic induction part cannot be collided.
4 Turbine Flow Meter Configuration and Calibration
The standard calibration method is the ten-point water calibration method, but different viscosity calibration values ​​are different, so the viscosity calibration curve is usually done.
5 Turbine flowmeter display instruments
The task of the display meter is to convert the number of output pulses per unit time and the total number of output pulses to instantaneous flow and total flow and display them.
The pulse signal output by the preamplifier is irregular in amplitude and waveform. After entering the display instrument, it must first be shaped into a regular rectangular electric pulse signal with a certain amplitude by the shaping circuit, and then through the frequency / The current conversion circuit converts the frequency signal into a corresponding current signal (4 to 20 mA) and then converts the instantaneous flow value, and the total amount is obtained by the conversion and integration circuit. Some display instruments are displayed locally, and some send DCS displays.
6 Precautions
(1) Before installing the turbine flow meter, clean the piping. When the measured media is unclean, add a filter. Otherwise, the turbine and bearing are easily stuck and no flow can be measured.
(2) When disassembling the flowmeter, the magnetic induction part cannot be collided.
(3) Set the meter factor before commissioning. Carefully check to make sure that the instrument wiring is correct and the grounding is good before sending power.
(4) When installing the turbine flowmeter, the front and rear duct flanges should be horizontal, otherwise the pipeline stress has a great influence on the flowmeter.
7 Development Prospects
With the continuous development of science, today's turbine transmitter has developed into a small, highly integrated module, design, powerful software, and has RS232 standard computer communication interface, which provides convenience for maintenance and overhaul. It can communicate with DCS. DCS replaces display instruments. For example, HOFFER flowmeter is more convenient and practical in the industrial production process.
In short, turbine flowmeter is a kind of speed flow meter. It has been widely used in chemical production due to its advantages of high measurement accuracy, fast response, wide measurement range, low price, and easy installation.