Turbine flowmeter principle and selection method mapping non-destructive testing

Analysis of turbine flowmeter

1 How the turbo transmitter works

The turbine flowmeter consists of a turbine, a bearing, a preamplifier, and a display instrument.
The fluid to be tested impinges on the turbine blade to rotate the turbine. The rotational speed of the turbine changes with the change of 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 the corresponding frequency by the magnetoelectric conversion device. After the preamplifier is amplified, it is sent to the display instrument for counting and display. The instantaneous flow rate and the accumulated flow rate can be obtained according to the number of pulses per unit time and the cumulative number of pulses.

The working principle of the turbo transmitter is that when the fluid flows along the axis of the pipe and impacts the turbine blades, a force proportional to the flow rate qv, the flow velocity V and the fluid density ρ is applied to the blades to push the turbine to rotate. While the turbine is rotating, the blades periodically cut the magnetic lines of force generated by the electromagnets to change the magnetic flux of the coils. According to the principle of electromagnetic induction, a pulsating potential signal is induced in the coil, and the frequency of the pulsating signal is proportional to the flow rate of the fluid to be measured.

The pulse signal output by the turbo transmitter can be measured by amplifying the signal after being amplified by the amplifier and sent to the display instrument.

2 Turbine flowmeter selection

(1) The flow meter body Zui good selection area is made of 316 stainless steel material to prevent corrosion. If it is an explosion-proof area, it must also be an explosion-proof result.
(2) Bearings generally have tungsten carbide, polytetrafluoroethylene, carbon graphite. Three specifications: the precision of tungsten carbide is high, it is used as a standard part of industrial control; PTFE, carbon graphite can be used for corrosion protection, generally in chemical industry. consider. The square of the life of the bearing is proportional to the inverse ratio, so the flow rate is better at 1/3 of the large flow rate of zui.
(3) The induction probe detects the motion of the rotating body and converts it into a pulse digital electrical signal. The voltage output value of the electromagnetic coil is close to a sinusoidal curve, and 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 2000 Ω, and greater than this value may be damaged.

3 Turbine flowmeter installation

(1) The power cable of the transmitter is made of 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 to avoid vertical installation, and to ensure that there are suitable straight pipe sections before and after, generally 10D in the front and 5D in the rear.
(3) Ensure that the flow direction of the fluid is consistent with the direction of the arrow on the instrument casing and must not be reversed.
(4) The measured medium should not corrode the turbine, especially at the bearing, otherwise measures should be taken.
(5) Note that the magnetic induction part cannot collide.

4 Turbine flowmeter configuration and correction

The standard calibration method is the ten-point water calibration method, but the viscosity is different from the calibration value, so the viscosity calibration curve is usually used.

5 display instrument of turbine flowmeter

The task of the display meter 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 front amplifier is irregular in amplitude and waveform. After entering the display instrument, it needs to be shaped by a shaping circuit to become a regular rectangular electric pulse signal with a certain amplitude. The current conversion circuit converts the frequency signal into a corresponding current signal (4-20 mA) and converts the instantaneous flow rate value, and the total amount is obtained by the conversion and integration circuit. Some display instruments are displayed on the spot, and some are sent to the DCS display.

6 Precautions

(1) Before installing the turbine flowmeter, the pipeline should be cleaned. When the measured medium is unclean, add a filter. Otherwise, the turbine and the bearing are easily stuck, and no flow can be measured.
(2) When disassembling the flowmeter, the magnetic induction part cannot collide.
(3) Set the meter factor before commissioning. Check carefully to make sure that the meter wiring is correct and the grounding is good before power can be sent.
(4) When installing the turbine flowmeter, the front and rear pipeline flanges should be horizontal, otherwise the pipeline stress will have a great influence on the flowmeter.

7 Development prospects

With the continuous development of science, today's turbo transmitters have developed into miniaturized, highly integrated modules, designs, powerful functional software, and RS232 standard computer communication interface, which provides convenience for maintenance and repair. It can communicate with the DCS, and the DCS replaces the display instrument. For example, the HOFFER flowmeter is more convenient and practical in the industrial proximity production process.

In short, the turbine flow meter is a speed type flow meter, which is widely used in chemical production because of its high measurement accuracy, fast reaction speed, wide measuring range, low price and convenient installation.