Electromagnetic flowmeter principle and structure and installation and use of vortex flowmeter

The device that measures the flow rate based on the principle of electromagnetic induction using the conductive properties of certain fluids is called an electromagnetic flowmeter. Electromagnetic flowmeters have a wide range of applications and can measure corrosive media such as acids, alkalis, and salt solutions as well as conductive slurries with suspended particles.

  1. Principle and structure of electromagnetic flowmeter

(1) Measurement principle of electromagnetic flowmeter Electromagnetic flowmeter is based on the principle of electromagnetic induction. The conductive fluid flows in the magnetic field perpendicular to the direction of the magnetic force line, and an induced electromotive force is generated on the electrodes on both sides of the flow channel. The magnitude of the induced electromotive force and the flow velocity of the fluid Relevant, the fluid flow can be obtained by measuring this electromotive force. The relationship between the induced electromotive force £ and the flow rate E = CBDu , where C is a constant; B is the magnetic induction intensity; D is the inner diameter of the pipe; u is the average velocity of the fluid.

When the instrument structure parameters are determined, the induced electromotive force and the flow rate r correspond to each other, then the volumetric flow rate of the fluid can be obtained. The flow equation can be written as follows: K is the instrument constant, and for the fixed electromagnetic flowmeter, K is the fixed value.

The direct-electromotive force S in the electromagnetic flowmeter generates electrolysis of the conductive liquid, thereby damaging the fluid and causing an error in the measurement. In order to overcome the electrolysis polarization phenomenon, magnetic field B of the electromagnetic flowmeter adopts alternating magnetic field _

Using an alternating magnetic field not only eliminates the influence of the dielectric polarization of the dielectric, but also facilitates the amplification of the output signal. However, using alternating magnetic fields increases the induction error.

(2) Structure of Electromagnetic Flowmeter The measurement body of the electromagnetic flowmeter is composed of a magnetic circuit system, a measurement catheter, an electrode, and an adjustment conversion device. The structure of the electromagnetic flowmeter is shown in Fig. 4-22. | The catheter made of non-magnetic material and the measuring electrode are embedded in the wall of the tube. If the catheter is a conductive material, the inner wall and the electrode must be insulated, usually in the entire The inner wall of the measuring duct is equipped with an insulating lining. The excitation coil on the periphery of the catheter is used to generate an alternating magnetic field; a yoke is also provided on the outside of the catheter and the coil so as to form a uniform magnetic field and have a large magnetic flux.

Electromagnetic flow meter conversion part of the output current /. It is proportional to the average flow rate.

2. Characteristics of electromagnetic flowmeter

   The electromagnetic flowmeter has no resistance in the measuring tube, and the pressure loss is very small; its flow rate measurement range is wide, 0.5~10m/s;

Range of up to 10:1; flow meter diameter can be from a few millimeters to several meters or more; flowmeter accuracy is 0.5 ~ 1_5 level; instrument response is rapid, the flow state of the impact of the indication is small, can measure the pulse flow and two phases Flow, such as the flow of mud and pulp. The lower limit of the conductivity of the fluid to be measured is determined by the input impedance of the converter. If the input impedance is 100MX1, the measured conductivity of the liquid must not be lower than 10 [iil/cm].

The electromagnetic flowmeter does not require a straight pipe section, and the length of the straight pipe section before the meter is 5D~10Z). The installation site should try to avoid severe vibration and strong AC and DC magnetic fields. In the vertical installation, the fluid flows through the instrument from bottom to top. When mounted horizontally, the two electrodes must be on the same plane. Make sure that the fluid, casing, and pipe are well grounded.

The electromagnetic flowmeter is selected according to the condition of the fluid under test to determine the appropriate lining and electrode material. The measurement accuracy is affected by the inner wall of the catheter, especially the fouling near the electrode. Care should be taken to maintain the cleaning.

Vortex flowmeter

Vortex flowmeter is a type of vortex flowmeter. It uses the principle of fluid oscillation to measure flow. When the fluid flows through the non-streamlined resistance element, a stable vortex array is generated. The frequency of the vortex generation has a definite correspondence relationship with the fluid flow velocity. When the frequency is measured, the flow rate of the fluid can be obtained.

1. Composition and flow equation of vortex flowmeter

Vortex flowmeter measurement body is a vortex generator. The vortex generator is a vertical cylinder with a non-streamlined cross section

Insert in the circulation section. As fluid flows through the vortex generating body, vortices are alternately generated on both sides of the generating body, and two columns of asymmetrical vortex columns are formed downstream of it. When the longitudinal distance between each two vortices ^ and the lateral distance I between the vortex columns satisfies a certain relationship, that is /i/Z = 0.281, the two vortex columns will be stable, called "Carmen. Vortex Street,"

A large number of experiments have shown that within a certain Reynolds number range, a stable vortex generation frequency/deterministic relationship with the flow velocity v at the vortex generating body is defined: In the formula, d is the characteristic size of the vortex generating body; the foot is called Strohal Number.

Relating to the shape of the vortex generator and the Reynolds number of the fluid, within a certain range of Reynolds number, the St value is basically unchanged. The shape of the 9 vortex generator body is a cylinder, a triangular column, a rectangular column, a T-shaped column, and a combination of the above simple cylinders. The resulting combination of cylinders, different cylinders \ different, such as the cylinder St = 0.21, corner cylinder St = 0.16. Among them, the vortex strength of the triangular column body is large, the stability is good, and the pressure loss is moderate, so it is used more.

When the body shape and size determination of the vortex can be generated by measuring the vortex frequency to measure the flow of the fluid, wherein the flow equation, K is the meter factor, typically measured experimentally.

There are many ways to detect the frequency of vortices, which can be divided into two types: body type and split type. ^ Body detection elements placed in vortex

In vivo, such as hot wire type, diaphragm type and thermistor type; split type detection element is installed downstream of the vortex generating element, such as piezoelectric type, pendulum type, and ultrasonic type. All of the above are measured using the fluctuations caused by the generation of vortices. Figure 4-24 shows a schematic diagram of a triangular column vortex flowmeter. The vortex frequency uses a thermistor detection method to embed two thermistors symmetrically at the inflow plane of the P-corner cylinder. resistance to a temperature slightly higher than the fluid, under the action of the vortex generated alternately, two resistors are periodically cooled, it changes the value of resistance, the resistance change detected by the bridge, can be measured vortex frequency generated To measure flow.

   2. The advantages and disadvantages of vortex flowmeters and their installation and use

(1) The advantages of vortex flowmeters

1) Vortex flowmeters are suitable for flow measurement of gases, liquids, and vapors. Their measurement is almost unaffected by changes in fluid parameters (temperature, pressure, density, and viscosity). In a certain Reynolds number range, the meter factor is only related to the shape and size of the vortex generator and the pipe.

2) The vortex flowmeter has no moving parts inside the instrument and has a long service life.

3) The structure is simple, firm, easy to install and maintain. There is no need for a pressure tube and a three-valve manifold to reduce leakage, blockages and freezing.

4) Low pressure loss.

5) The output is a frequency signal with a wide range (30:1).

6) The measurement accuracy is also relatively high. ± 0_5% ~ ± 1% 0

(2) Limitations of vortex flowmeters

1) It is sensitive to the mechanical vibration of the pipeline and should not be used for strong vibration.

2) The larger the aperture, the lower the resolution.

3) When the liquid temperature is too high, the sensor error is large

4) When the fluid has pressure pulsation or flow pulsation, the indication is significantly higher and has greater influence, so it is not suitable for pulsating flow.

(3) Installation of vortex flowmeter The vortex flowmeter can be installed horizontally or vertically. In vertical installation, the fluid must pass from bottom to top to fill the fluid with the tubing. A certain straight pipe section is required on the downstream and downstream of the instrument. The downstream length is 5D. The upstream length is determined according to the type of the resistance element. It is about 15D~40D, and the upstream flow control valve should not be provided.

Ultrasonic flowmeter

1. The principle of ultrasonic flowmeter

Ultrasonic flow meters use the propagation characteristics of ultrasonic waves in fluids to achieve flow measurements. When the ultrasonic wave propagates in the fluid, it will be affected by the velocity of the fluid, and the detected ultrasonic signal can be detected to determine the flow velocity, so as to obtain the fluid flow.

Ultrasonic measurement of flow has a variety of methods. According to the principle of action, there are propagation velocity difference method, Doppler effect, sound beam offset method, and correlation method. In industrial applications, the difference in propagation speed is the most common method.

The velocity differential method uses the variation of the velocity of the ultrasonic wave in the fluid to propagate downstream and countercurrently to measure the fluid velocity. The method of measuring the amount of time difference, phase difference and frequency difference method method: 9 Measurement principle propagation velocity difference method shown in Figure 4-25, in the pipe wall from the transmitting transducer, the downstream two 1 \, T2 issued Ultrasonic waves each reach the downstream and upstream receiving ultrasonic transducer I. When the fluid is stationary, the ultrasonic velocity of sound is C, and the sound velocity of the forward flow and the backward flow when the fluid flows will be different. The relationship between the two propagation times and flow rates can be written , where h is the downstream propagation time; the upstream propagation time; i is the distance between the two probes; о is the fluid average flow rate I. In general, C?? The relationship with the flow rate is that the measured time difference, A, is known as the flow rate.

When using frequency difference method, the relation between frequency and flow rate is listed as

The measurement by frequency difference method can not be influenced by the speed of sound, so it is not necessary to consider the influence of the temperature change of the fluid on the speed of sound.

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