Application and Troubleshooting of Electromagnetic Flowmeter in Acetic Acid Production

1 Type of electromagnetic flowmeter

Electromagnetic flowmeters can be classified in many ways, classified by excitation method, classified by sensors and converters, classified by connection method and classified by sensor and pipe connection method, and classified by application. This article mainly introduces the classification by excitation method. .

1) DC excitation type. This kind of electromagnetic flow meter has a very small quantity and is only used to measure the flow of liquid metal, such as mercury at room temperature and liquid sodium and potassium at high temperatures.

2) AC power frequency excitation type. The earlier electromagnetic flowmeters were excited with 50Hz power frequency and were gradually replaced by low frequency rectangular excitations because of their vulnerability to electromagnetic interference and zero drift. However, in the measurement of liquid-solid two-phase flow such as mud and slurry, the low-frequency rectangular wave excitation method cannot overcome the spike noise generated on the surface of the solid wiper electrode, while the instrument of the power frequency AC excitation does not have this disadvantage. Some electromagnetic flowmeters still use AC excitation.

3) Low frequency rectangular wave excitation type. The low-frequency rectangular wave excitation method has low power consumption and stable zero point, and is currently the main excitation method of the electromagnetic flowmeter. Its waveform has two kinds of "positive-negative" and "positive-zero-negative-zero" three values. Some electromagnetic flowmeter excitation frequency can be set by the user. Generally, the small-diameter instrument uses a higher frequency and the large-caliber instrument uses a lower frequency.

4) Dual-frequency excitation type. The waveform of excitation current is to superimpose a high-frequency rectangular wave on a low-frequency rectangular wave, which is mainly to overcome the presence of the slurry noise and flow noise of the square-wave excitation, and to improve the stability and response characteristics of the instrument.

2 electromagnetic flowmeter features

An electromagnetic flowmeter is a flow measurement instrument made according to Faraday's law of electromagnetic induction and is used to measure the volume flow of conductive liquid in a pipeline. Since the parts of the electromagnetic flowmeter have no rotating parts and no parts that impede the flow of fluid, there is almost no pressure loss; the liquid to be measured only contacts the pipe liners and electrodes of the electromagnetic flowmeter, so as long as the electrodes and lining materials are selected properly, Has good corrosion resistance, wear resistance [2]; easy to install the level, vertical, tilt can be installed; high measurement accuracy, can reach 0.5%; large range ratio, generally 1:20.

3 electromagnetic flowmeter fault analysis

Electromagnetic flowmeters may cause inaccurate measurement due to various faults during operation. In general, the electromagnetic flowmeters generated during operation may be classified into two types. One is the failure of the flowmeter itself, which is caused by damage to the components; the other is a failure caused by a change in the external conditions, such as an irrational installation that causes flow distortion, deposition, and scaling.

3.1 Media contains air bubbles that have failed to measure

The inhalation of gases from the outside of the medium or the conversion of dissolved gases in the medium into free gas bubbles are two ways of generating bubble gas in the liquid. If there is a large air bubble in the medium, the entire electrode is covered when the air bubble passes through the electrode, and the flow signal input circuit is opened instantaneously, so that the output signal will fluctuate. Judging the cause of such fluctuations can be done by cutting off the loop current of the magnetic field. If the flow meter still shows after cutting off and is still in a state of fluctuation, it is proved that the presence of bubbles in the medium may cause the electromagnetic flow meter to fluctuate. Using an analog multimeter to measure the resistance of the electromagnetic flowmeter electrode, you will find that the loop resistance of the electromagnetic flowmeter electrode is higher than the normal resistance value.

If the electromagnetic flowmeter installation position causes air to enter the measured medium, if the electromagnetic flowmeter is installed at the high point of the piping system and the gas is stored, or the flowmeter fluctuation due to the external air intake, the electromagnetic flowmeter needs to be replaced. The location modification is installed at the lowest point of the pipeline or U-tube installation. However, in some cases, due to the large diameter of the electromagnetic flowmeter or the location of the installation is not easy to change, a gas-collection bag and an exhaust valve may be installed upstream of the flowmeter to solve this problem [3].

3.2 The media is not full

Occasionally there will be non-full tube phenomenon in daily production. This phenomenon can be seen as a typical case of bubbles in liquids. When the electrode level is lower than the liquid level of the medium, it is ideal to use a straight pipe section before and after the flowmeter, and the measurement data is relatively stable. However, the gas volume in the upper half of the tube is also counted as the medium flow, so the measurement error is large in this case; when the electrode level is higher than the medium level, the measurement loop of the electromagnetic flowmeter is in an open state, and the measured data Seriously distorted. Dealing with the failure of this medium's non-full pipe can have the following methods: Install an electromagnetic flowmeter on a vertical pipe that flows from bottom to top; in the actual production, the electromagnetic flowmeter needs to be installed horizontally. In this case, it should be installed on the pipe. The lowest end of the electromagnetic flowmeter and the axis of the electrode is parallel to the horizon (otherwise the sediment will cover the lower electrode); in order to avoid negative pressure in the measurement tube, the sensor of the flowmeter should be installed downstream of the pump and controlled. Upstream of the valve; the installation port of the flowmeter sensor should have a certain back pressure and should be kept away from the direct discharge port [4].

However, the most important thing is that when the electromagnetic flowmeter is installed, it is better to prohibit the occurrence of the medium non-full pipe.

3.3 Electromagnetic flowmeter electrode is corroded

Since some corrosive media are exposed during the production of acetic acid, when the electrode material of the electromagnetic flowmeter is not properly selected, the medium will corrode the electrodes of the flowmeter and eventually cause the sensor to fail. So there will be fluctuations in the flowmeter output. Only when the flowmeter malfunctions after the electrode has been corroded can we find that the electrode material is not resistant to corrosion, and the performance of the material itself cannot be discerned before it is used. Therefore, only replace the new electrode to solve this problem. Therefore, the electrode corrosion failure judgment processing belongs to the method of post-maintenance processing.

3.4 The nature of the liquid to be measured leads to measurement failure

If the conductivity of the measured medium decreases, the output impedance of the electrode will increase. At this time, the input impedance of the converter will cause a load effect, and the flowmeter will produce a measurement error. If the electromagnetic flowmeter appears this fault, then only select the low-conductivity electromagnetic flowmeter that meets the requirements, or use an orifice flowmeter or other principle flowmeter.

3.5 Measurement fouling caused by electrode fouling or electrode short-circuit of the flowmeter

When the measured liquid contains metal, the electrodes of the flowmeter are prone to short circuit, and the measured value of the flowmeter is obviously small or tends to zero. This phenomenon does not happen often in daily production operations. When measuring high-viscosity media, because the media is easy to adhere and deposit on the wall of the pipe, if the conductivity of the measured liquid is lower than the conductivity of the attached media, the signal potential of the electrode will be precipitated and shunted and will not work properly, resulting in an electrode short circuit phenomenon. If the deposited medium is a non-conductive layer, it will cause the electrode open flow meter to not work properly. If the oxidized iron rust layer adheres to the lining tube wall, or the main component is a metal precipitate, its conductivity is greater than the liquid conductivity, and the actual flow rate value is higher than the flow rate value measured by the flow meter; if the precipitate is a scale layer such as calcium carbonate The measured conductivity of the liquid is higher than the conductivity of the precipitate. As a result, the measured flow value will be less than the actual flow rate.

In order to prevent the sediment in the fluid from affecting the operation of the flowmeter, the electrodes of the flowmeter are selected from those that are not easily attached to the protruding tip or hemisphere, and can be replaced or cleaned. The optional electrode can manually scrape the dirt outside the sensor on a regular basis. Alternatively, the measuring circuit may be temporarily disconnected and a small amount of low-voltage and large current may be used to indirectly remove grease deposits between the electrodes. It can also be used to increase the liquid flow rate to clean the wall of the adhesive layer.

3.6 Asymmetry of the medium to be measured

In the normal production situation, the flow velocity of the fluid in the pipeline is axisymmetric and the magnetic field is uniform. The non-axisymmetric flow velocity distribution of the fluid in the actual pipeline often occurs. At this time, the fluid flow can be divided into a straight flow along the axis of the pipeline. The volumetric flow of the liquid to be measured is its integral to the cross section of the pipeline; the other is the vortex flow. . Due to the effect of vortex flow on the output of the sensor, the flowmeter will produce an error [5]. In order to eliminate the influence of vortex flow on the flowmeter sensor, the upstream of the flowmeter should have a straight section long enough to distribute the fluid flow concentrically; the inner diameter of the pipe near the flowmeter should be the same as the inner diameter of the flowmeter. The flow velocity is evenly distributed; otherwise flow regulators can be installed to partially compensate for the lack of upstream straight sections.

3.7 Flowmeter lining distortion causes measurement fluctuations

The lining of the flowmeter is generally made of fluorine plastic, so that the lining of the flowmeter is easily deformed and metering failure occurs. There are two main reasons for the deformation of the lining: thermal diffusion of the fluoroplastic lining into the steam, usually the lining material, thickness, temperature difference inside and outside, the type of fluid and steam, pipeline pressure and many other factors determine the degree of penetration; Depending on the process structure of the fluoroplastic lining material, PTFE is generally used as the fluoroplastic lining material. The non-adhesive force of the PTFE material is only combined with the wall of the pressure, so the negative pressure pipeline does not use this. Kind of material [6].

In order to prevent lining deformation, we generally take the following measures: increase the thickness of the insulation between the flange and the coil box, reduce the fluid temperature difference and reduce the thermal diffusion, so that the temperature difference between the inside and outside of the lining is improved to the maximum extent, so that the permeability can be reduced and the measurement can be slowed down. Condensation of steam in the pipe wall; in addition, the thickness of the PTFE lining is thickened or another type of lining is replaced.

3.8 External electromagnetic interference

There are stray currents, static electricity, electromagnetic waves, and magnetic fields in the production site. The flow signal of the electromagnetic flowmeter is very small, and it is very easy to be interfered by external electromagnetic interference, which affects the normal operation of the electromagnetic flowmeter. The so-called electric field disturbance refers to an abnormal fluctuation of the output signal after the potential balance in the flowmeter measuring tube is destroyed by noise.

In order to reduce the interference of the external magnetic field on the flowmeter, we must install the electromagnetic flowmeter sensor away from the strong magnetic field source. In addition, enhanced shielding measures are taken to prevent interference from strong electric fields. It is also possible to insulate the connection between the electromagnetic flow sensor and the pipe.

3.9 Failure Caused by Other Causes

1) Lightning strikes. Electromagnetic flowmeters are susceptible to high voltages and currents in the line after being struck by lightning, damaging the flow agent.

2) Changes in environmental conditions. Once the environmental conditions of the flowmeter change and new interference sources occur during operation, the normal operation of the instrument will be disturbed and the output signal of the flowmeter will fluctuate.