Electromagnetic flowmeter error causes

Electromagnetic flowmeters have many advantages, but improper selection, installation, and use will cause increased errors, unstable indications, and even damage to the meter body.

(1) The liquid in the tube is not full. Due to insufficient back pressure or poor installation position of the flow sensor, the liquid in the measuring tube is not full. The fault phenomenon has different manifestations depending on the degree of underfill and the flow condition. If a small amount of gas flows in a stratified flow or a wavy flow in a water pipe, the fault phenomenon will be an increase in error, that is, the flow measurement value does not match the actual value; if the flow is bubble flow or plug flow, the fault phenomenon will be except that the measured value does not match the actual value. In addition, output sloshing will also occur due to the gas phase momentarily covering the electrode surface; if the flow cross-sectional area of ​​the gas phase increases in the stratified flow of the horizontal pipe, that is, the liquid is not full of the pipe, output sloshing will also occur. If the liquid is not full of the pipe, output sloshing will also occur. If the situation is serious enough that the liquid level is below the electrode, the output will be over full.

Example 1 A certain factory has an electromagnetic flowmeter measuring water flow. The operator reported that when the flow rate was zero after closing the valve, the output reached the full value instead. On-site inspection found that there was only a short section of pipe downstream of the sensor, and the water was discharged directly into the atmosphere. However, the stop valve was installed upstream of the sensor. After the valve was closed, all water in the sensor measuring pipe was emptied. Modify the valve to position 2 and the fault will be solved. The cause of this kind of failure is often encountered in the after-sales service cases of manufacturers, and it is undoubtedly an engineering design error.

(2) The liquid contains solids. The liquid contains solids such as powder, particles or fibers. Possible faults include: ① Slurry noise; ② Contamination of the electrode surface; ③ Conductive deposition layer or insulating deposition layer Covering the electrode or lining; ④ The lining is worn or covered with sediment, and the flow cross-sectional area is reduced.

Example 2 Short circuit effect of conductive deposition layer. If conductive material is deposited on the insulating lining of the electromagnetic flow sensor measuring tube, the flow signal will be short-circuited and the instrument will fail. Since conductive substances are gradually deposited, this type of fault usually does not appear during the commissioning period, but will appear after a period of operation.

On the electrolytic cutting process test device in a tool workshop of a diesel engine factory, the instrument measures and controls the flow of saturated salt electrolyte to obtain cutting efficiency. At first, the instrument operated normally, but after 2 months of intermittent use, the flow display value became smaller and smaller until the flow signal was close to zero. During the on-site inspection, it was found that a layer of yellow rust was deposited on the surface of the insulation layer. After wiping and cleaning, the instrument ran normally. The yellow rust layer is caused by the deposition of a large amount of iron oxide in the electrolyte.

This example is a fault during operation. Although it is not a common fault, if the ferrous metal pipeline is severely corroded and the rust layer is deposited, this short circuit effect will also occur. If it starts to run normally, but as time goes by, the flow rate becomes smaller and smaller, the possibility of such a failure should be analyzed.

(3) Electromagnetic flowmeters should be used with caution for liquids that may crystallize. Some chemical materials that are prone to crystallization can be measured normally when the temperature is normal. Since the conduits transporting fluids have good heat tracing and insulation, It will not crystallize during heat preservation work, but the measuring tube of the electromagnetic flow sensor is difficult to implement heat tracing and heat preservation. Therefore,When the fluid flows through the measuring tube, it is easy to cause a layer of solid to form on the inner wall due to cooling. Since flowmeter measurement using other principles also has crystallization problems, if there is no other better method, a "ring-shaped" electromagnetic flow sensor with a very short measuring tube length can be used, and the upstream pipe of the flowmeter can be Heat tracing and insulation are enhanced. In terms of the pipeline connection method, consider that the flow sensor is easy to disassemble and assemble, so that it can be easily removed and maintained once it crystallizes.

Example 3 It is not uncommon for electromagnetic flowmeters to fail to work properly due to liquid crystallization. For example, a smelting plant installed a batch of electromagnetic flowmeters to measure solution flow. Because the measuring tube of the electromagnetic flow sensor was difficult to implement heat tracing and insulation, a layer of crystals formed on the inner wall and electrodes after a few weeks, causing the internal resistance of the signal source to become very high. Large, the meter indicates that the value is abnormal. Because these electromagnetic flowmeters had a large diameter and were unbearable for frequent disassembly and cleaning, they were later switched to open channel flowmeters.

(4) Problems caused by improper selection of materials for electrodes and grounding rings. The parts of the electromagnetic flowmeter that are in contact with the medium are the electrodes and grounding rings that cause malfunctions due to the mismatch between the materials and the measured medium. Improper matching will eliminate the problem. In addition to corrosion resistance issues, as long as it is the electrode surface effect. Surface effects should include: ① chemical reaction (formation of passive film on the surface, etc.); ② electrochemical and polarization phenomena (generation of electric potential); ③ catalytic action (generation of aerosol on the electrode surface, etc.). Ground rings also have these effects, but to a lesser extent.

Example 4: A chemical (smelting) plant uses more than 20 Hastelloy B electrode electromagnetic flowmeters to measure a hydrochloric acid solution with a high concentration, and the output signal appears to be unstable and sloshing. On-site inspection confirmed that the instrument was normal and other interference causes that would cause output shaking were eliminated. However, other users have used the Hastelloy B electrode meter to measure hydrochloric acid and it has worked well in many places. When analyzing whether the cause of the failure is caused by the difference in hydrochloric acid concentration, there should be no experience in the effect of hydrochloric acid concentration on the electrode surface effect, and it is not yet possible to make a judgment. To this end, the instrument manufacturer and the user unit use the on-site conditions of the chemical plant to conduct real-flow tests to change the concentration of hydrochloric acid. The concentration of hydrochloric acid gradually increases. At low concentrations, the instrument output is stable. When the concentration increases to 15% to 20%, the instrument output begins to shake. When the concentration reaches 25%, the output shaking amount is as high as 20%. After switching to a tantalum electrode electromagnetic flowmeter, it runs normally.

(5) Problems caused by liquid conductivity exceeding the allowable range If the liquid conductivity is close to the lower limit, sloshing may occur. Because the lower limit value stipulated in the manufacturer's instrument specification is a low value that can be measured under various good conditions of use, and the actual conditions cannot be ideal, so low-strength distilled water or deionized water has been encountered many times. Its conductivity is close to the lower limit value 5 specified in the electromagnetic flowmeter specification, but the output oscillates when used. It is generally believed that the lower limit of conductivity that can be stably measured is 1 to 2 orders of magnitude higher.

The conductivity of liquids can be found in relevant manuals. If there is a lack of ready-made data, samples can be taken and measured with a conductivity meter. But sometimes samples are taken from the pipeline and sent to the laboratoryThe measurement is considered usable, but the actual electromagnetic flowmeter cannot work. This is because the liquid when measuring conductivity is different from the liquid in the pipeline. For example, the liquid has absorbed CO2 or NO in the atmosphere, generating carbonic acid or nitric acid, and the conductivity increases.

For noisy slurries produced by liquids containing particles or fibers, increasing the excitation frequency can effectively improve output sloshing. Table 9.4 shows an electromagnetic flowmeter with adjustable frequency, which measures corrugated cardboard slurry with a concentration of 3.5%. The displayed instantaneous flow sloshing amount is measured at different excitation frequencies on site. When the frequency is lower, 50/32Hz, the shaking is as high as 10.7%; when the frequency is increased to 50/2Hz, the shaking is reduced to 1.9%, and the effect is very obvious.