Causes of electromagnetic flowmeter errors

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 pipe is not full. Due to insufficient back pressure or poor installation of the flow sensor, the liquid in the measuring pipe is not full. The fault phenomenon varies depending on the degree of infilling and flow conditions. If a small amount of gas flows in a stratified or wavy flow in the water pipe, the fault phenomenon is an increase in error, that is, the flow measurement value does not match the actual value; if the flow is a bubble flow or a plug flow, in addition to the measurement value not matching the actual value, the output will also appear due to the gas phase covering the electrode surface for a moment; if the gas phase part of the flow cross-sectional area in the horizontal pipe stratified flow increases, that is, the degree of liquid not filling the pipe increases, and output shaking will also occur. If the liquid is not filling the pipe more seriously, so that the liquid level is below the electrode, the output will appear overfull.

Example 1 A shipyard has a DN80mm electromagnetic flowmeter to measure water flow. The operator reported that when the flow rate is zero after closing the valve, the output reaches the full value instead. On-site inspection revealed that there was only a short pipe downstream of the sensor, and the water was directly discharged into the atmosphere. However, the stop valve was installed upstream of the sensor. After the valve was closed, all the water in the sensor measuring tube was emptied. The fault was solved by modifying the valve to position 2. This type of fault is often encountered in after-sales service cases of manufacturers and should be attributed to engineering design errors.

(2) The liquid contains solid phase. The liquid contains solids such as powder, particles or fibers. Possible faults include: ① slurry noise; ② electrode surface contamination; ③ conductive deposition layer or insulating deposition layer covering the electrode or lining; ④ lining is worn or covered by sediment, and the flow cross-sectional area is reduced.

Example 2 Conductive deposition layer short-circuit effect. 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 the conductive material is gradually deposited, this type of fault usually does not appear during the commissioning period, but will only appear after a period of operation.

On the electrolytic cutting process test device in the tool workshop of a diesel engine factory, a DN80mm instrument is used to measure and control the flow of saturated salt electrolyte to obtain the best cutting efficiency. The meter was operating normally at first. After two months of intermittent use, the flow rate display value was felt to be getting smaller and smaller until the flow rate signal was close to zero. On-site inspection revealed a layer of yellow rust deposited on the surface of the insulation layer. After wiping and cleaning, the meter operated normally. The yellow rust layer is caused by the deposition of a large amount of iron oxide in the electrolyte.

This example is a failure during operation. Although it is not a common failure, if the ferrous metal pipeline is severely corroded and a rust layer is deposited, this short-circuit effect will also occur. Whenever the flow rate display becomes smaller and smaller over time after the meter starts to operate normally, 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 under normal temperature conditions. Since the conduits that convey the fluid have good heat insulation, they will not crystallize during the insulation work. However, it is difficult to implement heat insulation for the measuring tube of the electromagnetic flow sensor. 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 the crystallization problem also exists when using flowmeters based on other principles, in the absence of other better methods, an "oring" electromagnetic flow sensor with a very short measuring tube can be selected, and the upstream pipeline of the flowmeter can be heated and insulated. In terms of the pipeline connection method, the flow sensor should be easy to disassemble and install, so that it can be easily removed and maintained once crystallization occurs.

Example 3 It is not uncommon for electromagnetic flowmeters to fail to work properly due to liquid crystallization. For example, a smelter in Hunan installed a batch of electromagnetic flowmeters to measure the flow of solutions. Because the measuring tube of the electromagnetic flow sensor was difficult to heat and insulate, a layer of crystals formed on the inner wall and the electrode after a few weeks, causing the internal resistance of the signal source to become very large and the instrument reading to be abnormal. Because the diameter of this batch of electromagnetic flowmeters was large, frequent disassembly and cleaning were unbearable, so open channel flowmeters were finally used.

(4) Problems caused by improper selection of electrode and grounding ring materials The electromagnetic flowmeter parts that contact the medium due to material mismatch with the measured medium include electrodes and grounding rings. In addition to corrosion resistance problems, improper matching can also cause surface effects of electrodes. The surface effects should include: ① chemical reaction (formation of a blunt 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.). The grounding ring also has these effects, but the degree of influence is smaller.

Example 4 A chemical (smelting) plant in Shanghai used more than 20 Hastelloy B electrode electromagnetic flowmeters to measure a high concentration of hydrochloric acid solution, and the output signal was unstable and shaking. On-site inspection confirmed that the instrument was normal and ruled out other interference causes that would cause output shaking. However, other users in many places used Hastelloy B electrode instruments to measure hydrochloric acid and it worked well. When analyzing whether the cause of the failure was caused by the difference in hydrochloric acid concentration, there should be no experience in the influence of hydrochloric acid concentration on the electrode surface effect, and no judgment could be made. For this reason, the instrument manufacturer and the user unit used the on-site conditions of the chemical plant to conduct a real flow test to change the hydrochloric acid concentration. The hydrochloric acid concentration gradually increased. The instrument output was stable at low concentrations. When the concentration increased to 15% to 20%, the instrument output began to shake. When the concentration reached 25%, the output shaking was as high as 20%. After switching to a tantalum electrode electromagnetic flowmeter, the flowmeter operates normally.

(5) Problems caused by liquid conductivity exceeding the allowable range If the liquid conductivity is close to the lower limit, shaking may also occur. Because the lower limit specified in the manufacturer's instrument specification is the lowest value that can be measured under various good conditions, and the actual conditions cannot be ideal, we have encountered low-degree distilled water or deionized water many times, whose conductivity is close to the lower limit of 5 specified in the electromagnetic flowmeter specification, but the output shakes 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 the liquid can be consulted in the relevant manual. If there is no ready-made data, samples can be taken and measured with a conductivity meter. However, sometimes there are also cases where samples are taken from the pipeline and measured in the laboratory and are considered to be 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 to generate carbonic acid or nitric acid, and the conductivity increases.

For noisy slurries generated by liquids containing particles or fibers, the method of increasing the excitation frequency can effectively improve the output shake. The frequency-adjustable IFM3080F DN300 electromagnetic flowmeter was used to measure the instantaneous flow sway at different excitation frequencies on site when measuring 3.5% corrugated cardboard slurry. When the frequency was low, 50/32Hz, the sway was as high as 10.7%; when the frequency was increased to 50/2Hz, the sway was reduced to 1.9%, and the effect was very obvious. (end)