Causes of electromagnetic flowmeter errors

Causes of Errors of Electromagnetic Flowmeters I have mentioned the advantages of electromagnetic flowmeters before, and now I will talk about the causes of errors in the use of electromagnetic flowmeters. Electromagnetic flowmeters have many advantages, but if they are improperly selected, installed, and used, they will cause increased errors, unstable indications, and even damage to the meter body. The explanation is as follows: (1) The liquid in the tube is not full. Due to insufficient back pressure or the 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 paper mill has a CKY series LDBE-DN80mm electromagnetic flowmeter to measure water flow. The customer operator reported that when the flow rate was zero after closing the valve, the output reached the full value instead. After an on-site inspection by the company's technical staff, it was discovered 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 the water in the sensor measuring pipe was emptied. Modify the valve to position 2 and the fault will be solved. This type of failure is often encountered in our company's production and after-sales service cases. This case is 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 lining Being worn or covered by sediment, 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 CKY series DN100mm instrument is used to measure and control the flow of saturated salt electrolyte to obtain optimal 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.

These examples are all operational faults. Although they are not common faults, if the ferrous metal pipes are 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.