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There are many advantages to electromagnetic flowmeters. Improper selection and installation will cause errors.
2012-3-24 8:35:15

的故障类型 Failure type of electromagnetic flowmeter

For representative high temperature fluids that need to be measured but have some problems in measurement, we list the following three. (Vortex flowmeter, integrated orifice flowmeter) High-temperature gas, high-temperature steam as heat-conducting medium or energy source, strong thermal-conducting coal for high-speed breeder reactor-liquid metal sodium, etc .; It is these three fluids. Due to the difference in accuracy and materials used, the main points required in measuring the flow are different, and it is difficult to describe the common points in common. Therefore, in this chapter, only the practical problems of the throttling flow meter used for high temperature gas and high temperature steam and the electromagnetic flow meter for measuring liquid metal are described as the center to describe the flow measurement of high temperature fluid. For molten iron and molten steel, there are two methods for measuring the flow: one is the method of measuring the cross-sectional area of the flume using the high-frequency skin effect; the other is the electromagnetic flowmeter. Both methods can be used in continuous steelmaking equipment. In order to perform direct, non-contact measurement, various aspects of research and discussion have been conducted, but in fact, batch processing is generally performed by converting the flow rate to liquid column height or weight. Therefore, it is not described in this chapter. In addition, when measuring the flow rate of liquid metal sodium, the ultrasonic method of installing a detector outside the piping was tested, and it will be practical soon, and it will not be described here. Application examples of various devices for measuring the flow of high-temperature fluids with a throttling mechanism The departments that use high-temperature fluids are mainly electric power, steel, and chemical industries.

When measuring the flow rate of liquid, water vapor, gas, etc., people have first studied whether the differential pressure flow meter of the throttling mechanism is applicable from many types of flow meters with different principles and structures, and in fact, it has the most applications. This is because it has a simple structure and has rich data and practical experience. As a result, the throttling device, as a process detection instrument, is more valued than any other instrument. With the goal of high efficiency and large capacity, power generation equipment for enterprises and power plants for various plants are used to increase the steam temperature and pressure of boilers to critical temperatures and pressures. This kind of steam is high temperature and high pressure superheated steam, its temperature reaches 568 ° C, and its pressure reaches 240 kgf / cm2. The flow of high-pressure steam flowing from a boiler to a steam turbine is generally measured with a throttle device such as a nozzle. In steelmaking and ironmaking, the temperature of the combustion air is increased and then blown into a furnace such as a blast furnace, a converter, and a sintering furnace. The flow of these combustion air can be measured with throttling devices such as venturi tubes. In addition, the gas discharged from these furnaces also belongs to the high temperature range, and the flow rate can be measured with a venturi tube. Because the exhaust gas is high-temperature gas, it is corrosive and contains dust. When selecting materials, we must consider these characteristics and conditions of use, and choose materials that are corrosion-resistant and suitable for the purpose of use. In the chemical and petrochemical industries, for example: hydrogen production equipment and reforming furnaces using steam reforming methods, the temperature inside the reaction tube is also increased by about 800-900 ° C. Petroleum refining contact upgrading device. The temperature is 420-580 ° C, and the pressure is 15-50 kgf / cm2. Although the furnace temperature in these devices is high, it is because the flow rate before entering the furnace is measured and controlled. It is not necessary to measure such hot fluids. The above are just a few examples. There are many more high-temperature fluids to be measured, and the list is endless. When considering the use of a throttling device to measure the flow of high-temperature fluids, as well as the selection of valve materials, it is important to pay attention to pressure while paying attention to temperature. With the proper materials selected, flow rates in the high temperature range can be measured. However, from the point of view of material strength, the tensile strength and yield point decrease with increasing temperature. When the temperature reaches about 400 ° C or higher, creep occurs. Therefore, this temperature can be regarded as the highest temperature in the throttling device. .


At present, for industrial measuring instruments, different types have their own corresponding high temperature ranges. For example, the thermometer uses the range of 1500 to 2000 ° C as the high temperature range. For valves such as control valves and on-off valves, when referring to high temperatures, it means a temperature of 400 to 500 ° C or more. The flowmeter also stipulates different high temperature ranges according to the principle structure. The current practical upper limit of industrial temperature is about 540 ℃ for the differential pressure flow meter of the throttling mechanism, about 200 ℃ for the positive displacement flowmeter, and about 400 ℃ for the area type flowmeter.


Electromagnetic flowmeters have many advantages, but if they are selected, installed, or used improperly, errors will increase, the displayed value will be unstable, and even the meter body will be damaged.

(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 has not been filled. The failure phenomenon has different performances due to the degree of underfill and flow conditions. If a small amount of gas is stratified or wavy in the water pipe, the fault phenomenon will show an increase in error, that is, the measured value of the flow does not match the actual value; if the flow is a bubble flow or plug flow, the fault phenomenon will not match the actual value except the measured value In addition, there will be output sloshing due to the gas phase covering the electrode surface instantly; if the cross-sectional area of the gas phase in the horizontal flow of the layered pipe increases, that is, the degree of liquid is not full, the output sloshing will also occur. The situation is more serious, so that the liquid level is below the electrode, the phenomenon of output overfullness will occur.

测量水流量,运行人员反映关闭阀门后流量为零时,输出反而达到满度值。 Example 1 A shipyard has a SKLD series DN80mm electromagnetic flowmeter to measure the water flow. The operator reports that when the flow is zero after the valve is closed, the output actually reaches the full value. On-site inspection found that there was only a short tube downstream of the sensor, and water was directly discharged into the atmosphere, but a shut-off valve was installed upstream of the sensor. After the valve was closed, the water in the sensor measurement tube was completely drained. Refit the valve to position 2 and the fault is resolved. This kind of failure cause is often encountered in the case of after-sales service of the manufacturer, which is a mistake in engineering design.

(2) Liquid contains solid phase liquid contains powder, particles or fibers and other solids. Possible faults are: ① slurry noise; ② electrode surface contamination; ③ conductive or insulating deposition layer covers the electrode or lining; lining Covered by abrasion or sediment, the flow cross-sectional area is reduced. Example 2 Short circuit effect of conductive deposits. If a conductive substance is deposited on the insulating lining of the measuring tube of the electromagnetic flow sensor, the flow signal will be short-circuited and the instrument will fail. Due to the gradual deposition of conductive materials, this type of fault usually does not occur during the commissioning period, but only after a period of operation. On an electrolytic cutting process test device in a tool shop of a diesel engine factory, the SKLD series DN80mm instrument was used to measure and control the flow rate of saturated salt electrolyte to obtain the best cutting efficiency. At first, the meter was operating normally. After two months of intermittent use, the flow display value became smaller and smaller until the flow signal was close to zero. On-site inspection found that a layer of yellow rust was deposited on the surface of the insulation layer, and the meter operated normally after cleaning. The yellow rust layer is caused by a large amount of iron oxide deposition in the electrolyte. This example is a fault during operation. Although it is not a common fault, if the ferrous metal pipe is severely rusted and a rust layer is deposited, this short-circuit effect will also occur. Whenever it starts to run normally and the flow shows less and less over time, the possibility of such a failure should be analyzed.

(3) For liquids that may crystallize, electromagnetic flowmeters should be used with caution. Some chemical materials that are easy to crystallize can be measured normally under normal temperature conditions. Since the conduits that transport fluids have good heat insulation, they will not crystallize during thermal insulation work. However, it is difficult for the measuring tube of the electromagnetic flow sensor to implement heat tracing and insulation. Therefore, when the fluid flows through the measuring tube, it is easy to cause a solid layer on the inner wall due to cooling. Since there are also crystallization problems when using other flowmeters for measurement, so in the absence of other better methods, an "oring" electromagnetic flow sensor with a very short measuring tube length can be selected, and the flowmeter is The heat tracing and insulation of the upstream pipes are strengthened. In terms of pipeline connection, consider the convenience of disassembly and assembly of the flow sensor, which can be easily removed and maintained once it has crystallized. Example 3 It is not uncommon for an electromagnetic flowmeter to malfunction due to liquid crystals. For example, a smelter in Hunan installed a batch of electromagnetic flowmeters to measure the solution flow. Because the measuring tube of the electromagnetic flow sensor was difficult to implement heat tracing, 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, the meter indicates that the value is abnormal. Due to the large caliber of these batches of electromagnetic flowmeters, frequent dismantling and washing can't stand it, so they switched to open channel flowmeters in the end.

(4) Problems caused by improper selection of electrode and grounding ring materials Electromagnetic flowmeters that are in contact with the medium that cause failure due to the mismatch of materials and measured medium have electrodes and grounding rings. In addition to mismatching, except for corrosion resistance, as long as Electrode surface effect. The surface effects should be: ① chemical reactions (the formation of dull films on the surface, etc.); ② electrochemical and polarization phenomena (generating electric potential); ③ catalyst action (aerosol generation on the electrode surface, etc.). The ground loop also has these effects, but to a lesser extent. Example 4: A chemical (smelting) plant in Shanghai used more than 20 Hastelloy B-electrode electromagnetic flowmeters to measure hydrochloric acid solutions with a relatively high concentration, and the output signal was unstable. On-site inspection confirms that the meter is normal, and eliminates other causes of interference that may cause output jitter. However, it works well when other users use Hastelloy B electrode meter to measure hydrochloric acid. When analyzing whether the cause of the fault is caused by the difference in the concentration of hydrochloric acid, there should be no experience in the effect of the concentration of hydrochloric acid on the surface effect of the electrode, and no judgment can be made. To this end, the instrument manufacturer and the user unit use the on-site conditions of the chemical plant to conduct a real-flow test to change the concentration of hydrochloric acid. The concentration of hydrochloric acid gradually increases, and the output of the meter is stable at low concentrations. When the concentration increases to 15% to 20%, the output of the meter starts to shake. When the concentration reaches 25%, the output shaking amount is as high as 20%. After using the tantalum electrode electromagnetic flowmeter, it runs normally.

(5) Problems caused by the liquid conductivity exceeding the allowable range If the liquid conductivity is close to the lower limit value, the phenomenon of sloshing may occur. Because the lower limit value specified by the manufacturer's instrument specification is the lowest value that can be measured under various conditions of good use, and the actual conditions cannot be ideal, so I encountered low-level distilled water or Ionized water, whose conductivity is close to the lower limit value 5 specified by the electromagnetic flowmeter specification, but output jitter occurs during use. It is generally believed that the lower limit of the conductivity that can be measured stably is 1 to 2 orders of magnitude higher. The conductivity of the liquid can be consulted in the relevant manual, or the lack of ready-made data can be measured with a conductivity meter. But sometimes there are cases where samples are taken from the pipeline to the laboratory for determination, but the actual electromagnetic flowmeter cannot work. This is due to the difference between the liquid when measuring the conductivity and the liquid in the pipeline. For example, the liquid has absorbed CO2 or NO in the atmosphere to generate carbonic acid or nitric acid, which increases the conductivity. For noise slurry containing particles or fibrous liquid, the method of increasing the excitation frequency can effectively improve the output sloshing. ,测量浓度3.5%瓦楞纸板浆液,在现场以不同激励频率测量所显示瞬时流量晃动量。 Table 9.4 shows the SKLD DN300 electromagnetic flowmeter with adjustable frequency. It measures the corrugated cardboard slurry with a concentration of 3.5% and measures the instantaneous flow sloshing at different excitation frequencies in the field. When the frequency is low, 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.