Electromagnetic flowmeter measurement data output shake inspection and corresponding measures

When the electromagnetic flowmeter measures the flowing liquid medium, the real-time flow data obtained will become unstable due to the various conditions in the fluid and the unfavorable external measurement environment of the instrument. This will cause the output of the instrument to shake. The reasons for this are multifaceted. If we want to make the data we obtain stable and accurate, we need to understand the reasons for the existence of these unfavorable factors and actively eliminate them so that the measurement working conditions of the electromagnetic flowmeter can be as close to the ideal working conditions as possible. As an enterprise that has been operating in the electromagnetic flowmeter production industry for many years, Runzhong Instrument Co., Ltd. has a deeper understanding of how to improve the stability and accuracy of electromagnetic flowmeter measurement. In the process of long-term cooperation with customers, a set of effective solutions has been summarized. The output shaking problem of electromagnetic flowmeter during measurement needs to be paid attention to by every measurement technician. The following will introduce its causes and solutions to friends one by one:
 
First, analyze the causes of its failure.
　Output shaking can be generally summarized into five reasons for failure, which are:
(1) The flow itself is fluctuating or pulsating, which is not actually a fault of the electromagnetic flowmeter, but only reflects the flow condition truthfully;
(2) The pipeline is not full of liquid or there are bubbles in the liquid;
(3) Electrical and magnetic interference such as external stray current;
(4) Liquid physical properties (such as uneven liquid conductivity or slurry containing more variable particles/fibers, etc.);
(5) Improper matching of electrode materials and liquid.
 
　　Second, enter the corresponding inspection procedure.
　Figure 4 shows the process of checking the output shaking of the electromagnetic flowmeter. First, conduct a comprehensive preliminary investigation and judgment according to the flow chart, and then conduct a detailed inspection and try to eliminate the faults item by item. The order of inspection listed in the process is as follows: ① The inspections that can be observed or inquired that do not require major operations should be carried out first, that is, the easy ones should be carried out first, and then the difficult ones; ② According to past on-site maintenance experience, the ones that have a higher frequency of occurrence and a higher probability of occurrence in the future should be carried out first; ③ The order of inspection itself. If the initial investigation confirms that the latter few fault causes are sufficient, a detailed inspection can also be carried out in advance.
Figure 4 Electromagnetic flowmeter output shake inspection process

① Resistance method	●On and off of fuse ●On and off of signal cable and excitation cable ●On and off of excitation coil ●Electrode symmetry measurement ●Insulation resistance of electrode to ground ●Insulation resistance of excitation coil to ground	Troubleshooting process
②Current method	●Measure the excitation current ●Measure the output current
③ Voltage method	Determination: Is the working power supply (including power supply and converter power supply) correct?
④ Waveform method	Measure the waveform of key points based on familiarity with the line and identify the fault location

 
 
　3. After obtaining the cause and carefully checking the fault on site, further solutions need to be taken.
This section discusses the inspection methods and measures for the above five fault causes.
 
1. The pipeline is not filled with liquid or the liquid contains bubbles.
Check item 2 of the flow chart. This type of fault is mainly caused by poor design of the pipeline network engineering, which makes the sensor's measuring tube not filled with liquid or the sensor is improperly installed. Measures should be taken to avoid installing it in positions a, e as shown in Figure 3 and position b when discharging with a dotted pipe, and modify it to positions c, d.
 
There is no back pressure or insufficient back pressure downstream of the sensor. If it is installed in position e, the liquid flows through a very short section of the downstream pipe and is discharged into the atmosphere. If valve 2 is fully opened, the sensor measuring tube may not be filled with liquid. Sometimes the flow rate of the process is large enough to fill the meter and the meter operates normally. If the flow rate decreases, the liquid may not be full and the meter may malfunction.
 
There are two ways for the formation of bubbling gas in the liquid: inhalation from the outside and conversion of dissolved gas (air) in the liquid into free bubbles. The number of bubbles in the liquid is small and the bubble diameter is much smaller than the electrode diameter. Although the volume of some liquid is reduced, the electromagnetic flow output will not fluctuate. Larger bubbles can cover the entire electrode by rubbing against the electrode, causing the flow signal loop to open instantly, and the output signal will fluctuate more.
 
● Tiny bubbles in the liquid flow will gradually accumulate at high points or dead corners during the flow process. If the electromagnetic flowmeter is installed at a high point in the pipe system, the retained gas will reduce the liquid flow area in the sensor and affect the measurement accuracy. When there is more retention, interference signals will also be generated (see Case 3); if the sensor is installed at a high point, the gas accumulated at the high point exceeds the capacity or due to pressure fluctuations, the gas flows with the liquid in the form of bubbles or sheets, covering the electrode and causing output fluctuations.
 
● Common ways of inhaling air from the outside are mainly bubbles in the raw water of rivers in water supply utilities, or the water level at the suction port is too low (usually requiring a distance of 2-5 times the diameter of the suction port, depending on the suction flow rate), forming an inhaled vortex that draws in air. In the process industry, air is mixed into the proportioning mixing container during stirring, and air is sucked into the pump suction end or other places where the sealing is poor. This type of fault is often encountered in practice.
 
● Dissolved air in the liquid separates into free bubbles. When the pressure of the pipe system decreases, the originally dissolved air (or gas) will separate into free bubbles. For example, the valves at both ends of the pipe system filled with liquid are closed, and it gradually cools down after stopping operation. Due to the different thermal expansion coefficients, the liquid shrinks much more than the pipe system, and a contraction space is formed in the pipe system, forming a local vacuum state. The dissolved air in the liquid separates out to form bubbles and accumulates at the high point of the pipe system. Restart, the liquid with bubbles flowing over the electrode surface may cause the electromagnetic flowmeter output to shake. This may be one of the reasons for the phenomenon that the electromagnetic flowmeter output shakes at the initial stage of the pipe system startup and then tends to stabilize. For example, water can dissolve up to about 0.3% VN air at 1 atmosphere and 0°C. If the water temperature rises during the process, the air will separate into free bubbles (at 30°C, it can only dissolve about 0.15% at most). Accumulation may also cause faults.
 
2.
Check the first item of the flow chart for the fluctuation (or pulsation) of the flow itself. If the flow itself fluctuates, the shaking of the instrument output is a true reflection of the fluctuation. The inspection method can be to ask the operator and process technicians at the site of use or inspect whether there is a source of fluctuation. There are usually three reasons for the fluctuation (or pulsation) of the pipeline flow: (1) The flow power source upstream of the electromagnetic flowmeter uses a reciprocating pump or a diaphragm pump (often used for filling liquid in fine chemicals, food, medicine and water purification). The pulsation frequency of these pumps is usually between a few times and more than a hundred times per minute; (2) The flow characteristics and size of the control valve downstream of the instrument are not properly selected, resulting in hunting. This can be observed to see whether the control valve stem has oscillatory movement; (3) Other disturbance sources cause flow fluctuations, for example: whether there is a flow block (such as a fully open butterfly valve) in the upstream pipeline of the electromagnetic flowmeter to generate a vortex (such as the vortex column generated by the vortex generator of the vortex flowmeter, the gasket at the inlet end of the sensor extends into the flow channel, and the gasket strip fragments are suspended in the liquid flow and swing, etc.).
 
In pipelines with pulsating flow sources, in order to mitigate their impact on flow meter measurement, the flow sensor is usually placed away from the pulsating source and the flow resistance of the pipe flow is used to attenuate the pulsation; or a gas chamber buffer called a passive filter is installed at an appropriate position on the pipeline to absorb the pulsation.
 
3. External electromagnetic interference
Check item 3 of the flow chart. Electromagnetic flowmeters are easily affected by external interference due to their small flow signals. The interference sources are mainly pipeline stray current, static electricity, electromagnetic waves and magnetic fields.
 
● Pipeline stray currents are mainly protected by good grounding of electromagnetic flowmeters. Usually, the grounding resistance should be less than 100Ω, and the grounding should not be shared with other motors and electrical appliances. Sometimes the environmental conditions are good and the electromagnetic flowmeter can work normally without grounding, but we believe that it is still appropriate to do grounding even so. Because once the good environmental conditions no longer exist and the instrument fails, it will affect the use, and various inspections will bring a lot of trouble.
 
Sometimes , although the electromagnetic flowmeter is well grounded, the pipeline stray current is too strong (such as electrolysis process pipelines and cathode protection pipelines) to affect the normal measurement of electromagnetic flow juice. At this time, the electromagnetic flow sensor must be electrically insulated from the pipeline. For specific examples and their inspection and elimination process, please refer to Case 12.
 
● Static electricity and electromagnetic wave interference can be introduced through the signal line between the electromagnetic flowmeter sensor and the converter. Usually, if it is well shielded (such as using shielded cables for signal lines and placing cables in protective iron pipes), it can be prevented and controlled. However, there have been cases where strong electromagnetic wave prevention is ineffective. In this case, move the converter closer to the sensor, shorten the connected signal cable, or use an integrated instrument without external cables. For specific content of the example, please refer to Case 10.
 
● Magnetic field interference is usually only prevented by keeping the electromagnetic flow sensor away from the strong magnetic field source. The ability of the electromagnetic flowmeter to resist magnetic fields varies depending on the structural design of the sensor. For example, if the protective shell of the sensor excitation coil is made of non-magnetic materials (such as aluminum, plastic), the ability to resist the influence of magnetic fields is weak, while if it is made of steel, it is stronger.
 
4. Demonstration and verification of liquid properties
Check item 4 of the flow chart. There are 3 factors in the liquid properties that can cause the output to shake. They are: (1) the liquid contains solid particles or bubbles, (2) the two liquids in the two-component liquid have different conductivity and are not evenly mixed, or the chemical reaction in the pipeline has not been fully completed, (3) the conductivity of the liquid is close to the lower limit.
 
● The measured liquid contains more solid particles, which will cause the flow signal to have spike pulse noise, etc., just like the bubbles mentioned above, causing output shaking. If the solid phase is in powder form, it will usually not cause output shaking.
 
● In the fine chemical industry, food industry, pharmaceutical industry and water treatment projects, liquid medicine is often added to the main liquid, and the liquid medicine is usually injected by a reciprocating pump or a diaphragm pump in proportion to the main liquid flow rate. After the liquid medicine is injected, the upper liquid presents a series of segments separated by liquid medicine segments and non-liquid medicine segments. If the two liquids with different conductivity are not mixed evenly, the output of the electromagnetic flowmeter measuring the flow rate downstream will show shaking. In this case, the liquid addition point should be moved downstream, or the electromagnetic flowmeter should be moved upstream of the liquid addition point; if it is limited by on-site conditions or the modification work is too large, a mixer can be installed downstream of the liquid addition point to remedy it. However, after installing the static mixer, the liquid flow will produce a small rotation flow, which may cause an additional error of 1% or more. However, compared with the inability to measure the output shake, it is a measure that weighs the two disadvantages.
 
If the mixed liquid enters the electromagnetic flow juice measurement before the chemical reaction in the pipeline is completed, the output shake phenomenon may also occur. In this case, the only way is to change the position of the measurement point, so that the measurement position is upstream of the mixing point or downstream away from the mixing section. However, the distance away from the mixing section needs to be very long. For example, if the reaction time is 60s and the liquid flow rate is 3m/s, the distance is required to be 180m without considering the insurance factor.
 
●If the conductivity of the liquid is close to the lower limit, it may also cause shaking. 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 many times that the output shakes when measuring low-degree distilled water or deionized water, whose conductivity is close to the lower limit of 5×10-6S/cm specified in the electromagnetic flowmeter specification. It is generally believed that the lower limit of conductivity that can be stably measured should be 1-2 orders of magnitude.
 
The conductivity of the liquid can be found in the appendix or relevant manuals. If there is no ready-made data, it can be sampled and measured with a conductivity meter. But sometimes there are also cases where samples are taken from the pipeline and measured in the laboratory and it is 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 C02 or NOx in the atmosphere to generate carbonic acid or nitric acid, which changes the conductivity.
 
For noisy slurries containing particles or fibers, increasing the excitation frequency can effectively improve the output shake. Table 7-1 shows the frequency-adjustable IFM3080F DN300 electromagnetic flowmeter, which measures the instantaneous flow sway at different excitation frequencies when measuring the 3.5% concentration of corrugated low-board slurry on site. When the frequency is as low as 50/32Hz, the sway is as high as 10.7%; when the frequency is increased to 50/2Hz, the sway is reduced to 1.9%, and the effect is very obvious.
 
Table 1 Instantaneous flow sway at different excitation frequencies
 

Excitation frequency/Hz	Display flow rate (peak sway range)/m3•h-1	Percentage of the mean
50/32	180-223	10.7
50/18	200-224	5.6
50/6	190-220	7.3
50/2	255-265	1.9

 
5. Investigate the matching of liquid and electrode materials
Check item 5 of the flow chart. The selection of electrode materials should first consider the corrosion resistance of the measured liquid. However, improper selection will produce electrode surface effects, which will cause output shaking and other faults. Electrode surface effects include the formation of insulating layers such as passivation film or oxide film on the electrode surface, as well as polarization phenomena and electrochemistry. There is not enough information available for dielectric-core material matching like corrosion resistance. There are only some limited experiences, which need to be accumulated in practice.
 
Tantalum-water, alkali and other non-acidic liquids When measuring water flow, tantalum electrodes will form an insulating layer, causing the instrument to fail or produce a lot of noise after a short period of operation. In the process flow, even if the tantalum electrode is in contact with water or "non-acidic" liquid for a very short time, such as flushing the pipe with clean water, it will affect the normal use of the instrument. Alkali liquids such as sodium hydroxide should not be selected for tantalum electrodes.
 
Hastelloy B-high concentration hydrochloric acid Hastelloy B has several good applications for hydrochloric acid with low temperature concentration. However, when the concentration exceeds a certain value, noise will be generated, and tantalum electrodes should be used instead. There is also practical experience of similar effects in acid liquids such as nitric acid and sulfuric acid.
 
When platinum-hydrogen peroxide platinum electrode is used to measure low-pressure hydrogen peroxide (pressure less than 0.3MPa), the catalyst produces aerosol on the electrode surface, blocking the electrical path and affecting the work.
Mendelevium-hydrochloric acid with a concentration greater than 10% will produce noise in the electric field of platinum hydrochloric acid with a concentration greater than 10%, so tantalum electrode should be used instead.
Hastelloy B-aluminum sulfate solution water plant uses aluminum sulfate to mix with raw water to condense suspension. We have encountered Hastelloy B electrode measuring 15% aluminum sulfate solution, and the output shook. Later, we switched to acid-resistant steel electrode and obtained satisfactory results.