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Fault inspection and case analysis of yj-lde series electromagnetic flowmeter

Section 8 fault description and cases

 electromagnetic flowmeter

In this chapter, the fault type analysis and electromagnetic flowmeter During the fault inspection, some descriptions are made on the fault source. In this section, the fault source is further discussed, and 13 specific cases are cited. These cases are the practical experience of the author and his colleagues except for the notes. A fault source will show a variety of fault phenomena, which are summarized in table 7.2.


Fault source

Fault phenomenon

category

name

one No signal output

two Output shaking

three Zero point instability

four The measured values are inconsistent

five Output over full

1、 Piping system and related equipment

one Poor installation

 

 

two Not full tube

 

 

 

 

 

① A small amount of gas is dispersed

 

 

 

② The gas increases in a stratified or wavy flow

 

 

 

③ It is bubble flow or plug flow

 

 

 

 

④ The liquid level is below the electrode

 

 

 

 

three Trapped gas in piping system

 

 

 

four Piping suction bubbles

 

 

 

five Reciprocating pump or control valve oscillation produces pulsation

 

 

 

six Change of flow state during use

 

 

 

 

2、 Fluid

one There is gas in the liquid

 

 

 

two The liquid contains solids

 

 

 

 

 

① Slurry noise

 

 

 

 

② The electrode is contaminated

 

 

 

 

③ The electrode or lining is covered with conductive or insulating deposition layer

 

 

 

 

④ The lining is worn or deposited to change the flow area

 

 

 

 

three The conductivity is uneven or close to the threshold value

 

 

 

four Material mismatch with liquid contact

 

 

 

five Flow noise

 

 

 

 

3、 Environment

one Strong magnetic field

 

 

 

 

two Strong electromagnetic wave

 

 

 

 

three Stray current in pipeline

 

 

 

 

four Variation of ground potential

 

 

 

five Moisture immersion

 

 



1、 Piping system and related equipment


Caused by piping system and related equipment electromagnetic flowmeter The main fault sources are: 1. Improper installation; 2. Incomplete pipe; 3. Gas storage in the pipeline system; 4. Continuous inhalation of gas; 5. Pulsating flow generated by reciprocating pump or control valve oscillation; 6. Change of flow state during use.

1. Poor installation

Examples of improper installation include: (1) improper matching of inner diameter between flow sensor and connecting pipe, too large difference, (1s091 (4:1991 regulation: in the case of no manufacturer's recommendation, the inner diameter of connecting pipe shall not be less than the inner diameter of flow sensor, and shall not be greater than 3%); (2) gasket between flow sensor and pipeline protrudes into flow channel; (3) front deflection of adjacent flow sensor Serious flow velocity distribution distortion or rotating flow occurs in the flow parts, and the straight pipe section is insufficient. These reasons mainly cause the flow measurement value is inconsistent with the actual value, and sometimes the output shaking occurs.

Case 1 although this example is not directly caused by improper installation, it is the same as that of the gasket at the inlet end of the flow sensor.

Two dnl400 electromagnetic flowmeters in series are used to measure the finished water in two water pipes of a water plant in Shanghai, which are respectively controlled by the sender (water plant) and the receiver. The distance between the first sensor and the downstream sensor is about 5 meters and 2.5 meters respectively. The length of the upstream straight pipe is slightly insufficient. When it was put into operation in 1997, it was found that there was a difference of 15% between the measured values of two instruments of one pipeline, and the difference slightly decreased to 10% after half a year. In addition to not checking the internal condition of the sensor measuring tube, a comprehensive inspection and analysis was carried out, and the cause of the failure was not found out, which became an outstanding case. Until the sensor was removed in the second half of 1998, it was found that large pieces of cement lining of the inlet pipe fell off and accumulated at the inlet of the sensor with a height of 300-350mm (visual inspection). These deposits lead to serious distortion of the velocity distribution of the flow into the sensor, that is, there is a 0.25D arch section obstacle at the lower part of the pipe blocking the flow. Clear the obstacle and return to normal. The difference decreases from 15% to 10% after half a year, which can be explained by the fact that the stacking height is higher than 300-350mm at the beginning, and the height decreases with the flow scouring. When the large diameter flow sensor is removed from the pipeline or enters into the pipeline for inspection, the flow must be stopped, which involves a wide range of aspects. Generally, it can only be carried out at the end after eliminating the possibility of other faults.

2. Not full tube

Due to insufficient back pressure or poor installation position of flow sensor, the liquid in the measuring tube can not be filled, and the fault phenomenon has different performance due to the degree of underfilling and flow condition. If a small amount of gas is stratified flow or wavy flow in the horizontal pipeline, the fault phenomenon shows that the error increases, that is, the flow measurement value is inconsistent with the actual value; if the flow is bubble flow or plug flow, the fault phenomenon will not only be inconsistent with the measured value, but also output shaking due to the gas phase covering the electrode surface instantaneously. If the gas part of the flow area increases in the horizontal pipe stratified flow, that is, the liquid is not full, the output sloshing will also occur. If the liquid is not full and the surface of the liquid is below the electrode, the phenomenon of output overfilling will occur.

Case 2 a shipyard has a dn80mm electromagnetic flowmeter to measure the water flow, and the operator reflects that when the flow of the closing valve is zero, the output reaches the full value instead. On site inspection found that there was only a short pipe downstream of the sensor, and the water was directly discharged into the atmosphere, but the stop valve was installed upstream of the sensor (as shown in Figure 3, dotted line 1). After the valve was closed, the water in the sensor measuring tube was completely drained. If the valve is refitted to position 2, the fault will be solved. This kind of fault causes are often encountered in after-sales service cases, which is the error of engineering design.

3. Gas retention in piping system

One of the reasons for the gas retention in the pipeline system is that the air in the pipe can not be drained completely before it is put into operation, and the remaining gas accumulates at the high point of the pipeline system, and is entrained by the liquid and flows out in the form of bubbles; the other reason is that the small bubbles in the liquid gradually gather and remain at the high point of the pipeline system. The fault shows that the measured value of flow is inconsistent with the actual value and the output is shaking.

Case 3 a petrochemical plant in Nanjing uses dnl000mm pipeline to lead the water from Changjiang River. The pipeline is 10km long and passes through a hill along the undulating terrain. Dn700mm instruments are installed on the top of the hill. After the pipeline system was put into operation, the electromagnetic flowmeter could not work normally, and the abnormal phenomenon of water flow sound was found in the electromagnetic flow sensor and its vicinity. The preliminary analysis shows that the air in the pipeline can not be exhausted after the pipeline system is put into operation, and the exhaust valve is not installed at the high point in the engineering design. The measuring electrode signal is as high as 4MV (most of which is interference potential), so it is impossible to further check and remove the fault due to the failure of water cut-off. A few months later, the maintenance personnel of the manufacturer followed up again. At this time, there was no sound of water flow. After a period of time of flow, the remaining air was carried away with the water flow, and the normal operation could be achieved by re debugging.

4. Continuous inhalation

Case 4: a water plant in Guangxi set up a clear water tank on the mountain top in the suburb, and used the height potential energy of the pool to send the finished water. The operation personnel reflected that the dn700mm electromagnetic flowmeter measuring the water volume sometimes showed unstable flow. The shaking is more than 10% to 20%, and the error is large, and the estimated difference is about 20%. The field investigation found that if the flow sensor is installed in the pool as shown in Figure 7-9, bubbles will be involved if the water level is not high enough, and even the measuring pipe of the flow sensor can not be filled. If the water level drops to line a, although it is higher than the top of the suction inlet, but it is not much higher, there will be a vortex at C, which will draw the air on the surface of the water level to form bubbles, causing the display to shake; if the water level drops to B, the measuring pipe will not be satisfied with the pipe. We suggest to install an elbow as shown in the dotted line in the figure to expand the effective capacity of the pool, reduce the opportunity to inhale bubbles, and make up for the deficiency of the original design. This kind of instance is often encountered.


 Electromagnetic flowmeter 1
9 cases of bubble drawing


5. Reciprocating pump or control valve oscillation produces pulsating flow

When the reciprocating pump pumps the liquid flow and the measuring point is not far away from the pump, the pulsating flow will make the output of the electromagnetic flowmeter shake, and sometimes the measurement error will be produced. In order to reduce the impact of pulsation on the instrument, the excitation frequency of the electromagnetic flowmeter can be increased or the electrical damping can be increased, and damping devices such as air chamber can be installed in the pipe system. The mismatching of pipeline flow control makes the opening and closing oscillation of control valve also form pulsating flow.

6. Change of flow state during use

Usually, after the instrument is debugged and operated normally for a period of time, it will also fail due to the change of flow state. Although the probability of this kind of failure is not high, it should not be forgotten when analyzing the fault cause.

Case 5: This is a rare example of a striped liner suspended in a liquid flow, which forms a pulsating flow with the liquid manifold. Several electromagnetic flowmeters are installed in a copper mine in Jiangxi Province to measure the slurry containing powdery solid phase, which has been used normally for several years. By July 1998, users reported that one of the dn600m meters had output shaking, reaching 50% - 100% of the full scale value. The field inspection instrument itself is normal, and the inspection and inquiry have learned that the flow power source has not been changed, there will be no new flow fluctuation, and the possibility of introducing new interference due to the deterioration of the service environment is ruled out. The overall impression is that the instrument is normal and the installation and environmental conditions meet the requirements. At that time, because the flow could not be stopped, the hidden part of the flow sensor and its adjacent pipelines could not be removed and inspected, and the cause of the fault was not found for a moment. It was found that a large piece of rubber lining fell off from the U-shaped pipe with rubber lining at the downstream of the flow sensor and hung in the pipe. It swayed with the flow of liquid, resulting in flow fluctuation. The instrument reflected it truthfully and formed output shaking. After the new U-tube is replaced, the large output shaking will no longer appear.

Case 6 fish migrate in the pipe, and the output signal shows a big spike. This case is taken from a foreign report. It is a large-diameter electromagnetic flowmeter monitoring the cooling seawater flow of the condenser in thermal power plant in the United States. It can be seen from the recording paper that the instrument occasionally appears large spike signal. After several times of on-site inspection, the interference noise is transient, which can not be captured during the inspection, and no abnormal conditions such as electrical noise are found around. Some users once proposed the idea that there might be fish flowing in the flow sensor. At that time, they thought that the discharge valve was blocked by fine mesh and that there was no possibility of fish entering upstream of the flow sensor. However, an experiment was done afterwards. The electromagnetic flowmeter was installed in a small water tank and observed by goldfish. The goldfish drilled into the electromagnetic flow sensor, and the instrument really had a peak noise. Later, the questioner told us that fish shadow was indeed seen in the discharge channel, and the fry grew into adult fish after being drilled into the discharge pipe through the mesh grid.


2、 Fluid


The faults of electromagnetic flowmeter caused by fluid are as follows: 1. Gas in liquid; 2. Solid in liquid; 3. Uneven conductivity; 5. Flow noise.

1. There is gas in the liquid

The dissolved gas in the liquid will not affect the flow measurement, while the free gas (i.e. bubble) will affect the measurement and may cause fault. There are three sources of free gas in liquid: ① the air in the pipeline is not discharged completely; ② it is inhaled from outside the pipeline system; ③ the dissolved gas is transformed. The former two are described in the previous section, and the latter is caused by the change of liquid temperature and pressure in the pipe.

In the process industry, the pressure / temperature of pipeline liquid often changes. When the liquid pressure decreases or the temperature rises, the dissolved gas will be converted into free bubbles. For example, if the liquid is lower than room temperature and remains in the pipeline for a period of time (such as stopping), the dissolved air may be converted into bubbles; when the liquid is higher than room temperature, it will gradually cool and contract in the closed pipe at both ends, forming local truth Air bubbles are formed by dissolving air or vaporizing vapor. In the initial stage of process restart, the bubble often appears the phenomenon of output sloshing, which tends to be normal after a period of operation. When the opening of the control valve is very small, the easily gasified liquid will sometimes vaporize and form bubbles.

2. Liquid contains solid phase

The liquid contains powder, particle, or fiber and other solid, which may cause faults: ① slurry noise; ② electrode surface contamination; ③ conductive deposition layer or insulation deposit layer covering electrode or lining; ④ lining is worn or deposited, changing flow area.

Case 7 short circuit effect of conductive deposition layer if conductive material is deposited on the insulating lining surface of measuring tube of electromagnetic flow sensor, the flow signal will be short circuited and the instrument will fail. Due to the gradual deposition of conductive materials, this kind of fault usually does not appear in the commissioning period, but will show after a period of operation.

In order to obtain the best cutting efficiency, dn80mm electromagnetic flowmeter was used to measure and control the flow rate of saturated salt electrolyte in a diesel engine factory tool shop. At first, the instrument runs normally, and after two months of intermittent use, the flow display value becomes smaller and smaller until the flow signal is close to zero. On site inspection, it was found that a thin layer of yellow rust was deposited on the surface of the insulation layer. After wiping and cleaning, the instrument 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 fault during operation. Although it is not a common fault, if the ferrous metal pipe is seriously rusted and the rust layer is deposited, the short-circuit effect will also occur. The possibility of this kind of fault should be analyzed if the normal operation starts and the flow indication becomes smaller and smaller as time goes on.

Case 8: a waterworks in Shanghai uses rectangular pipes to divert water from the upper reaches of Huangpu River more than 30 km away, then pumps raw water into the plant with two dnl600mm circular pipes, and measures the water quantity with two dnl600mm electromagnetic flowmeters. It was put into use in 1991, but it was used normally, but in 1993, it felt that the measurement was reduced. After inspection, the fault reason of the open part of the instrument was eliminated. The resistance value of the two electrodes of the flow sensor was asymmetrical. It was analyzed that the possibility of the fault of the flow sensor was large, and the concealed part could not be checked because the current could not be cut off. It was only in April 1997 that we had the opportunity to enter the pipeline to check the internal condition of the flow sensor measuring pipe. The thickest part of the sludge deposited on the inner wall was more than 10 mm, and the electrode surface was also covered by the sludge deposit layer, which was flush with the surrounding sludge layer. After scrubbing, the electromagnetic flowmeter returned to normal operation. It was confirmed that the fault was caused by sludge deposited on the inner pipe wall.

This example reveals that the inner wall of the flow meter channel for raw water metering will always deposit silt, and whether it will affect the measurement is only a matter of time. In this case, the flow measurement value has been reduced after 3 years of operation under water quality conditions. Therefore, it is necessary to clean the electromagnetic flowmeter regularly. Other flow instruments, such as ultrasonic flowmeter and Venturi tube flowmeter, have the problem of reducing the flow area and affecting the measurement accuracy at least. The flow value of DNL 600mm pipe with deposition of 10mm should be changed by 1.2% - 2.5%. At the same time, in the engineering design, the countermeasures should be taken into account, such as increasing the flow rate at the measuring position to extend the cleaning period, and preset the inspection hole for cleaning in the pipe, etc.

This example is a common fault during operation

3. Uneven conductivity

In the proportioning process, the "dosing" liquid is often injected into the liquid, and the injection liquid is usually added by reciprocating pump. If the conductivity of the injected liquid is different from that of the main liquid, and the mixture has not been mixed evenly, if the electromagnetic flow sensor is installed downstream, the output of the instrument will shake because of the sharp change of the conductivity. Although the liquid conductivity is larger than the threshold value and changes slowly, it will not affect the normal measurement of the electromagnetic flowmeter. This phenomenon often occurs in the process of adding coagulant to raw water of water treatment project.

4. Improper matching with liquid contact material

There are electrode and grounding ring which have mismatching fault with liquid contact material. In addition to corrosion resistance, mismatching is mainly due to electrode surface effect. The surface effects are as follows: ① chemical reaction (passive film formed on the surface), ② electrochemical and polarization phenomena (generation of electric potential), ③ catalytic action (formation of gas mist on the surface of electrode, etc.), as described in Section 3 above (see page 10). Grounding rings also have these effects, but to a lesser extent.

Case 9 a chemical plant in Shanghai used more than 20 sets of Hastelloy B electrode electromagnetic flowmeter to measure high concentration hydrochloric acid solution, and the output signal was unstable. The on-site inspection confirmed that the instrument was normal, and other interference causes that could cause output shaking were eliminated. However, it works well when other users use Hastelloy Bi polarization meter to measure hydrochloric acid. In analyzing whether the cause of the fault may be caused by the difference of hydrochloric acid concentration, it is impossible to make a judgment because there is no experience in the influence of hydrochloric acid concentration on the electrode surface effect. For this reason, the instrument manufacturer and the user make use of the site conditions of the chemical plant to do the real flow test to change the concentration of hydrochloric acid. When the concentration of hydrochloric acid increases gradually, the output of the instrument is stable at low concentration. When the concentration increases to 15% - 20%, the output of the instrument begins to shake. When the concentration reaches 25%, the output shaking amount is as high as 20%. After using tantalum electrode electromagnetic flowmeter, it can work normally.


3、 Environment


The main faults of electromagnetic flowmeter caused by pipe environment are: 1. Strong magnetic field; 2. Strong electromagnetic wave; 3. Stray current of pipeline; 4. Ground potential change; 5. Moisture immersion.

1. Strong magnetic field

The practical experience of the influence of strong magnetic field is not much, because the installation should be far away from the strong magnetic field. In Section 6 of Chapter 5, several examples of application of external magnetic field are described.

2. Strong electromagnetic wave

The electromagnetic flowmeter shall meet the requirements of electromagnetic compatibility and work normally under the specified radiation electromagnetic field environment, which will not cause performance degradation or abnormal operation of the instrument. We have encountered cases of strong radio interference.

Case 10 a water plant in Fujian Province is equipped with multiple electromagnetic flowmeters, one of which has a large fluctuation in output. The transducer, which is 50m away from the instrument itself, is also placed in the conduit to check whether it meets the requirements of field installation. But the common mode interference signal is as high as 1.7V. The common mode signal is reduced to 0.6V, but the output fluctuation is not significantly improved.

After analyzing the environmental conditions of the site with the user, we know that there is a strong radio transmitting station near the flowmeter. In order to confirm whether the fault is caused by the interference source, the converter is temporarily moved to a place 3M away from the flow sensor. The common mode interference signal is less than 0.1mv. Although it is still too large, the instrument is running normally. The cause of the fault is that the electromagnetic wave is still introduced into the instrument even though the multi-layer shielded signal line is used.

This example reveals that when there is a large common mode interference in the field of separate electromagnetic flowmeter, it is necessary to consider whether the strong non electric wave is the interference source in the fault cause analysis. This example is a rare fault during debugging.

3. Stray current in pipeline

After the electromagnetic flowmeter is properly grounded, the influence of the stray current in the pipeline can be avoided. Sometimes according to the regulations, the flow sensor is bridged with thick wire and the grounding is improved. However, it will be affected by stray current, and other measures need to be taken.

Case 11 an aluminum smelter in Shandong uses DN80 to measure alkali liquor slurry. Grounding rings are installed at both ends of flow sensor, and wires are used for bridging and properly grounding. However, the instrument still can't work normally, and the stray current can't be isolated until two grounding points are set after 2m outward.

After the instrument was put into normal operation for a period of time, the output signal swayed again, which ruled out the possibility of flow fluctuation. The instrument itself was in good condition and initially judged as abnormal operation of the instrument. After several days of observation, it was found that the lunch break and night shift were running normally, but the output of day shift was shaking. According to this clue, it is found that the fault source is caused by welding on the same pipe system which is far away from the electromagnetic flow sensor.

Case 12: the electromagnetic flow sensor is insulated from the connecting pipe to eliminate the influence of large stray current

Two dn900 electromagnetic flow meters are installed in a water supply company in Zhejiang Province. One of them is in normal operation, and the other has a fluctuation of up to 50% FS in 1-2 hours. The user thinks that the operating conditions of the two instruments are similar, and the fault is caused by the instrument. The surrounding environment of the site was investigated. Two sections of 0.5m long unlined short steel pipes with good grounding were connected to the steel pipes with cement lining. At the same time, the possibility of pipeline flow pulsation is eliminated.

The distance between converter and sensor is about 10m. A three-phase transformer with hundreds of kV A is installed nearby, about 2m and 8m away from the converter and sensor respectively.

There are two possible reasons for the fault: (1) magnetic field interference caused by high-power transformer; (2) stray current interference on pipeline. In order to prove whether it is the influence of transformer magnetic field interference, it is arranged as the second step to check whether it is the interference of stray current in pipeline. The potential between the two poles is measured by oscilloscope without exciting current, and the value should be zero. However, the peak value of VPP is as high as 1V. It is preliminarily determined that even if the instrument is well grounded, the instrument is still affected by the stray current interference of the pipeline.

The electromagnetic flow sensor and two short steel pipes are electrically insulated from the pipeline network to make the flow sensor and liquid have the same potential. When the instrument is put into operation, the output display is stable and normal, and the influence of magnetic field interference of power transformer on flow measurement is eliminated. At the same time, the interference current is 60maac, and the current direction comes from the upstream of the flow sensor.

This measure is also applicable to the pipeline with cathodic protection current as a method to eliminate the influence of pipeline current interference.

4. Variation of ground potential

The change of ground potential will affect the flow measurement. For example, the ground potential of electromagnetic flowmeter will change due to the voltage drop on the grounding wire due to other equipment. If there is a large common mode interference, the measurement will also be affected.

5. Moisture immersion

When electromagnetic flowmeter is used in water supply and drainage industry, the flow sensor is often installed in the instrument well below the horizon, which is often immersed in the rain water which is not discharged in time, or even immersed in the water for a long time. Even if the enclosure protection level is IP67 (dust tight short-time immersion level) or IP68 (dust density continuous immersion level), such accidents often occur because the sealing gasket of terminal box cover or cable lead-in sealing ring is not compressed and sealed, the ferrule is not installed, or the ferrule is not matched with the outer diameter of the cable.

If the sealing gasket such as the terminal box cover of the flow sensor installed on the ground is not well sealed, the moisture will be absorbed and condensed into water by the breathing effect of temperature change. The cable lead-in device of terminal box is not installed with sealing ring or is not tightly pressed, and condensate water on the cable surface can easily enter into the terminal box. Such cases are also common. In the process of construction, the cable is cut off intentionally or unintentionally and then connected again, and sealed with tape. This hidden danger will not form a fault in the early stage of operation, but the package seal is aging for a long time, the connection is inhaled with moisture, and the cable insulation is reduced.

Water and moisture invade the terminal box, which reduces the insulation strength and insulation resistance. There will be no flow signal output in the flow signal circuit, and the excitation coil circuit will form zero offset or instability. If necessary, the sealing joint can be watered with silica gel.

The non hermetic structure of the excitation coil protection shell absorbs moisture due to respiration. If the liquid temperature is lower than the room temperature, it is easy to condense on the outer wall of the measuring tube. If the liquid temperature is lower than 0 ℃, it will frosting, which will make the flow signal circuit short circuit and failure.

Case 13: a dnl200 electromagnetic flowmeter is used to measure the Yellow River water from the plant, another dn900 meter is used to measure the groundwater in the plant, and two DNl000 electromagnetic flowmeters are connected in parallel to measure the finished water. After the system was put into normal operation for two years, it was found that the output water was 10% - 15% more than the incoming water. Observe the operation of the instrument without abnormal performance. Four electromagnetic current meters were tested by using portable ultrasonic flowmeter with external transducer (probe). It was proved that the output signal of two electromagnetic flow meters was high. The zero points of the two finished water meters were found to be significantly shifted when the zero points were closed respectively. According to experience, it is likely to be caused by water entering into the terminal box or the excitation coil being damped and insulation decreasing. When the water dew is wiped off and the terminal block of the junction box is dried by blowing with an electric blower, the ground resistance of the excitation terminal is recovered from 5-6mw to tens of MW, and the offset zero point immediately returns to the zero position, and the instrument operates normally.

The reason is that the insulation of the excitation coil circuit to the ground is decreased, which makes the electrode add a large insulation resistance and the signal internal resistance to divide the excitation voltage to form a larger common mode interference signal. However, the common mode rejection ratio of the preamplifier of the converter is limited, so that the zero point of the converter has output.



Section 9 special detection of electromagnetic flowmeter



1、 Measurement of electrode contact resistance

By measuring the contact resistance between electrode and liquid, the general condition of electrode and liner surface can be estimated indirectly without removing the flow sensor from the pipeline, which is helpful to analyze the fault cause. This is especially convenient for the inspection of large-diameter electromagnetic flowmeter. The inner surface condition of the flow sensor is estimated, such as whether there are deposition layers on the electrode and lining layer, whether the deposition layer is conductive or insulating, and whether the electrode surface is polluted.

The electrode contact resistance of the electromagnetic flow sensor should be measured and recorded immediately after the newly installed instrument is debugged. In the future, it should be measured once every maintenance. The analysis and comparison of these data will help to determine the cause of instrument failure in the future.

The contact area between the liquid electrode and the measured electrode depends on the contact area between the electrode and the liquid. When measuring distilled water with conductivity of 5 × 10-6s / cm, the resistance is 350kW, the domestic and industrial water with conductivity of 150 × 10-6s / cm is about 15kw, and the brine with conductivity of 1 × 10-2s / cm is about 200N.

Use a multimeter to measure the resistance between each electrode terminal and the ground when it is full of liquid. Experience shows that the difference of contact resistance between the two poles should be less than 10% - 20%, otherwise, it means there is a fault. Measuring electrode contact resistance with a multimeter is not a correct method to measure the accurate value of resistance, but to determine the general value. Accurate measurements must be made using an AC bridge, such as "kohlraush bridge".

The measured electrode to ground resistance has the following different trends compared with the original measured value

(1) The unbalance value of the two electrode resistances increases (i.e. the difference value increases),

(2) The resistance increases,

(3) The resistance decreases.

These three signs can be used to determine the following possible causes of failure:

(1) There is one electrode at the electrode part, and the insulation is greatly reduced,

(2) The electrode surface is covered with insulating layer,

(3) A conductive deposition layer is attached to the electrode surface and the lining surface.

The above failure probabilities can also be used as the precursors for predicting the occurrence of faults.

When measuring with a multimeter, pay attention to the following points:

(1) The maximum value of the pointer deviation should be read at the moment when the measuring rod contacts the terminal, and the measured value shall be subject to the first one. If re measurement, the measured values due to polarization are inconsistent;

(2) When the grounding electrode is the same, the negative electrode must be measured with the same electrode.

(3) The same type of multimeter should be used for measurement, and the same range should be used. The measuring gear with 1.5V battery working range is commonly used, such as: × LKN .


2、 Polarization voltage of electrode

The measurement of the polarization voltage between the electrode and the liquid will help to judge whether the fault of zero point instability or output sloshing is caused by the contamination or covering of the electrode.

The polarization voltage between the two electrodes and the ground is measured by using a digital multimeter with 2V dc gear (the electromagnetic flowmeter can be measured without power failure or power failure). If the electrode is contaminated or nearly equal, otherwise the electrode is covered. The polarization voltage depends on the "electrode potential" of the electrode material and the properties of the liquid, and the measured value may be in the range of several MV to several hundred MV.

In fact, it is impossible for the two electrodes to be completely and symmetrically polluted in operation, so the voltage on the two electrodes forms an asymmetric common mode voltage. Asymmetric common mode voltage becomes differential mode signal, resulting in zero offset.


3、 Measurement of signal cable interference

The signal cable will change the zero point of electromagnetic flowmeter due to the interference of external electrostatic induction and electromagnetic induction. In order to judge whether the zero point change is affected by the interference potential of signal cable, it is necessary to determine the general range of interference and the influence degree on electromagnetic flowmeter.

According to Fig.


 Electromagnetic flowmeter 2


Attention should be paid to the following points in the determination:

1. It should be noted that access to RA and Rb should not be induced by power interference;

2. When measuring, the zero point of the converter may change a little (usually rising) compared with that of the original connector, and it is generally considered that the change is not more than 5%;

3. Figure 7-10 only shows the lines of reconnecting the changed part, and the others are still the same;

4. When measuring, the environment around the sensor should be the same as that when the fault occurs. It would be better if the nearby motor can make the power on-off test.

4、 Determination of grounding potential

In the normal use of electromagnetic flowmeter, if the state of the electric machine near the sensor changes (such as leakage), the ground potential will change and cause the zero point change. To check whether there is such influence, short circuit the converter working grounding terminal C with the protective grounding g terminal, and judge whether there is grounding potential according to the change of zero point (or indicated value). If the zero point change exceeds the allowable value, contact the manufacturer and take necessary measures.

But this does not necessarily lead to the conclusion that there is no ground potential without zero (or indication) change.

5、 Discrimination of stray current flow direction in pipeline

Sometimes, in order to find the source of pipeline stray interference, the upstream of the flow sensor is still enough in the downstream, so as to narrow the search range and try to reduce or eliminate the influence of stray current interference.

 Electromagnetic flowmeter 3


Take case 12 in Section 8 (page 24) as an example to illustrate the specific approach. As shown in Figure 11, two points a and B of the upstream and downstream jumper wires and grounding wires of the flow sensor electrically insulated from the pipeline are connected to the ammeter respectively. The current measured at a ; point is 60maac, and the current at point B is zero, indicating that the interference source is upstream of the flow sensor.




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