Section 4 output shaking inspection and measures
1、 Cause of failure
Generally speaking, the output sloshing can be summarized into five fault causes
(1) The flow itself is fluctuating or pulsating, which is not the fault of electromagnetic flowmeter in essence, but only reflects the flow condition;
(2) There is no liquid in the pipe or the pipe is filled with liquid;
(3) External stray current and other electrical and magnetic interference;
(4) There are many reasons for the liquid (such as liquid / slurry) with uneven conductivity;
(5) The electrode material does not match the liquid properly.
2、 Inspection procedure
Figure 4 shows the inspection electromagnetic flowmeter Output the flow of shaking. First, make a preliminary investigation and judgment according to the flow chart, and then carefully check and try to remove the faults one by one. The principle of inspection sequence listed in the process is as follows: ① it can be understood by observation or inquiry that those without major operation are in the first place, that is, easy first and then difficult; ② according to the experience of on-site maintenance in the past, the one with higher frequency and higher probability of occurrence in the future is the first; ③ the sequence requirements of inspection itself. If the causes of the latter faults are confirmed by preliminary investigation, detailed inspection can also be carried out in advance.
Flow chart of electromagnetic output flow meter 4 shaking
3、 Failure inspection and measures
This section discusses the inspection methods and measures of the above five aspects.
1. The fluctuation (or pulsation) of the flow itself
Check flow chart item 1. If the output of the instrument fluctuates, it reflects the flow condition. The inspection method can ask the operator and process technician whether there is fluctuation source in the use site. There are three reasons for the fluctuation (or pulsation) of pipeline flow: (1) reciprocating pump or diaphragm pump (often used for filling liquid medicine in fine chemical industry, food, medicine and water purification) is used in the upstream flow power source of electromagnetic flowmeter, and the pulsation frequency of these pumps is usually between several times to more than 100 times per minute; (2) flow characteristics and size selection of control valve downstream of the instrument It can be observed whether the control valve stem has oscillatory movement; (3) other disturbance sources cause flow fluctuation, such as whether there are flow blocking parts (such as full open butterfly valve) in the upstream pipeline of electromagnetic flowmeter to generate vortex (such as vortex row generated by vortex generator of vortex street flowmeter, gasket at the inlet end of sensor extending into the flow channel, and gasket strip fragment) Hanging in the liquid flow, swinging, etc.).
In the pipeline with pulsating flow source, in order to reduce its influence on the measurement of flow instrument, the flow sensor is usually far away from the pulsating source, and the flow resistance of the pipe is used to attenuate the pulsation; or the air chamber buffer called passive filter is installed at the appropriate position of the pipeline to absorb the pulsation.
2. There are no bubbles in the liquid or the pipe is not filled
Check item 2 of the flow chart. This kind of fault is mainly caused by the poor design of pipe network engineering, which makes the measuring pipe of the sensor not filled with liquid or the sensor is installed improperly. Measures should be taken to avoid the installation of position a and E as shown in Figure 3 and position B when discharging with dashed line pipe to C and D positions.
There is no back pressure or insufficient back pressure downstream of the sensor. If the sensor is installed at position e, the liquid will flow through a short section of pipe downstream, which will be discharged into the atmosphere. If valve 2 is fully opened, the sensor measuring pipe may not be full of liquid. Sometimes the flow rate of the process is large and the instrument can be filled normally. If the flow rate is reduced, the liquid may be insufficient and the instrument will be out of order.
There are two ways to form bubbly gas in liquid, one is to inhale from outside and the other is to change dissolved gas (air) into free bubble. The number of bubbles in the liquid is small, and the diameter of bubble ball is far smaller than the diameter of electrode. Although the volume of some liquid is reduced, the output of electromagnetic flow juice will not slosh; the larger bubble can cover the whole electrode and make the flow signal circuit open instantaneously, then the output signal will shake more.
The micro bubbles in the liquid flow will gradually accumulate at the high point or dead angle during the flow process. If the electromagnetic flowmeter is installed at the high point of the pipeline system, the trapped gas will reduce the liquid flow area in the sensor, which will affect the measurement accuracy. If the sensor is installed at the high point, the accumulated gas at the high point exceeds the capacity or is subject to pressure fluctuation The electrode is covered by bubbly or sheet-like flow with liquid, resulting in output shaking.
The common ways to inhale air from the outside are as follows: the raw water of rivers contains bubbles, or the water level of the suction inlet is too low (usually it requires a distance of more than 2-5 times of the diameter of the suction inlet, depending on the suction flow rate), forming a suction vortex and entraining the air. In the aspect of process industry, air is mixed into mixing vessel when mixing, and air is inhaled in pump suction end or other places with poor sealing in pipeline system. This kind of fault is often encountered in practice.
The dissolved air in the liquid is separated into free bubbles, and the dissolved air (or gas) will separate into free bubbles when the pressure of pipe system is reduced. For example, the valve at the two ends of the pipe system filled with liquid is closed and cooled gradually after stopping operation. Due to the different coefficient of thermal expansion, the liquid shrinkage is much larger than that of the pipeline system, and a contraction space is formed in the pipeline system, forming a local vacuum state. The dissolved air in the liquid will be separated to form bubbles and accumulate at the high point of the pipe system. Restart, the liquid sandwiched with bubbles flowing through the electrode surface may make the output of electromagnetic flowmeter shake. This may be one of the reasons for the phenomenon that the output of the electromagnetic flowmeter shakes at the beginning of the start-up and then tends to be stable. Another example is that water can dissolve up to 0.3% VN air at 1 atmospheric pressure of 0 ℃. If the water temperature rises in the process, the air will be separated into free bubbles (up to 30 ℃, only about 0.15% can be dissolved). The accumulation may also cause failure.
3. External electromagnetic interference
Check flow chart item 3. Electromagnetic flowmeter is easy to be affected by external interference due to its small flow signal. The main interference sources are stray current, static electricity, electromagnetic wave and magnetic field.
The stray current of pipeline mainly depends on the good grounding protection of electromagnetic flowmeter, and the grounding resistance is generally less than 100Q, and it is not allowed to share the grounding with other motors and electrical appliances. Sometimes, the electromagnetic flowmeter can work normally without grounding under better environmental conditions, but we think that it is better to make grounding even so. Because once the good environmental conditions no longer exist, instrument failure, it will affect the use, and then make a variety of inspections will bring a lot of trouble.
Sometimes, although the electromagnetic flowmeter is well grounded, due to the strong stray current in the pipeline (such as electrolysis process pipeline and cathodic protection pipeline network) which affects the normal measurement of electromagnetic flow, it is necessary to make electrical insulation between the electromagnetic flow sensor and the pipeline. For specific examples and the inspection and exclusion process, please refer to case 12.
The static electricity and electromagnetic wave interference will be introduced through the signal line between the sensor and the converter of the electromagnetic flowmeter, which can be prevented by good shielding (such as shielded cable for signal line, and the cable is placed in the protective iron pipe). However, there have been some cases in which the strong electromagnetic wave prevention is ineffective. At this time, the converter is moved close to the sensor, the signal cable is shortened, or an all-in-one instrument without external cable is used. Please refer to case 10 for details.
Magnetic field interference usually only takes electromagnetic flow sensor away from strong magnetic field source. The ability of electromagnetic flowmeter to resist the magnetic field varies with the structure design of the sensor. For example, the protective shell of the excitation coil of the sensor is made of non-magnetic materials (such as aluminum and plastic), so the ability to resist the influence of magnetic field is weak, while that of steel is stronger.
4. Demonstration and verification of liquid properties
Check flow chart item 4. There are three factors in the liquid properties that can make the output slosh. They are: (1) there are solid particles or bubbles in the liquid, (2) the conductivity of the two liquids in the two-component liquid is different and the mixture is not uniform, or the chemical reaction in the pipeline has not been completed completely, (3) the conductivity of the liquid is close to the lower limit.
The measured liquid contains a lot of solid particles, which will cause the flow signal to appear sharp pulse noise like the bubble mentioned above, and cause the output to shake. If the solid phase is powdery, it usually does not form output sloshing.
In the fine chemical industry, food industry, medicine industry and water supply treatment engineering, the liquid medicine is often added into the main liquid, which is usually injected by reciprocating pump or diaphragm pump in proportion to the main liquid flow. If there is no uniform flow rate between the upper and the downstream of the flow meter, there will be no liquid sloshing in the flow meter. In this case, the filling point should be moved to the downstream or the electromagnetic flowmeter should be moved to the upstream of the filling point; if the site conditions are limited or the modification work is too large, a mixer can be installed downstream of the filling point for remedy. However, when the static mixer is installed, the liquid flow will produce a small rotating flow, which may cause an additional error of 1% or more. However, compared with the output sloshing which can not be measured, it is a measure to weigh the two disadvantages.
If the mixture enters the electromagnetic flow measurement before the end of chemical reaction in the pipeline, the output sloshing phenomenon may also occur. In this case, only the position of the measuring point can be changed so that the measuring position is upstream of the mixing point or far away from the downstream of the mixing section. However, the distance away from the mixing section needs a long distance, such as the reaction time of 60s, the liquid flow rate of 3m / s, and the distance of 180m without considering the safety factor.
If the conductivity of liquid is close to the lower limit, sloshing may also occur. Because the lower limit value specified in the instrument specification of the manufacturer is the lowest value that can be measured under good conditions, but the actual conditions can not be ideal. We have encountered many times that the conductivity of low-grade evaporated water or deionized water is close to 5 × 10-6s / cm of the electromagnetic flowmeter specification, but the output is shaking when it is used. It is generally considered that the lower limit of conductivity which can be measured stably should be 1-2 orders of magnitude.
The conductivity of liquid can be found in the appendix or related manual. If there is no ready-made data, it can be sampled and measured by conductivity meter. However, there are some cases that the electromagnetic flowmeter can not work when sampling from the pipeline to the laboratory is considered to be available. This is due to the difference between the liquid used in the measurement of conductivity and the liquid in the pipeline. For example, the liquid has absorbed CO2 or NOx in the atmosphere to generate carbonic acid or nitric acid, which changes the conductivity.
For the noise slurry containing particles or fiber liquid, increasing the excitation frequency can effectively improve the output sloshing. The yj-lde series DN300 electromagnetic flowmeter with adjustable full frequency is shown in table 7-1. The concentration of 3.5% corrugated low plate slurry is measured, and the instantaneous flow sloshing is measured at different excitation frequencies. When the frequency is 50 / 32 Hz, the sloshing is as high as 10.7%; when the frequency is increased to 50 / 2 Hz, the sloshing is reduced to 1.9%, and the effect is very obvious.
Table 1 instantaneous flow sloshing under different excitation frequencies
Excitation frequency /Hz
Display flow ( Peak sloshing range )/m
Percentage to average
one hundred and eighty － two hundred and twenty-three
ten point seven
five point six
seven point three
one point nine
5. Investigate the matching of liquid and electrode materials
Check flow chart item 5. The selection of electrode materials should first consider the corrosion resistance of the tested liquid. However, improper selection of electrode materials will cause electrode surface effect and cause output sloshing and other faults. The electrode surface effects include the formation of insulating layers such as passive film or oxide film, polarization and electrochemistry. There is not enough data for the matching of dielectric core materials as for corrosion resistance, and only some limited experience needs to be accumulated in practice.
Tantalum monohydrate, alkali and other non acid solutions
When tantalum electrode is used to measure water flow, insulation layer will be formed, which will cause instrument failure or noise after a short period of operation. In the process flow, even if the tantalum electrode is in contact with water or "non acid" liquid for a very short time, the normal use of the instrument will be affected if the tube is washed with clean water. The tantalum electrode can not be selected in alkaline solution such as sodium hydroxide.
Hastelloy B - high concentration hydrochloric acid
Hastelloy B has been applied to hydrochloric acid with low temperature and concentration. However, when the concentration exceeds a certain value, noise will be generated. Tantalum electrode, nitric acid, sulfuric acid and other acid solutions should be used instead.
Platinum hydrogen peroxide
When the platinum electrode is used to measure hydrogen peroxide at low pressure (the pressure is lower than 0.3MPa), the catalyst produces gas mist on the electrode surface, which blocks the electrical path and affects the work.
Platinum - hydrochloric acid with a concentration greater than 10%
When the concentration of hydrochloric acid is more than 10%, the platinum electric field will produce noise.
Hastelloy b-aluminum sulfate solution
Aluminum sulfate is mixed with raw water to agglomerate suspension. We have encountered the output shaking of Hastelloy B electrode when measuring 15% aluminum sulfate solution, and then use acid resistant steel electrode to obtain satisfactory results.
Section 5 zero point instability inspection and measures
1、 Cause of failure
The causes of zero point instability on human body can be summarized as
(1) The pipeline is not filled with liquid or the liquid contains bubbles;
(2) Subjectively, it is considered that there is no flow of liquid in the pipeline system, but there is actually a small flow; in fact, it is not electromagnetic flowmeter The fault is a misunderstanding that reflects the flow situation truthfully;
(3) The sensor is not well grounded and is subject to external interference such as stray current
(4) The reason of liquid (such as liquid conductivity uniformity, electrode pollution, etc.);
(5) The insulation of signal circuit decreases.
2、 Inspection procedure
Figure 5 shows the process of checking the instability of the zero point electromagnetic flowmeter. First, make a preliminary investigation and judgment according to the process, and then carefully check and try to remove the fault item by item. The sequence of inspection items listed in the process is as follows: (1) it can be understood by observation or inquiry that the items that do not need large operation are the first, that is, the easier is the later; (2) according to the experience of on-site maintenance in the past, the one with higher frequency and higher probability in the future is the first; and (3) the sequence requirements required by the inspection itself. If it is confirmed by preliminary investigation that it is the cause of the latter several faults, it can also be checked carefully in advance.
3、 Failure inspection and measures
This section discusses the inspection methods and measures of the above five aspects.
1. The pipe is not filled with liquid or the liquid contains bubbles
Flow chart check. This kind of failure is mainly caused by poor design of pipe network engineering or imperfect related equipment. Please refer to "2. The pipeline is not filled with liquid or the liquid contains bubbles" in page 9.
2. There is a slight flow in the pipe
Check flow chart item 2. Subjectively, it is considered that there is no flow in the flow sensor, but there is a micro flow in fact. The main reason for this kind of fault is the poor tightness of the stop valve of the pipeline, and the small leakage detected by the electromagnetic flowmeter is misunderstood as zero point change or zero point instability. Valve used for a long time or dirty liquid to make the valve airtight is often encountered, especially for large valves. Another common reason is that in addition to the upper pipe of the flow meter, there are several branch pipes, and the valve closing of these branches is forgotten or ignored. Sometimes, it is difficult to confirm that there is no flow in the piping system on site. At this time, as shown in Fig. 6, small diameter leakage monitoring valve 3 can be set between the stop valve 1 and 4 before and after the flow sensor 2 to observe whether there is leakage.
Figure 6 double valve closing and leakage monitoring
Flow sensor, 2, 5; stop valve
3. The imperfect grounding is affected by external interference and ground potential fluctuation
Check flow chart item 3. The external interference such as pipe stray current mainly depends on the good grounding protection of electromagnetic flowmeter. The grounding resistance is usually less than 1000, and it is not allowed to share the grounding with other electrical machines. Sometimes the environmental conditions are good and the electromagnetic flowmeter can work normally without grounding. However, once the good environment does not exist, the instrument will fail. At that time, it will bring a lot of trouble to check again.
The change of the state of the electric equipment near the flow sensor (such as the increase of leakage current) forms the change of grounding potential, which will cause the zero point change of electromagnetic flowmeter. Please refer to section 9 for inspection methods.
4. Investigation of liquid properties
Check flow chart item 4. When the liquid conductivity changes or is not uniform, the zero point will change when it is still, and the output will shake when it flows. Therefore, the flowmeter should be far away from the injection point or the downstream of the chemical reaction section of the pipeline, and the flow sensor should be installed upstream of these places.
If the liquid contains solid phase, or impurities deposit on the inner wall of the measuring tube, or scale on the inner wall of the measuring tube, or the electrode is polluted by grease, etc., the zero point change may occur. Because the inner wall surface fouling and electrode fouling degree can not be exactly the same and symmetrical, destroy the balance state of the factory initial setting. The positive measure is to remove the dirt and deposit scale layer; if the zero point changes little, you can try to reset the zero point.
5. Investigation of signal line insulation
Check flow chart item 5. When the insulation of signal circuit drops, zero point instability will be formed. The main reason for the insulation decline of signal circuit is that the insulation of electrode part is decreased, but the insulation decline or damage of signal cable and its wiring terminal can not be ruled out. Because sometimes the site environment is very harsh. If the instrument cover and wire connection are not sealed carefully, the moisture, acid mist or dust will invade the instrument junction box or the cable protection layer, which will reduce the insulation. The insulation resistance of signal circuit shall be checked according to the cable side and flow sensor side respectively, and the megger shall be used to test. Because the signal cable is easy to do first. The flow sensor is divided into two steps: full of liquid to measure the contact resistance of electrode surface and after tube measurement, and u-pole insulation resistance.
6. Check electrode contact resistance and electrode insulation resistance
Check item 6 of the flow chart in 2 steps.
(1) Fill the liquid measuring electrode surface spoon liquid contact resistance flow sensor, remove the signal cable wiring, use a multimeter to measure the resistance between each electrode and the grounding point, the left of the resistance value of the two electrodes to the ground should be 10% - 20%. The measurement of electrode contact resistance in Section 9 will be further explained.
(2) Empty pipe measuring electrode insulation vent measuring tube, dry the inner surface with dry cloth, after drying completely, use h500vdc megger to measure the resistance between each electrode and the ground, the resistance value must be above 100m Ω.
Figure 5 flow chart of zero point instability inspection of electromagnetic flowmeter