Sensor inspection
Test equipment: 500 Ω insulation resistance tester and multimeter.
Test steps:
(1) When the pipeline is filled with medium, measure the resistance between terminals A, B and C with multimeter, and the resistance between terminals A-C and B-C should be equal. If the difference is greater than 1 times, it may be that the electrode leaks electricity and the condensed water is adsorbed on the outer wall of the measuring tube or in the junction box.
(2) After the lining is dried, measure the insulation resistance between A-C and B-C with a mω meter (it should be greater than 200MΩ). Then use a multimeter to measure the resistance between the A and B terminals and the two electrodes in the measuring tube (it should be in a short circuit state). If the insulation resistance is very small, it means that the electrode is leaking, and the flowmeter should be returned to the factory for maintenance. If the insulation has decreased, but it is still above 50MΩ, the inspection result in step (1) is normal, and it may be that the outer wall of the measuring tube is wet, so the inside of the shell can be dried by a hot air blower.
(3) Measure the resistance between X and Y with a multimeter. If it exceeds 200 Ω, the excitation coil and its lead may be open or have poor contact. Remove the terminal board for inspection.
(4) Check the insulation resistance between X, Y and C, which should be above 200MΩ. If the temperature drops, blow dry the inside of the shell with hot air. In actual operation, the decrease of coil insulation will lead to the increase of measurement error and the instability of instrument output signal.
(5) If it is determined that the sensor is faulty, please contact the manufacturer of electromagnetic flowmeter. Generally, it can't be solved on site and needs to be repaired by the manufacturer.
Converter check
If it is determined that the converter is faulty and there is no problem due to external reasons, please contact the electromagnetic flowmeter manufacturer, who will generally replace the circuit board.
Maintenance of electrodes
1. Before using the electromagnetic flowmeter, the electromagnetic flowmeter should be calibrated with standard PH solution. After calibration and before operation, everyone must pay attention to cleaning the electrode of electromagnetic flowmeter with distilled water first, and then cleaning the electrode with measuring liquid.
2. If the electromagnetic flowmeter is not used, when the electrode of the electromagnetic flowmeter is to be removed, care should be taken not to let the tactile sensor of the electrode collide with a hard object, otherwise the use will be affected as long as the electrode is damaged.
3. After using the electromagnetic flowmeter, everyone should put the electrode sleeve of the electromagnetic flowmeter on it, and put less saturated solution in it, as long as the bulb of the electrode is wet, but remember not to soak it in distilled water.
4. Pay attention to keep the electrode clean at ordinary times, and don't short-circuit the two ends of its output, otherwise it will make the measurement inaccurate and affect the use of electromagnetic flowmeter. In fact, there are many ways to maintain the electrodes of electromagnetic flowmeter. Everyone should pay more attention in the use process, and don't let the electromagnetic flowmeter fail to work normally in the future because of a little negligence. 1. Failure during debugging
Debugging faults generally occur in the stage of instrument installation and debugging. Once eliminated, it will not happen again under the same conditions in the future. Common faults during debugging are usually caused by improper installation, environmental interference and the influence of fluid characteristics.
1. device
Usually, the fault is caused by the incorrect installation position of the electromagnetic flow sensor, for example, it is easy to accumulate gas when the sensor is installed at the highest point of the pipeline; Or installed on a vertical pipeline from top to bottom, which may lead to venting; Or there is no back pressure behind the sensor, and the fluid is directly discharged into the atmosphere, forming a non-full tube in the measuring tube.
1. Environmental aspects
Usually, it is mainly pipeline stray current interference, space strong electromagnetic wave interference, large motor magnetic field interference and so on. The stray current interference of pipelines can usually achieve satisfactory results by adopting good individual grounding protection. However, in the case of strong stray current (such as the pipeline in electrolysis workshop, sometimes the peak value of alternating current potential Vpp induced on two electrodes can be as high as 1V), extra measures should be taken and the flow sensor should be insulated from the pipeline. Space electromagnetic interference is generally introduced through signal cables, and single-layer or multi-layer shielding protection is usually adopted.
1. Fluid aspect
The measured liquid contains tiny bubbles, which are evenly distributed, and usually do not affect the normal work of the electromagnetic flowmeter, but with the increase of bubbles, the instrument will be damaged.
The output signal of the instrument will fluctuate. If the bubble is large enough to cover the whole electrode surface, the electrode loop will be instantly disconnected from the bubble flow passing through the electrode, making the output signal fluctuate more.
Electromagnetic flowmeter excited by low frequency square wave will also produce mud noise when measuring mud with excessive solid content, which will make the output signal fluctuate.
When measuring mixed media, the output signal will fluctuate if it enters the flow sensor for measurement before uneven mixing.
Improper selection of electrode material and measured medium will also affect normal measurement due to chemical action or polarization phenomenon. Electrode materials should be selected correctly according to instrument selection or related manuals.
1. Runtime failure
Running-time fault is the fault of electromagnetic flowmeter after debugging and normal operation for a period of time. Common runtime faults are basically caused by the adhesion layer on the inner wall of the flow sensor, lightning strike, changes in environmental conditions and other factors.
1. Adhesive layer on the inner wall of the sensor
Because electromagnetic flowmeter is often used to measure dirty fluid, after running for a period of time, adhesion layer often accumulates on the inner wall of sensor, which leads to failure. These faults are usually caused by too large or too small conductivity of the adhesive layer. If the insulation layer is attached, the electrode circuit will be broken and the instrument will not work normally; If the conductivity of the adhesive layer is significantly higher than that of the fluid, the electrode circuit will be short-circuited.
The instrument doesn't work properly either. Therefore, the scaling layer attached in the measuring tube of electromagnetic flowmeter should be removed in time.
1. Lightning strike
Lightning strike can easily induce high voltage and surge current in the instrument circuit, thus damaging the instrument. Mainly through the power line between the sensor and the inverter or excitation coil or flow signal line, especially from the control room power line.
1. Changes in environmental conditions
When debugging, because the environmental conditions are not bad (such as no interference source), the flowmeter works normally, and it is often easy to ignore the installation conditions (such as poor grounding). In this case, once the environmental conditions change and new interference sources appear in operation (such as welding pipes near the flowmeter and installing large transformers nearby), it will interfere with the normal work of the instrument and the output signal of the flowmeter will fluctuate. Typical fault diagnosis and treatment
1. No flow output. Check whether the power supply part is faulty and test whether the power supply voltage is normal; Test the on-off of the fuse; Check whether the arrow of the sensor is consistent with the flow direction of the fluid; If not, change the installation direction of the sensor; Check whether the sensor is full of liquid. If not, replace the pipe or install it vertically.
2. The signal is getting smaller and smaller or suddenly drops. Test whether the insulation between the two electrodes is damaged or short-circuited, and the resistance between the two electrodes is normally between (70 ~ 100)ω; Dirt may be deposited on the inner wall of the measuring tube, so clean the wiping electrode and don't scratch the inner lining. Measure whether the pipeline lining is damaged. If it is damaged, please replace it.
3. If the zero point is unstable, check whether the medium is full of the measuring tube and whether there are bubbles in the medium. If there are bubbles, a getter can be installed upstream. If it is installed horizontally, it can also be installed vertically. Check whether the instrument is well grounded. If not, it should be grounded in three levels (grounding resistance ≤100Ω); Check that the conductivity of the medium should not be less than 5 μ s/cm; Check whether the medium is deposited in the measuring tube, and be careful not to scratch the lining when cleaning.
4. The indicated flow value is inconsistent with the actual value. Check whether the liquid in the sensor fills the pipeline and whether there are bubbles. If there are bubbles, a getter can be installed upstream; Check whether the grounding conditions are good; Check whether there is a valve upstream of the flowmeter; If so, move it downstream or open it completely; Check whether the range setting of the converter is correct. If not, reset the correct range.
5. The indication fluctuates within a certain range. Check whether the environmental conditions have changed. If there are new interference sources and other magnetic sources or vibrations that affect the normal operation of the instrument, the interference should be eliminated or the flowmeter should be moved in time. Check the test signal cable, and treat the end with insulating tape so that the conductor, inner shielding layer, outer shielding layer and shell are not in contact with each other.
The fluid to be measured by electromagnetic flowmeter must be conductive, so the non-conductive substances such as gas, steam, oil, copper and propane cannot be measured by electromagnetic flowmeter. After preliminary debugging, after normal operation for a period of time, the common fault causes during operation are: adhesion layer on the inner wall of flow sensor, lightning strike, change of environmental conditions, etc.
1, inner wall adhesive layer
Because electromagnetic flowmeter has more opportunities to measure suspended solids or dirty bodies than other flowmeters, the failure probability of inner wall adhesive layer is relatively high. If the conductivity of the adhesive layer is close to that of the liquid, the instrument can still output signals normally, which only changes the flow area and causes the hidden danger of measurement error. If it is a highly conductive adhesive layer, the electromotive force between electrodes will be short-circuited; If it is an insulating adhesive layer, the electrode surface is insulated and the measuring circuit is disconnected. The latter two phenomena will make the instrument unable to work.
2. Lightning shock
Lightning impact induces instantaneous high voltage and surge current in the line, which will damage the instrument when it enters the instrument. There are three ways to introduce lightning detector: power line, flow signal line between sensor spoon converters and excitation line. However, from the analysis of the parts damaged by lightning stroke, it is found that most of the induced high voltage and surge current causing the fault are introduced from the power supply line of the control room, and the other two methods are less. It is also known from the scene of lightning accident that not only the electromagnetic flowmeter fails, but also other instruments in the control room often suffer lightning accidents at the same time. Therefore, users should understand the importance of setting lightning protection facilities for instrument power lines in the control room. At present, some design units know and explore to solve this problem, such as Qilu Petrochemical Design Institute [1].
3. Changes in environmental conditions
The main reason is the same as the fault environment during debugging in the previous section, except that the interference source did not appear during debugging and then intervened during operation. For example, an electromagnetic flowmeter with unsatisfactory grounding protection runs normally during debugging because there is no factory interference source. However, during the operation, new interference sources (such as pipeline welding near or far from the measuring point) interfere with the normal operation of the instrument, and the output signal fluctuates greatly. Maintenance of electromagnetic flowmeter
1, sensor check
Test equipment: 500 Ω insulation resistance tester and multimeter.
2. Check the converter
If it is judged that the electromagnetic flowmeter is a converter fault, please contact the manufacturer if there is no problem due to external reasons. Generally, the circuit board will be replaced.
Practice of measuring low conductivity medium with electromagnetic flowmeter
Electromagnetic flowmeter is used to measure the volume flow of conductive liquid medium with conductivity greater than 5μs/cm. The measuring principle of electromagnetic flowmeter is mainly based on Faraday's law of electromagnetic induction, that is, when the fluid passes through the measuring tube, cutting the magnetic field lines will induce electromotive force. The electromotive force is proportional to the magnetic flux density, and the product of the inner diameter of the tube and the average flow rate is measured. The electromotive force (flow signal) is detected by the electrode and sent to the converter through the cable. However, it is difficult to induce electromotive force when measuring weakly conductive media. Through field practice, we in Tolek summarized the following points for reference:
Firstly, it is necessary to determine whether the measured medium has conductivity;
Secondly, the electromagnetic flowmeter should be installed in strict accordance with the product instruction manual;
Thirdly, when the electromagnetic flowmeter is debugged, the electromotive force can be detected smoothly after the parameters of empty pipe alarm in the electromagnetic flowmeter converter are turned off.
Calculation and determination of aperture of electromagnetic flowmeter;
Electromagnetic flowmeter is mainly used to measure the volume flow of conductive liquid in closed pipeline. The electromagnetic flowmeter specifies that the minimum flow rate of fluid is not less than 0.5m/s, generally 2~4m/s, and the maximum flow rate is not higher than 8m/s. Therefore, when selecting the diameter of the electromagnetic flowmeter, the appropriate pipe size should be fully considered while ensuring the measurement accuracy of the electromagnetic flowmeter. So how to determine the caliber of electromagnetic flowmeter? Let me briefly introduce how to determine the caliber of electromagnetic flowmeter. I think it's 500 meters now. A pool of water needs to be pumped dry within 4 hours. How to determine what diameter pipe to use? Through the parameters required above, it can be determined that the flow range of the flowmeter is 500m? Divided by 4 hours is 125m? /H. The approximate range of pipe diameter can be calculated by flow, that is, πr? × speed (0.5~8m/s)= 125m? /h, through calculation, 125m is over? /h water, its caliber range is 0.075m~0.2975m, that is, DN80~DN300. Considering the accuracy requirements of electromagnetic flowmeter, the optimal flow rate is 2~4m/s, and its caliber is 0. 105m~0. 149m, that is, DN 100 ~ DN.
Installation steps of plug-in electromagnetic flowmeter
1. The plug-in electromagnetic flowmeter requires the user to set the pipeline horizontally, with at least 5DN straight pipe section in front of the sensor and at least 3DN behind the sensor. The flow regulating valve shall be located 3DN downstream of the sensor. The vibration of the pipeline should be obvious, and the inner wall of the pipeline has no obvious unevenness.
2. First, open a Ф 60-62 mm hole directly above the pipeline measuring point, and make sure that the edge around the circular hole is smooth and free from burrs and gas cutting scars.
3. Unscrew the mounting piece from the sensor and weld it reliably at the above opening. It is required that the lower end of the installation piece should be flush with the inner surface of the pipeline and ensure no leakage.
4. Loosen the three locking screws of the sensor and pull out the probe rod and probe head as a whole for later installation. (Note: The user must not open the connection between the probe and the insertion rod)
5. Wrap the thread on the upper end of the installation part with hemp lead oil or ptef, and then screw the ball valve and sealant locking mechanism on it.
6. Slowly insert the detection rod from above, tighten the lock nut with a little force, press the insertion rod, and measure L2 with the same size as recorded, and the installation is completed.
Influencing factors and matters needing attention in type selection
I. Influence of various media on measurement (1) The influence of velocity distribution can be known from fluid mechanics. When the liquid flows in the pipeline, the velocity of each point on the cross section of the pipeline is not equal, but whether it is laminar flow or turbulent flow, the velocity component can become axisymmetric distribution after passing through a certain distance of the straight pipe section, with the maximum velocity at the center of the pipe axis, zero at the wall of the pipe, and its average velocity is V-. As long as the velocity distribution is axisymmetric with respect to the center of the measuring pipe, it will be generated on the electrode. Therefore, the axial symmetry of velocity distribution is one of the working conditions that uniform magnetic field electromagnetic flowmeter must meet. If the velocity distribution is asymmetric with respect to the central axis of the tube, although the total flow is the same, the induced electromotive force near the electrode is large, so the measured signal is larger than the actual flow value. On the contrary, when the induced electromotive force is small at 90 degrees from the electrode, the obtained signal is smaller than the actual flow value, resulting in measurement error. Therefore, in order to make the velocity distribution symmetrical, it is necessary to add a straight pipe section in front of the flowmeter. ⑵ Influence of magnetic field edge effect on measurement If it is assumed that the magnetic field is always uniform along the direction of fluid flow, in fact, this means that the magnetic field along the tube axis is infinite? And the magnetic field of the actual flowmeter is limited? Therefore, the influence of edge effect caused by finite magnetic field on measurement must be considered. Assuming that the pipe wall is insulated, the magnetic field near the electrode is generally uniform, and the two ends gradually weaken, forming an uneven edge and finally dropping to zero. In this way, the electric field e inside the liquid is also uneven, which will produce eddy current. The secondary magnetic flux generated by eddy current in turn changes the working magnetic flux at the edge of the magnetic field? Thereby further destroying the uniformity of the magnetic field. At this time, the induced electromotive force measured on the electrode is different from that under the infinite magnetic field, resulting in errors. If the tube wall is conductive, the magnetic field edge effect will be more obvious due to the short-circuit effect of the conductive tube wall, and this effect will become more obvious with the change of the conductivity and wall thickness of the tube wall, which will lead to the increase of the loss of induced electromotive force on the electrode. For electromagnetic flowmeter, it is very necessary to measure the insulation of the pipe wall, so the pipe wall is usually coated with insulation. If the measured medium contains magnetic conductive substances, the magnetic field edge effect will be more complicated. Due to the existence of magnetic materials, the magnetic field is seriously distorted, which leads to the nonlinearity of measurement. Therefore, for liquid containing liquid metal, DC excitation is generally used to reduce the magnetic field edge effect. (3) The input impedance of the electromagnetic flowmeter converter is improved due to the influence of the conductivity of the measured medium. When measuring conductive liquid, there is generally no error due to the slight change of dielectric conductivity, but for a certain input impedance of the converter, the conductivity of the measured medium has a lower limit, which cannot be lower than this lower limit. The conductivity of the measured medium is not allowed to be too large. For example, when the conductivity exceeds about 10- 1S/cm, the flow signal will decrease and the indicated value will change, that is, the indicated flow value is less than the actual flow value. When the conductivity of the measured medium is large, the resistance of the external circuit is small. At this time, no matter how high the input impedance of the converter is, the result of parallel connection will depend on this part of the liquid external circuit, thus reducing the transmission accuracy between the transmitter and the converter. Therefore, for the electromagnetic flowmeter, the measurement is not affected by the conductivity of the medium, and the conductivity of the measured medium cannot be too large or too small. If the conductivity of the medium is extremely high, a large eddy current will be generated at the edge of the magnetic field, causing secondary magnetic flux, which will weaken and strengthen the magnetic fields on both sides of the working magnetic field edge respectively. Therefore, the medium with high conductivity should not be excited by AC, but by DC. With the development of electronic technology and the improvement of input impedance of converter, the lower limit of conductivity of measured medium will decrease. Third, the choice of flow sensor, the choice of electrode material for electromagnetic flowmeter? Improper selection of electrode material and measured medium will affect normal measurement due to chemical action or polarization phenomenon, and electrode material should be selected according to the corrosiveness of measured medium. Select the lining material of electromagnetic flowmeter according to the corrosiveness, abrasion and temperature of the measured medium. Try to choose electromagnetic flowmeter with lightning protection function. Four. Install the flow sensor 1. Installation site requirements. ? 1), when measuring mixed-phase fluid, choose a place that will not cause phase separation. When measuring two-component liquid, avoid installing it in the downstream position with uneven mixing. When measuring the chemical reaction pipeline, install it in the section where the reaction is completely completed. ? 2), as far as possible to avoid the measuring tube into negative pressure. ? 3) Choose places with small vibration, especially integrated instruments. 4) Avoid large motors and transformers nearby to avoid electromagnetic interference. ? 5) Places where it is easy to realize separate grounding of sensors. ? 6), try to avoid the surrounding environment with high concentration of corrosive gas. ? 7) The ambient temperature is in the range of -25-60℃ and the relative humidity is in the range of 10%-9O%, so direct sunlight should be avoided as far as possible. ? 8) The liquid should have the conductivity required for measurement, and the conductivity distribution should be generally uniform. Therefore, the installation of the flow sensor should avoid places where uneven conductivity is easy to occur, such as adding liquid medicine near its upstream, and the filling point is preferably located at the downstream of the sensor. 2. The electromagnetic flowmeter has lower requirements for the length of straight pipe sections. For 90o elbows, T-tees, concentric reducers and full-open gate valves, it is usually only necessary to stay away from the center line of the electrode, and it is not necessary to stay away from the connection surface at the inlet end of the sensor >: For straight pipe sections with a diameter and length of 5 times, valves with different openings need 1OD, and downstream straight pipe sections need 3D. When measuring the mixed liquid of different media, the distance between the mixing point and the flowmeter should be at least 30 d 3, and the installation position should be consistent with the flow direction. The installation direction of the sensor can be horizontal, vertical or inclined, which is not limited. But it is best to measure the solid-liquid two-phase fluid vertically and flow from bottom to top. This can avoid the disadvantages of serious local wear of the lower half of the lining when it is installed horizontally and solid phase precipitation at low flow rate. When installed horizontally, the axis of the electrode should be parallel to the horizon, so as to prevent occasional bubbles in the liquid from brushing the electrode surface and causing insulation? It is also possible to prevent the bottom electrode from being covered by deposits. When installed vertically, the flow direction should be upward, so that when there is no flow or small flow, the heavier solid particles in the fluid will sink, while the lighter fatty substances will rise and leave the sensor electrode area of the flowmeter. ?
4. The grounding sensor must be grounded separately, and the grounding resistance is below100Ω. In principle, the separation grounding should be on the sensor side, and the converter grounding should be at the same grounding point.