Section 1 fault types
Electromagnetic flux meter The first type of failure in operation is the failure of the instrument itself, that is, the failure caused by the damage of the instrument structure or components; the second type is the failure caused by external factors, such as improper installation, flow distortion, deposition and scaling. The second type of fault is discussed in this paper.
According to the fault occurrence period classification, it can be divided into: ① commissioning period fault; ② operation period fault. The main reasons for the failure during the commissioning period are improper selection or setting of instruments, improper installation, etc. The main reasons are that impurities in the fluid adhere to the electrode lining, and the environmental conditions change and new interference sources appear.
According to the external source analysis of the fault, it comes from three aspects: ① caused by pipeline system and installation; ② caused by environment; ③ caused by fluid. Source 1 is mainly shown in the commissioning period; source 2 and 3 will appear in both commissioning and operation periods.
1、 Failure during commissioning
This kind of fault appears in the initial installation and commissioning of electromagnetic flowmeter, but once the fault is eliminated after improvement, it will not appear again under the same conditions. The common faults in commissioning period mainly include improper installation, environmental interference and fluid characteristics.
1. Piping system and installation, etc
It is usually caused by the incorrect installation position of electromagnetic flow sensor. The common faults are as follows: the flow sensor is installed at the high point of the pipe network which is easy to accumulate trapped gas; there is no back pressure behind the flow sensor, and the liquid is directly discharged into the atmosphere, resulting in the non full pipe in the measuring pipe; when the flow sensor is installed on the vertical pipe with upward flow and downward flow, the evacuation may occur.
2. Environmental aspects
It is mainly the pipe stray current interference, space electromagnetic wave interference, large motor magnetic field interference and so on. The interference of stray current in pipelines can be measured satisfactorily by taking good independent grounding protection. However, if there is strong stray current in the pipeline (such as the pipeline in the electrolysis workshop), it is necessary to take measures to isolate the flow sensor from the pipeline (see case 12 below). Space electromagnetic wave interference is usually introduced through signal cable, and it is usually protected by single or multi-layer shielding, but it has been encountered that shielding protection can not be overcome (see case 10).
The liquid contains uniformly distributed small bubbles, which usually does not affect the normal measurement, but the measured volume flow rate is the sum of liquid and gas; the increase of bubble will make the output signal fluctuate. If the bubble is large enough to flow through the electrode and cover the whole electrode surface, the output signal circuit will be disconnected instantaneously, and the output signal will have greater fluctuation.
The low frequency (50 / 16 hz-50 / 6 Hz) rectangular wave excitation electromagnetic flowmeter will produce slurry noise and the output signal will fluctuate to a certain extent when the solid content in liquid exceeds a certain level.
When two or more liquids are used in pipeline mixing process, if the conductivity of the two liquids (or the potential between the two liquids and the electrodes) is different, they will enter the flow sensor for flow measurement before the mixing is not uniform, and the output signal will also fluctuate.
Improper selection of electrode material and measured medium, passivation or oxidation, formation of insulation film on the electrode surface, and electrochemical and polarization phenomena will hinder the normal measurement.
2、 Failure during operation
After initial debugging and normal operation for a period of time, the common fault causes are: inner wall adhesion layer of flow sensor, lightning shock, environmental conditions change.
1. Inner wall attachment layer
Because electromagnetic flowmeter has more chances to measure suspended solids or dirty bodies than other flow meters, the failure probability of inner wall adhesion layer is relatively high. If the conductivity of the adhesive layer is close to that of the liquid, the instrument can output signal normally, only changing the flow area to form a hidden fault of measurement error; if the conductivity of the adhesive layer is high, the electromotive force between the electrodes will be short circuited; if it is an insulating adhesive layer, the surface of the electrode will be insulated and the measuring circuit will be disconnected. Both of the latter two phenomena will make the instrument inoperable (see case study seven ）。
2. Thunder shock
Lightning strike induces instantaneous high voltage and surge current in the line, which will damage the instrument if it enters the instrument. Lightning damage instrument three There are three ways to lead in: power line, flow signal line and excitation line between sensor and converter. However, from the analysis of damaged parts in lightning fault, most of the induced high voltage and surge current are introduced from the control room power line, and the other two ways are less. It is also learned from the lightning accident site that not only the electromagnetic flowmeter fails, but also other instruments in the control room often have lightning accidents at the same time. Therefore, the user should realize the importance of lightning protection facilities for instrument power line in control room. At present, the Design Institute of Qilu Petrochemical Co., Ltd one ］。
3. Changes in environmental conditions
The main reason is that the fault environment in the commissioning period is the same as above, but the interference source does not appear in the commissioning period but intervenes again during the operation period. For example, for an electromagnetic flowmeter whose grounding protection is not ideal, the instrument operates normally because there is no interference source during the commissioning period. However, new interference sources (such as the pipeline near the measuring point or the pipeline welding in the distance) interfere with the normal operation of the instrument, and the output signal fluctuates greatly.
Section 2 fault phenomenon and inspection process
● electromagnetic flowmeter Common faults include:（ one ）There is no flow signal（ two ）Output shaking（ three ）The zero point is unstable（ four ）The flow measurement value is not consistent with the actual value（ five ）Mountain signal over full scale value five Class, which will be discussed in sections below.
The procedure of checking the whole measuring system and judging the fault is shown in the figure one As shown in the figure, the inspection link includes the sensor and converter of the electromagnetic flowmeter and the cable connecting the two, the upper process pipeline of the electromagnetic flowmeter, and the connecting cable of the lower (rear) display instrument.
The frequently used inspection means or methods and their inspection contents are listed as follows:
（ one ）General routine instrument inspection
（ two ）Substitution method
Based on the interchangeability between the converter and the sensor and between the components of the internal circuit board of the converter, the fault location can be identified by the substitution method.
（ three ）Signal trace method
In order to test the electromagnetic flow converter, the analog signal is used to replace the sensor and provide the flow signal when the liquid is not flowing.
Check whether the display instrument works normally or not, and carry out the inspection against the direction of flow signal transmission. The analog signal is used to test the converter to judge whether the fault occurs in the converter and its back position instrument or in the upper sensor of the converter. In case of converter failure, if possible, it is convenient to use converter or circuit board in converter for alternative debugging; if sensor fault needs to be replaced, the pipeline system must be shut down because of the need to stop operation, because of the wide range involved, it is often difficult to do so. In particular, the large diameter flow sensor has a large amount of work to try to replace. Usually, only after other inspections have been completed, can the determination be made to remove the pipeline, check the internal condition of the sensor measuring tube or replace it.
Section 3 no flow signal output inspection and measures
1、 Cause of failure
No flow signal output can be summarized as five The causes of these problems are
（ one ）Power failure such as power failure;
（ two ）Connecting cable (excitation circuit, signal circuit) system failure;
（ three ）Failure of liquid flow condition;
（ four ）The sensor parts are damaged or the inner wall adhesion layer is measured;
（ five ）Failure of converter components damage.
2、 Inspection procedure
chart two The flow chart of checking no flow signal output of electromagnetic flowmeter is shown. Make a comprehensive examination and troubleshooting according to the process. The principle of inspection sequence listed in the process is as follows: ① it can be understood by observation or inquiry that the one without large operation is the first, that is, the easier is the later; ② according to the past on-site maintenance experience, the frequency of occurrence is higher and the after-sales may occur; ③ the higher the inspection requirements, the earlier. 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.
① Analog signal device is specially designed for debugging and checking electromagnetic flowmeter, which simulates the output signal of flow sensor.
chart two Inspection process of electromagnetic flowmeter without flow signal output
3、 Failure inspection and measures
This section discusses them separately one On five Measures to check the cause of the failure
1. Check the power failure
Check flow chart No one , No two Item. First confirm that the power supply has been connected, and then check all parts of the power supply. Check the main power supply and excitation current fuse. If the new fuse meets the specified current value, it is necessary to find out the fault point. Check whether the output voltage of power circuit board is normal or try to replace the whole power circuit board.
2. Check the connection cable system failure
Check flow chart No three Item. Check whether the cable connecting excitation system and signal system is connected and whether the connection is correct.
3. Check the liquid flow direction and the liquid fullness in the pipe
Check flow chart No four , No five Item. The liquid flow direction must be consistent with the arrow direction on the sensor housing. For the electromagnetic flowmeter which can measure forward and backward direction, if the direction is inconsistent, it can still be measured, but it does not show that the flow direction is not consistent, so it must be corrected. If the workload of removing the sensor is heavy, the arrow direction on the sensor can be changed and the display instrument symbol can be reset.
If the pipeline is not filled with liquid, if the pipe network engineering design or sensor installation position is not appropriate, the sensor measurement pipe can not be filled with liquid. Measures should be taken to avoid installation as shown in the figure three As shown in Fig a ， e Position and discharge in dotted line pipe b Location, refit to c ， d Location.
chart three Sensor installation position
4. Check the integrity of the sensor and the condition of the inner wall of the measuring tube
Check flow chart No six , No seven , No eight , No nine , No ten Item. Mainly check the integrity of each terminal and excitation coil, as well as the condition of the inner wall of the measuring tube.
The faults of excitation coil and its system often include: ① the coil is disconnected; ② the insulation of coil or its terminal drops; ③ the turn to turn short circuit. Among the three kinds of faults, the frequency of insulation degradation is relatively high. When the coil is disconnected and the insulation is lowered, it can be easily checked by multimeter and megohmmeter. It is relatively complicated to check the inter turn short circuit. First of all, the DC resistance value and ambient temperature of the newly installed electromagnetic flowmeter shall be measured with the Wheatstone electrode before it is put into operation, and recorded as the reference value. When checking the fault, if there is a man-made inter turn short circuit, it can be judged by measuring the resistance with a multimeter;
; if there are a few inter turn short circuits, it is necessary to prove that there is no short circuit. It is also necessary to measure the temperature coefficient of copper resistance with an electric bridge.
The reason why the frequency of sensor excitation coil circuit insulation decline is relatively high is enough, and the protection level of electrical enclosure IP65 （ GB4203 － ninety-three ）The sensor is often immersed in water for a short time (e.g. when the sensor is installed at a lower position, there will be accident flooding around the sensor) IP65 It is only dustproof and spray proof, and it is easy to be immersed in water or moisture. even if it is IP67 (dust proof and short-term immersion) or IP68 (dust prevention and continuous water immersion) level, also often occurs after the completion of wiring, the cable sealing ring or the sealing gasket of terminal box cover does not meet the sealing requirements. Due to the negligence of operation, the faults caused by water entering into the sealing ring gasket are common.
When the insulation of the terminal is damped, the noise can be dried by hot air blowing to restore the insulation. The coil is damp. For the sensor with two half closed protective shell, the detachable shell cover is placed in the oven and dried at appropriate temperature; for the air tight type (i.e. the protective shell of welded structure), although the magnetic ring of the sensor is structurally guaranteed to be free from moisture, there are also cases of infiltration from the junction surface of cable and sealant.
The most reliable way to check and judge the condition of the inner wall of the measuring tube is to remove the sensor and observe the attached insulation layer or conductive layer directly, but the workload is heavy. On line indirect inspection method can also be used, that is, measuring the electrode contact resistance and electrode polarization voltage to estimate the adhesion layer condition. The specific operation of indirect method is shown in Section 9 of this chapter.
5. Check the failure of the converter
Check flow chart No eleven Item. modern Electromagnetic flux meter Circuit board spare parts and replacement method are often used to check the converter.