Section 6 inspection and measures for inconsistency between measured flow value and actual value
1、 Cause of failure
The failure causes of the measured flow not in accordance with the actual measurement can be summarized into the following six aspects.
(1) The set value of converter is not accurate;
(2) Improper installation position of sensor, incomplete tube or bubble in liquid:
(3) Signal cable is not handled well or cable insulation drops during use:
(4) The flow condition downstream of the sensor does not meet the requirements;
(5) The resistance between the electrodes of the sensor changes or the insulation of the electrodes decreases;
(6) There are inflow / outflow values not included in the assessment.
2、 Reference frame of comparison
Before checking its own fault phenomenon, it is necessary to evaluate and electromagnetic flowmeter The accuracy and correctness of the actual flow (i.e. the reference flow derived from each reference system) of the measured flow comparison. The reference flow is usually obtained from the following three aspects:
(1) From the material balance of the process system, that is, the quantity entering the system and the quantity leaving the system are compared;
(2) Comparison with other flow measurement values, such as the volume of the tank container or the flow rate of the external clamping ultrasonic flow juice;
(3) Compared with historical measurements.
According to these reference systems and their experience, the process operators at the process site put forward the view that the measurement value of the flow meter is not accurate. The instrument engineer should understand and analyze the basis, process and data of the judgment made by the operation personnel, understand the process, and go to the site for investigation when necessary, and make the next inspection after confirming its correctness.
In this paper, several common mistakes in accuracy evaluation of the clamp type ultrasonic flowmeter are given. (1) In the calculation of flow rate, the outer diameter and wall thickness of the pipe section are not measured by actual examples, but are only substituted according to the nominal size in the steel pipe specification table. Because of the tolerance of the outer diameter and wall thickness, the calculation error of the flow area is brought. For example, the difference between the nominal diameter of the tube and that of the nominal diameter of the tube is less than or equal to 2.5% of the nominal diameter. (2) If there is an error of (1-2)%, it will bring (1-2)% error to the flow measurement. (3) The thickness of cement lining layer, rust layer or dirt layer of old pipe is not included.
For example, the water plant often uses the volume of the clean water tank as a reference, so the accuracy of the pool area should be evaluated. It often occurs that the area of pool is calculated only according to the data of design drawing or as built drawing. As the as built drawing is only measured according to the engineering requirements but not measured according to the measurement requirements, it is inevitable that errors will be brought; the volume occupied by partition wall and pipeline in the pool and the volume of outflow / inflow of bypass pipeline may not be subtracted. In addition, it is necessary to confirm the sealing performance of the valve within the test time, which should be reviewed and evaluated.
3、 Inspection procedure
Figure 7 shows the process of checking that the measured flow rate of electromagnetic flowmeter is inconsistent with the actual flow rate. First, make a preliminary investigation and judgment according to the process, and then carefully check and try to remove the fault one by one. (2) it is not necessary to check the process in the past, that is to say, it is easier to check the process in the past. If the causes of the later faults are confirmed through preliminary adjustment, detailed inspection can also be carried out in advance.
4、 Failure inspection and measures
This section discusses the six aspects of fault cause inspection and measures shown in Figure 7-7.
1. Check the set value of the converter and check the zero point and full scale value
Check flow chart item 1. First of all, check whether the number of the matching sensor and converter is correct. Most of the current electromagnetic flow meters are calibrated at the manufacturer's factory, and the calibrated instrument constants are marked on the sensor name plate (or / or attached with the "instruction manual") and set in the matching converter. Therefore, it is necessary to check the instrument constant or whether the sensor number and the converter number match before debugging the newly installed instrument. Because such mismatches often occur, it is necessary to review the set values of calibre, measuring range and measurement unit. The zero point and range of the converter should be checked by the model of the electromagnetic flowmeter.
2. Check whether the pipeline is filled with liquid and contains bubbles
Check flow chart item 2. This kind of failure is mainly caused by the poor design of pipe network engineering or the imperfection of relevant equipment. Please refer to the section "2. The pipeline is not filled with liquid or the liquid contains bubbles" in Section 4 on page 9.
Fig. 7 flow chart for checking the inconsistency between the measured value of electromagnetic flowmeter and the application parameters
3. Check signal cable system
Check item 3 of the flow chart to see if the connecting cables match properly? Is the connection correct? Is the insulation reduced?
Usually people check electromagnetic flowmeter The cable system between the sensor and the converter is often ignored for the fault reason of the inconsistent flow measurement. However, in the fault cases in the process of debugging or inspection from the manufacturer to the field service, the frequency of the connecting cable is quite high. For example, the following cases are often encountered: (1) the whole attached cable is cut off and reconnected, and the connection will absorb moisture after using for a period of time, and the insulation will drop; (2) the signal wire end is not handled properly, and there is a short circuit between the inner shielding layer, the outer shielding layer and the signal core, or with the shell; (3) the cable with no specified model (or attached) is required; (4) the length of the cable exceeds the liquid current The upper limit of the length restricted by the conductivity system; (5) the liquid conductivity is low and the distance between the sensor and the converter is far away, and the driving shield cable is not used according to the regulations. When the length of some instrument cables is more than 30 m and the conductivity is lower than 10-4s / cm, a 2-core double shielded driving shielding layer is needed. Among the above five cases, (3) - (5) only occurred in the initial installation and commissioning period, and (2) also occurred in the initial installation and commissioning period.
4. Investigate the upstream flow condition of the sensor and check the inner wall of the channel measured by the sensor
Check flow chart item 4. Due to the limitation of installation space, the upstream flow condition of the sensor often deviates from the specified requirements. For example, if the flow is close to the obstruction and there is no enough length of straight pipe, these factors will affect the accuracy of measurement. The only way to improve the position of the upstream valve is to adjust the position of the upstream valve when the valve is not fully opened.
There is silting layer in the inner wall or the pipe wall is worn, which changes the flow area and affects the measured value. This kind of fault can only appear after a period of operation. Please refer to fault case 8.
Most of the reasons for the deviation of upstream flow condition of flow sensor are that the sensor is installed in an inappropriate position in engineering design; however, it has also happened that the installation of engineering design is good, but after running for a period of time, there are large errors. It seems impossible to judge the flow condition as poor according to common sense, but it does happen. Please refer to fault case 1.
5. The contact resistance between the electrode and the liquid and the electrode insulation are detected
Check flow chart item 5. The contact resistance between electrode and liquid mainly depends on contact area and liquid conductivity. When the conductivity of distilled water is 5 × 10-6s / cm, the resistance value is 350kd, the domestic and industrial water with conductivity of 150 × 10-6s / cm is about 15kn, and the brine with conductivity of 1 × 10-2s / cm is about 200N.
Although it is only a general value to measure the electrode contact resistance with a multimeter when it is filled with liquid, it is a convenient method to judge the condition of pipe wall. Accurate measurement must be done with AC bridge, such as "kohlraush bridge".
The electrode contact resistance of the electromagnetic flow sensor should be measured and recorded when the new instrument is debugged, and then it should be measured every time during maintenance (the measured resistance value of each time must be measured with a multimeter of the same model and the same measuring gear). The analysis and comparison will help to judge the instrument fault in the future, and it is unnecessary to remove the flow sensor from the pipeline for inspection.
If the measured electrode contact resistance is higher than before, it means that the electrode surface is covered or partially covered by the insulating layer; if the resistance value is lower than the previous value, it indicates that conductive deposition layer is attached to the electrode and lining surface.
Generally, the electrode insulation resistance should be greater than 100MN. If the inspection result is that the insulation is damaged, only the sensor can be replaced. The method to check the electrode insulation is to first remove the flow sensor, empty the liquid, wipe the inner surface of the lining with cloth, and dry it without liquid (water) stains. Then use 500VDC megohmmeter to test the resistance of the two electrodes to the ground. However, the reason of insulation decline is often caused by water immersion and dampness. Sometimes, the insulation can be restored by removing moisture by hot air blowing.
6. Check whether there is any outflow or inflow of manifold not included in the assessment
Check flow chart item 6. When the process technicians find that there is a big difference between the measured flow rate and the reference value, the analysis of various reasons often focuses on the flow meter and ignores the reasons for the outflow / inflow of the measurement pipeline manifold. When discussing with field service instrumentation engineers, process operators often say with certainty that there is no manifold outflow or inflow. However, field service experience shows that after a comprehensive inspection and elimination of various other failure possibilities, it is often the outflow or inflow of the manifold that leads to the discrepancy between the measured flow rate and the so-called "actual measurement". This example is not unique. Therefore, whether there are manifolds should also be investigated as an aspect. For example, it is necessary to investigate whether there is a manifold in the pipeline between the measuring point of the reference quantity (such as ultrasonic flowmeter, container and pool) and the electromagnetic flowmeter, and whether the valve is tightly closed. In addition, it is also necessary to check whether the container or pool is connected with other outflow and inflow sources.
(1) flow regulator (flow rectifier) is a kind of pipe fitting composed of porous plate or small tube bundle (or grid plate) in the pipeline, which is used to reduce (or eliminate) vortex and improve the distortion of velocity distribution, so as to shorten the length of straight pipe section.
Fig. 8 inspection process of electromagnetic flowmeter output signal over full scale value
Section 7 inspection and measures for output signal of electromagnetic flowmeter exceeding full scale value
1、 Cause of failure
The fault causes of the output signal exceeding the full scale value come from four aspects: the sensor, the connecting cable, the converter and the instrument connected to the converter output. There are many reasons for each aspect, which are as follows:
(1) In the aspect of sensors, there is no liquid communication between the electrodes, and the electric interference is introduced from the liquid;
(2) In terms of cable connection, the cable is disconnected;
(3) In the aspect of converter, the matching error with the sensor is wrong, and the setting is wrong
(4) In the aspect of rear position instrument, it is not electrically isolated, and the setting is wrong.
2、 Inspection procedure
Fig. 8 shows the flow of checking the output signal over full scale value. The inspection is sufficient to judge whether the cause of the fault comes from the converter before (i.e. upstream of the flow signal) or after the converter and the instruments after the converter, and then conduct preliminary investigation and judgment according to the process, and then carefully check and try to remove the faults one by one. The sequence of inspection items listed in the process is as follows: (1) it can be understood through observation or inquiry that there is no need for large operation first, i.e., easy first and then difficult; (2) according to the past on-site maintenance experience, the one with higher frequency and higher probability in the future is the first; and (3) the requirements of inspection itself. If it is confirmed by preliminary investigation that it is the cause of the latter several faults, it can also be checked in advance.
3、 Failure inspection and measures
This section discusses the inspection methods and measures of the above four aspects.
1. Judge whether the fault cause is in front of the converter or in the lower instrument after the converter
Check flow chart item 1. The fault occurs before the converter, that is, the signal cable between the sensor and the sensor / converter (the signal connecting line of the integrated electromagnetic flowmeter is in the internal of the instrument, generally, there is no fault); after that, it is in the sensor itself and its post integrator or flow computer.
First, water is supplied to the pipe system and flow sensor, and the two signal terminals of the converter and the functional or protective ground terminals are short circuited in the static state without flow. Observe whether the output signal of the converter is sufficient to zero. If the instrument can't be connected to zero and No. 6, the instrument can be checked before zero and No. 6.
2. Confirm the integrity of the signal cable and the full contact between the two electrode fields and the liquid
Check flow chart item 2. If the signal circuit is disconnected, the output signal will exceed the full scale value, so this inspection item is mainly to verify that the flow signal circuit is complete and unobstructed. The signal circuit includes cable and its connecting terminal, a pair of electrodes of flow sensor and liquid between electrodes. In addition to checking the circuit on and off, it is also necessary to verify the cable model, the connection correctness of each contact point, and whether the insulation meets the requirements. The electrode of the flow sensor does not contact with the liquid (two electrodes do not touch the liquid or one electrode does not touch the liquid, the signal cable is also disconnected. As shown in Fig. 3, the flow sensor must be refitted to the position where the liquid can be filled, etc., so as to eliminate the reason of the end contact between the electrode and the liquid.
3. Check the correctness of the converter settings, check the zero point and full point
Check flow chart item 3. When the separated electromagnetic flowmeter is delivered from the factory, the general converter and sensor shall be calibrated according to the caliber and flow rate specified in the contract and the set parameters, and the sensor and converter must be one-to-one corresponding. Therefore, first check whether the matching is correct, and then check whether the converter instrument constants and parameters are consistent. Then check the zero point with the analog signal. It is not necessary to check this item for all-in-one instruments.
4. Check the lower (rear) position instrument
Check flow chart item 4. The output flow signal of electromagnetic flow converter is transmitted to flow integrator, flow computer and other lower instruments. If the back position instrument is connected with electricity (i.e. active load), the power feedback on the load will damage the output circuit of the converter, and the phenomenon that the transmission signal exceeds the full scale value occurs, electrical isolation measures should be taken.
There are two types of output circuit of converter, which are allowed to be grounded and not allowed to be grounded. If the grounding is allowed, the output still exceeds the full scale value, and the converter is faulty; if the grounding is not allowed to be wrongly grounded, it can operate normally as long as the grounding is removed.
5. Check the introduction of electrical interference from the liquid
Check flow chart item 5. In the case of no exciting current, the interference potential is detected at the two electrodes with a multimeter or an oscilloscope. This fault often occurs in sodium chloride electrolysis process and cathodic protection pipeline in soda industry. The electromagnetic flow sensor can be insulated from the pipeline to make the electrode and liquid at the same potential. See case 12 (page 24).
6. Check the converter itself
The causes of the output signal exceeding the full scale value caused by the failure of the converter itself are more complex. It can be caused by a link in each unit circuit in the converter, and there are great differences in the type (analog or digital). For general users, the circuit board of modern electromagnetic flowmeter can be divided into interchangeable and independent units, and the standby circuit board (or the circuit board of other normal instruments of the same model) can be replaced to check and judge.
First check item 6 of the flow chart, i.e. input / output circuit. According to the characteristics of analog circuit converter or digital circuit converter, several links should be inspected emphatically. For analog circuit converter, the feedback circuit should be open and the output circuit should be damaged; for digital circuit converter, the nd conversion circuit and output circuit should be analyzed as the main inspection link. Then check item 7 of the flow chart, that is, check other circuits of the converter.