First, the cause of the failure
The cause of faults in the output signal's full-scale value comes from four aspects, namely, the sensor side, the connection cable side, the converter side, and the rear position meter connected to the converter output. Each aspect has its own multiple reasons, which are mainly listed below:
(1) Sensors: There is no liquid communication between the electrodes, and the introduction of electrical interference, but the instrument has an empty tube detection and alarm function design, but also often have empty tube zero function, the failure phenomenon does not occur;
(2) Connection cable - The cable is disconnected and the wiring is wrong;
(3) Converters - Mismatches with sensors, setting errors;
(4) Back-position meter - Not electrically isolated, setting error.
Second, check the program
Figure 7-7 shows the process of checking the output signal over-saturation value. The inspection is first to determine whether the cause of the fault is from the converter (ie upstream of the flow signal), or whether the converter is followed by a downstream instrument. Then, the process is thoroughly considered for preliminary investigation and judgment, and then detailed inspections and troubleshooting are carried out. The principle of the sequence of the inspection items listed in the process is: (1) It can be observed or inquired to know that it is difficult to do a large amount of operation first; that is, it is difficult to do it first; (2) According to the past experience of on-site maintenance, the frequency of occurrence is higher and may be more likely in the future. The person with higher probability of appearance is first; (3) Check one's own requirements. If it is confirmed after preliminary investigation that it is the cause of the next few faults, it can be checked in advance.
Third, flow meter failure inspection and take measures
This section discusses the inspection methods and measures for the causes of the above four aspects, respectively.
1. Determine the cause of the fault before the converter or the converter and the lower position
Check the first item of the flow chart. The fault is in front of the converter, that is, the signal cable between the sensor and the sensor/transducer (integrated electromagnetic flowmeter signal cable is generally within the instrument and rarely fails); after that, the sensor itself and its post-integrator Or flow computer and other lower instrument.
First, in the pipe system and flow sensor water, the static state without flow, the converter two signal terminals and functional or protective ground terminal short circuit, observe the converter output signal to zero. If it can reach zero, it can be initially judged that the fault is not in the converter itself and in the rear position before the conversion. In the next step, the connection cable and sensor (ie, the second item of the flow chart) can be checked first. If the value cannot reach zero, the center of gravity of the test should be the converter and the rear instrument (ie, items 3, 4, 6, and 7 of the flowchart).
2. Confirm signal cable integrity and both electrodes are in full contact with liquid
Check the flow chart item 2. If the signal loop is disconnected, the output signal will exceed the full-scale value. Therefore, the inspection item is mainly to verify the integrity of the flow signal circuit. The signal circuit includes the cable and its connection terminals, a pair of electrodes of the flow sensor and the liquid between the electrodes. In addition to checking the continuity of the circuit, it is also necessary to verify the type of cable, the correctness of the connection of each contact, and whether the insulation meets the requirements. The flow sensor electrode is not in contact with the liquid (both electrodes are not in contact with the liquid or one electrode is not in contact) and the signal cable is also disconnected. The flow sensor must be refitted to fill the liquid position as shown in Figure 6-17. The reason why the electrode is not in contact with the liquid is excluded.
3. Review the correctness of converter values, check zeros and full points
Check item 3 of the flowchart. Separate electromagnetic flowmeter factory, the general converter and sensor according to the contract caliber, flow and set parameters real flow calibration, sensor and converter must be one to one correspondence. Therefore, first check whether the matching is correct, and then check whether the converter meter constant and each parameter are in compliance, and then use the analog annunciator to review the zero point. Integrated meter does not need to check this item.
Figure 7-7 Check flow of the overshoot of the output signal of the electromagnetic flowmeter
Output signal oversizing check and take measures (2)
Figure 7-7 Check flow of the overshoot of the output signal of the electromagnetic flowmeter (continued)
4. Check the lower (post) meter
Check item 4 of the flowchart. Electromagnetic flow converter output flow signal is transmitted to the flow totalizer, flow computer and other lower instrument. If the rear instrument is electrically connected (active load), the power supply feedback on the load damages the converter output circuit and the output signal exceeds the full-scale value, and electrical isolation measures must be taken.
The output circuit of the converter has two types, grounding and grounding. If it is allowed to ground, the output still exceeds the full-scale value, the converter has a fault; if it is not allowed to ground the wrong ground, just remove the ground to run normally. Http://
5. Check the introduction of electrical interference from the liquid
Check item 5 of the flowchart. In the absence of excitation current, use a multimeter or an oscilloscope to detect the interference potential at both electrodes. This failure phenomenon often occurs in the alkaline industrial electrolysis process and the cathodic protection pipeline. It may take measures to insulate the electromagnetic flow sensor from the pipeline so that the electrode and the liquid are at the same potential.
6. Check the converter itself
The cause of the over-saturation of the output signal due to the fault of the converter itself is relatively complex. It can be caused by one of the elements in each element of the converter, and varies greatly depending on the type (analog or digital). For general use units, current electromagnetic flowmeter circuit boards can be divided into mutually interchangeable units, and alternative circuit boards (or temporarily borrowing the same type of other circuit boards that run normal meters) can be used for alternative inspections.
Check the flow chart item 6 first to check the input/output circuit. According to the respective characteristics of analog circuit converters or digital circuit converters, they focus on checking several aspects. For analog circuit converters, whether the feedback loop is open circuit or not, and whether the output circuit is damaged or not; The /D conversion circuit and output loop analysis are the main inspection steps.
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