Check flow output signal and full scale value and take action

Abstract: The information of flow output signal and full scale value inspection and measures to be taken are provided by excellent flowmeter and flowmeter manufacturers and quotation manufacturers. The failure causes of the output signal exceeding the full scale value come from four aspects, namely: the sensor side, the connection cable side, the converter side, and the rear instrument connected to the converter output. There are many reasons for each aspect, which are mainly listed as follows: (1) Biography. For more flowmeter manufacturers to select models and price quotations, you are welcome to inquire. The following is the details of the flow output signal and full scale value inspection and measures to be taken. The failure causes of the output signal exceeding the full scale value come from four aspects, namely: the sensor side, the connection cable side, the converter side, and the rear instrument connected to the converter output. There are many reasons for each aspect, which are mainly listed as follows: (1) Sensor aspects——There is no liquid connection between the electrodes, and electrical interference is introduced, but the instrument designed with empty pipe detection and alarm functions often has the empty pipe zero function, so this fault phenomenon will not occur; (2) Connecting cables——The cable is disconnected, the wiring is wrong; (3) the converter aspect——Wrong matching with sensor, wrong setting; (4) Rear instrument——Not galvanically isolated, wrong setting. 2. Inspection procedure As shown in Figure 7-7, it is the process of checking the output signal over-full scale value. Check first to determine whether the cause of the fault comes from before the converter (that is, upstream of the flow signal), or the instrument after the converter and after it, and then make a preliminary investigation and judgment according to the overall consideration of the process, and then carefully check and try to eliminate the fault one by one. The order of inspection items listed in the process is as follows: (1) It can be understood through observation or inquiry that it is not necessary to perform major operations first, that is, first easy and then difficult; (2) According to the past on-site maintenance experience, the frequency of occurrence is high and it may be possible in the future. The one with a higher probability of occurrence is first; (3) the requirements of the inspection itself. If it is confirmed by preliminary investigation that it is the cause of the latter faults, it can also be checked in advance. 3. Flowmeter failure inspection and measures This section discusses the inspection methods and measures for the above four fault causes. 1. Determine whether the cause of the failure is before the converter or in the converter and after the lower instrument check flow chart item 1. The fault is before the converter, that is, the signal cable between the sensor and the sensor/converter (the signal connection line of the integrated electromagnetic flowmeter is inside the instrument, and generally rarely fails); after that, the sensor itself and the totalizer behind it Or low-level instruments such as flow computers. First, pass water in the piping system and flow sensor, short-circuit the two signal terminals of the converter and the functional ground or protection ground terminal in a static state without flow, and observe whether the output signal of the converter reaches zero. If it can reach zero, it can be preliminarily judged that the fault is not in the converter itself and the rear instrument before the conversion. The next step is to focus on checking the connecting cables and sensors (i.e., item 2 in the flowchart). If it cannot reach zero, check the center of gravity on the converter and the rear instrument (i.e., items 3, 4, 6, and 7 in the flowchart). 2. Confirm that the signal cable is intact and that both electrodes are fully in contact with the liquid. Check item 2 of the flow chart. If the signal loop is disconnected, the output signal will exceed the full scale value, so this inspection item is mainly to verify that the flow signal loop is complete and unobstructed. The signal loop includes cables and their connecting 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 cable type, the correctness of the connection of each contact, and whether the insulation meets the requirements. The electrodes of the flow sensor are not in contact with the liquid (neither electrodes are in contact with the liquid or one electrode is not in contact), and the signal cable is also disconnected. The flow sensor must be modified to a position that can be filled with liquid as shown in Figure 6-17. Eliminate the reason why the electrode is not in contact with the liquid. 3. Check the correctness of the converter value, check the zero and full check flow chart item 3. When the separate electromagnetic flowmeter leaves the factory, the general converter and sensor are calibrated according to the caliber, flow rate and set parameters specified in the contract, and the sensor and the converter must be in one-to-one correspondence. Therefore, first check whether the matching is correct, then check whether the instrument constant of the converter and each parameter are consistent, and then use the analog signal device to check the zero point. Integral instruments do not need to check this item. Figure 7-7 Check flow of electromagnetic flowmeter output signal over-full value inspection and inspection process Check item 4 of the lower (rear) meter check flow chart. The output flow signal of the electromagnetic flow converter is transmitted to the lower instrument such as the flow totalizer and the flow computer. If the rear instrument is connected with electricity (that is, an active load), the power feedback on the load will damage the output circuit of the converter, and the output signal exceeds the full scale value, and electrical isolation measures should be taken. There are two types of converter output loops that allow grounding and those that do not allow grounding. 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 grounded by mistake, as long as the grounding is removed, it can operate normally. 5. Check item 5 of the flow chart for the introduction of electrical interference from liquids. In the case of no excitation current, use a multimeter or an oscilloscope to detect the interference potential at the two electrodes. This fault phenomenon often occurs in the electrolysis process of the alkali production industry and the cathodic protection pipeline. Measures can be taken to insulate the electromagnetic flow sensor from the pipeline, so that the electrode and the liquid are at the same potential.

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