Five field applications where it’s easy to mistake an issue for a malfunction in an electromagnetic flowmeter – how many of these have you encountered?

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This post was last edited by yuchenchf on 2015-11-20 at 16:55. Based on on-site experience, the following points have been summarized for everyone’s reference. 1. Operating pumps with different head capacities in parallel causes the real-time measurement values from the flow meters to be lower. A certain company uses several pumps in parallel to transport sulfuric acid, with flow meters installed in the pipelines downstream of each pump for measurement purposes. During operation, some flow meters indicated significantly lower flow rates, leading the customer to conclude that there was a fault with those flow meters. Cause analysis of the fault: The head values of the various pumps are not consistent; the pump with a higher head will suppress the output of the pumps with lower heads, resulting in lower readings from the flow meter. Conclusion: The flow meter is operating normally. Operating pumps with different head values in parallel causes the real-time measurement values of the flow meter to be lower by 2. The rated flow rate of the pump cannot be used to determine the accuracy of the flow meter. A certain company compared the rated flow rate of the pump with the flow rate measured by the electromagnetic flow meter, using whether these values matched as a criterion to determine the accuracy of the flow meter. This is a misunderstanding; the rated flow rate of the pump and the flow value measured by the flow meter can be used as references for each other, but they cannot serve as a basis for determining the accuracy of the flow meter. Fault cause analysis: The pumping volume of the pump varies depending on the operating load conditions. The rated flow rate indicated on the pump’s nameplate corresponds to a flow rate under specific conditions, with a tolerance of 4%-8%. In practice, when the pump is in operation, the output flow rate often deviates from the rated flow rate listed on the nameplate due to differences in the pipeline load. Conclusion: The rated flow rate of the pump cannot be used to determine the accuracy of the flow meter’s measurements. 3. Leakage in the valves of the bypass pipeline causes the flow meter to give lower measurement values. An enterprise installed electromagnetic flow meters in its water supply pipeline; these flow meters had bypass pipelines, and it was found that their measurement values were on the low side. Fault cause analysis: If the flow meter is equipped with a bypass pipe, leakage in the bypass valve will inevitably reduce the instrument reading. Moreover, minor leaks in the valve are difficult to detect, and this is often mistaken for inaccurate instrument measurements. 4. There is a discrepancy between the readings of the flow meter on the main saturated steam pipeline and those of the flow meters in the individual workshops. A tobacco manufacturing company installed intelligent vortex flow meters at the boiler’s saturated steam outlet to measure the total steam flow, while vortex flow meters were also installed in each workshop to measure the steam consumption in those areas. After some time of use, discrepancies appeared between the readings from the main meter and those from the individual meters; the company suspected that the vortex flow meters were faulty and not providing accurate measurements. Fault cause analysis: The significant difference between the reading of the flow meter at the outlet of the boiler’s saturated steam main and the reading of the flow meter in the workshop is caused by changes in the steam dryness. The steam dryness degree of the steam passing through the boiler outlet flow meter is generally high (above 0.95), whereas the steam passing through the plant flow meter experiences a significant decrease in its dryness degree due to saturated steam resulting from long-distance pipeline transmission and poor insulation, leading to the formation of condensate water and thereby increasing the discrepancy in the measurement values. Conclusion: When a flow meter is used to measure saturated steam, reading discrepancies between the main gauge and the sub-gauges do not indicate a malfunction of the flow meter. 5. A difference in readings exists between the main meter and the sub-meters when there is less flow in the branches. A certain company installed 10 smart electromagnetic flowmeters manufactured by Yunrun Instrument Manufacturing Co., Ltd.; one of these was used as the main meter, while the other 9 were used as sub-meters to measure the amount of raw materials used in various workshops. After being put into use for several years, they functioned properly and provided accurate measurements. In 2008, due to insufficient power supply, nearly half of the production lines came to a halt; significant reading errors occurred on the main meters and sub-meters, leading the users to suspect that there was a fault with the electromagnetic flowmeters. Analysis of the cause of the fault: Typically, flow meters are designed to operate at around 80%-90% of their full-scale flow rate. If half of the sub-meters are not in use (i.e., half of the flow paths are closed), it is possible that the main meter will operate at around 40% of its full-scale flow rate. If the basic error of the flow meter installed on-site is 1.5%, then the maximum difference between the reading of the main meter and those of the other sub-meters could be 5.6% of the measured value. If even fewer sub-meters are in use, the difference between the main meter’s reading and those of the sub-meters will be even greater. Conclusion: Due to the shutdown of branch flow, the discrepancy in readings between the main flow meter and the sub-meters does not indicate a malfunction of the flow meter.