The issue of selecting the measurement range for electromagnetic flowmeters
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I would like to ask the teachers: We have an electromagnetic flowmeter here. The pipe diameter selected according to the design was DN50, with a flow rate range of 25 cubic meters per hour. Later, after the project was completed, the flow rate was required to remain stable at 2 cubic meters per hour during the trial production phase. Is the range of this flow meter chosen to be too large? Is the flow meter accurate at 2 cubic meters per hour? If it is used for a long time at 2 cubic meters per hour, is it necessary to replace it with a flow meter that has a smaller diameter and a lower measurement range? Moreover, the data usage often fails to be displayed.
1. The recommended measurement range for electromagnetic flowmeters is 0.3~12 m/s; accuracy drops rapidly when the flow rate is below 1 m/s, whereas it remains high and the stability is good at higher flow rates； Based on a minimum flow rate of 0.3 m/s, the flow rate for DN50 is 2.1195 m3/h; 2. The range of the flow meter should be set higher considering the actual operating conditions ； 2. Is the flow meter accurate at a flow rate of 2 cubic meters per hour? The accuracy has decreased; the specific precision is unknown, but it can be estimated based on the attachment ； 3. If the accuracy requirement is not high, it’s not necessary to replace it ； 4. No traffic displayed: First, the low traffic volume may have been cut off; second, the sensor can no longer detect it. It is recommended to check the low-flow cutoff setting (set it to zero to disable cutoff); the flow rate display will fluctuate when it is below 2 m3/h.
This post was last edited by Shanghai Fuling on 2019-5-5 at 13:49. First of all, based on the actual flow rate, the diameter chosen by the poster is too large; it is designed for use at flow rates of ≤0.3 m/s. In cases where the operating conditions are favorable, such as with ordinary water, it’s not that this device cannot be used at such low flow rates, but issues such as no flow measurement or inaccurate readings can easily occur depending on the actual conditions (such as the type of fluid). Additionally, the poster can check the settings for low-flow cutoff and airway alarm to see if there is any improvement possible.
Firstly, if the process requirement is a flow rate of 2 m3/h, then this pipe is clearly too large; pipes with a diameter of 50 generally have a flow rate of 15–25 m3/h. Secondly, every electromagnetic flowmeter has a minimum flow limit; below a certain flow rate, the flowmeter is unable to make accurate measurements. If it is necessary to use such large-diameter pipes at such low flow rates, it is recommended to install a reducer before the flowmeter for measurement. Change the diameter back again. With a flow rate of 2 m3/h, if very thin pipes are used, DN15 or even smaller diameters may be necessary. It’s necessary to check with manufacturers to see if electromagnetic flowmeters of such small sizes are available. Additionally, if the pipe diameter is small and the pressure difference isn’t sufficient, the fluid is likely to cause blockages in the pipes.
Also, with such a low flow rate and such large pipes, it’s very likely that the pipes are not fully filled; if they’re not fully filled, the flow meter will give inaccurate readings.
Thank you very much for the suggestions. Is the flow meter installed vertically to ensure a full pipe? However, we did choose a larger diameter.
Thank you for the reply; it’s very helpful!