Electromagnetic flow failure and solutions

      The following is an example of an electromagnetic flowmeter installed in a device . This device uses more than 40 electromagnetic flowmeters as measurement units. After the device was officially put into operation, most of the electromagnetic flowmeters worked normally, but a few electromagnetic flowmeters did not work properly. After long-term observation and analysis by on-site operators, the following conclusions were drawn: these electromagnetic flowmeters that did not work properly were not caused by electromagnetic interference, but by interference from process fluid fluctuations. Here are some of the main problems summarized as follows. " v! |. F2 s$ O6 h
1. Interference of strong flow beam:. n# H! ~   H9 l8 a- A+ d% cThis
device has a very important electromagnetic flowmeter, and its design installation position is shown in Figure 2-19. The  electromagnetic flowmeter FT-112 is installed at the bottom A position of the curved pipe, and the length of the front and rear straight pipe sections meets the requirements.: ?/ S; v3 P& C# gHowever, after the device was put into operation, it did not work properly for a long time. From the DCS trend chart, it can be seen that the flow fluctuates greatly and the trend chart is messy. After repeated inspections, there are no problems with the instrument installation, electrodes and grounding. Later, the installation of the upstream pipeline of the process was checked. The flow of the main pipeline consists of three streams Fl, F2, and F3, of which F3 comes from a high-level tank and the flowmeter* q# w7 f; ], d& The elevation difference of the installation position is about 20m. The length of the D section pipeline is about 1m, and the length of the H section pipeline is about 1.5m. After the F3 flow stream comes down from the high-level tank, due to its huge potential energy converted into kinetic energy, F3 fails to mix well with F1 and F2 and directly passes through the electromagnetic flowmeter, that is, two fluids with different flow rates pass through the flowmeter. This flow stream interferes with the main flow, causing the flowmeter to indicate turbulent fluctuations. * W) \# Z+ M4 z% ]* d/ kAfter finding the cause, the electromagnetic flowmeter FT-ll2 was moved from position A to position B (see Figure 2-19), and position B is about 2m away from the original pipe bend. After improving the installation position, this problem that has plagued production for a long time has finally been solved. ( g! O; C# u0 M5 R  ^" D! [# [7 N 2. Interference of flow rate caused by changes in local resistance in the container: * t- X* L! r, W8 X5 LThere is another electromagnetic flowmeter in the device, and its original design installation position is shown in Figure 2-20. * m# _6 The length of the straight pipe section and grounding of  the electromagnetic flowmeter FT-377 meet the requirements, but the flow indication keeps jumping after starting, and the reason cannot be found.  ^% f3 v' ~5 k2 H) F  i0 aBy chance, the flow indication is stable after the agitator in the mother liquid tank is stopped. After inspection, it is found that the agitator is installed on the side wall, and its position is only about 1m away from the outlet of the flowmeter pipeline. Obviously, it is the waves raised by the agitator blades that change the resistance of the pipeline outlet. The distance D1 from the flowmeter outlet to the container wall is about 1.5m. Because the distance is too short, the stirring waves cause the pressure at the pipeline outlet to fluctuate, which makes the flow velocity at the flowmeter outlet unstable and causes the flow indication to jump. 9 p; D" a! w# @Then the flow meter was moved from position A to position B (see Figure 2-20), about 10m away from the original installation position, and the flow meter was able to operate normally. 3. Temperature interference on flow value: 6 e% i: f: Q' b2 d7 d0 There is a process line in the Y device as shown in Figure 2-21, in which FT-114, FT-126, and FT-127 are all electromagnetic flow meters.  The process fluid passes through the flow meter FT-114 and then enters the reactor through two flow meters FT-126 and FT-127. Under normal circumstances, the indication of FT-114 should be equal to the sum of the flow rates of FT-126 and FT-127, but sometimes it is found that the error is very large. With the cooperation of the process personnel, it was found that at the beginning of the feeding, a stream of fluid flowing through FT-127 had to pass through a heat exchanger E (depending on the process conditions, this stream of fluid sometimes needs to be heated to raise the original process medium of about 100°C to 180°C). The increase in the temperature of this stream of fluid causes the liquid volume to expand, which speeds up the flow through FT-127. 8 D8 ^: V, @$ }' ~ Since the electromagnetic flowmeter is essentially a velocity flowmeter, the flow value indicated by this flow is increased, so that the sum of the sub-flows greatly exceeds the indication of the total flowmeter. This flow is corrected according to the temperature conditions, so that the problem is solved.