Factors affecting the accuracy of insertion electromagnetic flowmeter

Case introduction

For the pipelines supporting the electromagnetic flowmeter, the main factors that affect the accuracy of the insertion electromagnetic flowmeter are the length of the straight pipe section and the inner diameter of the pipe:

1. Straight Pipe length: The plug-in electromagnetic flowmeter is a plug-in instrument. It can only measure the flow velocity at several points in the pipeline to estimate the flow rate, and has the nature of sampling. If high accuracy is required and the straight pipe section cannot meet the design requirements, the flow velocity distribution in the pipe cannot become concentric circles symmetrical to the axis. Measuring the flow velocity at only a few points in the diameter direction cannot fully reflect the flow velocity distribution of the entire cross-section. Therefore, it is necessary to There are strict requirements for flow velocity distribution. The straight pipe section should have a length of 15-25 times the inner diameter of the pipe. Only in this way can turbulent flow be fully developed and it is possible to reflect the flow velocity distribution of the entire section with several flow velocities. Since uniform velocity pipes are mostly used for large pipe diameters , it is generally difficult to ensure such a long straight pipe section on site, so it is difficult to improve the accuracy. However, as long as the flow is steady flow, that is, the flow velocity distribution is stable and does not change with time, the accuracy of measuring the flow velocity at only a few points is poor, but the repeatability is still good.

2. Pipe inner diameter: Few users carefully and accurately measure the pipe inner diameter. According to the provisions of the international standard ISO7145, when the inner diameter cannot be measured, the inner diameter of the pipe is also allowed to be determined based on the outer circumference of the pipe and the estimated wall thickness. Of course, the inner diameter determined in this way is difficult to be accurate. In addition, factors such as corrosion, scale, and condensation on the inner wall of the pipeline must also be considered. It is common for the inner diameter to have an error of 1 to 2 mm, but the impact on its flow rate cannot be ignored. The measurement principle of the insertion electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. The measuring tube of the flow meter is a short non-magnetic alloy tube lined with insulating material. Two electrodes penetrate the tube wall along the diameter direction and are fixed on the measuring tube. The electrode tip is basically flush with the inner surface of the liner. When the excitation coil is excited by a square wave pulse, a working magnetic field with a magnetic flux density B will be generated in the direction perpendicular to the axis of the measuring tube. At this time, if the fluid with a certain conductivity flows through the measuring tube. Cutting the magnetic field lines induces an electromotive force E. The electromotive force E is proportional to the magnetic flux density B, the product of the inner diameter d of the measuring tube and the average flow velocity v. The electromotive force E (flow signal) is detected by the electrode and sent to the converter through the cable. After the converter amplifies and processes the flow signal, it can display the fluid flow and output pulses, simulated current and other signals for flow control and regulation.