How to deal with A/D-converter saturation in electromagnetic flow meter

Abstract: The information on how to deal with the saturation of the A/D-converter in the electromagnetic flow meter is provided by the excellent flow meter and flow meter manufacturers and quotation manufacturers. Users often compare the material balance of the process system with historical measurement values ​​or with other reference flow measurement values, and feel that the flowmeter measurement is inaccurate during use, but remove the meter and check it on the flow standard device, except for a few errors in the meter itself (Such as debugging equipment. For more flow meter manufacturers to select models and price quotations, you are welcome to inquire. The following is the article details on how to deal with the saturation of A/D-converters in electromagnetic flow meters. Users often measure material balance from the process system, and historical measurements Value comparison or comparison with other reference flow measurement values, it is felt that the measurement of the flowmeter in use is not accurate. However, remove the meter and go to the flow standard device for calibration. Most of the instruments are normal. Most of the reasons are due to improper installation and arrangement of instruments and the mixing of different phases in the medium in the pipeline (such as condensation droplets in the gas, air bubbles in the liquid) and other errors in application. 1 Bad installation 1.1 Category 1 bad installation Bad installation with poor operation and improper arrangement, the common ones are: 1) The acute angle of the standard orifice plate is not installed on the upstream surface. 2) The inner diameter Dg of the sealing gasket between the instrument and the pipe is smaller than the inner diameter Dp of the pipe and the inner diameter Dm of the instrument to generate a beam. Dg should be slightly larger than Dm, such as Dg3) the gasket is eccentric (misaligned). The installation of the sealing gasket is eccentric, which covers part of the flow area and causes the velocity distribution to be seriously distorted and asymmetrical. Since the asymmetric flow occurs at the inlet of the flow sensor, that is, the length of the upstream straight pipe section is zero, it will bring measurement errors to the differential pressure, turbine, vortex, ultrasonic, target, electromagnetic and other instruments. For example, the eccentricity of the DN50mm electromagnetic flowmeter gasket is 10mm, and the measurement error is as high as 4% to 10%. 4) The flowmeter is in the wrong flow direction. 5) Install the instrument sensitive to vibration interference on the pipeline with vibration. 6) Lack of necessary protective accessories. These defects are well known or suggested by the instrument manufacturer and should be avoided. However, such mistakes are not uncommon because operators are not seriously trained and lack knowledge. Although the inner diameter of the gasket is too small or the installation is eccentric, although it has no or very little effect on the flow value of volumetric, float, Coriolis mass and other instruments, it will increase additional pressure loss. 1.2 Type 2 poor installation 1.2.1 Upstream disturbance sources The upstream disturbance sources include spiral welded pipes and various types of flow blocking fittings (such as elbows, reducers, branch pipes and valves), as shown in Figure 1 [2] . According to the type of turbulent flow, it is divided into two categories. The first type has velocity distribution distortion and secondary flow; the second type has vortex in addition to velocity distribution distortion and secondary flow. The most common types of pipe fittings are elbows and various combinations of elbows (like flat double elbows and three-dimensional double elbows). Various types of flow meters have different sensitivity to upstream flow disturbances, so their installation requirements must be put forward. Figure 1 Various pipe fittings classified according to the type of disturbed flow Among all kinds of flow instruments, the throttle differential pressure instrument has done the most perfect test for the length requirements of the upstream and downstream straight pipe sections of the throttle, and the typical results of the more mature choke have been It is specified in the international standard ISO5167. Other types of flow meters have not yet reached such a mature level; no matter the data provided by the standard specifications or the manufacturer's instruction manual, they are not as complete as those of the throttling differential pressure flow meter, and sometimes only serve as a reference. Due to the different structures of instruments of the same variety, the degree of influence varies greatly. For example, the turbine helical blades of the turbine flowmeter are much less affected by the vortex flow than the straight blades; the V method channel arrangement in the propagation time method ultrasonic flowmeter is less affected by the vortex flow than the Z method. 1.2.2 The downstream disturbance source usually thinks that once the fluid flows out of the flow meter, the flow state will not affect the meter, but it is just an illusion. In fact, disturbances to fluid flow caused by elbows, valves, etc. propagate upstream and can affect distances several times the diameter of the pipe. In most cases, 5 pipe diameters downstream is sufficient; some special cases may be slightly longer, but 10 pipe diameters downstream can be considered to be able to reliably cope with any disturbances caused by downstream pipe fittings. If the length of the straight pipe section cannot meet the requirements and the measurement accuracy is to be ensured, one of the following two workarounds can be adopted. 1) Calibrate under the field installation conditions, or calibrate on the laboratory real flow calibration device together with the disturbance blocker and the instrument under the same field installation conditions. 2) Install a flow conditioner as described in the following section upstream of the meter. 2 Flow regulators in the International Organization for Standardization Technical Committee draft ISO/CD 51671 'Measurement of flow with differential pressure devices installed in fluid-filled pipes of circular cross-section - Part 1———In the General Provisions [3], the nature of the data“Appendix C' classifies flow conditioners as flow straighteners and true flow conditioners.

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