Analysis and explanation of the limitations of several common flowmeters

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　　With the rise of mechanization and automation, flowmeters play a crucial role in industrial production when it comes to measuring instruments. When selecting a flowmeter, we need to take into account the physical and chemical properties of the medium being measured, as well as the industrial production process. It is important not only to understand the advantages of flowmeters but also their disadvantages, as this helps in making the right choice for a given application. Below is a summary of the disadvantages of several commonly used flow meters, aimed at helping everyone gain a better understanding of the limitations of these products. 　　I. Limitations of electromagnetic flowmeters We all know that electromagnetic flowmeters offer high accuracy, high safety levels, and stable performance when measuring conductive liquids. Below are their disadvantages: 1. Electromagnetic flowmeters can only measure the flow rate of conductive liquids; they cannot be used to measure the flow rate of non-conductive substances, such as gases and heated water that has undergone proper treatment. Additionally, under high-temperature conditions, the material of its lining needs to be considered. 　　2. An electromagnetic flowmeter determines the volumetric flow rate under operating conditions by measuring the velocity of the conductive liquid. According to the measurement requirements, for liquid media, the mass flow rate should be measured. Measuring the flow rate of a medium involves knowing the density of the fluid; different fluid media have different densities, and these densities change with temperature. If the electromagnetic flowmeter converter does not take into account the fluid density and only provides the volumetric flow rate at normal temperature, it is inappropriate. 　　3. The installation and calibration of electromagnetic flowmeters are more complex than those of other flowmeters, and stricter requirements are applied. Transmitters and converters must be used together; it is not possible to use instruments of two different models with each other. When installing the transmitter, from the selection of the installation location to the actual installation and commissioning, it is necessary to follow the requirements outlined in the product manual strictly. The installation location must be free from vibration and strong magnetic fields. During installation, it is necessary to ensure good contact between the transmitter and the pipeline, as well as proper grounding. The potential of the transmitter is at the same level as that of the fluid being measured. When in use, the gas remaining in the measuring tube must be completely expelled; otherwise, it will cause significant measurement errors. 　　4. When an electromagnetic flowmeter is used to measure viscous liquids containing dirt, the viscosity or deposits adhere to the inner wall of the measurement tube or the electrodes, causing changes in the electrical potential output by the transmitter and resulting in measurement errors. If the amount of dirt on the electrodes reaches a certain thickness, it may prevent the instrument from functioning properly. 　　5. Scaling or wear in the water supply pipes can change the inner diameter, affecting the intended flow rate and causing measurement errors. 　　6. The measurement signal from the transmitter is a very small potential signal in the millivolt range; in addition to the flow rate signal, it also contains various signals that are unrelated to the flow rate, such as the in-phase voltage, quadrature voltage, and common-mode voltage. 　　II. Limitations of ultrasonic flowmeters Ultrasonic flowmeters do not require cutting the pipeline to measure liquid media, thus they do not disrupt production processes in enterprises, and their performance is stable when measuring water-based fluids. Its disadvantages are as follows: 1. The ultrasonic flow meter has a limited temperature measurement range; it can generally only measure fluids with temperatures below 200°C. 2. It has poor resistance to interference. It is susceptible to interference from ultrasonic noise caused by bubbles, scaling, pumps, and other sound sources, which affects measurement accuracy. 　　3. Strict requirements are applied to the straight pipe sections: 20D at the front and 5D at the back. Otherwise, the discreteness will be poor and the measurement accuracy low. 　　4. The uncertainty in installation can lead to significant errors in flow measurement. 　　5. Scaling in the measurement pipeline can severely affect the accuracy of measurements, leading to significant errors; in severe cases, the instrument may not display any flow rate at all. 　　6. The reliability and accuracy level are not high, generally around grade 1.5 to 2.5, with poor repeatability. 　　7. It has a short service life; the accuracy can generally only be maintained for about a year. 　　8. An ultrasonic flow meter determines the volumetric flow rate by measuring the fluid velocity; for liquids, it is necessary to measure the mass flow rate. The instrument calculates the mass flow rate by multiplying the volumetric flow rate by a density value set manually. As the temperature of the fluid changes, its density also changes, and relying on a manually set density value does not ensure the accuracy of the mass flow rate measurement. The true mass flow rate can be obtained through calculations only if the fluid velocity is measured simultaneously with the fluid density. 　　III. Limitations of vortex flowmeters It is well known that vortex flowmeters are most widely used for measuring gases and steam; they offer accurate measurement, are easy to install, and are relatively inexpensive. It is undeniable that vortex flowmeters also have their own limitations: 1. There are many factors that can cause errors in flow measurement, such as measurement errors resulting from uneven flow velocities in the pipeline; it is not possible to accurately determine the density of the fluid when its operating conditions change; wet saturated steam is sometimes assumed to be dry saturated steam for measurement purposes. If these errors are not limited or eliminated, the overall measurement error of the vortex flow meter will be very large. 　　2. Poor vibration resistance. External vibrations can cause measurement errors in vortex flowmeters, or even prevent them from functioning properly. The high flow velocity of the channel fluid can cause additional vibrations in the cantilever of the vortex street generator, thereby reducing the measurement accuracy. The effect of larger pipe diameters is more pronounced. 　　3. Poor adaptability to measuring dirty media. The element of the vortex flow meter is highly susceptible to contamination by the fluid or entanglement by debris, which alters its geometric dimensions and has a significant impact on measurement accuracy. 　　4. High requirements are placed on the straight sections before and after the flow meter; theoretically, for a vortex flow meter, straight sections of 40D in front and 20D behind are necessary to meet the measurement requirements. 　　IV. Limitations of orifice plate flowmeters Orifice plate flowmeters are among the earlier types of flow meters; they are widely used and relatively easy to select. However, their disadvantages include the following: 1. The repeatability and accuracy of measurement are at a moderate level among flow meters, and it is difficult to improve accuracy due to the complex influence of numerous factors. 　　2. The range is narrow; since the flow coefficient is related to the Reynolds number, the typical range is only 3∶1 to 4∶1. 　　3. There is a requirement for a long straight pipe section, which is generally difficult to meet. The problem is even more pronounced, especially for larger pipe diameters; 4. High pressure loss: Typically, to keep an orifice flow meter operating properly, a pump needs additional power to overcome the pressure loss caused by the orifice. 　　5. The orifice plate relies on the sharp internal edges to ensure accuracy; as a result, it is sensitive to corrosion, wear, scaling, and dirt, making it difficult to maintain accuracy over time. 　　6. The use of flange mounting or flange connections can lead to problems such as leaks, spills, and drips, increasing the amount of maintenance work required. 　　There are a wide variety of flowmeters, each with different working principles and distinct advantages. Their strengths and weaknesses are also relative; no flowmeter is perfect. Therefore, the choice of flow meter must be based on the actual process conditions and operating environment in industrial production; each case needs to be analyzed individually to select the most suitable solution.