【Daily Question 20090417】Where are mass flow meters used, and what is the effectiveness of their use?

When capacity calculation is required for a product, such as for steam feed, ethylene feed, or when the finished product needs to be sent to the transportation department, accurate measurement is often necessary. Mass flow rate is generally used for measurement, rather than regular flow rate (volumetric flow rate); the accuracy aspect is also important here. When there is significant vibration, problems with the installation location can indeed affect the measurement results. Grounding is very important to prevent lightning strikes. In one incident, lightning damaged over a dozen Rosemount instruments – that’s quite expensive! (The cheapest ones seemed to cost around 160,000; this was the price five years ago.) This post was last edited by wopale3 on 2009-4-17 09:30

In addition to the need for high-precision mass flow meters in trade settlements, mass flow meters are also required in petrochemical plants for monitoring raw material consumption, product quality, and the products manufactured by those plants. The mass flow meters we use most frequently at the moment are those from German company E+H, with an accuracy ranging from ±0.1% to ±0.5%. In winter, the total flow rate of liquefied gas often encounters problems related to gas and liquid phases due to low ambient temperatures and the lack of insulation in the medium pipelines, which results in measurement errors; the situation improves after adding insulation to the sections before and after the mass flow meter.

Mass flow meters are generally used in trade measurement, as they offer high precision and low errors; E+H’s products are quite good.

Measurement principle The mass flow meter system consists of a sensor and a transmitter for signal processing. The flow tube in the sensor is driven by an electromagnetically driven coil installed at its end, causing it to vibrate in a manner similar to that of a tuning fork. The amplitude is generally less than 1 mm, and the frequency is around 80 Hz. When the fluid flows into the flow tube, it is forced to undergo the vertical movement of the flow tube. During half a cycle of the flow tube’s upward vibration (Figure B), the fluid and the force opposing the upward movement of the tube exert a downward force on the flow tube due to the increase in its vertical momentum. Conversely, the fluid flowing out of the flow tube exerts an upward force on the tube to counteract its upward vibration and thereby prevent a decrease in its vertical momentum. This leads to the distortion of the flow tube (Figure C). During the other half cycle of vibration, the flow tube vibrates downward, with the direction of twisting being opposite. This distortion phenomenon is known as the Coriolis effect (abbreviated as the Coriolis phenomenon). 　　The amount of twist in the flow tube is proportional to the mass flow rate passing through it. Electromagnetic signal detectors installed on both sides of the flow tube are used to detect the vibration of the detection tube. The magnitude of the mass flow rate is determined by the phase difference between these two signals; when no fluid flows through the flow tube, it does not twist, and the electromagnetic signals on both sides are in phase. When a fluid flows through the flow tube, it causes the tube to twist, which results in a phase difference between the two detection signals; this phase difference is directly proportional to the mass flow rate passing through. 　　Since the mass of the medium in the flow tube is density * volume of the flow tube, and the volume of the flow tube remains constant for sensors of any given diameter, the density of the medium can be determined by measuring the resonance frequency of the flow tube. 3 Selection of mass flow meters A mass flow meter is a flow measurement device that utilizes Coriolis technology; it consists of a sensor, a transmitter, and a display unit. The transmitter can be installed remotely or integrated with the sensor. In practical applications, users often experience confusion when selecting mass flow meters. Predicting the performance and reliability of instruments under specific environmental conditions is key to selecting the best flowmeter. Performance indicators include: the accuracy of the gauge, flow range, range utilization, pressure drop, etc.; therefore, these factors should be taken into consideration comprehensively when evaluating or selecting a mass flow meter. 3.1 Accuracy and Accuracy Description Accuracy refers to a gauge’s ability to measure actual values; it is the result of the combined effects of the gauge’s deviation, repeatability, and hysteresis. Accuracy description reflects the uncertainty of the measurement, and there are usually three ways to express it: percentage accuracy relative to flow rate, percentage accuracy relative to full scale, and percentage accuracy relative to flow rate with account for zero-point stability. The percentage accuracy of flow rate with zero-point stability is a combination of the percentage accuracy of flow rate and the percentage accuracy at full scale. Since zero-point stability describes the extent to which an instrument system is affected by disturbances or changes in the zero flow rate, and it represents the instrument’s ability to measure the true zero flow rate, accuracy is expressed as a percentage of flow rate along with the zero-point stability value; therefore, instruments with a low zero-point stability value are the best choice. 3.2 Flow range and range utilization The flow range refers to the upper and lower limits of flow; the lower limit is usually zero, while the upper limit is the rated flow. Range utilization is equal to the ratio of the rated flow to the instantaneous flow. Under conditions that meet the requirements for measurement accuracy and pressure drop, flow meters with a larger range width and higher range utilization exhibit superior performance. Sensors with different bore sizes exhibit varying performance characteristics such as rated flow rate, range utilization, zero-point stability, and pressure drop; therefore, it is crucial to select a sensor with an appropriate flange based on the actual process conditions.

Mass flow meters are often used in business handovers. . After all, the measurement accuracy is high, usually around 0.2%. When the material is unloaded, it exists in a gas-liquid phase, which causes severe distortion in the measurements. I’m looking for a solution; during the unloading process, air from an air compressor is used to pressurize the tank car, and the material then passes through a mass flow meter before entering the storage tank. . Seek a solution

Speaking of Coriolis mass flow meters, let me say a few more words. 1. Principle of operation of mass flow meters: The methods for measuring mass flow can be divided into two main categories: direct and indirect methods. The direct method involves directly measuring the parameter that has a functional relationship with the mass flow rate in order to determine the mass flow rate. These include the momentum method, vibration acceleration method (i.e., CMF, Coriolis mass flow meter), differential pressure method, and thermal method. The indirect method, also known as the derivation method, involves multiplying the volumetric flow rate QV by the fluid density ρ to obtain the mass flow rate QM, that is, QM = ρQV. There are two main categories of derived mass flow measurement methods: ① combinations of volume flowmeters and density flowmeters (thermometers and pressure gauges are used in place of density meters when measuring gases) ； ② Dual flowmeter combination. A Coriolis mass flow meter is a direct-type mass flow measuring instrument that operates on the principle that, as fluid flows through a vibrating tube, a Coriolis force is generated which is proportional to the mass flow rate. 2 The influence of the installation environment on the accuracy of flowmeters: Coriolis mass flowmeters have certain requirements regarding their installation environment. Firstly, there should be no vibrations at the installation site; otherwise, measures need to be taken to reduce such vibrations, such as using hoses at the inlet and outlet of the flowmeter to minimize the impact of pipeline vibrations on its accuracy. During installation, it should be placed as far away as possible from vibration sources; otherwise, vibration isolation devices should be used to minimize the impact of these sources on its measurement accuracy. The torque on the screws and pipes during installation is also one of the factors that cause inaccurate measurements by the flow meter. If hoses are not used to connect to the pipes during installation, but instead the pipes are bolted together, care should be taken to minimize stress as much as possible. ① It is necessary to ensure the concentricity of the pipeline and the parallelism of the flanges. The bolt holes of the takeover flanges should be aligned with one another so that the bolts can fit into the holes naturally. Try to avoid torsional stress to reduce zero drift. When multiple sensors are installed in series, resonance should be prevented. To this end, the supports for each sensor should be independent of one another, with a distance of more than 2 meters between sensors. ② The dedicated cables for the instruments should not be run in the same conduit as the power cables; the conduit used for these cables should be grounded. The flow sensor should be reliably grounded, with a grounding resistance of less than 4 ohms. If the grounding is poor and electromagnetic fields as well as RF interference are strong, the digital display of the flow meter will experience significant fluctuations. ③ The pulsation of the fluid can affect the Coriolis mass flow meter, thereby causing errors. ④ If zero drift is detected, zeroing should be performed again to avoid incorrect measurements. 3 Advantages and disadvantages of Coriolis mass flow meters  Advantages ① Directly measures the true mass flow rate, with high measurement accuracy. ② It can measure a wide range of liquids, including various highly viscous liquids, slurries containing solids, liquids with a small amount of gas evenly distributed within them, and gases with sufficient density (gases at high pressure). ③ The amplitude of the measuring tube is small, so it can be considered a non-moving component; there are no obstructions or moving parts within the measuring tube. ④ It is insensitive to the upstream flow velocity distribution, thus no straight pipe sections upstream or downstream are required. ⑤ The measurement value is insensitive to fluid viscosity, and the effect of fluid density on the flow rate is minimal. ⑥ It can be used for multi-parameter measurements, such as measuring density simultaneously.  Disadvantages: ① Most models of Coriolis mass flowmeters cannot be used to measure low-density media, such as low-pressure gases; a gas content in the liquid that exceeds a certain limit will significantly affect the measurement values. ② They are sensitive to external vibration disturbances; to prevent the impact of pipeline vibrations, most models of Coriolis mass flow sensors require specific installation conditions. ③ The pressure loss is relatively high, on par with positive displacement meters; some models of Coriolis mass flow meters even have a pressure loss 100% higher than that of positive displacement meters. ④Most models of Coriolis mass flow meters are heavy and large in size. (Repost)

For the starch slurry produced by our factory, a mass flow meter is needed to measure its density; subsequently, a control valve is used to achieve the desired density. Additionally, a mass flow meter is required for the output from the evaporator, in order to control the specific gravity of the output material.