Differences between various flow meters

1.2 Significance of research on the application of flow measurement instruments: There are a wide variety of flow measurement technologies and instrument types, and the complexity of the objects being measured contributes to the technical complexities associated with flow measurement instruments. It differs significantly from the use of traditional measuring instruments; simply installing a flow meter and putting it into operation does not necessarily ensure that the measurement goal will be achieved. Two experts conducted an investigation of the more than a thousand flow meters installed on site and found that approximately 60% of the chosen measurement methods were not the most suitable or incorrect. Among the remaining 40%, about half used appropriate measurement methods, but there were issues with the way they were arranged and installed on site; these problems led to corresponding measurement errors. Therefore, flow measurement is a type of measurement that relies heavily on the operating conditions. In the laboratory, flow meters can achieve extremely high precision, but in actual application sites, once there are significant changes in the fluid conditions or environmental conditions, not only is accuracy not guaranteed, but normal measurement becomes impossible as well. A flow meter is calibrated at the factory with an error margin of better than ±0.5%, but it is not uncommon for the error to increase to ±5%–±10% after the new meter is installed on-site and put into use. There are various reasons for this situation, such as improper selection, inappropriate range, insufficient length of straight sections upstream and downstream, incorrect installation, fluid properties deviating too much from the design values, operating conditions exceeding allowable limits, the effect of pulsating flow, vibrations, and other harsh environmental conditions; many more can be cited as well. Therefore, flow measurement is a systematic issue that includes detection devices, display devices, upstream and downstream straight sections, and auxiliary equipment. Research on application technologies also includes the measurement object itself; merely having a flow meter with good performance does not guarantee achieving the desired measurement results. The goal of research on the application technology of flow measurement instruments is to ensure their proper use, and it mainly includes the following specific aspects. (1) Improving the utilization rate. In the management of instrumentation equipment, the utilization rate is defined as: (total number of instruments – number of instruments not in normal use) / total number of instruments. Therefore, improving the availability rate means reducing the number of instruments that cannot be put into normal use. In design institutes, the meter reading rate for the measurement systems designed by the automation control team is one of the key indicators reflecting the quality of the designers’ work and their technical proficiency. Experienced and conscientious designers are able to achieve a meter reading rate of over 95%, or to bring it above 95% through necessary adjustments. However, under market economy conditions, engineering companies often adopt a turnkey contracting approach with clients, requiring a 100% standard rather than 95%. If the designed instrumentation system cannot be put into use properly and it is the responsibility of the engineering company, then repairs or replacement of the instruments are necessary, which results in financial losses. Therefore, research on instrument application technology holds practical economic significance. Setting an example represents a comprehensive reflection of the level of instrumentation application technology and the quality of the instruments themselves. The measurement methods and instruments, as well as their compatibility, coordination, and optimization with the object being measured and the operating environment, along with the design selection and installation calibration that take place prior to this, are all important factors that affect the accuracy of measurements. Over the years, the application technology of flow measurement instruments in our country has made great progress, with a significant improvement in the quality of such instruments. This is partly due to the overall improvement in the technical skills of the personnel working with these instruments, as well as an enhanced sense of responsibility among them. More importantly, the quality of these instruments has improved substantially compared to the planned economy era; the proportion of imported instruments and those manufactured using advanced foreign technologies is on the rise. Especially after these instruments became more intelligent, their measurement ranges could be adjusted more flexibly. Instruments that could not be used properly due to inappropriate measurement ranges could now generally be put into use by adjusting their range. (2) Ensuring measurement accuracy: The accuracy of flow measurement refers to the precision achieved by the flow measurement system, and it is distinct from the accuracy of the flow meter itself. Just because a flow meter has good performance and high accuracy does not necessarily guarantee high measurement precision. To ensure the accuracy of flow measurement systems, in addition to proper selection, correct installation and calibration, as well as timely maintenance, applying intelligent technologies to appropriately compensate for and correct the errors that may arise in the measurement section is also an effective method. For example, compensating for the temperature expansion coefficient of liquids, compensating for temperature, pressure, and compression coefficients of gases, compensating for the effects of Reynolds number and flow expansion coefficient on differential pressure flow meters, compensating for the non-linearity of the flow coefficients of various flow meters, compensating for the temperature effects on positive displacement flow meters and vortex flow meters, and compensating for the velocity distribution in ultrasonic flow meters, among others. This compensation and correction involve using a systematic approach to address the errors inherent in the detection section, errors that cannot be overcome by that section on its own, thereby eliminating or substantially reducing them. Practice has shown that this method is simple and effective, with great potential for development. This method will be described in detail in Chapter 8 of this book. Among the various methods to ensure measurement accuracy, online real-flow calibration holds an important position. In the past, offline methods were mostly used to calibrate flow meters. Flow meters calibrated using this method achieve high accuracy after error correction; however, since the reference conditions of the pipeline during calibration differ from those in actual use, the properties of the fluid used during calibration are different from those in actual operation, and the environmental conditions during calibration are not the same as those in the actual location where the instrument is used, these factors result in additional usage errors that reduce the measurement accuracy. The online real-time flow calibration method is an effective way to solve this problem. For example, oil metering stations are equipped with standard volume tube connection ports during the construction phase; once these standard volume tubes are connected, valve switching can be used to perform online real-flow calibration of the flow meters located in the metering station. Now, online real-flow calibration is also required at natural gas distribution stations. (3) Improve the reliability of the flow measurement system. If the flow meters used for safety interlock alarms are unreliable, they may fail to activate when interlock action is required, which can lead to accidents; or they may activate unnecessarily when they shouldn’t, resulting in unintended shutdowns and losses. If the fuel flow meter in industrial furnaces is unreliable and causes blockages in the flow path, it can easily lead to the furnace going out, resulting in accidents. If the flow meters used for process control are unreliable, they can send incorrect signals to the control system, leading to malfunctions in that system and disrupting the stability of the production process. This affects the quality, output, and material consumption of the products, resulting in losses. If the flowmeters used for financial settlement measurement are unreliable, it can lead to inaccurate measurements, resulting in measurement disputes and losses for the enterprise. It can be seen that the reliability of flow meters is extremely important. The main ways to improve the reliability of flow measurement are to enhance the reliability of the instruments themselves and to use instruments with high reliability for reliable design. In recent years, the reliability of flow measurement instruments has seen significant improvements, which are reflected in the following aspects. ①The reliability of the instrument itself has improved significantly. ②By improving the structural design of the instruments, the reliability of the system is enhanced. For example, by using a non-stop plug-in structure, the flow meter can be replaced without affecting the process operation. The clamp-type structure of ultrasonic flow sensors, the pressure-compatible replacement mechanism for the electrodes of electromagnetic flow sensors, and the structure that features ultrasonic probes installed outside the pipe in vortex flow sensors – all of these can help to reduce the repair time when instruments are damaged. ③Introduce redundancy techniques. If dual sensors are used and an automatic check is performed to determine whether the sensors are functioning properly, the signal from the faulty sensor will be excluded. ④Self-diagnosis technology is introduced, and the diagnostic results are sent via the field bus to the operation station or a dedicated Equipment Management System (AMS) for display and alarm generation, so as to detect faults promptly and take action early. (4) Cost savings: The costs mentioned here include not only the expense of purchasing the instruments, but also the costs associated with purchasing accessories, installation and commissioning, operation, spare parts, as well as maintenance and regular calibration. The depreciation cost resulting from the average lifespan of the instruments is also something that cannot be ignored. Some types of flowmeters, although having a lower purchase cost, require the addition of auxiliary equipment such as upstream and downstream shut-off valves and bypass valves; sometimes the cost of these auxiliary devices **exceeds the purchase cost of the flowmeter itself. When selecting instruments, one should avoid an excessive focus on high performance and high precision, as this not only increases the purchase cost but also often raises the costs associated with spare parts. The optimal design choice is the option that provides the highest reliability for the instrument, facilitates maintenance, and results in the lowest costs, all while meeting the required functional specifications. (5) Safety: Some of the fluids to be measured are flammable and explosive, and some locations where the instruments are installed are also flammable and explosive environments; therefore, the selection of instruments, system design, and installation must all comply with explosion-proof regulations. In addition to the aforementioned objectives, other requirements for use must also be met, such as pressure loss requirements, hygiene requirements, and protection requirements. Attention should also be paid to ease of maintenance, and in some cases, ease of carrying out mandatory inspections should also be considered.

The most commonly used classification methods for flowmeters currently include: differential pressure flowmeters, positive displacement flowmeters, float flowmeters, turbine flowmeters, electromagnetic flowmeters, vortex street flowmeters among fluid oscillation flowmeters, and mass flowmeters. The principles and characteristics of each type of flowmeter are briefly described here. 2. Differential pressure flow meter: A differential pressure flow meter measures flow rate by utilizing the differential pressure generated by flow detection elements installed in industrial pipelines; the flow rate is calculated based on known fluid properties as well as the geometric dimensions of the detection elements and the pipes. A differential pressure flow meter consists of a primary sensing element and a secondary instrument (a differential pressure converter or transmitter, along with a flow display instrument). Differential pressure flowmeters can be classified according to their sensing elements, including orifice flowmeters, venturi flowmeters, and average velocity tube flowmeters. Secondary instruments include various mechanical, electronic, and mechatronic differential pressure flowmeters, differential pressure transmitters, and flow display instruments. Differential pressure flow meters are the most widely used type among the various flow meter categories. They have been developed into standardized, modular products both domestically and internationally. Differential pressure flow meters can be used to measure flow rates alone, as well as other parameters such as pressure, level, and density. Based on their working principle, the sensing elements of differential pressure flow meters can be classified into several major categories: throttling devices, hydraulic resistance, dynamic head type, dynamic head gain and jet type, as well as centrifugal type. Test specimens are divided into two main categories: standard and non-standard. Standard-type sensing elements are designed, manufactured, installed, and used according to standard documents; their flow rate values can be determined and measurement errors can be estimated without the need for actual flow calibration. Non-standard detection elements are generally not yet included as detection elements in international standards. Differential pressure flowmeters are also the most widely used type of flow meter, occupying the top position among all types of flow meters in terms of usage. The main advantages are: (1) The orifice plate flow meter, which is the most widely used type, has a robust structure, stable and reliable performance, and a long service life; (2) It has a wide range of applications, and to date no other flow meter can compare with it ； (3) The test piece, transmitter, and display instrument are manufactured by different manufacturers, which facilitates economies of scale in production. The main disadvantages are: (1) generally low measurement accuracy; (2) narrow range, usually only 3:1 to 4:1 ； (3) High requirements for on-site installation conditions ； (4) High pressure loss (referring to orifice plates, nozzles, etc.). 3. Positive displacement flow meters: Positive displacement flow meters, also known as metering flow meters or simply PD flow meters, are the most accurate type of flow measurement instrument. It uses mechanical measuring elements to continuously divide the fluid into individual, known volume portions, and measures the total volume of fluid by counting the number of times each volume portion is filled and emptied in the measuring chamber. Positive displacement flowmeters can be classified according to their measuring elements: there are gear-type flowmeters, rotary piston flowmeters, reciprocating piston flowmeters, disc flowmeters, wet gas meters and diaphragm gas meters, as well as liquid-sealed rotameter types. Main advantages: (1) High measurement accuracy ； (2) The conditions of pipeline installation have no impact on measurement accuracy ； (3) Can be used for measuring high-viscosity liquids ； (4) Wide range ； (5) Direct-reading instruments can obtain cumulative and total values without external power, offering clear readings and simple operation. Main disadvantages: (1) Complex results and large size ； (2) There are significant limitations regarding the type of medium being tested, its diameter, and the operating conditions of the medium ； (3) Not suitable for high and low temperature environments ； (4) Most instruments are only suitable for clean single-phase fluids ； (5) Generates noise and vibration. 4. Float flow meter: The float flow meter, also known as a rotor flow meter, is a type of variable-area flow meter. In a vertical conical tube that widens from bottom to top, the gravity of a float with a circular cross-section is counteracted by the fluid dynamics, allowing the float to rise and fall freely within the conical tube. Float flow meters are a type of flow meter that is widely used after differential pressure flow meters, and they are suitable for measuring very small flow rates. Main advantages: (1) Simple structure, easy to use ； (2) Suitable for small pipe diameters and low flow rates ； (3) Lower pressure loss. Disadvantages: Low pressure resistance, and the glass tube is fragile. 5. Turbine flowmeter: The turbine flowmeter is one of the main types of velocity-type flowmeters. It consists of a multi-bladed rotor (turbine) that senses the average flow velocity of the fluid, thereby enabling the measurement of flow rate or total volume flow. Its structure consists of a sensor and a display unit, and it is available in both split and integrated versions. Turbine flowmeters, positive displacement flowmeters, and Coriolis mass flowmeters are collectively considered the three types of flowmeters with the best repeatability and accuracy. It is currently developing in the direction of a variety of products and multiple series. Main advantages: (1) High precision; it is the most accurate flow meter among all types of flow meters ； (2) Good repeatability ； (3) No zero drift, good interference resistance ； (4) Wide measurement range ； (5) Compact structure. Main drawback: (1) It cannot maintain its calibration characteristics over the long term ； (2) Fluid properties have a significant impact on flow characteristics. 6. Vortex shedding flow meter: The structure of a vortex shedding flow meter consists of a non-streamlined vortex generator placed in the fluid; it is a device in which fluid separates alternately on either side of the vortex generator, resulting in two sequences of vortexes that are arranged in a regular, alternating pattern. Vortex flowmeters are generally classified according to the frequency detection method, including stress-type, strain-type, capacitive, thermosensitive, photoelectric, ultrasonic, vibration-type, and others. Vortex shedding flowmeters are a type of new flow meter both domestically and internationally. Main advantages: (1) Simple and robust structure ； (2) Flow rate applicable to scenarios with multiple fluid types ； (3) Higher measurement accuracy ； (4) It has a wide measurement range and low pressure loss. Main disadvantages: (1) Not suitable for measuring fluids at low Reynolds numbers ； (2) A longer straight pipe section is required ； (3) Compared with turbine flowmeters, it has a lower coefficient of performance. 7. Electromagnetic flowmeter: An electromagnetic flowmeter consists of a sensor, a converter, a display, and other components. It is a flow meter used for measuring the flow rate of conductive fluids, and it is based on Faraday’s law of electromagnetic induction. Electromagnetic flowmeters possess unique advantages that no other type of flowmeter can match, making them particularly suitable for measuring dirty and corrosive fluids. In the 1970s and 1980s, significant technological advancements in electromagnetic flow measurement made it a widely used flow monitoring instrument in modern industrial applications. Main advantages: (1) Since the measurement channel is a section of smooth straight pipe, it will not get blocked, making it particularly suitable for liquid-solid two-phase fluids containing solid particles, such as pulp, sewage, sludge, etc ； (2) No pressure loss, excellent energy-saving effect ； (3) Unaffected by changes in fluid humidity, density, viscosity, pressure, and conductivity ； (4) Wide flow range and broad diameter range ； (5) Suitable for measuring corrosive fluids. Main disadvantages: (1) Not suitable for measuring fluids derived from released petroleum products ； (2) Not applicable to gases, vapors, and liquids containing large bubbles ； (3) Not suitable for high-temperature environments. 8. Ultrasonic flowmeters Ultrasonic flowmeters can be classified into methods based on the difference in propagation speed (direct time difference method, time difference method, phase difference method, and frequency difference method), wave speed shift method, Doppler difference method, cross-correlation method, spatial filtering method, and noise method, among others

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Additional note: The 9-target flowmeter was first used in industrial flow measurement in the 1960s, primarily to address the flow measurement of fluids with high viscosity and low Reynolds numbers. It has gone through two major development stages: pneumatic meters and electric meters. Suitable for measuring various media such as gases, liquids, and vapors. Advantages: (1) High resistance to vibration; not affected by pipeline vibrations. (2) Few leakage points, making maintenance easy. (3) No need for insulation or heating. (4) Low pressure loss. (5) Can be calibrated on-site, etc

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