Definitions and characteristics of various instrument flowmeters

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1 Overview: Instruments used to measure the flow rate of fluid in pipelines (the volume of fluid passing through per unit time). There are rotameters, throttle flow meters, slit flow meters, positive displacement flow meters, electromagnetic flow meters, ultrasonic flow meters, and weirs, among others. There are a wide variety of flow measurement methods and instruments, as well as numerous classification methods. To date, as many as 60 types of flow meters are available for industrial use. The reason for there being so many varieties is that to date, no flow meter has been found that is suitable for any fluid, any range, any flow condition, or any set of operating conditions. Of these more than 60 types of flow meters, each product has its specific applications as well as its limitations. Based on the object of measurement, they can be divided into two main categories: closed pipelines and open channels. Depending on the purpose of measurement, they can be further classified as volume measurement and flow rate measurement, with the corresponding instruments being volume meters and flow meters respectively. A total flow meter measures the volume of fluid that passes through a pipe over a certain period of time; it is expressed as the total volume that flows through in a short period divided by that time. In fact, flow meters usually also come equipped with an accumulation device for use as a total flow meter, and total flow meters likewise have a flow signaling device. Therefore, in a strict sense, flow meters and totalizers no longer have any practical significance. Based on the measurement principle, they can be classified into mechanical principles, thermal principles, acoustic principles, electrical principles, optical principles, atomic physics principles, etc. According to the most popular and widely used classification system at present, which includes positive-displacement flowmeters, differential pressure flowmeters, float flowmeters, turbine flowmeters, electromagnetic flowmeters, vortex flowmeters among fluid oscillation flowmeters, mass flowmeters, as well as insert-type and probe-type flowmeters, this text explains the principles, characteristics, general applications, and development trends of these various flowmeters both domestically and internationally. 1.1 Differential pressure flow meter A differential pressure flow meter is a device that calculates flow rate based on the differential pressure generated by a flow sensing element installed in the pipeline, along with the known properties of the fluid and the geometric dimensions of the sensing element and the pipeline. A differential pressure flow meter consists of a primary device (the sensing element) and a secondary device (the differential pressure conversion and flow display instrument). Differential pressure flowmeters are usually classified by their sensing elements, such as orifice flowmeters, venturi flowmeters, and average velocity tube flowmeters. Secondary devices include various mechanical, electronic, and mechatronic differential pressure gauges, differential pressure transmitters, and flow display instruments. It has evolved into a large category of instruments with a high degree of serialization, generalization, and standardization, featuring a wide variety of types and specifications. These instruments can be used to measure flow rates as well as other parameters such as pressure, level, density, etc. Based on their working principle, the sensing elements of differential pressure flow meters can be classified into several major categories: throttling devices, hydraulic resistance types, centrifugal types, dynamic head types, dynamic head gain types, and jet types. Test specimens can also be divided into two main categories based on their degree of standardization: standard and non-standard. The so-called standard test pieces are those whose flow rate values and measurement error can be determined without the need for actual flow calibration, as long as they are designed, manufactured, installed, and used in accordance with standard documents. Non-standard test pieces are those with a lower level of maturity and have not yet been included in international standards. Differential pressure flowmeters are the most widely used type of flowmeter, accounting for the largest share among all types of flow measurement devices. In recent years, due to the emergence of various new types of flow meters, its usage percentage has gradually declined; however, it remains the most important category of flow meters. Advantages: (1) The orifice plate flow meter, which is the most widely used type, features a robust structure, stable and reliable performance, and a long service life; (2) It has a wide range of applications, and no other type of flow meter can compare with it to date; (3) The sensing element, the transmitter, and the display instrument are manufactured by different manufacturers, which facilitates mass production for cost savings. Disadvantages: (1) The measurement accuracy is generally low; (2) The range is narrow, usually only 3:1 to 4:1; (3) High requirements are placed on the installation conditions in the field; (4) High pressure loss (referring to orifice plates, nozzles, etc.). Note: A new type of product – an intelligent probe-type flow meter – addresses the aforementioned drawbacks; it has almost no pressure loss and achieves a precision level of 0.2. Application overview: Differential pressure flow meters have a very wide range of applications. They are used for measuring flow rate in closed pipelines for various types of fluids, such as single-phase, mixed-phase, clean, dirty, and viscous flows. In terms of operating conditions, they can be used at normal pressure, high pressure, vacuum, normal temperature, high temperature, or low temperature. The pipe diameter can range from a few millimeters to several meters. As for flow conditions, they can handle subsonic, sonic, and pulsating flows. Its usage across various industrial sectors accounts for about 1/4 to 1/3 of the total usage of flowmeters. 1.2 Rotameter
A rotameter, also known as a variable-area flowmeter, is a type of variable-area flowmeter. In a vertical tapered tube that widens from bottom to top, the weight of a float with a circular cross-section is balanced by the hydrodynamic forces exerted by the liquid; this allows the float to move freely up and down within the tube. Float flowmeters are the type of flowmeter with the widest range of applications, second only to differential pressure flowmeters; they play a crucial role, especially in measuring small and micro flow rates. In the mid-1980s, sales in Japan, Western Europe, and the United States accounted for 15% to 20% of the total sales for flow meters. China’s production in 1990 was estimated to be between 120,000 and 140,000 units, of which over 95% were glass cone tube float flowmeters. Features: (1) The glass cone tube float flow meter has a simple structure and is easy to use, but its disadvantage is its low pressure resistance, as well as the significant risk of the glass tube breaking; (2) It is suitable for small pipe diameters and low flow rates; (3) It has low pressure loss. 1.3 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 instruments. 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. Based on their measuring elements, positive displacement flowmeters can be classified into elliptical gear flowmeters, scraper flowmeters, twin-rotor flowmeters, rotary piston flowmeters, reciprocating piston flowmeters, disk flowmeters, liquid-sealed drum flowmeters, wet gas meters, and diaphragm gas meters, among others. Advantages: (1) High measurement accuracy; (2) The conditions of the piping system in which it is installed have no impact on the measurement accuracy; (3) It can be used for measuring highly viscous liquids; (4) Wide measurement range; (5) As a direct-reading instrument, it can provide cumulative and total values without the need for external power, offering clear readings and simple operation. Disadvantages: (1) The results are complex and the device is large in size; (2) There are significant limitations regarding the type of medium being measured, its diameter, and the operating conditions of the medium; (3) It is not suitable for use in high or low temperature environments; (4) Most of these instruments are only applicable to clean, single-phase fluids; (5) They generate noise and vibration. Application overview: Positive displacement flowmeters, along with differential pressure flowmeters and float flowmeters, are among the three most commonly used types of flowmeters. They are frequently employed for measuring the total volume of expensive fluids such as oils and natural gas. In industrially developed countries, the sales value of PD flow meters (excluding household gas meters and domestic water meters) accounted for 13% to 23% of the total sales volume of flow meters in recent years; in China this figure is around 20%. In 1990, the production volume (excluding household gas meters) was estimated to be 340,000 units, of which elliptical gear-type and worm gear-type meters accounted for approximately 70% and 20% respectively. 1.4 Turbine flowmeters: Turbine flowmeters are one of the main types of velocity-type flowmeters. They use a multi-blade rotor (turbine) to detect the average flow velocity of the fluid, thereby determining the flow rate or total volume. Generally, it consists of a sensor and a display unit, or it can be designed as an integrated unit. Turbine flowmeters, positive displacement flowmeters, and Coriolis mass flowmeters are considered to be the three types of flowmeters with the best repeatability and accuracy. As one of the ten major types of flowmeters, these products have been developed into a range of varieties and series that are produced on a large scale. Advantages: (1) High precision – it is one of the most accurate flowmeters among all types; (2) Good repeatability; (3) No zero drift, and strong resistance to interference; (4) Wide measurement range; (5) Compact design. Disadvantages: (1) It cannot maintain its calibration characteristics over the long term; (2) The properties of the fluid have a significant impact on the flow characteristics. Application overview: Turbine flowmeters are widely used for measuring various fluids such as oil, organic liquids, inorganic liquids, liquefied gas, natural gas, and cryogenic fluids. In Europe and the United States, they are the second-most commonly used flowmeters for measuring natural gas, after orifice plate flowmeters. In the Netherlands alone, more than 2,600 gas turbine flowmeters of different sizes, operating under pressures ranging from 0.8 to 6.5 MPa, are used in natural gas pipelines; these flowmeters have proven to be excellent instruments for measuring natural gas. 1.5 Electromagnetic flowmeter: An electromagnetic flowmeter is a device for measuring conductive liquids, designed based on Faraday’s law of electromagnetic induction. Electromagnetic flowmeters possess a range of excellent features that enable them to address issues that are difficult for other types of flowmeters to handle, such as the measurement of dirty or corrosive fluids. In the 1970s and 1980s, significant technological advancements were made in electromagnetic flowmeters, which enabled them to become a widely used type of flowmeter; their share in the total number of flow measurement devices continued to increase. Advantages: (1) The measurement channel is a section of smooth straight pipe that does not get clogged, making it suitable for measuring liquid-solid two-phase fluids containing solid particles, such as pulp, sludge, and wastewater; (2) It does not cause pressure losses resulting from flow measurement, thus offering good energy-saving effects; (3) The measured volumetric flow rate is essentially unaffected by changes in fluid density, viscosity, temperature, pressure, or conductivity; (4) It has a wide range of flow rates and a broad range of pipe diameters; (5) It can be used with corrosive fluids. Disadvantages: (1) It cannot measure liquids with very low conductivity, such as petroleum products; (2) It cannot measure gases, vapors, and liquids containing large bubbles; (3) It cannot be used at high temperatures. Application overview: Electromagnetic flowmeters have a wide range of applications. Large-diameter models are commonly used in water supply and drainage projects; medium and small-diameter models are often utilized in situations that require high precision or present measurement challenges, such as in the control of cooling water for blast furnaces in the steel industry, for measuring pulp and black liquor in the paper industry, in highly corrosive liquids in the chemical industry, and in mineral slurries in the non-ferrous metallurgy industry. Small and ultra-small diameter models are typically used in industries such as pharmaceuticals, food processing, and biochemistry, where hygiene standards are important. 1.6 Vortex street flowmeter: A vortex street flowmeter is a device in which a non-streamlined vortex generator is placed within the fluid; the fluid separates on either side of this generator, resulting in two sequences of vortexes that are arranged in a regular alternating pattern. Vortex flowmeters can be classified according to their frequency detection methods into stress-type, strain-type, capacitive type, thermosensitive type, vibrating-body type, photoelectric type, and ultrasonic type, among others. Vortex flowmeters belong to the youngest category of flowmeters, but they have developed rapidly and are now become a common type of flowmeter. Advantages: (1) Simple and robust structure; (2) Suitable for a wide range of fluids; (3) High precision; (4) Wide measurement range; (5) Low pressure loss. Disadvantages: (1) Not suitable for measurements at low Reynolds numbers; (2) Requires a long straight pipe section; (3) Has a lower coefficient of performance (compared to turbine flowmeters); (4) Lacks experience in application with pulsating flows and multiphase flows. 1.7 Ultrasonic flowmeters Ultrasonic flowmeters are instruments that measure flow rate by detecting the effect of fluid flow on an ultrasonic beam (or ultrasonic pulse). Based on the principles of signal detection, 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), beam deflection method, Doppler method, cross-correlation method, spatial filtering method, and noise method, among others. Like electromagnetic flowmeters, ultrasonic flowmeters also belong to the category of obstruction-free flowmeters, as there are no obstacles in their flow channels. They are suitable for addressing difficulties in flow measurement, and they have particular advantages in measuring large-diameter flows. In recent years, they have been one of the types of flowmeters that have seen rapid development. Advantages: (1) It enables contactless measurement; (2) it is a measurement without flow obstruction, resulting in no pressure loss; (3) it can measure non-conductive liquids, serving as a complement to electromagnetic flowmeters that require unobstructed measurement. Disadvantages: (1) The propagation time method can only be used for clean liquids and gases, while the Doppler method can only be used to measure liquids containing a certain amount of suspended particles and bubbles; (2) The measurement accuracy of the Doppler method is not high. Application overview: (1) The propagation time method is applied to clean, single-phase liquids and gases. Typical applications include factory effluents, strange liquids, liquefied natural gas, etc.; (2) In terms of gas applications, there is good experience in using it in the field of high-pressure natural gas; (3) The Doppler method is suitable for two-phase fluids with a low content of heterogeneous components, such as untreated sewage, factory effluents, and dirty process fluids; it is generally not suitable for very clean liquids. 1.8 Coriolis Mass Flow Meter The Coriolis mass flow meter (hereinafter referred to as CMF) 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 that is proportional to the mass flow rate. The application of CMF in our country started relatively late. In recent years, several manufacturers (such as Taihang Instrument Factory) have developed their own products and supplied them to the market; other manufacturers have established joint ventures or utilized foreign technology to produce a range of instruments. 1.9 Open-channel flow meters differ from the previously mentioned types; they are instruments used to measure the flow rate of free-surface flow in open channels that are not completely filled. A water channel with non-full pipe flow is called an open channel, and a device used to measure the flow rate of water in such channels is known as an open channel flowmeter. In addition to circular shapes, open-channel flow meters also come in various other shapes such as U-shaped, trapezoidal, and rectangular. Applications of open-channel flow meters include urban water supply channels; intake and discharge channels in thermal power plants, as well as channels for sewage treatment and discharge; water discharge from industrial and mining enterprises; and channels used in hydraulic engineering and agricultural irrigation. Some estimates put the number at 1,995 units, accounting for about 1.6% of all flow meters, but there are no estimated figures available for their use in China. 2 Research and development of new principle flow meters 2.1 Electrostatic flowmeter The Tokyo Technical College in Japan developed an electrostatic flowmeter suitable for measuring the flow rate of low-conductivity liquids in oil transportation pipelines. The metal measuring tube of the electrostatic flowmeter is connected to the piping system in an insulated manner; the charge within the measuring tube can be determined by measuring the static charge on the measuring capacitor. They conducted actual flow tests using measuring tubes made of metals such as copper and stainless steel, as well as plastics, with inner diameters of 4–8 mm. The tests showed that there is a nearly linear relationship between flow rate and charge. 2.2 Combined effects meter: The working principle of this meter is based on the deformation caused by the momentum and pressure of the fluid acting on the meter’s chamber; the flow rate is determined by measuring this combined effect of deformation. This instrument was developed by the GMI College of Engineering and Management in the United States, and two patents have been applied for. 2.3 Tachmetric flowrate sensor: It was developed by the Industrial Instruments Company of the Russian Scientific and Engineering Center, and is based on the theory of the suspension effect. This instrument has been successfully applied in various field installations (for example, more than 2,000 units were installed in nuclear power plants to measure hot water flow, with continuous use over 8 years), and it is still being improved to expand its areas of application. 3 Several Applications and Development Trends of Flow Meters 3.1 Coriolis Mass Flow Meters (CMF) Abroad, more than 30 series of CMF devices have been developed. The technical focus in the development of these series includes: innovative design of the flow measurement tube structure; improvement of the instrument’s zero-point stability and accuracy; increasing the flexibility of the measurement tube to enhance sensitivity; optimizing the stress distribution within the measurement tube to reduce fatigue damage and improve resistance to vibration interference. 3.2 Electromagnetic Flow Meters (EMF) Since their introduction into industrial use in the early 1950s, EMF meters have seen an expanding range of applications; by the late 1980s, they accounted for 16%–20% of the total sales value of flow meters in various countries. Our country has seen rapid development in recent years; sales in 1994 were estimated to be between 6,500 and 7,500 units. In China, ENFs with the largest diameter of 2–6 meters have been produced, and there is also the capability to manufacture units with a diameter of 3 meters that have been tested under actual flow conditions. 3.3 Vortex Street Flowmeter (USF) The USF was introduced into industrial use in the late 1960s, and by the late 1980s it accounted for 4% to 6% of the total sales value of flowmeters worldwide. The estimated global sales volume in 1992 was 35,400 to 48,000 units, while domestic production during the same period was estimated at 8,000 to 9,000 units. 4 Conclusion As can be seen from the above, although flow meters have become increasingly mature over time, there are still an extremely large variety of them, and to this day no single type of flow meter exists that is suitable for all applications. Each type of flow meter has its applicable range as well as its limitations. This requires us to: (1) when selecting instruments, we must be familiar with both the instruments themselves and the objects being measured, while also taking other factors into consideration, so as to ensure accurate measurements; (2) strive to develop new types of instruments to further improve them on the existing basis.