Six commonly used flow meters in China’s industry

Thread Content

　　Currently, there is a wide variety of products available in our country for industrial flow measurement – ranging from general-purpose types to those designed for specific applications. For technicians choosing the right product, it’s easy to make a mistake if they are not careful, which can result in losses for the entire project. Below are my some thoughts on these products. I hope this can serve as a reference for everyone when making choices. 　　1. Ultrasonic flow meter: Ultrasonic waves carry information about the flow velocity of the fluid as they propagate through it. Therefore, the flow velocity of the fluid can be detected using the received ultrasonic waves, which can then be converted into flow rate. Based on the detection method, ultrasonic flowmeters can be classified into different types such as the velocity difference method (time difference method), Doppler method, beam deflection method, noise method, and related methods. An acoustic flow meter is a type of non-contact instrument that has come into use over the past decade or so, thanks to the rapid development of integrated circuit technology. It is suitable for measuring fluids that are difficult to access and observe, as well as flow rates in large-diameter pipes. It can be used in conjunction with a water level gauge to measure the flow rate of open-flow water. The use of ultrasonic flow measurement eliminates the need for measuring elements to be installed in the fluid; thus, it does not alter the fluid’s flow state or generate any additional resistance. Both the installation and maintenance of the meter can be carried out without disrupting the operation of production pipelines. Therefore, it is an ideal energy-saving flowmeter. 　　As is well known, current industrial flow measurement faces the challenge of measuring large-diameter flows with high volumes. This is because conventional flow meters encounter difficulties in manufacturing and transportation as the diameter of the measurement tube increases, resulting in higher costs, greater energy losses, and other drawbacks; ultrasonic flow meters can avoid all these issues. Since various ultrasonic flowmeters can be installed outside the pipe and perform non-contact flow measurement, their cost is basically independent of the diameter of the pipe being measured. In contrast, the cost of other types of flowmeters increases significantly as the diameter grows; therefore, for larger diameters, ultrasonic flowmeters offer a better cost-performance ratio compared to other types of flowmeters with similar functions. Considered to be a good flow meter for large-diameter flows, the Doppler ultrasonic flow meter is capable of measuring the flow rate of two-phase media, making it suitable for measuring dirty fluids such as those in sewers and wastewater systems. In power plants, using portable ultrasonic flowmeters to measure large-diameter flow rates such as the water inflow to turbines and the circulating water volume in steam turbines is much more convenient than using Pitot tubes in the past. Ultrasound flow meters can also be used for gas measurement. The applicable range of pipe diameter is from 2 cm to 5 m; it can be used in open channels and covered channels several meters wide, as well as in rivers up to 500 m wide. 　　Furthermore, the flow measurement accuracy of ultrasonic flow meters is hardly affected by parameters such as the temperature, pressure, viscosity, and density of the fluid being measured. Moreover, they can be designed as contactless and portable instruments, which enables them to address the flow measurement challenges associated with highly corrosive, non-conductive, radioactive, and flammable/explosive media, issues that are difficult to handle with other types of instruments. Furthermore, thanks to the characteristics of non-contact measurement and with the help of appropriate electronic circuits, one instrument can be used for measuring various pipe diameters as well as across a range of flow rates. The adaptability of ultrasonic flowmeters is also incomparable to that of other instruments. Ultrasonic flowmeters possess some of the advantages mentioned above; as a result, they are receiving increasing attention and are evolving toward standardized and generalized product lines. Standard, high-temperature, explosion-proof, and wet-type models with different sound frequencies have now been developed to accommodate flow measurement in various media, under different conditions, and in different pipeline setups. 　　2. Turbine flowmeter: A turbine flowmeter is a type of velocity meter that features low pressure loss, high accuracy, a low minimum flow rate, good resistance to shocks and pulsations in the fluid flow, a wide measurement range, and ease of maintenance. In this device, a turbine is placed within the fluid being measured; when gas enters the flowmeter, it is streamlined and accelerated under the action of a specially designed flow straightener. Within a certain flow range, the angular velocity of the turbine is proportional to the flow rate. Using the principle of electromagnetic induction, a pulse signal proportional to the volume flow rate of the fluid is generated. This signal is shaped by a pre-amplifier to determine the actual flow rate, which is then displayed on an LCD screen. If the signals detected by temperature and pressure sensors are also input into an intelligent flow calculation unit for processing, the flow rate under standard conditions can be determined and displayed on the LCD screen. The turbine flowmeter is a type of impeller-based meter, and its working principle is relatively simple. At the center of the pipe in the turbine flowmeter itself, there is a turbine that is supported at both ends by bearings. When fluid flows through the pipe, it strikes the turbine blades, generating a driving torque that causes the turbine to rotate despite the frictional torque and the resistance exerted by the fluid. Within a certain range of flow rates, and for a given viscosity of the fluid, the rotational speed of the turbine is proportional to the flow velocity of the fluid. Thus, the flow velocity can be determined from the rotational speed of the turbine, and the flow rate of the fluid passing through the pipe can be calculated accordingly. The rotational speed of the turbine is detected using sensor coils mounted outside the housing. When the turbine blades cut through the magnetic field lines generated by permanent magnets inside the housing, this causes changes in the magnetic flux in the sensor coils. The sensor coils transmit these periodic changes in magnetic flux to a pre-amplifier, where the signal is amplified and shaped to produce pulse signals that are proportional to the flow velocity. These pulse signals are then sent to a unit conversion and flow accumulation circuit, which calculates and displays the cumulative flow value. At the same time, the pulse signals are also sent to a frequency-to-current conversion circuit, where they are converted into an analog current value, thereby indicating the instantaneous flow rate. 　　A turbine flowmeter is a type of velocity-type flow meter; it is also known as a turbine meter based on its transliteration. Turbine flowmeters are further divided into gas turbine flowmeters and liquid turbine flowmeters, depending on the medium they measure. Among various flowmeters, the turbine flowmeter is the one with high repeatability and the best accuracy. Such as simple structure, few components required for processing, light weight, easy maintenance, high flow capacity (large flow rate possible for the same diameter), and the ability to operate under high parameters (high temperature, high pressure, and low temperature). Turbine flowmeters are widely used for measuring the following substances: oil, organic liquids, inorganic liquids, liquefied gas, natural gas, gas, and cryogenic fluids. At overseas transfer and gathering stations for liquefied petroleum gas, refined oil, light crude oil, etc., it is widely used for trade settlement at the beginning and end points of large-scale crude oil pipelines. 　　3. Electromagnetic flowmeter: The electromagnetic flowmeter is a new type of flow measurement instrument that developed rapidly in the 1960s along with the progress of electronic technology. It is based on Faraday’s law of electromagnetic induction and is used to measure the volumetric flow rate of conductive fluids. Due to its unique advantages, it is now widely used in the measurement of various conductive liquids in industry. For example, it is used to measure corrosive liquids such as various acids, bases, and salts; various flammable and explosive substances; as well as various industrial wastewater, pulp, sludge, etc. 　　The measurement principle is based on Faraday’s law of electromagnetic induction. That is, when a conductive liquid flows through an electromagnetic flowmeter, a voltage is generated in the conductive liquid that is proportional to the average flow velocity V (volumetric flow rate). This induced voltage signal is detected by two electrodes in contact with the liquid, transmitted via cables to an amplifier, and then converted into a standardized output signal. Based on the measurement principle of electromagnetic flowmeters, the flowing liquid is required to have a minimum level of conductivity. 　　The main advantages of electromagnetic flowmeters are as follows: ① The transmitter of an electromagnetic flowmeter has a simple structure; it contains no moving parts nor any throttling elements that could impede fluid flow. As a result, no additional pressure losses occur when the fluid passes through it, and issues such as wear or blockage do not arise. This makes it particularly suitable for measuring two-phase fluids containing solid particles, such as slurry and wastewater, as well as various highly viscous slurries. Additionally, due to the absence of moving parts, it is possible to incorporate corrosion-resistant insulating linings and use corrosion-resistant materials for the electrodes, thereby ensuring excellent corrosion resistance and allowing its use in measuring various corrosive media. ② An electromagnetic flowmeter is a device for measuring volumetric flow rate. During measurement, it is not affected by the temperature, viscosity, density, or conductivity of the fluid being measured (within certain limits). Therefore, after calibration using water, it can be used to measure the flow rate of other conductive liquids without the need for any additional corrections. ③ Electromagnetic flowmeters have a very wide range of measurement capabilities; the ratio of the measurement range for a single flowmeter can reach 1:10. Moreover, their measurement is proportional only to the average flow velocity of the fluid, regardless of whether the flow is laminar or turbulent. ④ Electromagnetic flowmeters lack mechanical inertia, allowing them to respond quickly. They can measure instantaneous pulsating flow rates with good linearity. Thus, the measurement signal can be directly converted into a standard signal using a converter, enabling local indication or remote transmission. Although electromagnetic flowmeters possess these excellent properties, they still have some limitations that restrict their use. The main drawbacks are as follows: ① Electromagnetic flowmeters cannot be used to measure gases, vapors, or liquids containing large amounts of gas. ② They cannot currently be used to measure liquid media with very low conductivity; the conductivity of the fluid must be at least 10-5 S/cm, which is equivalent to the conductivity of distilled water. They are also ineffective for measuring petroleum products or organic solvents. 　　③Due to the temperature limitations of the insulating lining materials in the measurement tube, industrial electromagnetic flowmeters are not yet capable of measuring high-temperature and high-pressure fluids. 　　④Electromagnetic flowmeters are affected by the flow velocity distribution; under axially symmetric conditions, the flow signal is proportional to the average flow velocity. Therefore, there must also be a certain length of straight pipe sections before and after the electromagnetic flowmeter. ⑤ Electromagnetic flowmeters are susceptible to external electromagnetic interference. 4. Vortex flowmeters: Vortex flowmeters are a new type of flowmeter that measures fluid flow in closed pipes based on the Karman vortex principle. Due to its excellent adaptability to different media, it can directly measure the volumetric flow rate of steam, air, gases, water, and liquids without the need for temperature or pressure compensation. Equipped with temperature and pressure sensors, it can also measure the volumetric flow rate and mass flow rate under standard conditions, making it an ideal alternative to throttle flow meters. A vortex flow meter involves placing one (or more) non-streamlined flow obstructing elements in the fluid; the fluid separates alternately on either side of these obstructing elements, resulting in the formation of two sets of regular vortices. Within a certain range of flow rates, the frequency of vortex separation is proportional to the average flow velocity within the pipe. By using various types of sensing elements to detect this vortex frequency, it is possible to calculate the volumetric flow rate. Vortex flow meters are suitable for measuring liquids, gases, or steam. 　　It has no moving parts, nor are there any issues with dirt. Vortex flow meters generate noise, and they require a high flow velocity of the fluid in order to create vortices. Due to the design in which the sensing probe of the sensor is separated from the vortex generator, and the high-temperature resistant piezoelectric crystal does not come into contact with the medium, vortex flowmeters feature a simple structure, good versatility, and high stability. 　　Vortex flow meters are primarily used for measuring the flow rate of fluids in industrial pipelines, including various media such as gases, liquids, and vapors. It is characterized by low pressure loss, a wide measurement range, and high accuracy; when measuring volumetric flow rate under operating conditions, it is hardly affected by parameters such as fluid density, pressure, temperature, and viscosity. There are no moving mechanical parts, thus it offers high reliability and low maintenance requirements. The instrument parameters can remain stable over the long term. It features high reliability and can operate within a temperature range of -20°C to +250°C. It provides both analog standard signals and digital pulse signals as outputs. 5. Float flow meter (rotary flow meter): A metal tube float flow meter is actually a variable-area flow meter. It usually consists of a vertical conical tube and a float that can move up and down freely within it depending on the flow rate. As the fluid flows upward through the conical tube, the kinetic energy of the fluid generates a thrust S on the float, and the buoyancy force A of the fluid causes the float to rise. As the annular flow area between the inner wall of the cone tube and the float increases, the thrust S generated on the float by the fluid kinetic energy decreases accordingly. When the sum of the thrust S and the buoyant force equals the weight G of the float itself, the float is in equilibrium and remains stable at a certain height; the scale corresponding to this height indicates the flow rate passing through the flow meter. The sensor converts the magnitude of the flow rate into the displacement of the float; through a magnetic coupling system, this displacement is transmitted to the converter, which then indicates the flow rate. 　　A float flow meter is a type of flow measurement instrument that changes area and is commonly used in industrial automation process control. It features a small size, a large detection range, and easy use. It can be used to measure the flow rate of liquids, gases, and vapors, and is particularly suitable for measuring the flow rate of media with low flow velocities and small flow rates. 　　6. Thermal (gas) mass flow meter: The thermal gas mass flow meter operates on the principle of thermal diffusion. Thermal diffusion technology offers excellent performance and high reliability under harsh conditions; its typical sensing elements include two thermistors (platinum RTDs), one of which serves as a velocity sensor, while the other acts as a temperature sensor to automatically compensate for changes in gas temperature. When these two RTDs are placed in the medium, the velocity sensor is heated to a constant temperature difference above the ambient temperature, while the other temperature sensor is used to detect the temperature of the medium. The mass flow rate of gas flowing through the velocity sensor is calculated based on the amount of heat transfer through the sensing element. As the gas flow rate increases, more heat is transferred from the sensor to the medium; therefore, more power is required. The power needed by the electronic unit to heat the RTD is proportional to the mass flow rate. 　　The thermal mass flow meter is a new type of instrument for measuring gas flow rates. Unlike other gas flow meters, it does not require pressure and temperature corrections; it measures the mass flow rate of gas directly. A single sensor can cover a range that spans from very low to very high values. It is suitable for the measurement of single gases and multi-component gases in fixed proportions. The principle it is based on is that the rate at which a fluid absorbs heat is directly related to its mass flow rate. Moving gas molecules absorb heat as they collide with the thermal resistor; the higher the flow rate, the more molecules come into contact with the thermal resistor, and thus the more heat is absorbed. Heat absorption is related to the number of molecules of a certain gas, its thermal properties, and its flow characteristics.