Thread Content
Flow meters are one of the instruments commonly used in wastewater treatment. There are many types of flow meters based on their working principles, and it is necessary to select the appropriate one according to the actual operating conditions. Next, let’s talk about the 6 commonly used flow meters in wastewater treatment. I. Electromagnetic flowmeter: An electromagnetic flowmeter is a flow measurement instrument that operates on the principle of electromagnetic induction. It consists of sensors and transmitters. 1. Well-known brands include E+H, Kolon, Yokogawa, Rosemount, Siemens, Emerson, ABB, and others. 2. Working Principle: The basic principle of an electromagnetic flowmeter is that a pair of magnetic poles, N and S, are installed outside a pipe made of a non-magnetic material in order to generate a magnetic field. When a conductive fluid flows through a pipe, a magnetic induced electromotive force is generated as the fluid cuts through the magnetic field lines. This induced potential is extracted through two electrodes positioned perpendicular to the magnetic poles. When the strength of the magnetic field remains constant and the diameter of the pipe is fixed, the magnitude of this induced potential depends solely on the flow rate. By transmitting this induced potential to a display instrument, it is possible to determine the flow rate. 3. Advantages: 1) There are no moving parts or flow obstructing elements inside the measurement conduit, so the pressure loss is very low; there is no mechanical inertia, resulting in a sensitive response ； 2) Wide measurable range: The range ratio is generally 10:1, with a maximum of 100:1; the flow velocity range is typically 1–6 m/s, and it can be extended to 0.5–10 m/s ； The flow rate range can be from 90 mL/h to over a hundred thousand m3/h ； The pipe diameter range can be from 2 mm to 2400 mm or 3000 mm ； 3) It can measure the volumetric flow rate of liquids with a certain electrical conductivity, such as those containing solid particles, suspended matter, or solutions of acids, bases, and salts. It can also measure pulsating flow and perform bidirectional measurements ； 4) There is a linear relationship between the flow signal and the volumetric flow rate of the fluid; therefore, the instrument features uniform scaling ； Furthermore, the volumetric flow rate of the fluid is independent of the physical properties of the medium and its flow state; therefore, after being calibrated with water, an electromagnetic flowmeter can be used to measure the volumetric flow rate of other conductive liquids without any need for adjustments ； 5) Compared with most other flow meters, the requirement for a straight pipe section ahead is lower. 4. Disadvantage 1) The operating temperature and pressure cannot be too high ； 2) It has a limited range of application and cannot be used to measure the flow rate of non-conductive fluids such as gases, vapors, and petroleum products, as well as fluids containing many large bubbles ； 3) When the flow rate and velocity distribution do not meet the specified conditions, significant measurement errors will occur ； 4) When the flow rate is too low, it is difficult to amplify and measure the induced electromotive force that is of an order of magnitude comparable to the interference signal, and the instrument is also prone to zero drift ； 5) The signal from electromagnetic flowmeters is weak, and even slight external interference can affect the accuracy of the measurements. II. Ultrasonic flowmeter: An ultrasonic flowmeter is a device that measures volumetric flow rate by detecting the effect of fluid flow on an ultrasonic beam (or ultrasonic pulse). 1. Well-known brands include Rosemount, E+H, Coriolis, Fluke, GE, Panasonic, and others. 2. Working principle: Sound waves propagate through a fluid; their speed increases in the direction of flow and decreases in the opposite direction, resulting in different propagation times for the same distance. The propagation time method utilizes the relationship between the difference in propagation speeds and the flow velocity of the liquid being measured to determine the flow velocity, and then combines this with the pipe diameter to calculate the flow rate. 3. Advantages: 1) Ultrasonic flowmeters enable non-contact measurement. For clamp-on type ultrasonic flowmeters, it is not necessary to stop the flow of fluid in the pipe; the transducer can be installed outside the pipe to be measured, allowing flow rate to be determined using an ultrasonic flowmeter on existing pipes where the flow cannot be stopped or holes cannot be drilled ； 2) Ultrasonic flowmeters enable measurement without flow resistance, resulting in no additional pressure loss ； 3) The instrument coefficient of the meter can be calculated from the actual geometric dimensions of the pipeline and sound channels, allowing for dry calibration; in most cases, no verification with actual flow is required, except in those models that include a measuring section ； 4) Ultrasonic flowmeters are suitable for large circular and rectangular pipes, and are not limited by pipe diameter in terms of principle; their cost is basically independent of the pipe diameter ； 5) Doppler ultrasonic flowmeters can measure liquids with a high solid content or those containing bubbles. 4. Disadvantages: 1) In the propagation time method, ultrasonic flowmeters can only be used for clean liquids and gases; they cannot measure liquids containing suspended particles or bubbles in quantities exceeding a certain limit ； Conversely, the Doppler LSF method can only be used to measure liquids containing a certain degree of heterophase ； 2) Ultrasonic flowmeters with externally mounted transducers cannot be used in pipes with thick linings or scale, nor in pipes where the lining (or rust layer) has separated from the inner pipe (as gas in the gap will severely attenuate the ultrasonic signals), or in pipes with severe corrosion that alters the ultrasonic path. 3) Doppler ultrasonic flowmeters generally do not provide high measurement accuracy ； 4) It cannot be used for pipes with a diameter smaller than DN25mm. III. Insertion-type turbine flow meter: The insertion-type vortex flow meter is mainly composed of an expansion ring, impeller, rear guide element, turbine head housing, limit plate, actuator mounting bolts, insertion rod, locking bolts, positioning rod, and signal transmission wires. To edit and search for images, please click to enter a description of the image. 1. Well-known brand insert-type turbine flowmeters: The main well-known brands include Famet, E+H, Yokogawa, etc. 2. Working principle: Inside the pipeline where fluid flows, an impeller that can rotate freely is installed. When the fluid passes through the pipeline under measurement, it drives the impeller in the turbine head to rotate. Over a wide range of measurement values, the rotation speed of the impeller is directly proportional to the flow rate in the pipeline. A magnetoresistive detector is used to detect the rotation speed of the impeller as well as the electrical pulse signals generated by it; these signals are amplified by a pre-amplifier and then sent to the display instrument, allowing for the measurement of both the instantaneous flow rate and the total flow volume. 3. Advantages: 1) The turbine head is in direct contact with the water, ensuring accurate measurements; the signal, after being amplified by the pre-amplification board, can be transmitted to the central control room ； 2) The flow meter has a small size, is capable of measuring high flow rates, and can also accurately measure the cumulative flow while measuring the flow rate ； 3) Low cost – a set of turbine heads costs only about 400 yuan. 4. The disadvantages include an inability to maintain a continuous count due to maintenance issues; the output signal ranges from 0 to 10 mA, and there is no serial communication interface. IV. Rotameter: A rotameter maintains a constant pressure difference across the rotor by changing the flow area of the fluid; hence it is also known as a flow-area-variable constant-pressure differential flow meter, or float flowmeter. 1. Well-known brands include German brands such as Meister, Yokogawa, Dewell, Cologne, Yokogawa, and Kroni, among others. 2. Working principle: The rotameter measures fluid flow based on the throttling principle. In a vertical conical tube that expands from bottom to top, the gravity of a float with a circular cross-section is supported by hydrodynamic forces, allowing the float to rise and fall freely within the conical tube. It moves up and down under the influence of flow velocity and buoyancy; once equilibrium is reached with the weight of the float, the flow rate is transmitted to the dial through magnetic coupling. 3. Advantages apply to small pipe diameters and low flow rates ； Reliable in operation, requires minimal maintenance, and has a long service life ； Lower requirements are imposed on the straight pipe section downstream ； It has a wide flow range of 10:1 ； The in-situ pointer indicates near-linearity ； The intelligent indicator features an LCD display that shows instantaneous and cumulative flow rates, and it can also output pulses as well as provide alarms ； With temperature compensation. 4. Disadvantage 1) There is a risk of the glass tube breaking ； 2) Most structured float flowmeters can only be installed in pipes with vertical flow from bottom to top ； 3) The application is limited to medium and small pipe diameters; conventional full-flow float flowmeters cannot be used for large pipe diameters ； 4) When a different fluid is used than the one used for factory calibration, a correction to the flow rate reading is necessary. If the density and viscosity of the actual fluid differ from those specified, the flow rate will deviate from the original calibrated value, and a conversion correction must be applied. V. Throttle Flow Meters Throttle flow meters are a type of differential pressure flow meter; they were the most commonly used instruments for measuring the flow rate of gases, liquids, and vapor streams in industrial production during earlier times. To edit and search for images, please click to enter a description of the image. 1. Well-known brands of throttle flowmeters include Bitobar, ABB, Emerson, Krohne, Siemens, and others. 2. Working principle: A orifice plate is installed in the fluid pipeline; a pressure difference transmitter is connected via pressure guide tubes to measure the pressure difference upstream and downstream of the orifice plate. The instantaneous flow rate is determined through calculations based on this measured pressure difference. 3. Advantages: 1) The standard orifice plate structure of the throttle element is easy to replicate; it is simple, robust, offers stable and reliable performance, has a long service life, and is inexpensive ； 2) Throttling applications are extremely wide-ranging; all single-phase fluids, including liquids, gases, and steam, can be measured. Some mixed-phase flows, such as gas-solid, gas-liquid, and liquid-solid flows, can also be measured. There are products available for various production processes, pipe diameters, and operating conditions (pressure, temperature) ； 3) Accessories from various manufacturers are interchangeable; if they conform to international standards, no calibration is required ； 4) It is easy to install and maintain; unlike ordinary differential flow meters, it does not have long pressure lead pipes, which are prone to leakage, blockage, and freezing ； 5) There is no issue with zero drift. 4. Disadvantages: 1) The repeatability and accuracy of measurements are at a moderate level ； 2) The range is narrow, as the instrument signal is proportional to the flow rate in a quadratic relationship; generally, the range can only reach 3:1 to 5:1 ； 3) The on-site installation requirements are quite high; for example, long straight pipe sections are needed, which is difficult to meet ； 4) The pressure lead pipes are weak links, prone to failures such as leakage, blockage, freezing, and signal distortion ； 5) High pressure loss. VI. Target Flow Meters Target flow meters began to be used in industrial flow measurement in the 1960s, mainly to address the flow measurement of fluids with high viscosity and low Reynolds numbers. 1. Well-known brand target flowmeters: The main well-known brands include Omega, Hilton, Siede, tecfluid, etc. 2. Working principle: When the medium flows through the measuring tube, its kinetic energy creates a pressure difference with the target plate, resulting in a force acting on the target plate that causes it to move slightly. The magnitude of the force exerted is proportional to the square of the fluid flow rate. The force acting on the target plate is transmitted through the target rod, causing a slight change in the elastomer of the sensor; this disruption of the balance of the bridge formed by the surface-mounted capacitors results in a voltage signal that corresponds to the force exerted by the flow rate on the target plate. 3. Advantages: 1) Wide measurement range and low flow rate, making it particularly suitable for measuring gases or liquids with large diameters, at high or low pressures ； 2) Features integrated temperature and pressure compensation, with direct output of mass or standard cubic flow rate ； 3) Features optional small-signal rejection, nonlinear correction, and selectable filtering time ； 4) Simple and convenient to install, with good repeatability, fast measurement, and easy maintenance ； 5) Highly resistant to dirt, high temperatures, with high sensitivity; also exhibits strong resistance to blockages, interference, and impurities ； 6) Low pressure loss. 4. Disadvantages: 1) Not suitable for applications where the fluid switch is used very frequently; it works better in situations of continuous operation ； 2) The accuracy is not very high; it is generally used as a measuring instrument for process control, and should be used with caution in trade settlements ； 3) Narrow range; most instruments have a ratio of 10:1 ； 4) Due to the weight of the target disc and target rod in the target flow meter, the zero point must be reset after installation.