There is no doubt that flowmeters are the most widely used measuring instruments in sewage treatment plants. Therefore, discussions on the "installation location of flowmeters" have naturally become "lively": Some netizens believe that "how to install a flowmeter" should be based on the location recommended by the equipment manufacturer. However, other netizens think that one should not overly trust manufacturers. Reliable manufacturers are better than what is written in books, but if you encounter unreliable ones, you will have to deal with the aftermath. So the question arises: How should flowmeters be installed? What are the precautions? Next, the editor will select 3 commonly used flowmeters for discussion, analyze their advantages, disadvantages, and installation, hoping to be helpful to everyone. ### 1. Electromagnetic Flowmeter #### 1.1 Principle of Electromagnetic Flowmeter The electromagnetic flowmeter is made based on Faraday's law of electromagnetic induction and is used to measure the volume flow of conductive liquids. Its working principle is: in a closed pipeline, a magnetic field perpendicular to the flow direction is set, and the flow rate is deduced by measuring the induced electromotive force generated by the movement of the conductive liquid in the magnetic field. #### 1.2 Advantages and Disadvantages of Electromagnetic Flowmeter ##### 1.2.1 Advantages - Measurement is not affected by fluid density, viscosity, temperature, pressure, and other working conditions. - It has good corrosion resistance and wear resistance. - Compared with other types of flowmeters, it has relatively low power consumption, stable zero point, and high accuracy. - There are no throttling elements in the measuring tube, so the medium will not cause pressure loss when flowing through, and the requirement for straight pipe sections is relatively lower than that of other types of flowmeters. - Various installation forms are available for users to choose, such as integral type, split type, wall-mounted type, etc. - The flowmeter can measure bidirectionally, display positive and negative flow, and has multiple output options, such as current, pulse, digital communication, HART, etc. ##### 1.2.2 Disadvantages - It is basically impossible to measure liquid media with very low or no conductivity. - It is easily affected by surrounding strong magnetic fields. - The price is relatively expensive, especially for large-caliber ones. #### 1.3 Installation Essentials of Electromagnetic Flowmeter 1. Choose the installation point at the lowest part of the pipeline. After confirming the flow direction of the measured substance, the installation point should be selected such that the measuring tube is always filled with liquid at any time to prevent errors caused by the indication not being at zero due to no liquid in the measuring tube. 2. Ensure the straight pipe sections are 5D before and 3D after (D is the nominal diameter of the sensor). To obtain the measurement accuracy of the instrument, the length of the straight pipe section on the upstream side of the sensor should not be less than 5D, and that on the downstream side should not be less than 3D. If there are more than two elbows or other flow-blocking parts on the upstream side of the sensor, the front straight pipe section should be greater than 10D. 3. The signal electrodes should be in the horizontal direction. When installed horizontally, the electrode axis should be parallel to the horizon, because the electrode at the bottom is easily covered by sediments, and occasional bubbles in the measured medium may wipe and cover the electrode surface, causing fluctuations in the output signal. 4. Try to avoid vibration sources and magnetic sources (such as high-power motors, transformers, etc.). Since the measured induced voltage of the electromagnetic flowmeter is very small and the voltage is low, it is easily affected by external electromagnetic noise. Therefore, reliable grounding connections should be made during installation. ### 2. Ultrasonic Flowmeter #### 2.1 Principle of Ultrasonic Flowmeter Like the electromagnetic flowmeter, the ultrasonic flowmeter belongs to the unobstructed flowmeter because there are no obstacles in the instrument's flow channel, and it is suitable for measuring large-diameter and inaccessible fluids. The ultrasonic flowmeter measurement is an indirect method, mainly including two methods: Doppler method and time-difference method. - The time-difference method is suitable for measuring pure fluids. Fluids with more bubbles or impurities will hinder the normal propagation of ultrasonic waves and affect the accuracy of measurement results. Its principle is: in the fluid, the ultrasonic signal propagates faster when flowing with the current and slower when flowing against the current. By measuring the time difference between the ultrasonic signal propagating with and against the current in the fluid, the fluid velocity is indirectly measured, and then the fluid flow can be calculated through the velocity. - The Doppler flowmeter is suitable for measuring turbid fluids but can only measure fluids with impurities of about 50 mg/L or more. Its principle is: the ultrasonic receiver receives the signal scattered by suspended substances or tiny bubbles encountered during the propagation of ultrasonic waves. The frequency of the transmitted signal changes to a different signal frequency due to the Doppler effect. According to the quantitative relationship between the frequency difference and the fluid velocity, the fluid flow can be calculated. #### 2.2 Advantages and Disadvantages of Ultrasonic Flowmeter ##### 2.2.1 Advantages - The ultrasonic flowmeter can be used for non-contact measurement. The clamp-on transducer ultrasonic flowmeter can be installed without stopping the flow and pipe, as long as the transducer is installed outside the pipeline to be measured, that is, it can be used to measure the flow on existing pipelines that cannot be shut off or drilled. - The ultrasonic flowmeter is a non-flow-obstructing measurement with no additional pressure loss. - The instrument coefficient of the meter can be calculated from the geometric dimensions of the actual measuring pipeline and sound channel, that is, dry calibration can be used, and generally, no actual flow calibration is required except for the measuring pipe section type. - The ultrasonic flowmeter is suitable for large circular and rectangular pipelines, and in principle, it is not limited by the pipe diameter, and its cost is basically independent of the pipe diameter. - The ultrasonic flowmeter can measure liquids with a high solid content or containing bubbles. ##### 2.2.2 Disadvantages - In the time-of-flight method, the ultrasonic flowmeter can only be used for clean liquids and gases, and cannot measure liquids with suspended particles and bubbles exceeding a certain range; conversely, the Doppler method can only be used for measuring liquids with certain heterogeneous phases. - The ultrasonic flowmeter with external clamp-on transducers cannot be used for pipelines with thick linings or scaling, nor for pipelines where the lining (or rust layer) is peeled off from the inner pipe (if there is gas in the interlayer, it will seriously attenuate the ultrasonic signal) or severely rusted (changing the ultrasonic path). - It cannot be used for pipelines with a diameter less than DN25mm. #### 2.3 Installation Essentials of Ultrasonic Flowmeter 1. Ensure the fluid fills the pipeline to avoid bubbles and impurities. When installing in a vertical pipeline, choose a pipeline where the fluid flows from bottom to top. 2. There should be a sufficiently long straight pipe section in front of the meter. The straight pipe section should be sufficient, with the upstream straight pipe section at the installation position greater than 10D and the downstream straight pipe section greater than 5D; the distance from the installation position to the upstream water pump should be greater than 30D. 3. Confirm the geometric dimensions of the pipeline (fully consider the thickness of the scale on the inner wall of the pipeline). It is known from the relationship between flow and pipe diameter that a 1% error in pipe diameter will cause an approximately 3% flow error, so the inner diameter of the pipeline must be measured accurately. In addition, pipeline scaling will not only cause pipe diameter errors but also scatter sound waves, reducing the measurement accuracy of the flowmeter. For severe scaling, the section of the pipeline where the transducer is installed should be replaced with a new stainless steel pipeline. 4. Set an exhaust valve in the pipeline upstream of the flowmeter. The time-difference ultrasonic flowmeter is particularly sensitive to bubbles mixed in water. Bubbles in the fluid will cause instability in the flowmeter's indication. If the accumulated gas coincides with the installation position of the probe, the flowmeter will not work. 5. Avoid strong magnetic fields and vibration interference. The flowmeter sensor, transmitter, connecting cable, and other parts should be far away from water pumps, high-power radios, and other equipment with strong magnetic fields and strong vibrations. It is worth mentioning that the sensor of the ultrasonic flowmeter has three installation methods as follows: 1. Z-method installation: Z-method installation is single-path measurement, which means that the sound wave completes the measurement after passing through one sound path, and a strong signal can be obtained. 2. V-method installation: V-method installation is double-path measurement, which means that two sensors are installed in the same direction, and the sound wave is received after being reflected by the opposite pipeline, with a total of 2 sound paths. 3. W-method installation: W-method installation is multi-path measurement, generally referring to the measurement method with 3 or more sound paths. Its advantage is that higher measurement accuracy can be obtained, but the disadvantage is that the signal is weak, which is only suitable for pipelines with good conditions. ### 3. Parshall Flume Flowmeter #### 3.1 Principle of Parshall Flume Flowmeter The Parshall flume, also known as the巴氏槽, its measurement principle is: by scientifically and effectively contracting the cross-section of the channel's water passage, the water level is forced to rise, creating a stable backwater surface between the upstream and downstream of the weir flume, converting the non-uniform flow upstream of the weir flume into uniform flow. Thus, the ultrasonic sensor can more accurately and intuitively determine the channel water level. Through instruments, the water flow is determined using the fixed functional relationship between water level and water volume, and then the water diversion of the channel is finally determined by accumulating according to the linear relationship between flow and time. #### 3.2 Characteristics of Parshall Flume Flowmeter Water measuring weirs and flumes are generally divided into: thin-plate weir, broad-crested weir, triangular profile weir, flat V-shaped weir, Parshall flume, and throatless flume. Among them, the Parshall flume is most commonly used in sewage measurement. The Parshall flume has the following characteristics compared with the stilling tank: - High accuracy in measuring water level. Due to the stable backwater surface formed by the Parshall flume, it is slightly better than the stilling tank in terms of measurement accuracy. - It can operate normally and satisfactorily under the conditions of high submergence coefficient and low head loss. - The installation technical requirements are relatively complex. Installing the Parshall flume on the original channel requires the channel to be in a non-water diversion state, which has strict requirements on the construction period, while the stilling tank can be directly installed without draining the water in the channel. - The installation cost is relatively high. The construction of the Parshall flume is more complex, and there are many auxiliary equipment. - The installation of the Parshall flume changes the flow state of the original channel, thereby reducing the water flow of the original channel and lowering the water diversion capacity of the channel. - In channels with high sediment content, due to long-term operation and water scouring, the wetted perimeter of the Parshall flume will be worn (especially at the throat), and the measurement accuracy of the channel will gradually decrease. - The maintenance cost is relatively high. To ensure the normal operation of the facility and its measurement accuracy, daily inspections such as regular derusting, desilting, and equipment inspection of the Parshall flume body are required. #### 3.3 Installation Essentials of Parshall Flume Flowmeter The construction and installation of the Parshall flume mainly include three parts: selection of installation location, installation of the flume body, and installation of the water level gauge. ##### 3.3.1 Selection of Installation Location The following principles should be followed: - Choose a straight section of the channel with unchanged interception size, avoid or stay away from bends, and the length of the straight channel is generally not less than 10 times the throat width. - Choose a location with gentle water flow, away from water with hydraulic jump. - In channels with large longitudinal slopes and complex water flow conditions, a flow retardation plate or an energy dissipation pool can be set upstream of the flume to change the water flow state. ##### 3.3.2 Installation of the Flume Body The following points should be noted: - There should be a 1:4 slope between the bottom plane of the channel and the flume body connection. The channel side wall and the flume body should be connected with a curved surface or a flat surface (curved surface: curvature > 2 times the maximum water level; the straight surface should form a 45° angle with the center line). - The inlet contraction section should be installed horizontally. - The channel center line and the flume body center line should be on the same axis. - After setting the flume body, the water level in the upstream channel rises. To prevent the channel water from overflowing, methods such as raising the channel wall or deepening the channel can be adopted. However, when deepening the channel, attention should be paid to avoiding reverse flow or submerged flow from the outlet of the flume body. - The side walls on both sides of the throat should be parallel to each other and vertically vertical. - When using concrete backfill to reinforce the flume body, on the premise of ensuring firmness, the flume body should be horizontally level on the left and right and symmetrical along the water flow axis to avoid inclination or deformation of the flume body. ##### 3.3.3 Installation of the Water Level Gauge The following installation essentials should be noted: - The water level measurement position should be at the intersection of the flume body center line and the vertical line of the water inlet. The installation position of the ultrasonic sensor of the water level gauge should be as close as possible to this intersection. - The measurement range of the water level gauge should cover all water level ranges, and the installation height must be greater than the maximum water level. - Ensure that the water level gauge is installed firmly, add protective measures, eliminate interference from the external environment on the sensor, and avoid errors in water level reading. ### 4. General Precautions Sewage flowmeters play a significant role in the environmental protection field. In actual use, improper installation may affect the normal use of sewage flowmeters. Today, the flowmeter manufacturer Qingtian Instrument mainly explains some installation requirements for sewage flowmeters, hoping to help everyone. 1. Location: The installation location of the flowmeter should avoid large temperature changes and direct sunlight. The ambient temperature should be between -25～60℃. If the instrument installation location is subject to thermal radiation from heat sources, thermal insulation or ventilation facilities should be provided. Avoid atmospheric environments with strong corrosiveness and places containing explosive gases. The straight pipe sections upstream and downstream of the flowmeter installation location generally follow the principle of 10D before and 5D after, requiring the front straight pipe section to be not less than 10D and the rear straight pipe section to be not less than 5D; if other chemical substances are injected into the upstream pipe section, it is easy to cause uneven conductivity, and the flowmeter should be as far away from the main inlet as at least 20D; if the upstream and downstream of the sewage flowmeter adopt reducers, the center cone angle α of the reducer should be less than 15°. 2. Avoid magnetic field interference: The sewage flowmeter should not be installed near motors, transformers, or other power sources that are likely to cause electromagnetic interference. 3. Length of straight pipe section: To ensure the measurement accuracy of the flowmeter, the length of the straight pipe section upstream of the sensor should be at least 5 times the pipe diameter, and that downstream should be at least 3 times the pipe diameter. 4. Maintenance space: The flowmeter should be installed in a place that meets the necessary maintenance space. 5. Support for the flowmeter: Do not install the flowmeter孤立 on a freely vibrating pipeline. Use an installation base to fix the measuring pipe. When the flowmeter needs to be installed underground, supporting objects should be set for the inlet and outlet pipes, and a metal protective plate should be installed above the flowmeter. 6. Clean inner wall: To ensure the measurement accuracy of the sewage flowmeter, the inner walls of the upstream and downstream pipe sections where the instrument is located should be clean, without obvious dents, scaling, or peeling. Article sources: Environmental Protection Water Circle, Flowmeter Manufacturer's Lecture Hall, Beijing Zhongruineng Instrument