Flowmeter is one of the instruments that often used in sewage treatment. There are many kinds of flowmeters according to the working principle. When selecting the type, it is necessary to choose the appropriate flowmeter according to the actual working conditions.

1. Electromagnetic Flowmeter

An electromagnetic flowmeter is a flow-measuring instrument that works on the principle of electromagnetic induction. It consists of sensors and transmitters.

Working Principle

The basic principle of the electromagnetic flowmeter is: a pair of magnetic poles N and S are installed outside the pipe made of general non-magnetic materials to generate a magnetic field. When the conductive liquid flows through the pipe, the magnetic induction electromotive force is generated because the fluid cuts the magnetic field line. The induced electric potential is drawn by two electrodes perpendicular to the magnetic pole. When the strength of the magnetic field is constant and the diameter of the pipe is constant, the induced electric potential The size is only related to the flow rate, and the flow rate can be read by passing the size of the induced potential to the display instrument.

Advantage

1) There are no moving parts and bluffs in the measuring conduit, so the pressure loss is very small, and there is no mechanical inertia, so the response is sensitive;

2) Wide measurable range: turndown ratio is generally 10: up to 100; flow velocity range is generally 1-6m/s, can be extended to 0.5-10m/s; flow range can be from 90mL/h to hundreds of thousands of m3/h; The pipe diameter can range from 2mm to 2400mm or 3000mm;

3) It can measure the volumetric flow rate of liquids containing solid particles, suspended matter, or acid, alkali, and salt solutions with a certain conductivity, and can also measure pulsating flow rates, and can perform two-way measurement;

4) There is a linear relationship between the flow signal and the volume flow of the fluid, so the meter has a uniform scale, and the volume flow of the fluid has nothing to do with the physical properties and flow state of the medium, so the electromagnetic flowmeter can be used to measure other Volume flow of conductive liquids without correction;

5) Compared with most other flow meters, the requirements for the front straight pipe section are lower.

Shortcoming

1) The operating temperature and pressure should not be too high;

2) The scope of application is limited, and it cannot be used to measure the flow rate of non-conductive fluids such as gases, steam and petroleum products, and fluids containing many large bubbles;

3) When the flow rate and velocity distribution do not meet the set conditions, large measurement errors will occur;

4) When the flow rate is too low, it is difficult to amplify and measure the induced potential of the order of magnitude opposite to the interference signal, and the instrument is also prone to zero point drift;

5) The signal of the electromagnetic flowmeter is relatively weak, and a slight external interference can affect the measurement accuracy.

2. Ultrasonic Flowmeter

Ultrasonic flowmeters are instruments that measure volume flow by detecting the action of ultrasonic beams (or ultrasonic pulses) when fluid flows.

Working Principle

The sound wave propagates in the fluid, the propagation speed of the sound wave will increase in the downstream direction, and decrease in the upstream direction, and the same propagation distance will have different propagation times. The propagation time method is to use the relationship between the difference in the propagation velocity and the flow velocity of the measured liquid to obtain the flow velocity and combines the pipe diameter to obtain the flow rate.

Advantage

1) The ultrasonic flowmeter can be used for non-contact measurement. The clamp-mounted transducer ultrasonic flowmeter can be installed without stopping the flow-carrying tube, as long as the transducer is installed outside the pipeline to be tested, it can be used when the flow cannot be interrupted or cannot be opened. Measure the flow rate with an ultrasonic flowmeter on the existing pipeline of the hole;

2) The ultrasonic flowmeter is measured without flow resistance and has no additional pressure loss;

3) The instrument factor of the measuring meter can be calculated from the geometric dimensions of the actual measuring pipe and sound channel and can be calibrated by the dry method. Except for the type with the measuring pipe section, real flow calibration is generally not required;

4) Ultrasonic flowmeters are suitable for large circular and rectangular pipes and are not limited by pipe diameter in principle, and their cost is basically independent of pipe diameter;

5) The Doppler ultrasonic flowmeter can measure liquids with more solid content or bubbles.

Shortcoming

1) The ultrasonic flowmeter in the propagation time method can only be used for cleaning liquids and gases, and cannot measure liquids with suspended particles and bubbles exceeding a certain range; on the contrary, the Doppler method LSF can only be used to measure liquids containing certain heterogeneous phases;

2) Ultrasonic flowmeters with external transducers cannot be used for pipelines with thick lining or scaling, and cannot be used for stripping the lining (or rust layer) from the inner pipe (if there is gas in the interlayer, the ultrasonic signal will be seriously attenuated) Or pipes with severe corrosion (changing the ultrasonic path);

3) In most cases, the Doppler ultrasonic flowmeter has low measurement accuracy;

4) It cannot be used for pipes with a diameter smaller than DN25mm.

3. Plug-in turbine flowmeter

The plug-in vortex wheel gauge is mainly composed of an expansion ring, an impeller, a rear guide, a turbine head casing, a limit cutting handle, a transmitter mounting bolt insertion rod, a locking bolt positioning rod, and a signal transmission line.

Working Principle

A freely rotating impeller is installed in the pipeline where the fluid flows. When the fluid flows through the pipeline under test, the impeller in the turbine head is driven to rotate. In a wide measurement range, the rotation speed of the impeller is the same as that in the pipeline under test. The flow rate is proportional, and the impeller speed and the electric pulse signal of the speed are detected by the magnetic resistance group detector. After being amplified by the preamplifier, it is transmitted to the display instrument, and the instantaneous flow rate and the total amount can be measured.

Advantage

1) It is more accurate to measure the turbine head directly in contact with the water, and the signal can be transmitted to the central control room after being amplified by the pre-amplifier board;

2) The flowmeter is small in size, can measure large flow, and can also measure the cumulative flow more precisely while measuring the flow;

3) The cost is low. A set of turbine heads only costs about 400 yuan.

Shortcoming

Due to maintenance problems, the counting cannot be continuous, the output signal is 0-10mA, and there is no serial communication interface.

4. Rotameter

The rotameter changes the flow area of the fluid to keep the differential pressure above and below the rotor constant, so it is also called a variable flow area constant differential pressure flowmeter, also known as a rotameter.

Working Principle

Rotameters measure fluid flow based on the throttling principle. In a vertical conical tube that expands from bottom to top, the gravity of the float with a circular cross-section is borne by liquid power, and the float can rise and fall freely in the conical tube. It moves up and down under the action of flow velocity and buoyancy, and after balancing with the weight of the float, it is transmitted to the dial to indicate the flow through magnetic coupling.

Advantage

Suitable for small pipe diameter and low flow rate; reliable work, low maintenance, long life; low requirements for downstream straight pipe section; wide flow range of 10:1; local pointer indication is close to linear; intelligent indication The device has LCD liquid crystal display, which can display instantaneous and cumulative flow, and can also output pulse and output alarm; it has temperature compensation.

Shortcoming

1) There is a risk that the glass tube is fragile;

2) Most structural rotameters can only be used for pipeline installations with vertical flow from bottom to top;

3) The application is limited to small and medium pipe diameters, and ordinary full-flow rotameters cannot be used for large pipe diameters;

4) When the used fluid is different from the factory calibration fluid, the flow indication value must be corrected. If the actual used fluid density and viscosity are different, the flow rate will deviate from the original division value, and a conversion correction must be made.

5. Throttling Flowmeter

Working Principle

Add an orifice saving part in the fluid pipeline, introduce the differential pressure transmitter through the pressure guiding tube to measure the pressure difference between the upstream and downstream of the saving part, and calculate the instantaneous value of the flow rate according to the measured pressure difference.

Advantage

1) The standard orifice plate structure of the throttling part is easy to copy, simple, firm, stable, and reliable in performance, long in service life, and low in price;

2) The throttling type has a wide range of applications. All single-phase fluids, including liquid, gas, and steam, can be measured, and some mixed-phase flows, such as gas-solid, gas-liquid, liquid-solid, etc., can also be used. The general production process and pipe diameter, There are products in the working state (pressure, temperature);

3) All manufacturers of accessories can be used in common, and if it is an international standard, it does not need to be calibrated;

4) It is easy to install and maintain, unlike ordinary differential flow meters, which have long pressure pipelines that are easy to leak, block, and freeze;

5) There is no zero drift problem.

Shortcoming

1) The repeatability and accuracy of measurement belong to the middle level;

2) The range is narrow because the instrument signal and the flow are in a square relationship, the general range can only reach 3:1~5:1;

3) The requirements for on-site installation conditions are relatively high, and it is difficult to meet the requirements of long straight pipe sections;

4) The pressure induction pipeline is a weak link, which is prone to failures such as leakage, blockage, freezing, and signal distortion;

5) Large pressure loss.

6. Target Flowmeter

Target flowmeters began to be used in industrial flow measurement in the 1960s, mainly to solve the flow measurement of high viscosity and low Reynolds number fluids.

Working Principle

When the medium flows in the measuring tube, because of its own kinetic energy and the pressure difference between the target plate and the force on the target plate, the target plate produces a small amount of displacement. The magnitude of the force is proportional to the square of the flow velocity of the medium. The force on the target plate is transmitted through the target rod, which causes a slight change in the elastic body of the sensor, thus breaking the balance of the bridge composed of chip capacitors, resulting in a gap between the target and the flow rate. The voltage signal corresponds to the force on the plate.

Advantage

1) Wide measurement scale and low flow rate, especially suitable for the measurement of large pipe diameter, high and low-pressure gas or liquid;

2) With integrated temperature and pressure compensation, directly output mass or standard flow;

3) With the optional small signal cut, non-linear correction, filter time can be selected;

4) Simple and convenient installation, good repeatability, fast measurement, and easy maintenance;

5) Dirt resistance, high-temperature resistance, high sensitivity, anti-blocking, anti-interference, and anti-impurity capabilities are extremely strong;

6) The pressure loss is small.

Shortcoming

1) It is not suitable for very frequent fluid switching conditions, and it is better for continuous operation;

2) The accuracy is not very high, and it is generally used as a process control meter, and it should be used with caution in trade settlement;

3) The measuring range is narrow, and the range of general instruments is 10:1;