What flow meters are commonly used in wastewater treatment?

Time:2022-08-10 18:01:49  PV:98

Flow meter is one of the instruments often used in wastewater treatment. There are many kinds of flow meters according to the working principle, and the selection needs to be based on the actual working conditions to choose the right flow meter. Here we will talk about the 6 kinds of flow meters commonly used in sewage treatment.

 

1. Electromagnetic flowmeter

Electromagnetic flowmeter is based on the principle of electromagnetic induction and the work of the flow measurement instrument. It is composed of sensors and transmitters.

(1) working principle

Electromagnetic flowmeter basic principle is: in general non-conductive material made of pipe outside, installed with a pair of magnetic poles N and S, to generate a magnetic field. When the conductive liquid flow through the pipe, because the fluid cuts the magnetic lines of force and the magnetic potential, this induced potential from the two electrodes perpendicular to the direction of the magnetic poles, when the strength of the magnetic field remains unchanged, the diameter of the pipe is certain, the size of this induced potential is only related to the flow rate, the size of this induced potential is transmitted to the display instrument, you can read the flow.

(2) Advantages

① measuring conduit without moving parts and resistive fluid, so the pressure loss is very small, no mechanical inertia, so the response is sensitive.

② wide range of measurement: range ratio is generally 10:1, up to 100:1, flow rate range is generally 1-6m / s, can be extended to 0.5-10m / s; flow range can be from 90mL / h to more than 100,000m3 / h; pipe diameter range can be from 2mm to 2400mm or 3000mm.

③It can measure the volume flow of liquid containing solid particles, suspended matter or acid, alkali and salt solutions with certain conductivity, and also pulsating flow, and can be measured in both directions.

④ flow signal and fluid volume flow rate has a linear relationship between the instrument has a uniform scale; and the volume flow rate of the fluid and the physical properties of the medium, flow state, so the electromagnetic flowmeter only needs to be calibrated with water, can be used to measure the volume flow rate of other conductive liquids without correction;.

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

(3) disadvantages

① the use of temperature and pressure can not be too high.

(2) limited application range, can not be used to measure the flow of gases, steam and petroleum products and other non-conductive fluids and fluids containing more larger bubbles.

③When the flow rate and velocity distribution do not meet the set conditions, it will produce a large measurement error.

④ when the flow rate is too low, to amplify and measure the induced potential with the interference signal to the order of magnitude is more difficult, and the instrument is also prone to zero point drift; ⑤ electromagnetic flowmeter signal is relatively weak.

⑤ electromagnetic flowmeter signal is relatively weak, the outside world slightly interference can affect the accuracy of the measurement.



 


Second, ultrasonic flowmeter

Ultrasonic flowmeter is through the detection of fluid flow on the ultrasonic beam (or ultrasonic pulse) to measure the volume flow of the instrument.

(1) working principle

Sound waves in the fluid propagation, downstream direction of sound wave propagation speed will increase, counter-flow direction is reduced, the same propagation distance will have a different propagation time. Propagation time method is the use of the difference between the propagation speed and the measured liquid flow rate relationship to obtain the flow rate, and combined with the diameter of the pipe to derive the flow rate.

(2) advantages

① ultrasonic flowmeter can be non-contact measurement, clip-on transducer ultrasonic flowmeter can be installed without stopping the flow carrier tube, as long as the pipe to be measured outside the installation of the transducer can be, that is, you can not break the flow or can not punch the existing pipeline with ultrasonic flowmeter to measure the flow.

② ultrasonic flowmeter for no flow resistance radial measurement, no additional pressure loss.

③ measurement meter instrumentation coefficient can be calculated from the actual measurement pipeline and sound channel geometry to obtain, that is, the use of dry calibration, in addition to with the measurement pipe section type generally do not need to make the actual flow calibration;.

④ ultrasonic flowmeter for large round and rectangular pipe, and the principle is not limited by the diameter of the pipe, its cost basically has nothing to do with the diameter of the pipe;.

⑤ Doppler ultrasonic flowmeter can measure more solid phase content or contain bubbles of liquid.

 

(3) disadvantages

① propagation time method in the ultrasonic flowmeter can only be used for clean liquids and gases, can not measure suspended particles and bubbles over a range of liquids; conversely, Doppler method LSF can only be used to measure liquids containing certain heterogeneous phases.

② external clamped transducer ultrasonic flowmeter can not be used for lining or scaling very thick pipeline, as well as can not be used for lining (or rust layer) and the inner tube stripping (if the sandwich with gas will seriously attenuate the ultrasonic signal) or rust serious (change the ultrasonic path] pipeline.

③ Doppler method ultrasonic flowmeter in most cases the measurement number is not high.

④ can not be used for pipe diameter less than DN25mm pipeline.


 



Third, the insertion type turbine flowmeter

Insertion type vortex wheel flowmeter mainly by the up circle, impeller, rear guide parts, turbine head shell, limit piece felling handle, transmitter installation bolts inserted into the rod locking bolt positioning rod signal transmission line.

(1) working principle :

In the fluid flow in the pipeline, installed a free rotation of the impeller, when the fluid flow through the measured pipeline, push the turbine head in the impeller rotation, in a wide range of measurement, the impeller rotation speed and the measured flow in the pipeline is proportional to the use of resistive group detector to detect the impeller speed and speed of the electrical impulse signal, amplified by the preamplifier and transmitted to the display instrument, you can measure the instantaneous flow and The total amount can be measured.

(2) Advantages

① turbine head direct contact with water measurement is more accurate, the signal can be transmitted to the central control room after amplification by the preamplifier board.

② flowmeter volume is small, can measure the large flow, and in the measurement of flow can also be more precise measurement of cumulative flow.

③ low cost, a set of turbine head as long as about 400 yuan.

(3) disadvantages

Due to maintenance problems counting can not be continuous, the output signal 0 a 10mA, no serial communication interface.

 

Fourth, the rotor flowmeter

Rotor flowmeter is to change the fluid flow area to maintain a constant differential pressure above and below the rotor, so also known as variable flow area constant differential pressure flowmeter, also known as float flowmeter.

(1) working principle

Rotor flowmeter is based on the throttling principle to measure the flow of fluid. In a vertical cone tube expanded from the bottom up, the gravity of the float of the circular cross-section is borne by the liquid power, the float can rise and fall freely in the cone tube. In the flow rate and buoyancy under the action of up and down, and float weight balance, through the magnetic coupling to pass to with the dial to indicate the flow.

(2) Advantages

① Suitable for small pipe diameter and low flow rate.

② reliable work, low maintenance, long life.

③For the downstream straight pipe section requirements are not high.

④With a wide flow range of 10:1.

⑤In-place type pointer indication is close to linear.

⑥Intelligent indicator with LCD liquid crystal display, can display instantaneous, cumulative flow, also can output pulse, output alarm.

(7) with temperature compensation.

(3) Disadvantages

① risk of fragile glass tube.

② most of the structure float flowmeter can only be used for the installation of vertical flow from the bottom up pipeline.

③ application is limited to small and medium pipe diameter, ordinary full-flow float flowmeter can not be used for large pipe diameter.

④ the use of fluid and factory calibration fluid is different at the same time, to make flow rate correction, such as the actual use of fluid density, viscosity and the difference, the flow rate to deviate from the original index value, to be converted to amend.

 

Fifth, the throttle flowmeter

Throttle flowmeter is a typical differential pressure flowmeter, is used to measure the flow of gases, liquids and vapors in the pre-industrial production of the most commonly used a flow meter.

(1) working principle

In the fluid pipeline to participate in an orifice plate saving parts, through the introduction of pressure transmitter to measure the pressure difference between the saving parts upstream and downstream, based on the measured pressure difference through the calculation that the instantaneous value of the flow.

(2) Advantages

① standard orifice plate structure of the throttle is easy to copy, simple, solid, stable and reliable performance, long service life and low price.

② throttling application range is extremely wide, all single-phase fluid, including liquid, gas, steam can be measured, part of the mixed-phase flow, such as gas-solid, gas-liquid, liquid-solid can also be applied, general production processes and pipe diameter, working conditions (pressure, temperature) are products.

③ Accessories can be common to all manufacturers, if the international standard and can be used without calibration.

④ easy to install and maintain, unlike ordinary differential flowmeter, a longer lead pressure line, easy to leak, easy to plug, easy to freeze.

⑤ there is no zero point drift problem.

(3) disadvantages

① measurement repeatability, accuracy is a medium level.

(2) Narrow range, because the instrument signal and flow rate into a square relationship, the general range can only reach 3:1 ~ 5:1.

③The site installation conditions are high, if a long straight section is needed, it is difficult to meet.

④The pilot line is the weak link, easy to produce leakage, blockage, freezing and signal distortion and other failures.

⑤ large pressure loss.

 

Six, target flowmeter

Target flowmeter in the sixties began to be used in industrial flow measurement, mainly for solving the high viscosity, low Reynolds number fluid flow measurement.

(1) working principle

When the medium flows in the measurement tube, because of its own kinetic energy and the target plate pressure difference, and the force on the target plate, so that the target plate produces a small amount of displacement. The size of the force is proportional to the square of the medium flow rate, the force on the target plate is transmitted by the target rod, so that the elastomer of the sensor produces a slight change, which breaks the balance of the bridge composed of patch capacitors, and produces a voltage signal corresponding to the force on the target plate with the flow.

(2) Advantages

① wide measurement scale, low flow rate, especially suitable for large pipe diameter, high and low pressure gas or liquid measurement.

②With integrated temperature and pressure compensation, direct output of mass or standard square flow.

③With optional small signal cut, non-linear correction, filtering time can be selected.

④Simple and easy installation, good repeatability, fast measurement, and extremely easy maintenance.

⑤ Dirt resistance, high temperature resistance, high sensitivity, anti-blocking and anti-interference, resistance to impurities and other capabilities are particularly strong.

⑥Small pressure loss.

(3) Disadvantages

① Not suitable for fluid switching very frequent working conditions, the application is better in the case of continuous work.

② accuracy is not very high, generally used as a process control type measurement instrument, trade settlement caution.

③ narrow range, the general instrument are 10:1 range degree.

④ due to the target flowmeter target sheet and target rod has a self-weight, must be reset after installation of the zero point.