Sewage flow meter design and application

News Author: Source Automation Equipment Co., Star Lake: Lake Star Automation Equipment Co., Ltd.

1, ultrasonic sewage flow meter measurement principle

1.1 Doppler ultrasonic sewage flowmeter

The Doppler effect principle was used to determine the fluid flow, as shown in Figure 1. Transducer A transmits a continuous ultrasonic signal with a frequency of fA to the fluid. Suspended particles and bubbles in the irradiated pipe generate ultrasonic Doppler frequency shifts and reflect to the receiving transducer B, fA and fB at the frequency of fB. The difference is the Doppler frequency difference fD.

Let the fluid velocity be V, the ultrasonic velocity be C, and the Doppler frequency shift fD be proportional to the fluid velocity V, ie


V =C/2·fA·sinθ?fD

When the pipeline condition, transducer installation position, transmission frequency, and sound velocity are determined, C, fA, fB, and θ are constants, and the flow velocity of the fluid is proportional to the Doppler frequency shift, and the flow velocity of the fluid can be measured by measuring the frequency shift. , And then get the flow of fluid.

Doppler ultrasonic sewage flow meter features: Instrument accuracy is usually between ± (3 ~ 7)% FS, suitable for measuring particles containing a suitable amount of strong reflection signal and bubbles. Its suspension content is about lg/L~5g/L. It is generally used for the discharge of untreated sewage and return sludge from the factory, and should not be used for flow measurement of clean liquids.

1.2, time difference type multi-channel ultrasonic sewage flowmeter

Measuring principle: Measuring the instantaneous flow rate Q of the fluid in a closed full pipe is based on the increase of the propagation speed of the ultrasonic signal in the direction of the water flow, and the decrease of the direction of the counter flow; at the same propagation distance, it has different propagation speeds and Propagation time; The propagation time difference can be calculated from the forward propagation time t1 and the reverse propagation time t2, and then the average line velocity can be calculated. The principle is shown in Figure 2.

The monophonic flow equation can be expressed as:

In the downstream direction, the propagation time is: t1=L/C+VCosθ
In the upstream direction, the propagation time is: t2=L/C-VCosθ.

Δt=t2-t1=2Lvcosθ/C2-V 2cos2θ

Due to C>>V, so


V = C2/2Lcosθ×Δt


Where: to measure the line average velocity on the acoustic path; L is the effective path length of the measured acoustic path; t1 is the forward propagation time of the acoustic wave; t2 is the reverse propagation time of the acoustic wave; △t is the forward propagation time and inverse propagation time difference of the acoustic wave C is the speed of ultrasonic sound; θ is the angle between the axis of the sound path and the flow channel; S is the cross-sectional area of ​​the measurement circuit; q is the volume flow rate.

If it is four-way:

In the formula: In order to measure the average velocity of the cross-section line; Ki is the weighted integral coefficient of the i-th channel; V i is the average flow velocity of the i-th channel; S is the cross-sectional area of ​​the cross-section; Ki is the weighted integral coefficient, which is the coefficient of the transducer Sound path layout and installation to determine.

The project selected Swiss company RISONIC2000 time difference type four-channel ultrasonic sewage flowmeter, the sound path layout and weight coefficients are listed in Table 1 [1]. The Gaussian-Jacobi integral method in the table recommends the use of rectangular and trapezoidal structural channels. Gauss-Legendre recommends the use of circular structural channels. W is the weighted integral constant of the ultrasonic sewage flowmeter and Ki is the proportional constant. Sound path layout shown in Figure 3.

RISONIC2000 time difference type four-channel ultrasonic sewage flowmeter circuit principle shown in Figure 4.

Figure 4 shows a phase-locked loop (PLL) ultrasonic flowmeter that generates a frequency proportional to the reciprocal of the ultrasonic pulse propagation time. Among them, the transmission frequency of the forward oscillator VCO(1) is divided into N equal parts, the cycle of which is almost the same as the propagation time of the ultrasonic waves in the liquid. The difference in propagation time between the electrical pulse output by the frequency divider circuit and the receiving ultrasonic pulse transmitted through the water is detected by the time difference detection circuit. The output is converted into DC and added to the VCO (1) to control its oscillation frequency so that the time difference voltage is automatically zeroed. At this time, the output frequency of the VCO is fixed. Therefore, in the steady state, the formation process of the VCO(2) oscillation frequency is exactly the same as that of the VCO(1), except that the ultrasonic waves propagate backward. As long as the difference between these two frequencies is calculated, a value proportional to the flow velocity can be obtained.

2RISONIC2000 Time Difference Multi-channel Ultrasonic Sewage Flowmeter Features

1. The installation method of time-differential ultrasonic sewage flowmeter of this project adopts plug-in type, and the measurement accuracy is relatively high. For example, the accuracy of factory calibration of four-channel time-difference ultrasonic sewage flowmeter is 0.5%.
2. Box and culvert measurements are straight-through tubes. The transducers have minimal projections and no flow blockers in the box culverts, so that there is no external pressure loss in the fluid. At the same time, it is a meter that will not disrupt the flow field of the fluid and measure the fluid.
3, the measurement accuracy is not affected by the physical and chemical properties of the medium such as conductivity, viscosity, temperature, pressure, composition changes and other factors, it is also suitable for high viscosity liquid flow measurement.
4, can under the operating conditions (non-stop flow) with pressure replacement transducer, easy maintenance.
5. The Reynolds number (Re) of the fluid has little influence on the measurement. Under normal use conditions, the instrument manufacturer specifies that the measured flow rate range of the instrument is 0.003m/s to ±20m/s. The project design flow rate is much greater than the minimum flow rate. A technical indicator.
6. The range of fluid flow measurement (range ratio) is wide, up to 1:300.
7. The ultrasonic sewage flowmeter itself has no mechanical inertia and can be quickly sampled. The flow can be calculated by averaging multiple samples, which can eliminate the effects of measurement caused by fluctuations in the flow velocity of the fluid.
8, multi-channel ultrasonic sewage flow meter in order to ensure accurate measurement of propagation time, take the following effective measures:
The counter circuit uses a high-stability quartz crystal oscillator of 80 MHz or more, and the time base error is controlled within ±12.5 ns.
Accurately measure the propagation delay of the signal in the electronic circuit and transducer and subtract it from the measured total propagation time to improve the instrument measurement accuracy.
Multi-channel ultrasonic sewage flowmeter has a complete signal quality detection circuit, which can effectively solve the impact of noise interference, signal attenuation and circuit parameters on the measurement accuracy.

9. RISONIC2000 has a powerful self-diagnostic function. In the detection process, the validity of the flow rate of each sound path and sound path is checked. When the invalid sound speed and the invalid sound path flow rate appear for the first time, the system will use Instead of previous valid values, if multiple occurrences occur, these measurements will remain frozen and replaced with previous measurements until the data is updated after a valid measurement has been identified. When it appears continuously, it will display the invalid sound path speed and invalid flow rate error status alarm. At the same time, the sound path arrangement will automatically switch, that is, when the above fault occurs in a certain measurement sound path of the cross section, it will automatically switch to single-section measurement. When the above fault occurs in a single cross-section measuring sound path, the sound path will be closed. Use other sound paths instead.

10. As the instrument is used in the industrial field, and there are high-frequency signals and high-voltage emission signals of the ultrasonic transducer in the circuit, there is more serious interference. The system takes measures from both software and hardware to greatly improve the system's anti-jamming capability. That is, the software adopts a modular structure design. It uses instruction redundancy and software trap programs to prevent runaway. The hardware at the reset end of the microcomputer processing is composed of peripheral circuits to prevent programs. Runaway watchdog circuit.

3, time difference ultrasonic sewage flow meter design program

3.1. Selection of ultrasonic sewage flow meter measurement principle

In time-difference ultrasonic water flowmeter measurement, the liquid to be measured must be able to be penetrated by acoustic waves, that is, the liquid can not contain bubbles, suspended particles and sediments of too high concentration, otherwise the acoustic waves will be refracted, affecting the measurement accuracy. When the time difference type ultrasonic sewage flow meter measures, its suspended matter content is limited to within 1% of the volume content. If it exceeds the content, the Doppler principle ultrasonic flowmeter is selected. The suspended matter content of this project is only within 500ppm/L. Therefore, it is advisable to choose time-difference ultrasonic sewage flowmeter.

3.2, ultrasonic sewage meter transducer sound path selection

Ultrasonic sewage flow meter measurement principle is a speed type flow meter, the fluid flow state has a greater impact on the accuracy of the meter. If a single-channel transducer is used, the accuracy of the instrument is only 5%. Based on the measurement accuracy requirements in the choice of acoustic path, it can be selected as a combination of 1, 2.4, and 8 sound paths and 1 to 4 measurement sections. It can effectively solve the influence of asymmetric distribution of fluid flow in one direction. In this selection of 4 sound paths, the measurement accuracy can reach 0.5% under the conditions of the straight sections of the front and back of the measurement section.

3.3, Ultrasonic Sewage Meter Measurement Section Range Selection

Closed pipe 150 to 14000mm (round pipe and rectangular box culvert)
Open channel 250mm~15m (up to 150m)

3.4. Selection of ultrasonic sewage flowmeter transducer

The main three types of transducers are available depending on the installation site and installation method.

1. In-Paste Transducer: When there is no exposed pipe section that can only be installed internally, an internal-paste transducer can be used to mount it directly on the inner wall of the pipe. This kind of installation is simple and can also arrange 1 to 8 sound paths. However, when the transducer is replaced, it must be provided with the current-breaking condition. The acoustic path angle is designed to be 45o, and the maximum distance between the transducer and the host can reach 300m.

2. Plug-in type transducer: For the inside and outside of the tube, it is a good solution to use the plug-in transducer. As long as the hole is made in the pipe wall, the transducer is inserted into the hole and fixed to the pipe by a nut and sealed with a sealing gasket. The plug-in transducer can be installed with constant flow of the tubing and the transducer protrusions are minimized. The sound path angle is designed to be 45o, and the maximum distance between the transducer and the host can reach 300m.

The plug-in transducer and the measuring tube segment are integrated into one body. This is the segment-type ultrasonic sewage flowmeter. The transducer is accurately positioned and factory-calibrated in the production plant, so the measurement accuracy is higher than that of the on-site production. If an E-shaped valve is installed The group can be carried out as a whole, making it easy to disassemble, overhaul and calibrate.

3, open channel transducer: This transducer is used for open channels (rectangle, trapezoidal, circular) and pressure tunnel, can be installed and maintained in the case of a broken flow, it is also a type of in-cell transducer, sound The corner is designed for 45o. When installed in a rectangular open channel, the acoustic angle design is effective between 35o and 70o.

The minimum distance between the transducer and the host is 150 meters, and the width of the measurement channel is 0.25 to 15 meters or even 50 meters. At this time, the operating frequency of the selected ultrasonic transducer decreases, the frequency decreases, the penetrating ability thereof increases, the measurement channel width increases, and the operating frequency of the transducer can be selected from 1 MHz to several hundred kHz. The accuracy of the measurement is then reduced. In this case, the method of increasing the ultrasonic path can be used to improve the measurement accuracy.

3.5, ultrasonic sewage meter accuracy choice

According to the requirements of GB/T17167-1997 “Guidelines for the Equipping and Management of Enterprise Energy Measuring Instruments”, the accuracy of the sewage metering device is 5.0%. Taking into account the large flow rate of sewage, the characteristics of large caliber box culvert, ultrasonic flowmeter flowmeter is difficult to carry out real-flow calibration, in the future to be used on-line large-flow sewage ultrasonic flowmeter design and application of Huihui flow dry calibration method for calibration . At the same time, the uncertainty of the whole measurement system of the ultrasonic sewage flow meter not only depends on the accuracy of the flow measurement instrument itself, but also involves the determination of the cross-sectional area of ​​the box and culvert, the precise positioning of the transducer installation, the measurement of the acoustic path length, and many other factors affecting the measurement accuracy. . The ultrasonic flowmeter for trade settlement uses a single-section, four-channel, time-difference ultrasonic sewage flowmeter. The accuracy of the flowmeter is 0.5%, ensuring that the uncertainty of the entire flowmeter system is within 2.5%.

3.6. Selection of Integral Mathematical Model for Ultrasonic Sewage Flowmeter

This project is that the pressure box culverts are geometric shapes with rectangular inner cross-sections and four corners. Therefore, the four-channel installation position and the fluid flow distribution integration method of the ultrasonic transducer adopt the sound path approved by the IEC-41 (1991.11) regulations. Arrange (1E4P) and Gauss-Jacobi mathematics model.

4. Main technical indicators of the selected time difference ultrasonic sewage flowmeter

1, ultrasonic meter measurement principle: time difference method, sound path layout: 1E4P
2. Accuracy: 0.5 level 3. Installation method: Waste tank culvert installed external type transducer 4. Host: Integrated user screen, large screen LC display 51/2 bit flow, instantaneous flow, cumulative flow and other signals 5, Signal Processing Technology: DSP (speed, area quadratic integral)
6, power: 220 volts AC, and installation of lightning protection module.
7. Instrument self-diagnosis function: normal state of sound path, strong and weak signals, no signal received by transducer, no use of sound path, strong and weak reference signal, invalid measurement time of propagation, invalid sound velocity, invalid flow (change in flow rate) Rate exceeds the limit), measurement flow is greater than or less than the flow rate and other information.
8. Reverse flow measurement can be performed: Measurable bi-directional flow 9. Permissible volume content of suspended solids in the measurement range: ≤ 1%
10. Protection class: IP68 transducer; host IP65
10, sensor pressure level: ≥ 2MPa
6, sensor cable: waterproof, shielded, strong electromagnetic interference resistant enhanced cable 10, ambient temperature: the host is -20 °C ~ +60 °C transducer is -30 °C ~ 70 °C
10, the output signal: 4 ~ 20mA; RS-232 standard communication interface; Profibus-DP: communication upload positive and negative instantaneous, cumulative flow, acoustic time, self-diagnostic information. The information is uploaded to the FCS system (fieldbus control system) via the fieldbus, the host computer configuration monitoring, data management screen.

5, ultrasonic sewage meter construction technical requirements

The large-diameter multi-channel ultrasonic sewage flowmeter is usually composed of a flow meter main unit and a transducer mounted on a box culvert or a pipeline. Therefore, the measurement error of the flow meter is not only related to the technology and quality of the flow meter itself, but also to the user's box culvert, The condition of the pipeline is related to the precise positioning of the transducer. First of all, the installation of the transducer must ensure that the front and back straight sections, followed by the integration method and the arrangement of acoustic transducers approved by the International Electrotechnical Commission (IEC) regulations can control the flow distribution and integration calculation error to ± 0.1% Within.

In addition, the spatial position error of the transducer and the measurement error of the cross-sectional area of ​​the tank culvert constitute an important cause of the measurement error of the ultrasonic flowmeter. The accurate positioning and cross-sectional area measurement of the laser theodolite must be used.

5.1. Technical Requirements for Ultrasonic Sewage Meter Installation

1. Select the measurement section: In order to achieve the measurement performance, the flow state of the fluid must be taken into consideration, and the measurement section and the acoustic path arrangement must be selected at the same time. First select the actual installation length of the transmitting section or receiving transducer of the measurement section to be twice the width of the culvert. However, in order to ensure good flow conditions and flow measurement accuracy of the measurement section, the section of the installed transducer requires not only adequate direct current in the upstream and downstream of the transducer but also in the downstream. Segment, in addition to double the culvert width. In this section, the unevenness of the four-walled culvert should not exceed 1 cm, and the concrete surface should be smooth and clean.
2. When the transducer is installed, its axial direction and theoretical direction are controlled within 5o.
3, the length of the cable between the ultrasonic sewage flowmeter host to the transducer does not exceed 300m.
4, ultrasonic sewage flowmeter has a good independent grounding, grounding resistance <4Ω.
5, the transducer and the host's installation site should avoid the electromagnetic interference of strong electric cable, frequency converter and electric welding machine.
6. The transducer installation site should avoid strong sound sources and vibrations to prevent strong acoustic wave vibrations.
7. The lightning protection module is installed in the host, and the UPS power supply can be added in case of on-site conditions permitting to meet the measurement requirements during the power outage.
8. The transducer is installed in the design position. If the offset is large, then the value of the integral number coefficient will be involved, and the Owics technology of the ultrasonic sewage flowmeter can be used to correct it to the theoretical installation position, and the geometric deviation of the pipeline can be compensated to improve its measurement accuracy.

5.2 Installation and adjustment of ultrasonic sewage flowmeter

Use professional installation tools and instruments (such as laser theodolites, laser sights) for installation and commissioning, including the following aspects:

1. Accurately measure the section size of box culverts. Measurement accuracy is controlled within millimeters.
2. The use of laser theodolite for accurate transducer channel arrangement and positioning of the transducer allows the measurement error of the acoustic angle θ to be controlled within 0.03o.
3. Install the transducer and use a torque wrench for even tightening to prevent fluid leakage.
4. Correctly set the key data of ultrasonic sewage flowmeter: box culvert equivalent height, width, sound path length, channel angle, integral weight coefficient, signal strength level, section number, measurement unit, viscosity, forward and reverse flow And other parameters. And twice review, confirmed to be formally put into operation.

6. Usage

Nine sets of ultrasonic sewage flow meters have been in trial operation for nearly three months, and the supply and demand sides have conducted on-site acceptance inspections of six sets of ultrasonic sewage flow meters used for trade settlement, and formally served as the basis for trade settlement between the two parties. The data during the trial run showed that the uncertainty of the nine ultrasonic sewage flow meters reached and exceeded the expected design level 2.5 requirements, and the results were satisfactory. In the 10-day assessment operation in July 2004, the average relative error between the sum of the total sewage flow of the 4 inlets (untreated sewage) and the total sewage flow of the 2 outlets (treated sewage) is within +2%, and the data is stable. reliable.

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