Application of electromagnetic flowmeter in sewage treatment plant
With the rapid growth of the global population and rapid industrial development, the state of global water resources is rapidly deteriorating, and the "water crisis" is becoming increasingly serious. To cope with this situation, the Chinese government will invest 300 billion yuan to build and rebuild several dry-sewage treatment plants during the "11th Five-Year Plan" period to promote the treatment and utilization of urban sewage and reduce pollution. Against this background, Huangshan City, Anhui Province is investing in the construction of a sewage treatment plant. The first phase of the daily water treatment 150,000 0IT13 sewage project has been completed. It is responsible for the important tasks of domestic and industrial sewage purification and improving the environmental quality of water bodies. Make Guangzhou a city of clean water.
Before 2004, there were basically no large-scale sewage treatment plants and other facilities in Guangzhou, let alone a flow meter for sewage flow detection. In the sewage treatment process, the flow of raw sewage is a key parameter that needs to be detected. Because raw sewage is a corrosive medium containing particles, suspended solids, and solid-liquid mixtures, the biggest feature of electromagnetic flow meters is that they can measure the flow of acids, alkalis, salt solutions, and solid particles (such as mud) or fibrous liquid. The sewage treatment plant uses two electromagnetic flow meters for the flow measurement of raw sewage.
Working principle of electromagnetic flowmeter
The basic principle of the electromagnetic flowmeter is the Faraday's law of electromagnetic induction, that is, when a conductor cuts magnetic motion in a magnetic field, the induced electromotive force E at its two ends is:
In the formula, K is the instrument constant; B is the magnetic induction intensity (T); v is the average speed of conductor movement m / sD is the inner diameter of the pipe ㈣.
The working principle and structure of the electromagnetic gage is shown in Figure 1. It consists of two parts: flow sensor and converter.
When measuring the flow rate, the conductive liquid corresponds to the conductive metal rod in the Faraday test, and flows at a mean speed v through a magnetic field perpendicular to the flow direction. The flow of the conductive liquid induces a voltage proportional to the average flow velocity at the measurement electrode. Its induced voltage signal E is detected by one or more pairs of electrodes in direct contact with the liquid.
According to the formula of the fluid's volume flow, Q = 1/4 D [D2v (2)
It can be known from formula (2) that Q and v are a one-to-one correspondence function that is proportional to each other, then E and Q are also a one-to-one correspondence function that is proportional to each other. The induced voltage E is also measured as the volume flow rate Q of the medium.
The measured induced voltage E is sent to the converter by the cable, and processed intelligently, and then displayed on the LCD, or converted into a standard signal 4 ～ 20mA output.
2. Characteristics of electromagnetic flowmeter
(l) There is no resistance element in the measuring pipe, no additional pressure loss, it is not easy to be blocked, and it has significant energy saving significance;
(2) There are no moving parts in the measuring pipe, which is not easy to wear, so the sensor has a long life;
(3) The short straight pipe section required by the sensor is convenient for installation;
(4) Reasonable selection of electrodes and lining materials, which can resist corrosion and wear;
(5) Two-way measurement system, which can measure forward and reverse flow;
(6) The measurement of flow is volume flow, which is not affected by changes in flow density, viscosity, temperature, pressure, and conductivity;
(7) The sensor voltage signal has a linear relationship with the average flow rate, high measurement accuracy (± 0. 3r-- -0.5%), and a wide range ratio (k 150).
3. Selection of electromagnetic flowmeter
The selection of the meter is very important. The relevant data show that 2/3 of the failure of the meter in actual application is caused by the wrong selection of the meter or the improper use. Therefore, after determining the selection of the electromagnetic flowmeter, it should also be based on different processes, Comprehensive consideration of environment, measurement, and media.
3. 1 Caliber selection
According to the process pipe diameter, pipe pressure and sewage flow rate provided by the sewage treatment plant, an electromagnetic flowmeter with a diameter of D-6 0mrn is selected, and no shrinkage and expansion are needed on site.
3.2 Selection of lining and electrode materials
The electromagnetic flowmeter is mainly used to measure the fluid flow with a conductivity of ≥ 5U S / cm. It should be based on the characteristics of the corrosiveness, abrasion, temperature, and condensability of the measured substance and its price tolerance. Different linings and electrode materials should be selected.
The lining materials of electromagnetic flowmeters are: neoprene materials with medium corrosion resistance and resistance to low concentrations of acids, alkalis and salts. Polyurethane rubber with strong abrasion resistance and general corrosion resistance has strong corrosion resistance. And suitable for high temperature polytetrafluoroethylene, chemical properties equivalent to polytetrafluoroethylene tensile, compression polyfluoroethylene propylene, resistance to dilute acid, alkali, salt polyethylene <60 ℃ and temperature <100 ℃ polyphenylene sulfide and so on.
The electrode material is installed on the inner wall of the pipe and is in direct contact with the test substance, so it should be selected based on the corrosivity of the test substance. Electromagnetic flowmeter electrode materials are: titanium (Ti) electrode resistant to salt and less than 50% alkali solution, tantalum (Ta) electrode resistant to acid and salt, precious metal platinum electrode with strong corrosion resistance, Hastelloy not suitable for hydrochloric acid C electrode and Hastelloy B electrode which is not suitable for nitric acid, stainless steel 316L, etc. with low corrosion resistance but low price.
According to the sewage treatment plant's main treatment of domestic wastewater and industrial wastewater, and considering the price acceptable to users, chloroprene rubber lining and stainless steel 316L material electrodes were selected.
3.3 Selection of protection level
According to national standards, the sensor has two protection levels: IP65 water spray type and IP68 submersible type. According to the actual installation location of the user in a low shaft, the sensor is IP68 dustproof submersible type.
3.4 Selection of additional functions
(l) For the selected basic type, the electromagnetic flowmeter has LCD display, 4-20mA current output and 0-lKHz frequency output. According to the requirements of communication between the flowmeter and the computer, the RS-485 communication port function needs to be added.
(2) For sensors installed below the ground, the split type should be selected.
4. Installation and commissioning of electromagnetic flowmeter
4.1 Installation essentials
(1) Select the installation point at the lowest point of the pipeline
After confirming the flow direction of the measured substance, the installation point should be selected to be filled with liquid at any time to prevent the error caused by the indication of zero position due to the absence of liquid in the measurement tube.
(2) Straight pipe section is guaranteed before 5D and after 3D
In order to obtain the measurement accuracy of the instrument, the length of the straight pipe section on the upstream side of the sensor is not less than 5D and the downstream side is not less than 3D (D is the nominal diameter of the sensor). If there are more than two elbows or other obstructions on the upstream side of the sensor, the front straight pipe section should be greater than 10D.
(3) Signal electrode is horizontal
When installing horizontally, the electrode axis should be parallel to the horizon, because the electrode at the bottom is easily covered by the sediment, and occasional air bubbles in the measured medium will wipe and cover the electrode surface, causing the output signal to fluctuate.
(4) Try to avoid vibration sources, high-power motors and transformers with magnetic sources)
Because the measurement induced voltage of the electromagnetic flowmeter is very small, the voltage is low, and it is easily affected by external electromagnetic noise. Therefore, a reliable grounding connection should be performed during installation. SINIER's 4-electrode design can effectively optimize the system and save costs.
4. 2 converter installation and commissioning
The integrated electromagnetic flowmeter does not have a separate installed converter. The split converter is installed near the sensor or the instrument room. The place to choose is larger and the environmental conditions are better. The distance between the converter and the sensor is subject to the conductivity of the measured medium and the type of signal cable, that is, the distributed capacitance of the cable, the cross-section of the cable, and the number of shielding layers. Use the signal cable of the specified type attached by the instrument of the manufacturer. When the conductivity is low and the transmission distance is long, a two-layer shielded cable is used.
In order to avoid interfering with the signal, the signal cable must pass through the grounded protective steel pipe separately. The signal cable and the power cable cannot be installed in the same steel pipe.
During the functional debugging, the intelligent converter reads the parameters of the instrument, the threshold alarm test, etc., and then writes the flow coefficient value.
5. Instrument commissioning and troubleshooting
Before the electromagnetic flowmeter is put into operation, the pipeline and the flowmeter should be thoroughly inspected, including cleaning of pipeline debris, and the electrodes should be dried. It also includes the inspection of electrical lines. First, the grounding of the line should be detected. After ensuring the reliable grounding, insulation resistance and grounding resistance tests are also required.
The electromagnetic flowmeter has been in normal operation for more than a year, and basically has no failures. However, after one year, abnormal operation has occurred. The specific manifestation is that the measured value becomes larger or smaller, or it constantly fluctuates. The possibility of unsatisfactory pipes and gas in the medium has been ruled out. It is found that the occurrence of such problems may be related to cable faults. Because the flowmeter is buried in the ground with the pipeline, the sensor has an IP68 protective structure, and the converter is installed in the meter box or indoor. The two are connected by cables. Due to changes in the site conditions such as ground subsidence and humid mountain environment, the sensor and converter are not completely Do the moisture (waterproof) and other treatments at the cable joints. Just when the connector is in a humid environment, such as meter wells, cable trenches, etc., moisture intrusion into the cable connector may cause the following:
(l) The signal line's insulation to the ground decreases, causing signal attenuation, and the final measurement result is too small;
(2) The contact resistance at the signal cable connection becomes larger, making the measured value smaller. If the contact resistance is unstable, the measured value cannot be stable, and it is easy to introduce interference;
(3) The insulation of the excitation coil to the ground is reduced, resulting in a small measurement result;
(4) The insulation performance of the signal line excitation line to the ground is reduced, making the measurement result much larger than normal data. If this interference is unstable, the impact on the instrument will also vary, and then fluctuate;
(5) The contact resistance at the connection of the excitation circuit cable becomes larger, so that the excitation circuit of the converter is in a non-constant current operating area, and the excitation current decreases, which also causes the measurement result to be too small. If the contact resistance is unstable, the measurement value will fluctuate.
It is difficult to judge this kind of abnormal situation because it occurs in the signal loop. It only has to go deep into the field. After eliminating other possible failures of the flowmeter, you should check the cable for problems. Compare the measured value with the data provided by the manufacturer, test the grounding resistance, determine the problem step by step, and then take measures, or replace the entire cable, or keep the cable connector dry.
6. Concluding remarks
Due to the complicated operating environment of the electromagnetic flowmeter and many factors affecting its normal operation, the debugging technology needs to be continuously summarized and updated. In short, as long as we select and install the electromagnetic flowmeter reasonably, and take effective measures and ant maintenance, we can avoid or reduce the occurrence of failures, prolong the service life of the instrument, and provide accurate and reliable measurement data for production.
In the modern sewage treatment process, equipped with flow detection instruments, constitute a reliable automatic control system, so that the quality of sewage water meets conventional discharge standards, benefiting mankind. Therefore, Guangzhou Dichuan Instrument Co., Ltd. continuously strives for better instruments.
Reprinted please indicate this article from Guangzhou Dichuan Instrument Co., Ltd.