Abstract: This paper describes the basic role of automated instrumentation in water treatment production, and the general requirements for the classification and selection of commonly used instruments. Focused on the analysis of the working principle, precautions and application scope of the four types of flowmeters, and the types and precautions of turbidity meters. Summarizes the importance of automated meters in the modern management of the water treatment industry, and the importance of professional and technical personnel in the use of meters and management controls.
Key words: automation; electronic instrumentation; water treatment; process management 1 The basic role of automation instruments in water treatment production: Take a water purification plant in Qinhuangdao as an example: The main process is basically related to the corresponding instruments and automation technology. The electronic instrument continuously detects the process data and manually or automatically controls the data according to these real-time data, so as to coordinate the relationship between the supply and demand, the components of the system, and the respective water treatment processes, so that the water treatment equipment and facilities can be further improved. Full and reasonable use. At the same time, the value measured by the measuring instrument can be continuously compared with the set value. When deviation occurs, the adjustment is carried out immediately to ensure the quality of the water treatment. According to the parameters detected by the instrument, it can further automatically adjust and control the dosage of the medicine, ensure the reasonable operation of the water pump unit, make the management more scientific, and achieve the purpose of economic operation. Because the instrument has the function of continuous detection and over-limit alarm, it is convenient to deal with the accident in time.
2 Classification of Commonly Used Instruments for Municipal Water Treatment Process The instruments used in municipal water treatment processes can be roughly divided into two categories: one is the instruments that monitor the physical parameters of the production process, such as temperature, pressure, liquid level, and flow rate. This kind of instrument uses a part of the domestically produced watch, its performance and quality can basically meet the requirement. The other type is an analytical instrument for detecting water quality, such as detecting water turbidity, pH value, dissolved oxygen content, conductivity, residual chlorine, and SCD value. These special instruments developed relatively late in our country. Therefore, the basic selection of foreign imported E+H products is more economical and reliable in terms of actual results.
The quality of the instrumentation is directly related to the automation of the water plant and even the safety of the water supply. In the water plant instrument design selection, from the instrument performance, quality, price, spare parts, after-sales service and other aspects of repeated comparisons, and finally using imported instruments and domestic instruments combined method.
3 General requirements for the selection of water treatment instruments (1) Accuracy: refers to the accuracy of the instrument's measurement results under normal conditions of use. The smaller the error, the higher the accuracy. The accuracy of the physical detection instrument in the production process is ±1%, the accuracy of the water quality analysis instrument is ±2%, and the accuracy of the turbidity meter for measuring the turbid water is ±5%. (2) Response time: When measuring the measured value, the indicator value of the instrument will be displayed after a certain period of time. This time is the response time of the instrument. A meter can not quickly reflect the changes in the parameters, is an important indicator. The response time for water quality analysis instruments should not exceed 3 minutes. (3) Output signal: The analog output of the instrument should be 4mA ~ 20mADC signal, the load capacity is not less than 600Ω. (4) The degree of protection of the instrument should meet the requirements of the environment in which it is located. It should generally be no less than IP65. The instrument used for the pharmaceutical dosing system is required to be resistant to corrosion. (5) The four-wire instrument power supply is mostly 220VAC and 50Hz, and the two-wire instrument power supply is 24VDc. (6) On-site monitoring instrument should use digital display. (7) The working power of the instrument should be independent, and it should not share the power supply with the computer to ensure that the power supply does not interfere with each other during the failure and maintenance, so that each can operate stably and reliably. (8) In order for the computer to detect the abnormal signal of the voltage transformer and the current transformer and alarm, the input voltage and current transducer input voltage should be designed to be larger than the current and voltage transformers, that is 0~6A respectively. And 0 ~ 120V. (9) It should select instrument manufacturers who can provide reliable services and have rich experience.
4 level gauge selection and precautions Selection type should pay attention to the nature of the measurement object, such as the physical and chemical properties of the measured medium, and the working pressure and temperature, installation conditions, the speed of the liquid level changes; also take into account the measurement And control requirements, such as measurement range, measurement or control accuracy, display method, on-site instructions, remote indication, interface with the computer, safety and corrosion protection, reliability, and construction convenience.
The level gauges commonly used in existing municipal water supply and drainage works are as follows:
(1) Float level gauge: Put a hollow float in the liquid. When the liquid level changes, the float will produce the same displacement as the liquid level change. The displacement of the float ball can be measured mechanically or electrically, and its accuracy is low. This level gauge is not suitable for high-viscosity liquids and the output is switch-controlled. In the design of our company's water purification plant and new wastewater treatment plant, this level switch was mostly used in the sump pit between underground facilities to control the automatic start-stop of the sewage pump.
(2) Static pressure or differential pressure level gauge: Since the static pressure of the liquid column is proportional to the liquid level, the liquid level can be measured by using a pressure gauge to measure the static pressure of the liquid column on the reference surface. Calculate the pressure or pressure difference range according to the density of the measured medium and the measurement range of the liquid, and then select the suitable pressure gauge or differential pressure gauge for range and accuracy. This level gauge has an accuracy of ±0.5% to 2%.
(3) Capacitance type liquid level gauge: When the liquid level changes, the internal medium of the electrode changes, and the capacitance between the electrodes or between the electrode and the container wall also changes. The change in the capacitance is then converted into standardization. DC signal. Its accuracy is ±0.5% to 1.5%.
Capacitance type liquid level meter has the following advantages: The sensor has no mechanically movable part, and the structure is simple and reliable; the accuracy is high; the detection end consumes less power, and the dynamic response is fast; the maintenance is convenient and the service life is long. The disadvantage is that the dielectric constant of the fluid being measured can cause errors. Capacitive level gauges are generally used to adjust the level measurement of tanks, clear water tanks, etc. When the measuring range does not exceed 2m, rod-shaped, plate-shaped, coaxial electrodes are used; when it exceeds 2m, cable-type electrodes are used. When the medium to be measured is water, an electrode with polyethylene available for insulation is used.
(4) Ultrasonic level gauge: The ultrasonic level gauge sensor consists of a pair of transmitter and receiver transducers. The transmitting transducer faces the liquid surface to emit ultrasonic pulses, and the ultrasonic pulses are reflected back from the liquid surface and received by the receiving transducer. According to the time from launch to reception, the distance between the sensor and the liquid level can be determined, which can be converted into liquid level. Its accuracy is ±0.5%. This level gauge has no mechanical movable part, high reliability, simple and convenient installation, is non-contact measurement, and is not affected by the viscosity, density, etc. of the liquid, so it is often used in drug pools, medicine tanks, mud tanks, etc. Level measurement. However, this method has a certain blind area, and the price is more expensive.
5 Flow meter selection and precautions Flow measurement is divided into two types, one for flow detection, and participate in process control, in order to achieve the purpose of improving production automation, improving production process conditions, and improving product quality and output. Another kind of measurement used for flow measurement not only measures the output of products, but also serves as the basis for calculation of major technical and economic indicators of water supply companies. Among the eight major basic economic indicators of water supply companies, four are based on data measured by flow meters.
The following factors should be considered when selecting the type of flowmeter: Any type of flowmeter must have a certificate verified by the national metrology department before it can be used. The pressure loss of the flowmeter itself is small. According to industry requirements, the accuracy of the flowmeter should not be lower than 2.5. The installation site conditions should meet the requirements of the selected flowmeter for straight sections. The selected flow meter should be able to adapt to the installation site environmental conditions such as temperature, humidity, and electromagnetic interference. The selected flow meter should be suitable for the liquid medium to be tested.
At present, in the water supply and drainage engineering design, the electromagnetic flowmeter and the ultrasonic flowmeter are the most used.
(1) Electromagnetic flowmeter: The principle of electromagnetic flowmeter is to apply Faraday’s law of electromagnetic induction, which consists of sensors and transducers. In the measurement, the liquid itself is a conductor, and the magnetic field is generated by two coils installed in the pipeline. The coil is excited by an AC or DC power source, and the magnetic field acts on the liquid flowing in the pipeline to generate a voltage in the pipeline corresponding to the average flow velocity V of the fluid to be measured, and the voltage is independent of the flow velocity distribution of the fluid. The two electrodes insulated from the pipe monitor the induced voltage of the liquid. The direction of the magnetic field, the direction of fluid flow, and the relative positions of the two detection electrodes are perpendicular to each other.
Advantages of electromagnetic flowmeters: The measurement is independent of the temperature, pressure, or viscosity of the liquid being measured. No pressure loss. Continuous measurement, high measurement accuracy. The scope and measurement range of the caliber are large, and the measurement range is continuously adjustable. Independent of the flow velocity distribution. Straight sections are short before and after, front straight section is 5D (D is the diameter of the instrument), and rear straight section is 3D. Good stability, the output is a standardized signal, can easily enter the automatic control system. The inner wall of the transmitter conduit is lining material, with good corrosion resistance and wear resistance. The converter has small size, low power consumption, and strong anti-interference performance, which is convenient for on-site observation. The lining material of the electromagnetic flowmeter used in the water treatment system is mostly made of neoprene, because it has better wear resistance.
Installation should pay attention to the source of the electromagnetic field away from the outside world, so as not to affect the sensor's working magnetic field and flow signal, the sensor is installed horizontally, the center axis of the two electrodes is required to be in the horizontal state, to prevent the deposition of particulate impurities, affecting the electrode work. The measuring tube should be full and not allow a lot of bubbles to pass through the sensor. When the conditions cannot be met, appropriate measures should be taken. In order to make the instrument work reliably and improve the measurement accuracy without interference from external parasitic potentials, the sensor should have a good separate grounding wire, and the grounding resistance should be less than 10Ω, especially when it is installed on the cathodic protection pipeline.
Under the premise of satisfying the installation environment and usage requirements, the distance between the converter and the sensor and the connecting cable should be as short as possible to save the investment and reduce the interference of the strong electric signal that may be generated.
(2) Ultrasonic Flowmeters: In the past decade or so, due to the development of electronic technology, ultrasonic flowmeters have been used for flow measurement. There are many ways to use the ultrasonic flow meter to measure, among which the time difference method and Doppler method are typical. The time-difference flowmeter is often used in water purification plants. The method is to install two transducers on the measuring pipeline. Because of the difference in flow velocity between the forward flow and the reverse flow, the time difference from launching to receiving is measured, and the flow velocity is measured accordingly.
Ultrasonic flowmeter's main advantages: easy installation and maintenance. With the widespread use of clip-on sensors, ultrasonic flowmeters can be mounted and maintained without the need for perforating or cutting off the flow in the pipeline, allowing easy installation in existing applications, especially for large-diameter pipeline inspection systems. . The scope of the caliber is large, and the price is not affected by the pipe diameter. High measurement reliability. No pressure loss. Not affected by the fluid parameters. Output standard DC signal, can easily enter the automatic control system.
Ultrasonic flowmeters should pay special attention to the installation error of the sensor, the fouling of the inner wall of the pipeline and the uniformity of the anticorrosion layer. These factors have great influence on the measurement results. According to the measurement principle of ultrasonic flowmeter, only when the flow velocity is evenly distributed can the accuracy of the measurement be ensured. Therefore, there must be enough straight sections upstream and downstream of the flowmeter. Refer to the various materials and the flowmeter's manual for use. Less than 10D, downstream greater than 5D.
Because the tap water industry is a continuous production, continuous measurement is very important, so the flow meter generally installed on the pipeline can not be dismantled and sent for inspection. The general approach is to use a portable ultrasound flowmeter with high accuracy and send it to the national certification on a periodic basis. The unit is calibrated as a standard equipment of the company, and then the online flow meter is periodically checked for comparison.
(3) Open channel flowmeter: The essence of the existing open channel flowmeter of the company's Eastern District Wastewater Treatment Plant is the use of ultrasonic level gauges in conjunction with Parshall troughs. Parshall trough can turn the flow in the open channel into liquid level. The Parshall trough has the advantages of low water level loss, little precipitation even if there is solid matter in the water, and less influence on the water level on the downstream side, so it is used to measure the flow of other liquids such as agricultural water and industrial water.
Ultrasonic Open Channel Flowmeter Features: It can measure the flow of non-full tubes (circular tubes, egg-shaped tubes or other special-shaped tubes). Measure the flow of channels (circular, rectangular or other irregular channels). Measure natural rivers and streams. Sewage discharge channels or pipeline (sewer) flow can be measured. Positive and negative flow rates and flows can be measured. Instantaneous flow and cumulative flow values ​​are available. Output signal: RS-485, Modbus, 4 ~ 20Ma current signal and multi-way switch. Sensors can work long hours under harsh conditions and sewage water quality. Optional telematics or GPRS wireless module for remote telemetry. The sensor housing is polycarbonate with protection class IP68. Built-in automatic temperature compensation. The blind spot can be adjusted to shield the signal near the probe.
Ultrasonic open channel flowmeter accuracy has two concepts: ultrasonic probe and transmitter processing liquid level accuracy, and liquid level signal, area, time after the integral calculation accuracy, generally 0.2% to 0.5%.
6 Selection of Turbidity Meters and Their Attentions Turbidity is a measure of the degree of turbidity of a water body. A water purification plant is responsible for supplying domestic and industrial water for residents. The quality of water supply directly relates to people’s health and safety, as well as food, brewing, medicine, and textiles. , printing and dyeing, electricity and other industries in the normal production and product quality. Turbidity is a very important water quality indicator, so the choice of turbidimeter is particularly important.
Turbidity meter can be divided into two categories: visual turbidity meter and photoelectric turbidity meter. Optoelectronic turbidity meter can be divided into process monitoring for continuous measurement of turbidity meter and laboratory fixed and portable turbidity meter for its use. The design principle can be divided into transmission turbidity meter and scattered light turbidity meter.
Because the scattered light turbidity meter has high sensitivity to low turbidity of water, high accuracy, small relative error, good repeatability, water color does not indicate turbidity, and the ratio of scattered light to incident light intensity can be linear Relationship, so in September 1992 the World Health Organization published "Guidelines for Drinking Water Quality," the provisions of the scattered light turbidity meter as a measuring instrument.
The E+H scattered light turbidity meter was used in our company's water purification plant process, and the water quality laboratory and the second phase of Longhai Road used the HACH scattered light turbidity meter. The focused beam is projected downward from the sensor head assembly into the water sample in the body of the turbidimeter. The photocells immersed in the water sample measure the scattered light in the 90° direction of the suspended solids in water. The amount of scattered light and the turbidity of the water sample Degree is proportional to.
The selection of turbidity sampling points should be closely combined with the process specialty. The most representative point should be selected. The sampling hole should not be opened on the top of the sampled pipe, avoiding the bubbles in the pipe being drawn into the sampling pipe and affecting the measurement of the turbidity meter. Accuracy, the water sample extraction is best sampled with a small sampling pump to ensure that there is a certain flow rate within the sampling tube, not easy to fouling the inner wall of the pipeline. The caliber of the sampling line should be determined based on the total amount of water sampled by the meter. The turbidity meter for measuring the filtered water is installed in the filter station pipe gallery. It is wall-mounted and cabinet-mounted. The measurement of the factory water is performed in the water pump room to set the water quality meter, and the turbidity meter and other water quality detection meters are placed in the instrument room. , and then lead the signal to the monitoring station.
7 The grounding and lightning protection grounding of automatic instrument systems can be divided into protective earthing and working earthing. The protective grounding is to avoid the risk of electric shock and the safety of the protection equipment when workers are damaged due to the insulation of the equipment or the insulation performance is reduced. The working ground is to ensure the stable and reliable operation of the instrument. The TN-S system is used for the grounding of the general water purification plant instrument system, that is, three phase lines A, B, and C, and one neutral line N is the protection line PE. The exposed electrically conductive part of the power equipment is connected to the PE wire. The advantage is that the PE wire does not show current during normal operation. Therefore, the exposed conductive part of the equipment does not show the voltage to ground and it is easy to cut off the power in the event of an accident. Strong electromagnetic compatibility avoids the interference of higher harmonics. The principle of working grounding is single point grounding. Due to the existence of ground potential difference, if there is more than one grounding point, a ground loop will be formed and the interference will be introduced into the instrument. Therefore, the same signal loop and the same shielding layer can only have one grounding point.
The instrument working ground can be set separately or share the same grounding body with the protective ground. From the engineering practice experience, the grounding resistance should generally not exceed 1Ω. The facilities of the water purification plant are scattered, the structure is low, and the terrain is flat and open. In particular, some metering wells are located outside the plant area. In this case, the lightning strike rate of the instrumentation equipment increases. In practice, in April 2009, there was a lightning strike that damaged the equipment in the water purification plant. Therefore, it is an indispensable protection measure to install the arrester with good quality and reliable operation.
8 Summary of Selection and Installation of Automatic Instrumentation and Management Measures (1) To realize the modern management of the water treatment industry, automation instruments must be used. (2) The instrument installer should have professional skills. For the sake of users, when designing and selecting the instrument, it should be: stable and reliable, simple to operate, easy to install, inexpensive, continuous measurement, responsive, and highly interchangeable. Easy to maintain and so on. (3) The water treatment enterprise should have professional and technical personnel to manage it. At ordinary times, it should pay attention to the collection and arrangement of technical data so as to facilitate digestion and absorption. After the instrument is correctly put into use, the technical personnel shall make more visits to the site and do a follow-up survey on the use of the instrument to understand the working condition of the instrument and sum up experience in a timely manner so as to facilitate the improvement of the management work in the future.
Key words: automation; electronic instrumentation; water treatment; process management 1 The basic role of automation instruments in water treatment production: Take a water purification plant in Qinhuangdao as an example: The main process is basically related to the corresponding instruments and automation technology. The electronic instrument continuously detects the process data and manually or automatically controls the data according to these real-time data, so as to coordinate the relationship between the supply and demand, the components of the system, and the respective water treatment processes, so that the water treatment equipment and facilities can be further improved. Full and reasonable use. At the same time, the value measured by the measuring instrument can be continuously compared with the set value. When deviation occurs, the adjustment is carried out immediately to ensure the quality of the water treatment. According to the parameters detected by the instrument, it can further automatically adjust and control the dosage of the medicine, ensure the reasonable operation of the water pump unit, make the management more scientific, and achieve the purpose of economic operation. Because the instrument has the function of continuous detection and over-limit alarm, it is convenient to deal with the accident in time.
2 Classification of Commonly Used Instruments for Municipal Water Treatment Process The instruments used in municipal water treatment processes can be roughly divided into two categories: one is the instruments that monitor the physical parameters of the production process, such as temperature, pressure, liquid level, and flow rate. This kind of instrument uses a part of the domestically produced watch, its performance and quality can basically meet the requirement. The other type is an analytical instrument for detecting water quality, such as detecting water turbidity, pH value, dissolved oxygen content, conductivity, residual chlorine, and SCD value. These special instruments developed relatively late in our country. Therefore, the basic selection of foreign imported E+H products is more economical and reliable in terms of actual results.
The quality of the instrumentation is directly related to the automation of the water plant and even the safety of the water supply. In the water plant instrument design selection, from the instrument performance, quality, price, spare parts, after-sales service and other aspects of repeated comparisons, and finally using imported instruments and domestic instruments combined method.
3 General requirements for the selection of water treatment instruments (1) Accuracy: refers to the accuracy of the instrument's measurement results under normal conditions of use. The smaller the error, the higher the accuracy. The accuracy of the physical detection instrument in the production process is ±1%, the accuracy of the water quality analysis instrument is ±2%, and the accuracy of the turbidity meter for measuring the turbid water is ±5%. (2) Response time: When measuring the measured value, the indicator value of the instrument will be displayed after a certain period of time. This time is the response time of the instrument. A meter can not quickly reflect the changes in the parameters, is an important indicator. The response time for water quality analysis instruments should not exceed 3 minutes. (3) Output signal: The analog output of the instrument should be 4mA ~ 20mADC signal, the load capacity is not less than 600Ω. (4) The degree of protection of the instrument should meet the requirements of the environment in which it is located. It should generally be no less than IP65. The instrument used for the pharmaceutical dosing system is required to be resistant to corrosion. (5) The four-wire instrument power supply is mostly 220VAC and 50Hz, and the two-wire instrument power supply is 24VDc. (6) On-site monitoring instrument should use digital display. (7) The working power of the instrument should be independent, and it should not share the power supply with the computer to ensure that the power supply does not interfere with each other during the failure and maintenance, so that each can operate stably and reliably. (8) In order for the computer to detect the abnormal signal of the voltage transformer and the current transformer and alarm, the input voltage and current transducer input voltage should be designed to be larger than the current and voltage transformers, that is 0~6A respectively. And 0 ~ 120V. (9) It should select instrument manufacturers who can provide reliable services and have rich experience.
4 level gauge selection and precautions Selection type should pay attention to the nature of the measurement object, such as the physical and chemical properties of the measured medium, and the working pressure and temperature, installation conditions, the speed of the liquid level changes; also take into account the measurement And control requirements, such as measurement range, measurement or control accuracy, display method, on-site instructions, remote indication, interface with the computer, safety and corrosion protection, reliability, and construction convenience.
The level gauges commonly used in existing municipal water supply and drainage works are as follows:
(1) Float level gauge: Put a hollow float in the liquid. When the liquid level changes, the float will produce the same displacement as the liquid level change. The displacement of the float ball can be measured mechanically or electrically, and its accuracy is low. This level gauge is not suitable for high-viscosity liquids and the output is switch-controlled. In the design of our company's water purification plant and new wastewater treatment plant, this level switch was mostly used in the sump pit between underground facilities to control the automatic start-stop of the sewage pump.
(2) Static pressure or differential pressure level gauge: Since the static pressure of the liquid column is proportional to the liquid level, the liquid level can be measured by using a pressure gauge to measure the static pressure of the liquid column on the reference surface. Calculate the pressure or pressure difference range according to the density of the measured medium and the measurement range of the liquid, and then select the suitable pressure gauge or differential pressure gauge for range and accuracy. This level gauge has an accuracy of ±0.5% to 2%.
(3) Capacitance type liquid level gauge: When the liquid level changes, the internal medium of the electrode changes, and the capacitance between the electrodes or between the electrode and the container wall also changes. The change in the capacitance is then converted into standardization. DC signal. Its accuracy is ±0.5% to 1.5%.
Capacitance type liquid level meter has the following advantages: The sensor has no mechanically movable part, and the structure is simple and reliable; the accuracy is high; the detection end consumes less power, and the dynamic response is fast; the maintenance is convenient and the service life is long. The disadvantage is that the dielectric constant of the fluid being measured can cause errors. Capacitive level gauges are generally used to adjust the level measurement of tanks, clear water tanks, etc. When the measuring range does not exceed 2m, rod-shaped, plate-shaped, coaxial electrodes are used; when it exceeds 2m, cable-type electrodes are used. When the medium to be measured is water, an electrode with polyethylene available for insulation is used.
(4) Ultrasonic level gauge: The ultrasonic level gauge sensor consists of a pair of transmitter and receiver transducers. The transmitting transducer faces the liquid surface to emit ultrasonic pulses, and the ultrasonic pulses are reflected back from the liquid surface and received by the receiving transducer. According to the time from launch to reception, the distance between the sensor and the liquid level can be determined, which can be converted into liquid level. Its accuracy is ±0.5%. This level gauge has no mechanical movable part, high reliability, simple and convenient installation, is non-contact measurement, and is not affected by the viscosity, density, etc. of the liquid, so it is often used in drug pools, medicine tanks, mud tanks, etc. Level measurement. However, this method has a certain blind area, and the price is more expensive.
5 Flow meter selection and precautions Flow measurement is divided into two types, one for flow detection, and participate in process control, in order to achieve the purpose of improving production automation, improving production process conditions, and improving product quality and output. Another kind of measurement used for flow measurement not only measures the output of products, but also serves as the basis for calculation of major technical and economic indicators of water supply companies. Among the eight major basic economic indicators of water supply companies, four are based on data measured by flow meters.
The following factors should be considered when selecting the type of flowmeter: Any type of flowmeter must have a certificate verified by the national metrology department before it can be used. The pressure loss of the flowmeter itself is small. According to industry requirements, the accuracy of the flowmeter should not be lower than 2.5. The installation site conditions should meet the requirements of the selected flowmeter for straight sections. The selected flow meter should be able to adapt to the installation site environmental conditions such as temperature, humidity, and electromagnetic interference. The selected flow meter should be suitable for the liquid medium to be tested.
At present, in the water supply and drainage engineering design, the electromagnetic flowmeter and the ultrasonic flowmeter are the most used.
(1) Electromagnetic flowmeter: The principle of electromagnetic flowmeter is to apply Faraday’s law of electromagnetic induction, which consists of sensors and transducers. In the measurement, the liquid itself is a conductor, and the magnetic field is generated by two coils installed in the pipeline. The coil is excited by an AC or DC power source, and the magnetic field acts on the liquid flowing in the pipeline to generate a voltage in the pipeline corresponding to the average flow velocity V of the fluid to be measured, and the voltage is independent of the flow velocity distribution of the fluid. The two electrodes insulated from the pipe monitor the induced voltage of the liquid. The direction of the magnetic field, the direction of fluid flow, and the relative positions of the two detection electrodes are perpendicular to each other.
Advantages of electromagnetic flowmeters: The measurement is independent of the temperature, pressure, or viscosity of the liquid being measured. No pressure loss. Continuous measurement, high measurement accuracy. The scope and measurement range of the caliber are large, and the measurement range is continuously adjustable. Independent of the flow velocity distribution. Straight sections are short before and after, front straight section is 5D (D is the diameter of the instrument), and rear straight section is 3D. Good stability, the output is a standardized signal, can easily enter the automatic control system. The inner wall of the transmitter conduit is lining material, with good corrosion resistance and wear resistance. The converter has small size, low power consumption, and strong anti-interference performance, which is convenient for on-site observation. The lining material of the electromagnetic flowmeter used in the water treatment system is mostly made of neoprene, because it has better wear resistance.
Installation should pay attention to the source of the electromagnetic field away from the outside world, so as not to affect the sensor's working magnetic field and flow signal, the sensor is installed horizontally, the center axis of the two electrodes is required to be in the horizontal state, to prevent the deposition of particulate impurities, affecting the electrode work. The measuring tube should be full and not allow a lot of bubbles to pass through the sensor. When the conditions cannot be met, appropriate measures should be taken. In order to make the instrument work reliably and improve the measurement accuracy without interference from external parasitic potentials, the sensor should have a good separate grounding wire, and the grounding resistance should be less than 10Ω, especially when it is installed on the cathodic protection pipeline.
Under the premise of satisfying the installation environment and usage requirements, the distance between the converter and the sensor and the connecting cable should be as short as possible to save the investment and reduce the interference of the strong electric signal that may be generated.
(2) Ultrasonic Flowmeters: In the past decade or so, due to the development of electronic technology, ultrasonic flowmeters have been used for flow measurement. There are many ways to use the ultrasonic flow meter to measure, among which the time difference method and Doppler method are typical. The time-difference flowmeter is often used in water purification plants. The method is to install two transducers on the measuring pipeline. Because of the difference in flow velocity between the forward flow and the reverse flow, the time difference from launching to receiving is measured, and the flow velocity is measured accordingly.
Ultrasonic flowmeter's main advantages: easy installation and maintenance. With the widespread use of clip-on sensors, ultrasonic flowmeters can be mounted and maintained without the need for perforating or cutting off the flow in the pipeline, allowing easy installation in existing applications, especially for large-diameter pipeline inspection systems. . The scope of the caliber is large, and the price is not affected by the pipe diameter. High measurement reliability. No pressure loss. Not affected by the fluid parameters. Output standard DC signal, can easily enter the automatic control system.
Ultrasonic flowmeters should pay special attention to the installation error of the sensor, the fouling of the inner wall of the pipeline and the uniformity of the anticorrosion layer. These factors have great influence on the measurement results. According to the measurement principle of ultrasonic flowmeter, only when the flow velocity is evenly distributed can the accuracy of the measurement be ensured. Therefore, there must be enough straight sections upstream and downstream of the flowmeter. Refer to the various materials and the flowmeter's manual for use. Less than 10D, downstream greater than 5D.
Because the tap water industry is a continuous production, continuous measurement is very important, so the flow meter generally installed on the pipeline can not be dismantled and sent for inspection. The general approach is to use a portable ultrasound flowmeter with high accuracy and send it to the national certification on a periodic basis. The unit is calibrated as a standard equipment of the company, and then the online flow meter is periodically checked for comparison.
(3) Open channel flowmeter: The essence of the existing open channel flowmeter of the company's Eastern District Wastewater Treatment Plant is the use of ultrasonic level gauges in conjunction with Parshall troughs. Parshall trough can turn the flow in the open channel into liquid level. The Parshall trough has the advantages of low water level loss, little precipitation even if there is solid matter in the water, and less influence on the water level on the downstream side, so it is used to measure the flow of other liquids such as agricultural water and industrial water.
Ultrasonic Open Channel Flowmeter Features: It can measure the flow of non-full tubes (circular tubes, egg-shaped tubes or other special-shaped tubes). Measure the flow of channels (circular, rectangular or other irregular channels). Measure natural rivers and streams. Sewage discharge channels or pipeline (sewer) flow can be measured. Positive and negative flow rates and flows can be measured. Instantaneous flow and cumulative flow values ​​are available. Output signal: RS-485, Modbus, 4 ~ 20Ma current signal and multi-way switch. Sensors can work long hours under harsh conditions and sewage water quality. Optional telematics or GPRS wireless module for remote telemetry. The sensor housing is polycarbonate with protection class IP68. Built-in automatic temperature compensation. The blind spot can be adjusted to shield the signal near the probe.
Ultrasonic open channel flowmeter accuracy has two concepts: ultrasonic probe and transmitter processing liquid level accuracy, and liquid level signal, area, time after the integral calculation accuracy, generally 0.2% to 0.5%.
6 Selection of Turbidity Meters and Their Attentions Turbidity is a measure of the degree of turbidity of a water body. A water purification plant is responsible for supplying domestic and industrial water for residents. The quality of water supply directly relates to people’s health and safety, as well as food, brewing, medicine, and textiles. , printing and dyeing, electricity and other industries in the normal production and product quality. Turbidity is a very important water quality indicator, so the choice of turbidimeter is particularly important.
Turbidity meter can be divided into two categories: visual turbidity meter and photoelectric turbidity meter. Optoelectronic turbidity meter can be divided into process monitoring for continuous measurement of turbidity meter and laboratory fixed and portable turbidity meter for its use. The design principle can be divided into transmission turbidity meter and scattered light turbidity meter.
Because the scattered light turbidity meter has high sensitivity to low turbidity of water, high accuracy, small relative error, good repeatability, water color does not indicate turbidity, and the ratio of scattered light to incident light intensity can be linear Relationship, so in September 1992 the World Health Organization published "Guidelines for Drinking Water Quality," the provisions of the scattered light turbidity meter as a measuring instrument.
The E+H scattered light turbidity meter was used in our company's water purification plant process, and the water quality laboratory and the second phase of Longhai Road used the HACH scattered light turbidity meter. The focused beam is projected downward from the sensor head assembly into the water sample in the body of the turbidimeter. The photocells immersed in the water sample measure the scattered light in the 90° direction of the suspended solids in water. The amount of scattered light and the turbidity of the water sample Degree is proportional to.
The selection of turbidity sampling points should be closely combined with the process specialty. The most representative point should be selected. The sampling hole should not be opened on the top of the sampled pipe, avoiding the bubbles in the pipe being drawn into the sampling pipe and affecting the measurement of the turbidity meter. Accuracy, the water sample extraction is best sampled with a small sampling pump to ensure that there is a certain flow rate within the sampling tube, not easy to fouling the inner wall of the pipeline. The caliber of the sampling line should be determined based on the total amount of water sampled by the meter. The turbidity meter for measuring the filtered water is installed in the filter station pipe gallery. It is wall-mounted and cabinet-mounted. The measurement of the factory water is performed in the water pump room to set the water quality meter, and the turbidity meter and other water quality detection meters are placed in the instrument room. , and then lead the signal to the monitoring station.
7 The grounding and lightning protection grounding of automatic instrument systems can be divided into protective earthing and working earthing. The protective grounding is to avoid the risk of electric shock and the safety of the protection equipment when workers are damaged due to the insulation of the equipment or the insulation performance is reduced. The working ground is to ensure the stable and reliable operation of the instrument. The TN-S system is used for the grounding of the general water purification plant instrument system, that is, three phase lines A, B, and C, and one neutral line N is the protection line PE. The exposed electrically conductive part of the power equipment is connected to the PE wire. The advantage is that the PE wire does not show current during normal operation. Therefore, the exposed conductive part of the equipment does not show the voltage to ground and it is easy to cut off the power in the event of an accident. Strong electromagnetic compatibility avoids the interference of higher harmonics. The principle of working grounding is single point grounding. Due to the existence of ground potential difference, if there is more than one grounding point, a ground loop will be formed and the interference will be introduced into the instrument. Therefore, the same signal loop and the same shielding layer can only have one grounding point.
The instrument working ground can be set separately or share the same grounding body with the protective ground. From the engineering practice experience, the grounding resistance should generally not exceed 1Ω. The facilities of the water purification plant are scattered, the structure is low, and the terrain is flat and open. In particular, some metering wells are located outside the plant area. In this case, the lightning strike rate of the instrumentation equipment increases. In practice, in April 2009, there was a lightning strike that damaged the equipment in the water purification plant. Therefore, it is an indispensable protection measure to install the arrester with good quality and reliable operation.
8 Summary of Selection and Installation of Automatic Instrumentation and Management Measures (1) To realize the modern management of the water treatment industry, automation instruments must be used. (2) The instrument installer should have professional skills. For the sake of users, when designing and selecting the instrument, it should be: stable and reliable, simple to operate, easy to install, inexpensive, continuous measurement, responsive, and highly interchangeable. Easy to maintain and so on. (3) The water treatment enterprise should have professional and technical personnel to manage it. At ordinary times, it should pay attention to the collection and arrangement of technical data so as to facilitate digestion and absorption. After the instrument is correctly put into use, the technical personnel shall make more visits to the site and do a follow-up survey on the use of the instrument to understand the working condition of the instrument and sum up experience in a timely manner so as to facilitate the improvement of the management work in the future.
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