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Author of Application of Automated Instruments in Water Treatment Systems: Yang Zuping, Tianjin Water Supply and Drainage Research and Design Institute With the deepening of my country's reform and opening up policy and the increasing number of foreign loan projects, computer measurement and control management systems have generally entered the field of water purification plant automation. At present, the most commonly used automatic control system in domestic water purification plants is a multi-level distributed computer measurement and control management system composed of industrial computers (IPC) + programmable logic controllers (PLC) + automated instruments. The important position of automated instruments in water treatment systems In modern water purification plants, every production process is always related to corresponding instruments and automatic control technology. The instrument can continuously detect various process parameters and perform manual or automatic control based on the data of these parameters, thereby coordinating the relationship between supply and demand, between various components of the system, and between various water treatment processes, so that various equipment and facilities can be used more fully and rationally. At the same time, since the value measured by the detection instrument can be continuously compared with the set value, when a deviation occurs, adjustments can be made immediately to ensure the quality of water treatment. According to the parameters detected by the instrument, the dosage of chemicals can be further automatically adjusted and controlled to ensure the reasonable operation of the water pump unit, make management more scientific, and achieve the purpose of economic operation. Because the instrument has the functions of continuous detection and limit-limit alarm, it is convenient to handle accidents in a timely manner. Instruments are also a prerequisite for computer control. Therefore, automated instruments play a very important role in advanced water treatment systems. Classification of commonly used instruments in secondary water treatment systems Instruments used in water supply projects can be roughly divided into two categories: One category belongs to instruments that monitor physical parameters of the production process, such as detecting temperature, pressure, liquid level, flow, etc. This type of instrument uses domestically produced instruments, and its performance and quality can basically meet the requirements. The other category belongs to analytical instruments that detect water quality, such as detecting water turbidity, pH value, dissolved oxygen content, residual chlorine, SCD value, etc. These special instruments developed relatively late in our country. Therefore, advanced foreign products are usually used, which is more economical and reliable in the long run. The quality of detection instruments is directly related to the effect of water supply automation. During the engineering design process, repeated comparisons are made from aspects of instrument performance, quality, price, spare parts availability, after-sales service, etc. We generally use a combination of imported instruments and domestic instruments. The composition model and monitoring parameters of the monitoring system of the third water purification plant 1. The composition model of the monitoring system of the water purification plant The monitoring system of the water purification plant is generally composed of a two-level system of the water plant management layer and the on-site monitoring layer, and is monitored according to the principles of centralized management and decentralized control. In the engineering design, the plant-level computer system (i.e., the main station) is located in the central control room of the water plant. The number and location of each on-site monitoring station (i.e., the sub-station) are determined according to the process flow and the location and degree of dispersion of the structures. Generally, the setting of on-site substation of surface water plant is: Water inlet pump room substation, reaction precipitation and chlorine dosing substation, filtration substation, water delivery pump room and power transformation and distribution room substation, sludge treatment substation. The data from each monitoring instrument are sent to the computer system, which can be displayed, controlled, printed, recorded, and alarmed on the industrial computer at the monitoring station. 2. Monitoring parameters of each sub-station a. Monitoring parameters of sub-station in water inlet pump room Water quality parameters: Source water turbidity, pH value, water temperature, dissolved oxygen, etc. Operating parameters: Adjust the water level of the pool, the water level of the suction well, the flow rate of the source water, the power of the pump, the total power of the pumping station, etc. b. Water quality parameters of reaction precipitation, chlorination and dosing substations: Turbidity at the outlet of the sedimentation tank, residual chlorine after filtration, and SCD value. Operating parameters: Water level in the sedimentation tank, flow rate before sedimentation, liquid level in the mixing tank, liquid level in the chemical tank, concentration of the chemical liquid, and mud level in the sedimentation tank. c. Filter substation water quality parameters: Water turbidity and residual chlorine after filtration. Operating parameters: Filter water level, head loss, backwash water flow, flushing tank water level. d. Water quality parameters in the water supply pump room and sub-station in the transformer and distribution room: Factory water flow and residual chlorine. Operating parameters: Factory water pressure, flow rate, water level in clear water tank, water level in suction well, AC voltage, AC current, electricity, etc. e. Operating parameters of sludge treatment substation: Backflow tank water level, water volume, concentration tank water level, return water turbidity. Four issues that should be paid attention to in the selection and design of commonly used instruments in water treatment systems 1. General requirements for instrument selection (1) Accuracy: It refers to the accuracy of the instrument measurement results under normal use conditions. The smaller the error, the higher the accuracy. The accuracy of the physical detection instruments in the production process is ±1%, and the accuracy of the water quality analysis instruments is ±2% (the accuracy of the turbidity meter for measuring high turbid water is ±5%). (2)Response time: When measuring the measured value, it will always take a period of time for the instrument indication value to be displayed. This period of time is the response time of the instrument. Whether an instrument can respond to parameter changes as quickly as possible is a very important indicator. The response time required for water quality analysis instruments should not exceed 3 minutes. (3)Output signal: The analog output of the instrument should be a 4~20mA DC signal, with a load capacity of not less than 600Ω. (4) The protection level of the instrument should meet the requirements of the environment where it is located, and generally should not be lower than IP65. The detection instrument used in the chemical dosing system must be corrosion-resistant. (5) The power supply of four-wire instruments is mostly 220V AC, 50Hz, and the power supply of two-wire instruments is 24V DC. (6) On-site monitoring instruments should use digital display instruments. (7) The working power supply of the instrument should be independent and should not share the power supply with the computer to ensure that the power supplies do not interfere with each other during failure and maintenance, so that each can operate stably and reliably. (8) In order for the computer to detect abnormal signals of voltage transformers and current transformers and alarm, the input signals of the optional voltage and current transmitters should be larger than those of the current and voltage transformers, that is, 0~6A and 0~120V respectively. (9) Instrument manufacturers who can provide reliable services and have rich experience should be selected. 2. The following factors should be considered when selecting a level meter for water level measurement:: (1) Measurement objects, such as the physical and chemical properties of the measured medium, as well as working pressure and temperature, installation conditions, speed of liquid level change, etc. ； (2) Measurement and control requirements, such as measurement range, measurement (or control) accuracy, display mode, on-site instructions, remote instructions, interface with computer, safety and anti-corrosion, reliability and construction convenience. The liquid level gauges commonly used in water supply projects and their selection points are as follows:: a. The float-type liquid level meter puts a hollow float into the liquid. When the liquid level changes, the float will produce the same displacement as the liquid level changes. The displacement of the float can be measured mechanically or electrically, with an accuracy of ±(1~2)%. This type of liquid level meter is not suitable for high-viscosity liquids. Its output end has switch control and continuous output. In the design of water purification plants, this kind of liquid level gauge is often used for liquid level measurement in water collection wells to control the automatic start and stop of drainage pumps. b. Static pressure (or differential pressure) type liquid 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 differential pressure range based on the density of the medium being measured and the liquid measurement range, and then select a pressure gauge or differential pressure gauge with appropriate range, accuracy and other properties. The accuracy of this liquid level gauge is ±(0.5~2)%. c. The capacitive liquid level meter inserts an electrode into the container. When the liquid level changes, the medium inside the electrode changes, and the capacitance between the electrodes (or between the electrode and the container wall) also changes. The change in capacitance is then converted into a standardized DC signal. Its accuracy is ±(0.5~1.5)%. Capacitive level gauge has the following advantages: The sensor has no mechanical movable parts and has a simple and reliable structure. ； High accuracy ； The detection end consumes little power and has fast dynamic response. ； Easy maintenance and long life. The disadvantage is that the unstable dielectric constant of the liquid being measured will cause errors. Capacitive liquid level gauges are generally used for liquid level measurement in regulating pools, clean water pools, etc. When the measurement range does not exceed 2m, rod-shaped, plate-shaped, and coaxial electrodes are used ； When it exceeds 2m, cable electrodes are used. When the measured medium is water, an electrode with an insulating layer (polyethylene can be used) is used. d. Ultrasonic liquid level meter The sensor of the ultrasonic liquid level meter consists of a pair of transmitting and receiving transducers. The transmitting transducer emits ultrasonic pulses facing the liquid surface, and the ultrasonic pulses are reflected back from the liquid surface and are received by the receiving transducer. The distance between the sensor and the liquid level can be determined based on the time from transmitting to receiving, which can be converted into liquid level. Its accuracy is ±0.5%. This kind of liquid level meter has no mechanical movable parts, is highly reliable, is simple and convenient to install, is a non-contact measurement, and is not affected by the viscosity and density of the liquid, so it is mostly used for liquid level measurement in medicine pools, medicine tanks, mud drainage pools, etc. However, this method has certain blind spots and is more expensive. 3. Flow measurement There are two types of flow measurement. One is used for flow detection and participates in process control to achieve the purpose of improving the level of production automation, improving production process conditions, and improving product quality and output. Another type of measurement used for flow rate not only measures the output of products, but is also the basis for calculating the main technical and economic indicators of water supply enterprises. Among the eight most important economic indicators of water supply enterprises, three indicators are based on data measured by flow meters. The following factors should be considered when selecting a flow meter:: (1) Any type of flow meter must have * * The certificate issued by the metrology department can be used. (2) The pressure loss of the flow meter itself should be small. (3) According to industry requirements, the accuracy of the flow meter should be no less than level 2.5. (4) The installation site conditions should meet the requirements of the selected flow meter for the straight pipe section. (5) The selected flow meter should be able to adapt to the environmental conditions of the installation site such as temperature, humidity, electromagnetic interference, etc. (6) The selected flowmeter should be suitable for the liquid medium to be measured. At present, electromagnetic flowmeters and ultrasonic flowmeters are most commonly used in water supply engineering design. a. Electromagnetic flowmeter The principle of electromagnetic flowmeter is to apply Faraday’s law of electromagnetic induction and consists of a sensor and a converter. In the measurement, the liquid itself is a conductor and the magnetic field is generated by two coils installed in the pipe. The coil is excited by an AC or DC power supply, and the magnetic field acts on the liquid flowing in the pipe, generating a voltage in the pipe corresponding to the average flow velocity V of the fluid being measured, and this voltage has nothing to do with the flow velocity distribution of the fluid. Two electrodes insulated from the pipe monitor the induced voltage in 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 flowmeter: (1) The measurement is not affected by the temperature, pressure or viscosity of the liquid being measured. (2) No pressure loss. (3) It can measure continuously and has high measurement accuracy. (4) The caliber range and measurement range are large, and the measurement range is continuously adjustable. (5) It has nothing to do with flow velocity distribution. (6) The front and rear straight pipe sections are short, the front straight pipe section is 5D (D is the diameter of the instrument), and the rear straight pipe section is 3D. (7) Good stability, the output is a standardized signal, and it can be easily entered into the automatic control system. (8) The inner wall of the transmitter conduit is lined with material, which has good corrosion resistance and wear resistance. (9) The converter has small size, low power consumption, strong anti-interference performance, and is convenient for on-site observation. The lining material of electromagnetic flowmeters used in water treatment systems is mostly neoprene because of its good wear resistance. When installing, attention should be paid to keeping away from external electromagnetic field sources to avoid affecting the working magnetic field and flow signal of the sensor. When the sensor is installed horizontally, the central axes of the two electrodes are required to be in a horizontal state to prevent the deposition of particle impurities and affect the work of the electrodes. The measuring tube should be full and a large number of bubbles should not be allowed to pass through the sensor. When the conditions cannot be met, corresponding measures should be taken. In order to make the instrument work reliably, improve measurement accuracy, and not be interfered by external parasitic potentials, the sensor should have a good independent grounding wire, and the grounding resistance should be less than 10Ω, especially when installed on a cathodic protection pipeline. For example, the electromagnetic flowmeter installed on the factory main pipe of Tianjin Water Source Plant uses cathodic protection, and the inner and outer walls of the pipe to protect against electrolytic corrosion are insulated. The measured medium has no ground potential. Therefore, the sensor grounding ring is installed on both ends of the sensor and is insulated from the flange of the connecting pipe. The sensor is connected to the ground ring with a ground wire and leads to the ground electrode. The pipe flanges are connected by cables to each other but not to the sensor. Flange connection bolts are isolated with insulating bushings and washers. Since this electromagnetic flowmeter was put into production, the effect has been good. The converter should be installed in a place that meets the requirements of its protection level. Under the premise of meeting 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 investment and reduce the interference of possible strong electric signals. b. Ultrasonic flowmeter In the past ten years, due to the development of electronic technology, ultrasonic flowmeter has been applied to flow measurement. There are many measurement methods using ultrasonic flowmeters, among which the more typical ones are the transit-time method and the Doppler method. Water purification plants often use transit time flowmeters. The method is to install two transducers on the measuring pipe. Due to the influence of the difference in flow velocity between downstream and countercurrent, the time difference from transmitting to receiving is measured, and the flow rate is measured accordingly. Main advantages of ultrasonic flow meters: (1) Easy to install and maintain. With the widespread use of clamp-on sensors, ultrasonic flowmeters can be easily installed in existing applications without drilling holes in the pipeline or cutting off the flow during installation and maintenance. It is especially suitable for large-diameter pipeline detection systems. (2) The caliber range is large, and the price is not affected by the pipe diameter. (3) High measurement reliability. (4) No pressure loss. (5) Not affected by fluid parameters. (6) Output standardized DC signal, which can easily enter the automatic control system. When choosing an ultrasonic flowmeter, special attention should be paid to the installation error of the sensor, scaling on the inner wall of the pipeline, and the uniformity of the anti-corrosion layer. These factors have a great impact on the measurement results. In addition, according to the measurement principle of the ultrasonic flowmeter, measurement accuracy can only be ensured when the flow velocity is uniformly distributed. Therefore, there must be sufficient straight pipe sections upstream and downstream of the flowmeter. Refer to various information and the flowmeter manual, it is required that the upstream is at least not less than 10D and the downstream is greater than 5D. Since the tap water industry is in continuous production, uninterrupted measurement is extremely important. Therefore, flow meters generally installed on pipelines cannot be frequently disassembled and sent for inspection. The general practice is to use portable ultrasonic flow meters with higher accuracy and send them periodically. * * The certification unit performs calibration and uses it as the enterprise's standard instrument, and then regularly tests the online flow meter through comparison. This requires designers to reserve space for comparison measurements based on the requirements of the user unit and the needs of future production management during design to facilitate users. That is, the flow meter well should be made slightly larger. In addition to installing a fixed flow meter, a space for portable flow meter measurement should also be reserved as shown in Figure 1. Figure 1 Schematic diagram of the reserved measurement space of the portable flow meter 4. Measurement of turbidity Turbidity is a measure of the turbidity of the water body, that is, the presence of finely dispersed suspended particles in the water body, which reduces the water transparency. A turbidity meter is an instrument that measures the turbidity of water and is mainly used for monitoring and managing water quality. Water purification plants are responsible for supplying domestic and industrial water to residents. The quality of water supply directly affects people's health and safety, as well as the normal production and product quality of various industries such as food, brewing, medicine, textiles, printing and dyeing, and electric power. Turbidity is a very important water quality indicator, so the choice of turbidity meter is particularly important. Turbidimeter can be divided into two categories: visual turbidity meter and photoelectric turbidity meter. Photoelectric turbidimeter can be divided into process monitoring (continuous measurement) turbidity meter and laboratory (including portable) turbidity meter according to its purpose. According to its design principle, it can be divided into transmitted light turbidity meter and scattered light turbidity meter. Since the scattered light turbidity meter has high sensitivity to low turbidity of water, high accuracy, small relative error, and good repeatability, the color of water does not show turbidity, and the intensity ratio of scattered light to incident light can show a linear relationship, the "Guidelines for Drinking Water Quality" published by the World Health Organization in September 1992 stipulated that the scattered light turbidity meter should be used as a measuring instrument. At the same time, the "Technological Progress Development Plan for the Water Supply Industry in 2000" has clearly stipulated that the water turbidity index of the first-class water pipe network is 1NTU. HACH Company's 1720D and SS6 series turbidity meters (scattered light turbidity meters) are commonly used in the design of water purification plants. In the measurement of filtered water and factory water, the 1720D (originally 1720C) series turbidity meter is generally used. When in use, the water sample continuously flows into the turbidity meter, flows through the deaerator to evacuate the bubbles in the water flow, then enters the middle column of the turbidity meter, rises to the measurement chamber and overflows its edge into the discharge port. The condensed beam is projected downward from the sensor head assembly into the water sample in the main body of the turbidity meter. The photocell immersed in the water sample measures the scattered light in the 90° direction of the suspended solids in the water. The amount of scattered light is proportional to the turbidity of the water sample. 1720D does not require a sample cell, which can reduce stray light and improve measurement accuracy. The accuracy of the 1720D is: It is ±2% in the range of 0~40NTU, ±5% in the range of 40~100NTU, the resolution is 0.001NTU, and the response time is 75s. Turbidimeters for measuring filtered water are mostly installed in the pipe gallery of the filter station, and can be wall-mounted or cabinet-mounted. For the measurement of factory water, a water quality instrument room is generally set up in the water delivery pump room. The turbidity meter and other water quality testing instruments are placed in the instrument room, and then the signal is directed to the monitoring station. Although the measurement range of 1720D is 0~100NTU, it is best not to use it to measure pre-filtered water, because although 100NTU can be measured optically, it will cause a lot of inconvenience in production and use. The SS6 series surface scattering turbidity meter is often used to measure source water and pre-filtered water. It emits a beam on the liquid surface and measures the scattered light from the liquid surface. It avoids direct contact between the optical system and the water sample and eliminates signal loss caused by cleaning the flow cell, as shown in Figure 2. Figure 2 Measurement principle diagram of SS6 surface scattering turbidity meter. The measurement range of SS6 series is 0~9999NTU. Generally, the source water of surface water plants is within this range. Its accuracy is ±5% in the range of 0~2000NTU, and ±10% in the range of 2000~9999NTU. The selection of the turbidity meter sampling point should be closely combined with the technical expertise, and the most representative point should be selected. It is best not to open the sampling hole at the top of the sampled pipe to avoid pumping air bubbles in the pipe into the sampling pipe and affecting the measurement accuracy of the turbidity meter. It is best to use a small sampling pump to extract water samples to ensure a certain flow rate in the sampling pipe and to prevent scaling on the inner wall of the pipe. The diameter of the sampling pipe should be determined based on the total demand for water sampled by the instrument. 5. Selection of display instruments. Generally, intelligent display instruments are used in water purification plant projects. They have complete functions and can perform digital signal processing and realize control functions. The measured values are displayed in liquid crystal, which is easy to operate, can save data, and has self-diagnosis function. Although after being networked with the computer system, its advantages were not fully realized and were replaced by the computer system. However, in the current construction of water purification plants, intelligent display instruments are used as auxiliary instruments when the computer system is not debugged and put into operation or when a failure occurs, which can also meet the requirements of on-site control and display. In some cases, local display and remote transmission are required at the same time. In this case, it is not advisable to adopt the signal series connection method. Instead, a signal distributor should be used, that is, one input and two outputs. One output is sent to the display instrument, and the other output can be input to the PLC, such as the commonly used WS15242, as shown in Figure 3. Figure 3 Wiring diagram of signal distributor 6. The grounding and lightning protection grounding of the instrument system can be divided into protective grounding and working grounding. Protective grounding is to prevent workers from suffering electric shock due to equipment insulation damage or degradation of insulation performance and to protect the safety of equipment. The working grounding is to ensure the stable and reliable operation of the instrument. Generally, the grounding of the instrument system of water purification plants adopts the TN-S system, that is, three phase wires A, B, and C, and one neutral wire N, which is the protective wire PE. The exposed conductive part of the electrical equipment is connected to the PE line. The advantage is that the PE line does not show current during normal operation. Therefore, the exposed conductive part of the equipment does not show voltage to ground and can easily cut off the power supply in the event of an accident. It has strong electromagnetic adaptability and avoids interference from high-order harmonics. The principle of working grounding is single point grounding. Due to the existence of the ground potential difference, if more than one grounding point appears, a ground loop will be formed and interference will be introduced into the instrument. Therefore, the same signal loop and the same shielding layer can only have one grounding point. The working grounding of the instrument can be set separately or share the same grounding body with the protective grounding. Judging from engineering practice experience, the grounding resistance should generally not exceed 1Ω. Generally, water purification plants have scattered facilities, low structures, and flat and open terrain. In particular, some flow meter wells are located outside the plant area. In this case, the lightning strike rate of the instrument equipment increases. In practice, the author has repeatedly encountered incidents where instruments were damaged by lightning strikes or were damaged for unknown reasons. Therefore, installing a lightning arrester with good quality and reliable action is an indispensable protective measure. For example, the ESP series lightning arrester from the German Pepperl+Fuchs company is used to protect the signal and power supply of the flow meter, and the effect is good. Five Summary (1) To realize the modern management of water purification plants, automated instruments must be used. (2) Designers should stand from the user's perspective and consider the user. When designing and selecting instruments, they should: Stable and reliable, simple to operate, easy to install, high quality and low price, continuous measurement, sensitive response, strong interchangeability and easy maintenance. (3) Designers should usually pay attention to the collection and organization of technical data to facilitate digestion and absorption. (4) After the water purification plant is completed and the instruments are put into use correctly, designers should go to the site frequently to conduct follow-up investigations on the use of the instruments, understand the working conditions of the instruments, and sum up experience in a timely manner to facilitate the improvement of future design work.