Application of automatic meters in waterworks: 3.4 SCD instrument: Â
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Automated online instrumentation is the most direct detection device on the water plant's production site. It is called the “eye†of automation, and its accuracy directly affects the accuracy of automated control. The use of automated online instrumentation in waterworks for monitoring can reduce consumption, ensure water supply safety, ensure the stability of water quality and pressure, and produce good economic and social benefits.
1. The important position of the automation instrument in the water treatment system:
In a modern water purification plant, each production process is always related to the corresponding instrumentation and automation technology. The instrument can continuously detect various process parameters, and perform manual or automatic control according to the data of these parameters, so as to coordinate the relationship between the supply and demand, between the various components of the system, and the respective water treatment processes, so that various 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 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 accidents in time. Instrumentation is still a prerequisite for computer control. Therefore, in advanced water treatment systems, automated instruments have a very important role.
2. Classification of commonly used instruments for water treatment systems:
The instruments used in water supply engineering 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 adopts the domestically produced watch, its performance and the quality basically can satisfy the request. The other type is an analytical instrument for detecting water quality, such as detecting water turbidity, pH value, dissolved oxygen content, residual chlorine, SCD value, and the like. These special instruments have developed relatively late in China. Therefore, they often use foreign advanced products. From a long-term perspective, they are more economical and reliable. The detection of the instrument is directly related to the effect of water supply automation. In the engineering design process, we repeatedly use the combination of imported instrumentation and domestic instrumentation to repeatedly compare instrument performance, quality, price, spare parts, and after-sales service.
3 should pay attention to the problem of application:
3.1 Industrial PH meter
Industrial PH meter is a commonly used instrument and equipment. It is mainly used to accurately measure the pH value of liquid medium. The corresponding ion selection electrode can also measure the MV value of ion electrode. It is widely used in industry, agriculture, scientific research, environmental protection, etc. field. This instrument is also a necessary inspection equipment for QS and HACCP certification of food factories and drinking water plants.
When the measured temperature changes greatly, the continuous flow measurement using the industrial pH meter can automatically perform temperature compensation (normally the temperature is increased by 10° C., the pH decreases by 0.05), but only the influence of the temperature on the measurement part is eliminated. At different temperatures, the pH of the water sample also changes with temperature due to changes in acid-base dissociation constants. In view of the large influence of the water sample temperature on the measurement results of the instrument, when the automatic compensation system fails to achieve the compensation effect, special attention should be paid to the adjustment work in use. It is advisable to adopt constant temperature measures for the acidity meter with large measurement error. Reduce the measurement error. The service life of industrial PH meter probes is generally one year, and should be replaced in time.
3.2 Online turbidity meter:
The online turbidity meter generally measures the turbidity of water by detecting the scattering intensity of suspended particles in the water sample. At present, there are more Haze Turbidimeters in the water supply industry.
The on-line turbidity meter sample chamber should be cleaned regularly, and the flow rate of the sample should be kept constant. Excessive flow will flush the sediment in the sample chamber or produce bubbles that will not reflect the true situation. When using the toolbox calibration method (using two standard turbidity to set the proportional curve for the meter), the inverted standard turbidity fluid with a good standard should be lighter to prevent air bubbles and cause incorrect calibration. Turbidimeter light bulbs are easier to burn and spare parts must be purchased. In addition, seals should be properly handled in the water sample chamber to prevent entry of gecko and insects and cause turbidimeter measurement errors.
3.3 chlorine analyzer:
The chlorine analyzer consists of a monitor, a flow module and a probe. Among them, when the diaphragm of the liquid storage cavity is dirty, it can be reused after dipping with dilute acid, but it cannot be cleaned with cotton to prevent clogging; in general, the liquid cavity should be replaced every 3 months to 6 months. The electrolyte in one time ensures that the residual chlorine meter works properly. When the probe with no liquid reservoir is in use, it is repeatedly calibrated but is still far from the measured value. It can be soaked in 2%~5% acetic acid for two hours and then soaked in clean water for about one hour. Normally it can work normally.
In addition, the time interval between two calibrations is generally more than one hour, so that the residual chlorine instrument has a proper setting time is good; need to pay attention to the sampling pipeline should not use galvanized steel, or the pipeline is easily corroded by lv gas. The lifetime of the probe of the residual chlorine meter is generally two years. Pay attention to the replacement.
Flow current detector also known as SCD instrument. The premise of using the SCD instrument is that the particles in the water sample under test completely cover the surface of the probe piston. Unintended electrical interference can form a particulate “membraneâ€, and the SC value is proportional to the potential §. However, when harmful substances interfere, strong adsorption and passivation, serious deposits in the test chamber, and corrosion of the electrodes, the linear relationship between the flowing current and the colloidal state will be misaligned, resulting in poor results; on the other hand, the total delay time of the system should be suitable. A well-stirred representative measurement sample taken after 2 minutes of dosing often gives the best results.
Therefore, we must pay attention to the regular cleaning of pipelines, probes, calibration of the instrument; and combined with the observation of the effects of dosing personnel on the effects of the injection, carefully fill in the operating table of the SCD instrument, and strengthen management.
3.5 Flowmeter:
Ultrasonic flowmeters and electromagnetic flowmeters are commonly used in waterworks, and ultrasonic flowmeters have better measurement accuracy than electromagnetic flowmeters, but they are expensive.
In the water supply industry, sensors and probes using flowmeters are installed in instrument wells below the ground, often immersed in rainwater that cannot be discharged in time, or even soaked in water for a long time, even if the enclosure protection class is IP67 (dust tight short-term submergence level ) or IP68 (dust continuous water immersed level), often due to the terminal cover seals or cable leads into the sealing ring is not pressed tightly, or the ferrule and cable do not match and damp, resulting in the terminal insulation oxide layer, increase The large resistance value weakens or fluctuates the induced potential, and even directly interferes with the excitation terminal inside the terminal, inducing an abnormal potential, resulting in unstable display values ​​and even more complicated faults. Therefore, it is better to use a reinforced concrete structure in the instrument well, and install a small submersible pump to automatically pump water away to ensure that the sensor and probe are not immersed in water, but also need to pay attention to fastening the wiring point.
3.6 Salinity meter:
The chloride instrument, also known as salinity meter, is an instrument for measuring salinity of seawater. It is widely used in water plants to monitor whether there is salty tide. However, when the chloride concentration is low, the salinity meter is susceptible to other ions and there is a large error. Therefore, the laboratory staff should appropriately use the analytical method (AgNO3 titration method) and compare it with the on-site salinity meter measurement results to find the gap. In addition, a suitable amount of filter sand can be placed in the sampling water-stabilized water tank to purify the sample water and prevent scale or mud deposits on the probe surface.
3.7 Other issues requiring attention:
(1) Designate full-time staff to maintain and manage online instruments. Periodically clean instruments, meter probes and pipes, regularly calibrate various instruments, and replace probes on schedule to ensure the accuracy of meter measurements. In addition, the laboratory staff is required to use analytical methods to analyze the corresponding test items, and to compare with the on-site instrument monitoring results. If the deviation is too large, the instrument should be corrected in time to ensure accuracy.
(2) Different brands of instruments, their parts (such as probes, light bulbs) are generally not interchangeable.