Application of Automation Instruments in Water Treatment Systems

With the deepening of China'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 widely 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) + automatic instruments.

The important position of automatic instruments in water treatment systems

In modern water purification plants, every production process is always related to corresponding instruments and automatic control technologies. The instrument can continuously detect various process parameters and perform manual or automatic control based on the data of these parameters, so as to coordinate the relationship between supply and demand, between various components of the system, and between various water treatment processes, so as to make various equipment and facilities more fully and reasonably used. At the same time, since the values ​​measured by the detection instrument can be continuously compared with the set values, when deviations occur, adjustments are made immediately to ensure the quality of water treatment. According to the parameters detected by the instrument, the dosage of the reagent 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. Since the instrument has the functions of continuous detection and over-limit alarm, it is convenient to deal with accidents in a timely manner. The instrument is also a prerequisite for realizing computer control. Therefore, in advanced water treatment systems, automation instruments play a very important role.

2. Classification of commonly used instruments in water treatment systems

The instruments used in water supply projects can be roughly divided into two categories: one is the instrument for monitoring physical parameters of the production process, such as temperature, pressure, liquid level, flow, etc. This type of instrument uses domestic instruments, and its performance and quality can basically meet the requirements. The other is analytical instruments for detecting water quality, such as turbidity, pH value, dissolved oxygen content, residual chlorine, SCD value, etc. These special instruments were developed relatively late in China. Therefore, advanced foreign products are usually selected, which is more economical and reliable from a long-term perspective.

The quality of the detection instrument is directly related to the effect of water supply automation. In the process of engineering design, we repeatedly compare the performance, quality, price, spare parts, after-sales service, etc. of the instrument. We generally use a combination of imported instruments and domestic instruments.

3. Composition mode and monitoring parameters of the monitoring system of the water treatment plant

1. Composition mode of the monitoring system of the water treatment plant

The monitoring system of the water treatment plant is generally composed of two-level systems: the water plant management layer and the on-site monitoring layer, and is monitored according to the principle of centralized management and decentralized control. In the engineering design, the plant-level computer system (i.e., the main station) is set up in the central control room of the water plant, and the number and location of each on-site monitoring station (i.e., substation) are determined according to the process flow and the location and dispersion of the structure. The general setting of the on-site substation of the surface water plant is: water inlet pump room substation, reaction sedimentation and chlorination and dosing substation, filtration substation, water delivery pump room and substation, sludge treatment substation. The data of each monitoring instrument are sent to the computer system, which can be displayed, controlled, printed, recorded, and alarmed on the industrial computer of the monitoring station.

2. Monitoring parameters of each substation

a. Monitoring parameters of water inlet pump room substation

Water quality parameters: source water turbidity, pH value, water temperature, dissolved oxygen, etc.

Operation parameters: regulating tank water level, water level of water well, source water flow, pump power distribution, total power of pump station, etc.

b. Water quality parameters of reaction sedimentation and chlorination and dosing substation

: sedimentation tank outlet turbidity, residual chlorine after filtration, SCD value.

Operation parameters: sedimentation tank water level, flow before sedimentation, mixing tank level, medicine tank level, medicine liquid concentration, sedimentation tank mud level.

c. Filtration substation

water quality parameters: filtration water turbidity, residual chlorine.

Operation parameters: filter tank water level, head loss, backwash water flow, flushing water tank water level.

d. Water quality parameters of water supply pump room and transformer substation

: outgoing water flow, residual chlorine.

Operation parameters: outgoing water pressure, flow, clear water tank water level, suction well water level, AC voltage, AC current, power, etc.

e. Sludge treatment substation

operation parameters: return tank water level, water volume, concentration tank water level, return water turbidity.

Four issues that should be noted in the selection and design of commonly used instruments in water treatment systems

1. General requirements for instrument selection

(1) Accuracy: refers to the accuracy of the instrument measurement results under normal use conditions. The smaller the error, the higher the accuracy.

The accuracy of physical detection instruments in the production process is ±1%, and the accuracy of water quality analysis instruments is ±2% (the accuracy of turbidity meters for measuring highly turbid water is ±5%).

(2) Response time: When measuring the measured value, the instrument indication value always takes a period of time 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, and the load capacity should not be less than 600Ω.

(4) The protection level of the instrument should meet the requirements of the environment, generally not less than IP65, and the detection instrument used in the reagent dosing system is required to 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) Digital displays should be used for on-site monitoring 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 in the event of a fault or maintenance, so that each can operate stably and reliably.

(8) In order for the computer to detect abnormal signals of the voltage transformer and current transformer and alarm, the input signal of the voltage and current transmitter should be larger than that of the current and voltage transformer, that is, 0~6A and 0~120V respectively.

(9) Instrument manufacturers that can provide reliable services and have rich experience should be selected.

2. Water level measurement

The following factors should be considered when selecting a level meter: (1) Measurement object, such as the physical and chemical properties of the measured medium, as well as the working pressure and temperature, installation conditions, and the speed of liquid level change; (2) Measurement and control requirements, such as measurement range, measurement (or control) accuracy, display mode, on-site indication, remote indication, interface with the computer, safety and corrosion resistance, reliability, and construction convenience.

The commonly used level gauges and selection points in water supply projects are as follows:

a. Float-type level gauges

place 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 can be measured by mechanical or electrical methods, with an accuracy of ±(1~2)%. This level gauge is not suitable for high-viscosity liquids, and its output end has switch control and continuous output.

In the design of water treatment plants, this type of 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) level gauges

Since the static pressure of the liquid column is proportional to the liquid level, the liquid level can be measured by measuring the static pressure of the liquid column on the reference plane using a pressure gauge. The pressure or pressure difference range is calculated based on the density of the measured medium and the liquid measurement range, and then a pressure gauge or differential pressure gauge with appropriate range, accuracy and other performance is selected. The accuracy of this level gauge is ±(0.5~2)%. [page]

c. Capacitive level gauges

insert electrodes into containers. When the liquid level changes, the medium inside the electrodes changes, and the capacitance between the electrodes (or between the electrodes 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 gauges have the following advantages: the sensor has no mechanical moving parts, the structure is simple and reliable; the accuracy is high; the detection end consumes less power and has a fast dynamic response; it is easy to maintain and has a long service life. The disadvantage is that the dielectric constant of the measured liquid is unstable, which will cause errors. Capacitive level gauges are generally used for level measurement in regulating tanks, clear water tanks, etc.

When the measuring 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, electrodes with an insulating layer (polyethylene can be used) are used.

d. Ultrasonic level gauge

The sensor of the ultrasonic level gauge 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 received by the receiving transducer. The distance between the sensor and the liquid surface can be determined based on the time from transmission to reception, which can be converted into the liquid level. Its accuracy is ±0.5%.

This type of liquid level meter has no mechanical moving parts, high reliability, simple and convenient installation, non-contact measurement, and is not affected by the viscosity and density of the liquid. Therefore, it is mostly used for liquid level measurement in medicine pools, medicine tanks, sludge pools, etc. However, this method has certain blind spots and is relatively 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. The other type is used for flow measurement, which not only measures the output of products, but also serves as 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 selection of flow meters should consider the following factors:
(1) Any type of flow meter must have a certificate issued by the national metrology department before it 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 not be lower than level 2.5.
(4) The installation site conditions should meet the requirements of the selected flow meter for straight pipe sections.
(5) The selected flowmeter should be able to adapt to the environmental conditions at the installation site, such as temperature, humidity, and electromagnetic interference.
(6) The selected flowmeter should be suitable for the liquid medium to be measured.

At present, electromagnetic flowmeters and ultrasonic flowmeters are the 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. It 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 pipeline. The coil is excited by an AC or DC power supply, and the magnetic field acts on the liquid flowing in the pipeline, generating a voltage in the pipeline corresponding to the average flow velocity V of the measured fluid, and this voltage is independent of the flow velocity distribution of the fluid.

Two electrodes insulated from the pipeline monitor the induced voltage of the liquid. The direction of the magnetic field, the flow direction of the fluid, and the relative position 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 measured liquid.
(2) There is no pressure loss.
(3) It can measure continuously with high measurement accuracy.
(4) The caliber range and measurement range are large, and the measurement range is continuously adjustable.
(5) It is independent of the flow velocity distribution.
(6) The front and rear straight pipe sections are relatively 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 easily enter the automatic control system.
(8) The inner wall of the transmitter tube is lined with material, which has good corrosion resistance and wear resistance.
(9) The converter is small in size, consumes little power, has strong anti-interference performance, and is convenient for on-site observation.

The lining material of the electromagnetic flowmeter used in the water treatment system is mostly chloroprene rubber because of its good wear resistance. When installing, you should pay attention to stay away from the external electromagnetic field source to avoid affecting the working magnetic field and flow signal of the sensor. When the sensor is installed horizontally, the central axis of the two electrodes is required to be in a horizontal state to prevent the deposition of particulate impurities and affect the operation of the electrodes. The measuring tube should be full, and a large number of bubbles are not 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 the measurement accuracy, and not be interfered by external parasitic potential, the sensor should have a good separate 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 main pipeline of Tianjin Water Source Plant, because the pipeline adopts cathodic protection, the inner and outer walls of the pipeline that protects against electrolytic corrosion are insulated, and the measured medium has no ground potential, so the sensor grounding ring is installed on the two end faces of the sensor, and is insulated from the flange connected to the pipeline. The sensor is connected to the grounding ring with a grounding wire and leads to the grounding electrode. The pipeline flanges are connected with cables but not connected to the sensor. The flange connection bolts are isolated with insulating bushings and washers. The electromagnetic flowmeter has been effective since it was put into production. The

converter should be installed in a place that meets the requirements of its protection level. On the premise of meeting the installation environment and use 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 strong electric signals that may be generated.

b. Ultrasonic flowmeter

In the past decade, due to the development of electronic technology, ultrasonic flowmeters have been used in flow measurement. There are many methods for measuring using ultrasonic flowmeters, among which the more typical ones are the time difference method and the Doppler method. Water treatment plants often use time difference flowmeters, which are based on the installation of two transducers on the measuring pipe. Due to the difference in flow velocity between the upstream and downstream, the time difference from transmission to reception is measured, and the flow velocity is measured accordingly.

The main advantages of ultrasonic flowmeters are:

(1) Easy installation and maintenance. With the widespread use of clamp-on sensors, ultrasonic flowmeters do not need to be drilled or cut off when installing and maintaining them. They can be easily installed in existing applications, and are particularly suitable for large-diameter pipeline inspection 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) Outputs standardized DC signals that can be easily entered into the automatic control system.

When selecting ultrasonic flowmeters, special attention should be paid to the installation error of the sensor, scaling on the inner wall of the pipeline, and whether the anti-corrosion layer is uniform. These factors have a great impact on the measurement results. According to the measurement principle of ultrasonic flowmeter, the accuracy of measurement can only be guaranteed when the flow velocity is evenly distributed. Therefore, there should be enough straight pipe sections upstream and downstream of the flowmeter. According to various materials and the manual of the flowmeter, the upstream should be at least 10D and the downstream should be greater than 5D.

Since the tap water industry is a continuous production, it is extremely important to carry out uninterrupted measurement. Therefore, the flowmeter installed on the pipeline cannot be disassembled and inspected frequently. The general practice is to use a portable ultrasonic flowmeter with higher accuracy, send it to the national certification unit for calibration on a periodic basis, and use it as the standard instrument of the enterprise to regularly detect the online flowmeter by comparison. This requires designers to reserve space for comparison measurement according to the requirements of the user unit and the needs of future production management during design to facilitate users, that is, to make the flowmeter well slightly larger. In addition to installing a fixed flowmeter, space for portable flowmeter measurement should be reserved as shown in Figure 1.

Figure 1 Schematic diagram of reserved measurement space for portable flow meter [page]

4. Turbidity measurement

Turbidity is a measure of the turbidity of water, that is, the degree to which the water transparency is reduced due to the presence of finely dispersed suspended particles in the water. Turbidimeter is an instrument for measuring the turbidity of water, and is mainly used for monitoring and management of water quality.

Water purification plants are responsible for supplying domestic and industrial water to residents. The quality of water supply is directly related to 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 electricity. Turbidity is a very important water quality indicator, so the selection of turbidity meters is particularly important. Turbidimeters can be divided into two categories: visual turbidity meters and photoelectric turbidity meters. Photoelectric turbidity meters can be divided into process monitoring (continuous measurement) turbidity meters and laboratory (including portable) turbidity meters according to their uses, and can be divided into transmitted light turbidity meters and scattered light turbidity meters according to their design principles.

Since the scattered light turbidity meter has high sensitivity to low turbidity of water, high accuracy, small relative error, good repeatability, the color of water does not show turbidity, and the ratio of scattered light to incident light intensity can be linear, the "Guidelines for Drinking Water Quality" published by the World Health Organization in September 1992 stipulates that the scattered light turbidity meter should be used as a measuring instrument. At the same time, the "Technical Progress and Development Plan for the Water Supply Industry in 2000" has clearly stipulated that the turbidity index of the first-class water company's pipe network water is 1NTU.

HACH's 1720D and SS6 series turbidity meters (scattered light turbidity meters) are often used in the design of water treatment plants.

In the measurement of filtered water and factory water, the 1720D (formerly 1720C) series turbidity meter is generally used. When in use, the water sample flows continuously into the turbidity meter, flows through the deaerator to empty the bubbles in the water flow, and then enters the middle column of the turbidity meter, rises to the measurement chamber and overflows its edge into the discharge port. The focused light beam is projected downward from the sensor head assembly into the water sample in the main body of the turbidity meter. The photoelectric tube 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. The 1720D does not require a sample pool, which can reduce stray light and improve measurement accuracy. The accuracy of the 1720D 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.

The turbidity meter for measuring filtered water is mostly installed in the pipe gallery of the filter station. It can be wall-mounted or cabinet-mounted. The measurement of factory water is generally set up in the water quality instrument room in the water delivery pump room. The turbidity meter and other water quality detection instruments are placed in the instrument room, and then the signal is led to the monitoring station.

Although the measurement range of the 1720D is 0~100NTU, it is best not to use it to measure the water before filtration, because although it can measure 100NTU optically, it will bring many inconveniences in production use. The SS6 series surface scattering turbidity meter is often used to measure source water and pre-filter water. It projects a light beam onto the liquid surface and measures the scattered light from the liquid surface, avoiding direct contact between the optical system and the water sample and eliminating signal loss caused by cleaning the circulation pool, as shown in Figure 2.

Figure 2 SS6 surface scattering turbidity meter measurement principle diagram

The measurement range of the SS6 series is 0~9999NTU, and the source water of general surface water plants is within this range. Its accuracy in the range of 0~2000NTU is ±5%, and the accuracy in the range of 2000~9999NTU is ±10%.

The selection of sampling points for turbidity meters should be closely combined with the process specialty, and the most representative points should be selected. It is best not to open the sampling hole at the top of the sampled pipe to avoid drawing 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 that there is a certain flow rate in the sampling pipe and it is not easy to scale on the inner wall of the pipe. The caliber of the sampling pipe should be determined according to the total amount of water sampled by the instrument.

5. Selection of display instruments

Generally, intelligent display instruments are used in water purification plant projects. They are fully functional, can perform digital signal processing, realize control functions, and the measured values ​​are displayed on LCDs. They are easy to operate, can save data, and have self-diagnosis functions. Although its advantages have not been fully utilized after being connected to the computer system and it has been replaced by the computer system, in the current construction of water purification plants, the use of intelligent display instruments as auxiliary instruments in the uncommissioned and commissioned stages of the computer system or when a failure occurs 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. At this time, it is not appropriate to adopt the signal series connection method, but a signal distributor should be used, that is, 1 input and 2 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 Signal distributor wiring diagram

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 the staff from suffering from electric shock danger and protect the safety of the equipment when the insulation of the equipment is damaged or the insulation performance is reduced. Working grounding is to ensure the stable and reliable operation of the instrument. Generally, the grounding of the instrument system of the water purification plant adopts the TN-S system, that is, 3 phase lines A, B, C, and 1 neutral line N, which is the protective line PE. The exposed conductive part of the electrical equipment is connected to the PE line. The advantage is that the PE line does not present current during normal operation, so the exposed conductive part of the equipment does not present a voltage to the ground and it is easy to 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 potential difference to the ground, if there is more than one grounding point, a ground loop will be formed, which will introduce interference into the instrument. Therefore, there can only be one grounding point for the same signal loop and the same shielding layer.

The instrument working grounding can be set separately or share the same grounding body with the protective grounding. From the perspective of engineering practice experience, the grounding resistance should generally not exceed 1Ω.

Generally, water treatment plants have scattered facilities, low structures, flat and open terrain, and some flow meter wells are located outside the plant area. In this case, the lightning strike rate of instrument equipment increases. In practice, the author has encountered many incidents of lightning damage to instruments or unexplained damage to instruments. Therefore, lightning arresters with good installation quality and reliable operation are indispensable protection measures. For example, the ESP series lightning arrester from Pepperl+Fuchs of Germany is used to protect the signal and power supply of the flow meter, which has a good effect.

Conclusion

(1) To realize the modern management of water treatment plants, automatic instruments must be used.
(2) Designers should stand in the user's perspective and consider the user. When designing and selecting instruments, they should achieve the following: stable and reliable, simple operation, easy installation, good quality and low price, continuous measurement, sensitive response, strong interchangeability, and easy maintenance.
(3) Designers should pay attention to the collection and organization of technical information to facilitate digestion and absorption.
(4) After the water treatment plant is completed and the instruments are put into use correctly, the designers should go to the site more often to conduct follow-up investigations on the use of the instruments, understand the working conditions of the instruments, and summarize the experience in a timely manner, so as to facilitate the improvement of future design work. (end)