Flowmeter, level gauge, temperature sensor in water purification plant

Not in the modern water purification plant, every 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.

I. Classification of commonly used instruments in 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 measuring 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.

Second, the composition and monitoring parameters of the water purification plant monitoring system

The monitoring system of a water purification plant is generally composed of a two-tier system of the water plant management and the on-site monitoring layer, and is monitored in accordance with the principles of centralized management and decentralized control. In the engineering design, the plant-level computer system ( ie, the main station ) is located in the central control room of the water plant . The number and location of the field monitoring stations ( ie, substations ) are determined according to the process flow and the location and degree of dispersion of the structures. The site substation settings for general surface water plants are: inlet pump station substations, reaction precipitation and chlorination dosing substations, filtration substations, pumping pump substations, substations of substations and substations, and sludge treatment substations. The data of each monitoring instrument is sent to the computer system, which can be displayed, controlled, printed, recorded, and alarmed on the industrial control computer of the monitoring station.

2. Monitoring parameters of each substation

a. Monitoring parameters of sub-station water pumping house parameters: source water turbidity, pH value, water temperature, dissolved oxygen, etc. Operating parameters: adjusting the water level of the pool, the water level of the water absorption well, the source water flow, the power of the pump, and the total power of the pump station.

b. Reaction sedimentation, chlorination plus sub-station water quality parameters: turbidity at the sedimentation tank outlet, residual chlorine after filtration, and SCD value. Operating parameters: sedimentation tank water level, pre-sediment flow, stirring tank liquid level, drug tank liquid level, liquid chemical concentration, sedimentation tank mud position.

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 of the pump station and the substation of the power distribution room : the factory water flow and residual chlorine. Operating parameters: factory water pressure, flow, clear water level, suction well water level, AC voltage, AC current, electricity and so on.

e. Sludge treatment station operating parameters: water level in the return tank, water volume, water level in the concentrate tank, and turbidity of the return water. Third, the water treatment system commonly used instruments in the selection and design should pay attention to the question

1. General requirements for instrument selection

(1) Accuracy: refers to the accuracy of the meter'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% , and the accuracy of the water quality analysis instrument is ± 2% ( the accuracy of the turbidity meter for measuring the turbidity 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 4 ~ 20mADC signal, the load capacity is not less than 600 Ω.

(4) The degree of protection of the instrument should meet the requirements of the environment, and generally should not be lower 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 of the optional design should be larger than the current and voltage transformers, ie, 0 to 6A respectively. And 0 to 120V .

(9) It should select instrument manufacturers who can provide reliable services and have rich experience.

2. Water level measurement

The following factors should be considered when selecting the level gauge:

(1) Measurement objects, such as the physical and chemical properties of the measured medium, 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 method, on-site instructions, remote indication, interface with the computer, safety and corrosion protection, reliability, and construction convenience.

The main points of level gauges and selections commonly used in water supply engineering are as follows:

a.Floating ball level gauge

Put a hollow ball 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 mechanically or electrically and its accuracy is ± (1 to 2)% . This level gauge is not suitable for high-viscosity liquids, and its output has switch control and continuous output. In the design of a water purification plant, such level gauges are often used for level measurement of a catchment well to control the automatic start and stop of the drain pump.

b. 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. The accuracy of this level gauge is ± (0.5 to 2)% .

c. Capacitive level gauge

Insert the electrode in the container, when the liquid level changes, the medium inside the electrode changes, the capacitance between the electrodes ( or between the electrode and the container wall ) also changes, and the change of the capacitance is converted into a standardized 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 precision is high ; the detection end consumes small electric energy, 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 more than 2m , cable electrodes are used. When the measured medium is water, an electrode with an insulating layer ( polyethylene available ) is used.

d. Ultrasonic level gauge

The ultrasonic level gauge sensor consists of a pair of transmitter and receiver transducers. The transmitting transducer faces the 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 mechanically 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.

Flow measurement is divided into two types, one for flow detection and participation in process control, so as to achieve the goal of improving production automation, improving production process conditions, and improving product quality and yield. 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 main economic indicators of water supply companies , three are based on data measured by flow meters.

Flow meter selection should consider the following factors:

(1) Any type of flowmeter must have a certificate verified by the national measurement department before it can be used.

(2) The pressure loss of the flowmeter itself should be small.

(3) According to industry requirements, the accuracy of the flowmeter should not be less than 2.5 .

(4) The installation site conditions should meet the requirements of the selected flowmeter for straight pipe sections.

(5) The selected flow meter should be able to adapt to the installation site environmental conditions such as temperature, humidity, electromagnetic interference.

(6) The selected flow meter should be suitable for the liquid medium to be tested.

At present, in the water supply engineering design, electromagnetic flowmeters and ultrasonic flowmeters are the most used.

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 pipe to generate a voltage in the pipe corresponding to the average flow velocity V of the measured fluid , 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.

Electromagnetic flowmeter advantages:

(1) The measurement is not affected by the temperature, pressure or viscosity of the liquid to be measured.

(2) No pressure loss.

(3) Continuous measurement, high measurement accuracy.

(4) The scope and range of measurement are large, and the measurement range is continuously adjustable.

(5) Irrespective of the flow velocity distribution.

(6) The length of the straight pipe is short before and after, and the straight pipe length is 5D (D is the diameter of the instrument ) , and the straight pipe is 3D .

(7) The stability is good, the output is a standardized signal, and it can easily enter the automatic control system.

(8) The inner wall of the transmitter conduit is lining material, with good corrosion resistance and wear resistance.

(9) The converter has a 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 central 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 10 Ω, especially when it is installed on the cathodic protection pipeline. For example, electromagnetic flowmeters installed on the main trunk of Tianjin Shuiyuan Plant, because the pipeline uses cathodic protection, the insulation between the inner wall and the outer wall of the pipeline that protects the electrolytic corrosion is insulative, and the measured medium has no ground potential, so the sensor grounding ring is installed. On both sides of the sensor, it is insulated from the flange of the connecting pipe. The sensor is connected to the grounding ring with a grounding wire and led to the grounded pole. Pipe flanges are connected by cables but not to the sensor. The flange bolts are isolated from the bushings and washers. The electromagnetic flowmeter has been effective since it was put into production.

The converter shall be installed in a place that meets the requirements of its degree of protection. Under the premise of meeting the installation environment and the requirements for use, the distance between the converter and the sensor and the connecting cable should be as short as possible to save investment and reduce the potential for strong electricity. Signal interference.

In the last decade or so, due to the development of electronic technology, ultrasonic flow meters 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 and reverse flow, the time difference from launching to receiving is measured, and the flow velocity is measured accordingly.

The main advantages of ultrasonic flowmeters:

(1) 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 shutting off the flow in pipes, allowing easy installation in existing applications, especially for large-diameter pipe inspection systems. .

(2) The scope of the caliber is large, and the price is not affected by the pipe diameter.

(3) High measurement reliability.

(4) No pressure loss.

(5) Not affected by the fluid parameters.

(6) The output of the standardized DC signal can be easily entered into 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 cycle. The unit is calibrated as a standard equipment of the company, and then the online flow meter is periodically checked for comparison. This requires designers to design according to the requirements of the use of units, consider the need for future production management, set aside space for comparison, to facilitate the user, the flow meter well to do slightly larger, in addition to the installation of fixed flowmeter outside The space for portable flow meter measurements should also be reserved as shown in Figure 1 .

Turbidity is a measure of the degree of turbidity in a water body, ie, the presence of finely dispersed, suspended particles in the water, to the extent that water transparency is reduced. The turbidity meter is an instrument for measuring the degree of turbidity in a water body and is mainly used for the monitoring and management of water quality. The water purification plant is responsible for supplying domestic water and industrial water for residents. The quality of water supply directly involves the people's health and safety, and 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 turbidimeter is particularly important. Turbidity meter can be divided into two categories: visual turbidity meter and photoelectric turbidity meter. Optoelectronic turbidity meters can be divided into process monitoring ( continuous measurement ) turbidity meters and laboratory ( including portable ) turbidity meters for their use. The design principles can be divided into transmission light turbidity meters and scattered light turbidity meters.

Because the scattered light turbidity meter has high sensitivity to low turbidity of water, the accuracy is high, the relative error is small, and the repeatability is good. The chromaticity of water does not show 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. At the same time, the "Water Supply Industry's 2000 Technology Development and Development Plan" has clearly specified that the water turbidity index of a water company's pipe network is 1 NTU . HACH 's 1720D and SS6 turbidimeters (which belong to the scattered light turbidity meter ) are commonly used in water purification plants . In the measurement of filtered water and factory water, 1720D ( formerly 1720C) turbidimeters are generally used . During use, the water sample continuously flows into the turbidimeter and flows through the deaerator to drain the air bubbles in the water stream. Then it enters the center column of the turbidimeter and rises to the measurement chamber and overflows its edge into the discharge port. The light beam is projected downward from the sensor head assembly into the water sample in the turbidimeter body. The photocells immersed in the water sample measure the scattered light in the 90 ° direction of the suspended solids in water , and the amount of scattered light and the turbidity of the water sample. Degree is proportional to. The 1720D eliminates the need for a sample cell, which reduces stray light and improves measurement accuracy. Accuracy of the 1720D is: ± 2% in the range of 0 to 40NTU , ± 5% in the range of 40 to 100NTU , resolution of 0.001NTU , and response time of 75s .

The turbidity meter for measuring the filtered water is installed in the filter station pipe gallery. It can be wall-mounted or cabinet-mounted. The measurement of the outgoing water is generally set in the water pump room to set the water quality meter, and the turbidity meter and other water quality detection meters are placed on the meter. In the room, the signal is then directed to the monitoring station.

Although the 1720D has a measurement range of 0 to 100 NTU , it is best not to use it to measure the pre-filter water, because although it can measure 100 NTU optically , it will cause a lot of inconvenience in production and use. The SS6 series surface scattering turbidity meter is used to measure the source water and the pre-filter water . It irradiates the light beam on the liquid surface to measure the scattered light from the liquid surface, avoiding direct contact between the optical system and the water sample, and eliminating the cleaning flow cell. Loss of signal when brought.

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

The selection of turbidity meter 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 turbidity meter. The accuracy of the measurement, the water sample extraction is best to use 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 general water purification plant project uses multiple intelligent display instruments. Its functions are complete, digital signal processing can be performed, and the control function is realized. The measured value is displayed on the liquid crystal, and the operation is convenient, the data can be saved, and the self-diagnosis function is provided. Although the advantage of the computer system was not fully realized after being connected to the computer system, it was replaced by the computer system. However, in the current construction of the water purification plant, the intelligent display instrument was used as the computer system during the commissioning or failure of the computer system. The auxiliary instrument 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 a signal series method. Instead, a signal distributor should be used, that is, one input, two outputs, one output and display instrument, and the other output can be input to the PLC. , such as the commonly used WS15242 .

Grounding 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 1 Ω.

Generally, the facilities of the water purification plant are scattered, the structure is low, and the terrain is flat and open. Especially, some flowmeter wells are located outside the plant area. In this case, the lightning strike rate of the instrumentation equipment increases. In practice, the author has repeatedly encountered lightning damage damage to the meter or meter unexplained damage. Therefore, the installation of a lightning arrester with good quality and reliable operation is an indispensable protection measure. For example, the Pepperl+Fuchs company's ESP series lightning protection grid is used for signal and power supply protection of the flowmeter, and the effect is good.

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