Selection of sewage and wastewater flow metering
Water resources include industrial water after primary filtration, filtered water after re-precipitation of raw water, desalinated water after deionized treatment of filtered water, sewage discharged from various production units and recycled water after treatment of sewage, etc. In order to further carry out energy-saving and water-saving work, various types of flow meters are used on production units. The author focuses on the main points of the selection of sewage flow metering instruments to meet the requirements of use and improve the efficiency and cost ratio.
It is difficult to measure sewage flow. Many flowmeters cannot be used for sewage flow measurement. After practical verification, there are four kinds of flowmeters available: open channel flowmeters, such as weir flowmeter, trough flowmeter and flow rate-water level flowmeter; Electromagnetic flowmeter adopts electrical principle and is divided into ordinary type and plug-in type. Ultrasonic flowmeter adopts acoustic principle, commonly used with time difference type and Doppler type, etc. Differential pressure flowmeter, use dynamic pressure principle, such as uniform tube flowmeter or Weiliba flowmeter.
Instrument selection should avoid one-sided pursuit of high performance and high accuracy, which not only increases the purchase cost, but also increases the cost of spare parts. Design selection is to meet the requirements of the use of the premise, select good reliability, easy maintenance of the instrument.
1 sewage flow measurement characteristics and instrument selection points
Sewage can be divided into domestic sewage and industrial sewage. Its water quality is dirty (sometimes it contains oil, solid particles or suspended solids, and even solid particles with larger diameter), and it also has certain corrosiveness. The fluid working conditions change greatly (sometimes the medium conditions change every day), the measurement range is wide, the reliability requirements are different, the accuracy requirements are high and low, and the purchasing ability of users is different, for different working conditions of the sewage selection of a useful and economical flow meter needs to consider a lot of factors.
SelectSewage flow meterQuantity should also consider some other factors, such as: flow measurement is the least intuitive, and it is difficult to directly determine its accuracy on site; Sewage pipelines often use large pipe diameters, and low flow rates bring great difficulties to flow measurement. Sewage flow metering instruments are directly connected with pipelines and cannot be disassembled online usually. When the instruments fail, they are not allowed to cut off and cannot be repaired. Therefore, the selection of sewage flow measurement is regarded as a difficult point at home and abroad.
Selection points of sewage flow metering instrument include :
a. Flange standards, including Chinese national standard GB, European system standard DIN and American system standard ANSI, etc;
B. measuring pipe, pipe diameter usually refers to the outer diameter, the same specifications of the pipe diameter has different wall thickness;
C. flowmeter material selection, usually stainless steel material 316L can basically meet the needs;
d. Determine the purpose of the flowmeter, which must be clear about precise measurement, general measurement, flow control and observation of instantaneous quantities without requiring accumulation;
E. performance requirements for enterprise energy measuring instruments (GB/T17167), the accuracy of sewage discharged by enterprises shall reach 5% × FS(FS is full scale);
F. the physical and chemical properties of the medium, the state of the medium, chemical properties, pressure loss and physical properties must be clear;
g. Flow measurement range, different flow meters have clear requirements for the minimum flow rate and the maximum flow rate;
h. Installation environment and conditions, the diameter of pipelines and instruments, whether the location and space can meet the installation and maintenance requirements, the upstream and downstream straight pipe sections and the location of valves, whether the installation environment has vibration, whether there is power supply conditions, whether there is electromagnetic interference around the installation, temperature/humidity and explosion-proof requirements of the installation environment must be clearly defined;
I. The output type of the flowmeter must be clear for on-site direct reading, digital pulse or analog output and output with HART protocol.
j. The cost of installation, operation, calibration, maintenance and spare parts, service life, cost performance and technical services must be specified.
2 Characteristics and Selection of Several Types of Sewage Flowmeters
2.1 weir flowmeter and trough flowmeter
If the supply/drainage channel is an open channel with free surface natural flow, or some drainage channels and sewer channels, although not open, flow under non-pressurized and non-full water conditions, are actually open channels, and weir flow meters and trough flow meters can be used for measurement. Its advantages are: high measurement accuracy, good reliability, and no special requirements for liquids. The solid matter in the water where the trough flowmeter is located hardly precipitates and is discharged with the water flow. The channel where the weir flowmeter is located should be intercepted, but the structure of the detection element is relatively simple, the straight pipe section is required to be shorter, and the water level elevation of the trough flowmeter is smaller than that of the weir flowmeter. It is suitable for channels that do not allow large drop. However, its disadvantages are: the head loss is large and cannot be used for channels close to flat ground; the straight pipe section upstream of the weir flowmeter is longer than that of the trough flowmeter; the solid matter is easy to accumulate upstream of the weir, which needs to be cleaned regularly; the channel section where the trough flowmeter is located needs to be loaded into the trough, but the structure of the detection piece is more complicated, etc. The flow channel of weir flowmeter includes right angle triangular weir (60 ° or 90 °), rectangular weir and equal width weir. There are many forms of trough flow meter flow trough, mainly including P trough of rectangular open channel (commonly used) and PB trough of circular open channel. Weir flowmeter and trough flowmeter both use liquid level meter to indirectly measure flow, so non-contact ultrasonic liquid level meter is required to continuously measure liquid level, and the flow value is calculated according to the relationship between flow and liquid level. For example, the flow measurement accuracy of a certain ultrasonic liquid level meter and triangular weir can reach (1. 00~3. 00)% ×FS, the flow measurement accuracy with rectangular weir can reach ± (1. 00~5. 00)% ×FS, and the flow measurement accuracy with Parscher trough (I. e. P trough) can reach ± (3. 00~4. 00)% ×FS. Note that in order to ensure the normal use of the ultrasonic level gauge, the measuring point must avoid a large amount of foam on the surface.
2.2 flow-level flow meter
Velocity-water level flowmeter is actually a combination of liquid level meter (usually pressure type, static pressure type and ultrasonic liquid level meter) and flow meter (usually time difference method ultrasonic flow meter, Doppler ultrasonic flow meter and electromagnetic flow meter). It also belongs to open channel flow meter and is often placed in rectangular open channel. Liquid level meter is used to measure liquid level (because the width of rectangular open channel is known, the flow area can be known), use a flow meter to measure the flow rate (using the time difference method of ultrasonic flow measurement is the line average flow rate, Doppler ultrasonic flow measurement is the measurement point flow rate or local small area average flow rate), the two can be multiplied to obtain the flow value. In a general sense, the sewage to be treated at the inlet of the sewage treatment device is turbid, the failure rate of the pressure type or static pressure type liquid level meter is high, and the Doppler ultrasonic flowmeter is more suitable. After the sewage is purified at the outlet of the sewage treatment device, the static pressure type liquid level meter and the time difference method ultrasonic flowmeter are more suitable. Electromagnetic flowmeter
Speed can be applied (in a sense, it is not full tube electromagnetic flowmeter). Its advantages are: small head loss, wide measurement range, measurable reverse flow, solid substances in water almost no precipitation and discharge with water flow, no need to change the channel. The disadvantage is that the upstream/downstream straight pipe section should have a long enough straight channel, and the upstream straight pipe section is longer than the weir flowmeter and the trough flowmeter.
2.3 electromagnetic flowmeter
Electromagnetic flowmeter is an ideal volume flowmeter for measuring conductive fluids, so it is suitable for the measurement of sewage flow. The sewage enters the centrifugal dewatering machine after precipitation and concentration. The sludge flowmeter at the inlet of the centrifugal dewatering machine can be obtained by using electromagnetic flowmeter. It is well proved that in this sense, electromagnetic flowmeter is a universal flowmeter.
Its advantages are: the pipeline at the sensor installation place is a smooth straight pipe, no blockage, no wear, little pressure loss, no movable parts, high reliability, good long-term stability and maintenance-free; The measurement range is large, and the measurement range to ensure accuracy can generally reach 40: 1; The requirements for fluid condition are not high, and the requirements for front/rear straight pipe sections are low (e.g. front 5D and rear 2D); The high precision can reach 0.20% ×FS; large caliber range (DN1 ~ DN3 000mm); It can measure the flow rate in the forward/reverse direction. There are many choices for electrodes and lining materials in contact with fluid. The response speed can reach 10ms; Self-cleaning, etc.
Its disadvantages are: there is a limit of minimum conductivity; wear and scaling will affect the measurement accuracy; fluid needs to be filled with pipes (there are also unsatisfactory tube-type electromagnetic flowmeters, but the accuracy is low); large-diameter products have high prices, and the larger the diameter, the higher the price.
Electromagnetic flowmeter can be divided into DC excitation (not adopted at present), AC excitation (power frequency 50Hz) and low frequency rectangular wave excitation (mainstream products) according to excitation mode. According to the installation and connection mode, it is divided into flange connection (flange type with measuring pipe or wafer type), threaded connection and plug-in type, etc. According to the structure form, it is divided into integrated type and split type, etc. According to the medium state, it is divided into full tube type. The advantages of AC excitation are large magnetic induction intensity, high signal-to-noise ratio and suitable for two-way media, etc. The disadvantage is that it is easy to introduce power frequency interference, easy to produce noise (such as eddy current in liquid and flow signal phase noise, electrode pollution formation noise and phase shift noise, etc.) and large zero drift. The main points of electromagnetic flowmeter selection are first of all precision selection, full tube type (0. 50~2. 00) × FS level, full tube type (1. 50~2. 00) × FS level, insertion type (2. 00~4. 00) × FS level; The second is flow rate selection, with a flow rate range of 0. 5~10. 0 m/s, note that easy adhesion, deposition and scaling should be greater than 2 m/s, containing particle media should be less than 3 m/s. Large-diameter electromagnetic flowmeters are more expensive. The solution is to use plug-in flowmeters (the price basically does not change with the pipe diameter). Its advantages are light weight, small pressure loss, easy installation and maintenance, and the lower limit of the measured flow rate is unlimited., The blocking effect can be ignored, but the error is large. Because the flow rate of large diameter sewage pipeline is generally low, sediment and dirt are easy to deposit on the surface of the probe, so it is often equipped with a ball valve to realize that the probe can be removed for maintenance and inspection under the condition of continuous flow. The probe of the plug-in electromagnetic flowmeter is generally located at a specific position in the pipeline (generally 1/8 of the inner diameter of the pipeline, I .e. the average flow rate of the pipeline), measures the local flow rate at the position, and then calculates the flow rate according to the flow rate distribution in the pipeline and the geometric size of the sensor. However, the front/rear straight pipe section requires higher requirements than those of the ordinary electromagnetic flowmeter (e.g. 10D and 5D), and 5D respectively. In addition, the pipeline must be filled with liquid, in order to prevent the gas in the liquid from accumulating in the pipeline, an exhaust valve should be installed at the highest point of the pipeline to realize automatic exhaust or manual regular exhaust.
2.4 ultrasonic flowmeter
The transducer of the ultrasonic flow meter may not directly contact the sewage. According to the measurement principle, there are propagation time method (time difference method, etc.), Doppler effect method and other methods, of which time difference method and Doppler effect method are used more. According to the installation method, it can be divided into mobile installation (external clamp transducer) and fixed installation (with measuring tube or online installation transducer); According to the number of channels, mono and multi-channel are divided. The advantage is that there is no resistance loss, low flow limit and suitable for large pipelines. The disadvantage is that it can only be used for clean media, can not be used for small diameter measurement and multi-channel and expensive. At the same time, the accuracy of the flowmeter varies greatly. In the time difference ultrasonic flowmeter, the accuracy of the large-diameter multi-channel ultrasonic flowmeter with measuring tube is the highest, reaching (0. 50~1. 00)% × FS, and some are as high as 0. 15% × FS. However, the installation requirements are extremely high. It must be accurately installed under the guidance of professionals and can be installed online, the accuracy of Doppler ultrasonic flowmeter is ± (3. 00~10. 00)% ×FS, and when the solid content is basically unchanged, it can reach ± (0. 50~3. 00)% ×FS.
Time difference ultrasonic flowmeter is mainly used to measure clean fluid flow. In addition, it can also measure uniform fluid with low solid impurity content (generally, the solid particle content is less than 10g/L and the particle diameter is less than 1mm), but it cannot measure fluid flow with continuous mixed bubbles, high solid particle content or large solid particle diameter that affect ultrasonic propagation. Therefore, the impurity content of solid particles or the diameter of solid particles in the measured medium should have an upper limit, specifically: the bubble content in the measured medium should not exceed 5%; Turbidity cannot exceed 5,000 mg/L, and solid content greater than 10% is not recommended. There cannot be a large number of continuous flocculent suspended solids; If it contains a large amount of sediment and other particles, its particle diameter cannot be greater than 3mm; the medium should not produce large amounts of crystals in the tube wall. Doppler ultrasonic flowmeter relies on the reflection of solid particles in sewage to measure the flow rate of sewage, so it is suitable for dirty water or slurry with more solid particles or large diameter of solid particles, such as municipal sewage, sludge and factory process liquid with stable impurity content, etc., and can measure the liquid continuously mixed with bubbles. The measured medium must contain a certain number of scatterers (particles or bubbles), otherwise, the instrument cannot work normally. It should be noted that the impurity content of solid particles or the diameter of solid particles in the measured medium should have a lower limit.
2.5 differential pressure flowmeter
Sewage flow metering also retains part of the orifice flowmeter, and the new device has been basically not selected. Its advantages are: simple production, stable performance, insensitive to vibration and strong anti-interference ability. Its disadvantages are: the measurement accuracy belongs to the medium level, the scope is narrow, the pressure loss is large, the installation requirements are high, the leakage point is many, the pressure guide pipe is easy to be blocked by the solid particles in the sewage and other debris (sometimes forced to adopt isolation measures) and the maintenance amount is large, etc.
3 Suggestions on Selection of Sewage Flow Metering Instrument
The sewage flow transported by open channels can be measured by weir flowmeter, trough flowmeter and flow rate-water level flowmeter, and the accuracy can meet the measurement requirements. For those requiring small head loss, the flow rate-water level flowmeter should be selected, but the appropriate combination of liquid level meter and flowmeter should be selected for different water quality to form the flow rate-water level flowmeter. For those with large allowable head loss, weir flowmeter or trough flowmeter can be selected. Among them, weir flowmeter can be selected which is cheaper, allows large drop and can regularly clean up solid sediments on the upstream side. Otherwise, trough flowmeter is selected, but when selecting trough flowmeter, submerged flow and reverse flow should be prevented. The sewage flow transported by pipeline can be measured by electromagnetic flowmeter or ultrasonic flowmeter, but attention should be paid to the cleaning of measuring pipes, electrodes and transducers. Generally speaking, when the pipe diameter is not more than 200mm, the ultrasonic flowmeter is more expensive than the electromagnetic flowmeter, and the electromagnetic flowmeter can be selected. When the pipe diameter is not less than 250mm, the ultrasonic flowmeter is cheaper than the electromagnetic flowmeter, and the ultrasonic flowmeter can be selected. Note that it is not necessary to choose a multi-channel ultrasonic flowmeter (such as a mono ultrasonic flowmeter) in pursuit of high performance. In terms of performance, the time difference ultrasonic flowmeter and the Doppler ultrasonic flowmeter can measure the sewage flow with the impurity content of solid particles or the diameter of solid particles within a certain range (that is, their applicable scope has a certain coincidence area), but below or beyond this range, only the time difference ultrasonic flowmeter or the Doppler ultrasonic flowmeter can be used for measurement respectively. In this sense, a comprehensive understanding of the medium condition of the measured sewage is the key to instrument selection. When the medium condition of the measured sewage cannot be confirmed, electromagnetic flowmeter should be selected as much as possible, but attention should be paid to the low accuracy of Doppler ultrasonic flowmeter. In addition, plug-in electromagnetic flowmeter, movable installation (external clamping transducer) and fixed installation (online installation transducer) ultrasonic flowmeter can be selected on large pipelines, and the accuracy can fully meet the measurement requirements, however, plug-in electromagnetic flowmeter or movable installation (external clamping transducer) ultrasonic flowmeter should be preferred. Due to the high installation requirements of differential pressure flowmeter, many leakage points, easy blockage of pressure guide pipe by solid particles and other debris in sewage and large maintenance amount, it is not selected as far as possible in sewage flow measurement.
The selection of flow measuring instrument in sewage treatment system is a difficult problem, the author introduces the characteristics of four kinds of sewage measuring instrument selection points, through understanding the performance of these types of measuring instruments, according to the process of production needs to choose the appropriate measuring instrument, so that the measurement and control of sewage flow to meet the requirements of accuracy and reliability.