The instrument selection shall avoid one-sided pursuit of high performance and high accuracy, which not only increases the purchase cost, but also the cost of spare parts. The optimal design and selection is to select instruments with good reliability and convenient maintenance on the premise of meeting the use requirements.

　　1. Characteristics of sewage flow measurement and key points of instrument selection

　　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 large diameter). It is also corrosive. The fluid working conditions change greatly (sometimes the medium changes every day), the measurement range is very different, the reliability requirements are different, and the accuracy requirements are high and low, The purchasing ability of users is different. Many factors need to be comprehensively considered to select a flow instrument that is easy to use and cost-effective for sewage under different working conditions.

　　Some other factors should also be considered in the selection of sewage flow measurement, such as: the flow measurement is not intuitive, and it is difficult to directly determine its accuracy on site; Sewage pipelines often adopt large pipe diameter and low flow rate, which brings great difficulties to flow measurement; The sewage flow measuring instrument is directly connected with the pipeline, which can not be disassembled online. When the instrument fails, it is not allowed to cut off and cannot be repaired. Therefore, the selection of sewage flow measurement is regarded as a difficulty at home and abroad.

　　Key points of type selection of sewage flow metering instrument include:

　　a. Flange standards, including Chinese national standard GB, European system standard DIN and American system standard ANSI;

　　b. Measuring pipe, pipe diameter usually refers to the outer diameter, and the pipe diameter of the same specification has different wall thickness;

　　c. For the selection of flowmeter materials, generally, stainless steel 316L can basically meet the requirements;

　　d. Determine the purpose of the flowmeter. It must be clear about accurate measurement, general measurement, flow control and only observing instantaneous quantity without accumulation;

　　e. Performance requirements for energy measuring instruments of enterprises (GB / t17167) shall reach ± 5% for sewage discharged by enterprises × Accuracy of FS (FS is full scale);

　　f. The physical and chemical properties of the medium must be clear about the state, chemical properties, pressure loss and physical properties of the medium;

　　g. Flow measurement range, different flow meters have clear requirements for minimum flow rate and maximum flow rate;

　　h. Installation environment and conditions: whether the diameter and location space of pipelines and instruments can meet the installation and maintenance requirements, the situation of upstream and downstream straight pipe sections and the position of valves, whether the installation environment has vibration, whether there are power supply conditions, whether there is electromagnetic interference around the installation, temperature / humidity and explosion-proof requirements of the installation environment must be clear;

　　i. The output type of flowmeter must be clear for on-site direct reading, digital pulse or analog output and HART protocol output;

　　j. The costs of installation, operation, calibration, maintenance and spare parts, service life, cost performance and technical services must be clear.

　　2 characteristics and type selection of several types of sewage flow meters

　　2.1 weir flowmeter and trough flowmeter

　　If the water supply / drainage channel is an open channel with free surface natural flow, or some drainage channels and sewer channels, although not open, but flow under non pressure and non full water, they actually belong to open channels, which can be measured by weir flowmeter and trough flowmeter. The utility model has the advantages of high measurement accuracy, good reliability and no special requirements for liquid. The solid substance 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 closed, but the structure of the detection element is simple, the straight pipe section is required to be short, and the water level rise of the trough flowmeter is smaller than that of the weir flowmeter. It is suitable for channels where large drop is not allowed. The disadvantages are: the head loss is large and can not be used in the channel close to the flat ground. The straight pipe section upstream of the weir flowmeter is longer than that of the trough flowmeter. Solids are easy to accumulate upstream of the weir, which should be cleaned regularly. One section of the channel where the trough flowmeter is located should be installed in the groove, but the structure of the detection parts is complex, etc.

　　The flow measuring groove of weir flowmeter includes right angle triangular weir (60 ° or 90 °), rectangular weir and equal width weir. There are many forms of flow measuring tank of slot flowmeter, mainly including P tank of rectangular open channel (commonly used) and Pb tank of circular open channel. Weir flowmeter and trough flowmeter use liquid level gauge to measure flow indirectly. Therefore, non-contact ultrasonic liquid level gauge shall be equipped to continuously measure liquid level, and the flow value shall be calculated according to the relationship between flow and liquid level. For example, the flow measurement accuracy of a certain ultrasonic liquid level gauge and triangular weir can reach ± (1.00 ~ 3.00)% × FS, matched with rectangular weir, the flow measurement accuracy can reach ± (1.00 ~ 5.00)% × FS, which is matched with basher tank (i.e. P tank), the flow measurement accuracy 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 rate level flowmeter

　　Flow rate water level flowmeter is actually a combination of liquid level meter (pressure type, static pressure type and ultrasonic liquid level meter are often used) and flowmeter (time difference ultrasonic flowmeter, Doppler ultrasonic flowmeter and electromagnetic flowmeter are often used). It also belongs to open channel flowmeter and is often placed in rectangular open channel, Use a liquid level gauge to measure the liquid level (because the width of the rectangular open channel is known, the flow area can be known), and use a flowmeter to measure the flow rate (the time difference ultrasonic flowmeter measures the line average flow rate, and the Doppler ultrasonic flowmeter measures the flow rate at the measuring point or the average flow rate in a small area). The flow value can be obtained by multiplying the two. Generally speaking, the sewage to be treated at the inlet of the sewage treatment unit is turbid, and the failure rate of the pressure or static pressure level gauge is high, so the Doppler ultrasonic flow meter is suitable; The purified sewage at the outlet of the sewage treatment unit is relatively clear, and the static pressure liquid level meter and time difference ultrasonic flow meter are suitable; The electromagnetic flowmeter can be used to measure the flow rate (in a certain sense, it is a non full tube electromagnetic flowmeter). The utility model has the advantages of small head loss, wide measurement range, reverse flow measurement, almost no precipitation of solid substances in the water and discharge with the water flow, and there is no need to change the channel. Its 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 weir flowmeter and trough flowmeter.

　　2.3 electromagnetic flowmeter

　　Electromagnetic flowmeter is an ideal volume flowmeter for measuring conductive fluid, so it is most suitable for the measurement of sewage flow. It can be well proved that the electromagnetic flowmeter is used for the sludge flowmeter at the inlet of the centrifugal dehydrator after the sewage is precipitated and concentrated. In this sense, the electromagnetic flowmeter is a universal flowmeter. The utility model has the advantages that the pipeline at the installation position of the sensor is a smooth straight pipe, no blockage, no wear, small 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; Low requirements for fluid conditions and low requirements for front / rear straight pipe sections (e.g. front 5D and rear 2D); High accuracy up to ± 0.20% × FS; Large diameter range (DN1 ~ dn3000mm); Flow in positive / negative direction can be measured; There are many choices of electrode and lining materials in contact with fluid; The response speed can reach 10ms; Self cleaning, etc. Its disadvantages are: it has the limitation of minimum conductivity; Wear and scaling will affect the measurement accuracy; The fluid needs to fill the pipeline (there are also dissatisfied tubular electromagnetic flowmeter, but the accuracy is low); The price of large caliber products is high. The larger the caliber, the higher the price.

　　According to the excitation mode, the electromagnetic flowmeter can be divided into DC excitation (not used at present), AC excitation (power frequency 50Hz) and low-frequency rectangular wave excitation (mainstream products); 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 connection, etc; It can be divided into integrated type and split type according to the structural form; It can be divided into full tube type and insufficient tube type according to the medium state. It has the advantages of high signal-to-noise ratio and high bidirectional magnetic induction; The disadvantages are easy to introduce power frequency interference, easy to produce noise (such as the same phase noise of eddy current and flow signal in liquid, noise caused by electrode pollution and phase shift noise, etc.) and large zero drift.

　　The key points of electromagnetic flowmeter selection are accuracy selection, full tube type ± (0.50 ~ 2.00) × FS grade, less than pipe type ± (1.50 ~ 2.00) × FS grade, plug-in type ± (2.00 ~ 4.00) × FS level; The second is the selection of flow rate. The flow rate range is 0.5 ~ 10.0m/s. Note that those easy to adhere, deposit and scale should be greater than 2m / s, and those containing granular media should be less than 3m / s.

　　The price of large-diameter electromagnetic flowmeter is expensive. The solution is to select plug-in flowmeter (the price basically does not change with the pipe diameter). Its advantages are light weight, small pressure loss, easy installation and maintenance, unlimited lower limit of measured flow rate, negligible blocking effect, but large error. Because the flow velocity 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 ball valve, so that the probe can be removed for maintenance and inspection under the condition of continuous flow. The probe of 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 velocity of the pipeline), measure the local velocity, and then calculate the flow according to the velocity distribution in the pipeline and the geometric size of the sensor. However, the requirements of the front / rear straight pipe section are higher than those of ordinary electromagnetic flowmeter (such as front 10d and rear 5d), 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 shall be installed at the highest point of the pipeline to realize automatic exhaust or manual periodic exhaust.

　　2.4 ultrasonic flowmeter

　　The transducer of ultrasonic flowmeter can not directly contact sewage. According to the measurement principle, there are propagation time method (time difference method, etc.), Doppler effect method and other methods, among which time difference method and Doppler effect method are more used; According to the installation mode, it can be divided into mobile installation (external clamp transducer) and fixed installation (with measuring tube or on-line transducer); It is divided into mono and multi-channel according to the number of channels. The utility model has the advantages of no resistance loss, low lower flow limit and is suitable for large pipelines. The disadvantages are that it can only be used for clean media, can not be used for small-diameter measurement and multi-channel, and the price is expensive. At the same time, the accuracy of the flowmeter varies greatly. Among the time difference ultrasonic flowmeter, the multi-channel ultrasonic flowmeter with large diameter measuring pipe has the highest accuracy, up to ± (0.50 ~ 1.00)% × FS, some up to ± 0.15% × FS, but it requires extremely high installation requirements. It must be accurately installed under the guidance of professionals and can be installed online. The accuracy of Doppler ultrasonic flowmeter can reach ± (3.00 ~ 10.00)% × FS, when the solid content is basically unchanged, it can reach ± (0.50 ~ 3.00)% × FS。

　　The time difference ultrasonic flowmeter is mainly used to measure the clean fluid flow. In addition, it can also measure the 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 can not measure the fluid flow with continuous mixed bubbles affecting ultrasonic propagation, high solid particle content or large solid particle diameter. Therefore, the impurity content of solid particles or the diameter of solid particles in the measured medium shall have an upper limit, specifically: the bubble content in the measured medium shall not exceed 5%; The turbidity shall not exceed 5000mg / L, and the solid content shall not exceed 10%, which is not recommended; There shall be no continuous large amount of flocculent suspended solids; If there is a large amount of sediment and other particles, the particle diameter shall not be greater than 3mm; The medium shall not produce a large amount of crystals in the pipe wall.

　　Doppler ultrasonic flowmeter relies on the reflection of solid particle impurities in sewage to measure the sewage flow rate. Therefore, it is suitable for dirty water or slurry with large solid particle impurity content or large solid particle diameter, such as urban sewage, sludge and factory process liquid with stable impurity content, 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

　　Some orifice meters are reserved for sewage flow measurement, and the new devices are basically not selected. The utility model has the advantages of simple manufacture, 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, there are many leakage points, the impulse pipe is easy to be blocked by solid particles and other sundries in the sewage (sometimes forced to adopt isolation measures) and the amount of maintenance is large.