/div> & nbsp;& nbsp;& nbsp;& nbsp; Flow rate is a common process control parameter in the production process of sewage treatment industry, and it is an important production technical index of sewage treatment industry. Among many types of flow meters, electromagnetic flow meters are mostly used in sewage treatment industry because of its advantages as follows:
 & nbsp;& nbsp;& nbsp;& nbsp; 1) The structure is simple, the sensor is equipped with movable parts, there is no measurement error and instrument failure caused by mechanical movement wear or debris winding.
 & nbsp;& nbsp;& nbsp;& nbsp; 2) The electromagnetic flowmeter has no throttling device and will not produce pressure loss. The electromagnetic flowmeter has high precision, good linearity and stable operation, which is suitable for sewage treatment industry.
 & nbsp;& nbsp;& nbsp;& nbsp; Detection and verification of 1 flowmeter
 & nbsp;& nbsp;& nbsp;& nbsp;& nbsp; Nanjing Jiangxinzhou sewage treatment plant is a modern large-scale sewage treatment plant with a designed daily treatment capacity of 640000 tons. Electromagnetic flow meters are used for sewage flow meters and sludge flow meters in the plant, including 4 large-diameter flow meters for inlet and outlet water, and 3 flow meters with diameters of 1600mm, 1600mm and 2000mm produced by Kaifeng instrument factory. Since it was installed and used in 2003, there has been no failure and it has been working normally; Small diameter flowmeter mainly adopts e + H, Danfoss and other brands.
 & nbsp;& nbsp;& nbsp;& nbsp; According to the requirements of ISO9001 quality management system, the electromagnetic flowmeter, as a measuring instrument, needs to be inspected regularly. According to its different use occasions, the electromagnetic flowmeter can be divided into mandatory verification and non mandatory verification. The flowmeter used for fund settlement in sewage treatment plant is a compulsory verification measuring instrument. Its verification should be carried out by the legal metrological verification institution in accordance with the relevant national metrological verification regulations. The three inlet flow meters and one outlet flow meter of Jiangxinzhou sewage treatment plant belong to this kind of instruments, which need to be verified by the legal metrological verification institution.
 & nbsp;& nbsp;& nbsp;& nbsp; The traditional verification method is the laboratory volumetric method, which is not easy to use due to the limitation of site, production and other conditions. The ultrasonic flowmeter is an instrument to measure the flow by detecting the effect of fluid flow on the ultrasonic beam (or ultrasonic pulse). According to the principle of signal detection, ultrasonic flowmeter can be divided into propagation velocity difference method (direct time difference method, time difference method, phase difference method and frequency difference method), beam migration method, Doppler method, cross-correlation method and noise method. Like electromagnetic flowmeter, ultrasonic flowmeter belongs to unimpeded flowmeter because there is no obstruction in the flow channel of the instrument. It is a kind of flowmeter suitable for solving the problem of difficult flow detection, especially in large diameter flow measurement. It has prominent advantages and broad development prospects.
 & nbsp;& nbsp;& nbsp;& nbsp; 2 implementation
 & nbsp;& nbsp;& nbsp;& nbsp; This paper analyzes the verification method, advantages and disadvantages of ultrasonic flowmeter by taking a 1600mm diameter class 0.5 electromagnetic flowmeter produced by Kaifeng instrument factory as an example.
 & nbsp;& nbsp;& nbsp;& nbsp; The 1600mm diameter flowmeter is installed on the water inlet pipeline of Jiangxinzhou sewage treatment plant. Due to the characteristics of the industry, it can not be cut off and out of service. Even if it is cut off and out of service, because it is installed indoors and the space is narrow, the electromagnetic flowmeter can not be disassembled and moved. Therefore, the traditional method of return calibration can not be used, and the online detection method can only be used for verification. In January 2004, Nanjing Municipal Bureau of quality and technical supervision's flow steel tape measurement verification station went to the site for on-line detection. The standard meter is a Doppler ultrasonic flowmeter with accuracy of 0.5.
 & nbsp;& nbsp;& nbsp;& nbsp; The verification test data are as follows: & nbsp;
The tested scale (M & sup3/ s)
Standard scale (M & sup3/ h)
Basic error (%)
one point eight nine
six thousand eight hundred and forty
-0.53
one point nine nine
seven thousand one hundred and ninety-eight
-0.47
two
seven thousand two hundred and forty
-0.55
 & nbsp;& nbsp;& nbsp;& nbsp; Verification conclusion: qualified (Level 1)
 & nbsp;& nbsp;& nbsp;& nbsp; After analysis, the external clip Doppler ultrasonic flowmeter with good performance is selected as the on-line detection instrument of large diameter flowmeter, which has the following advantages:
 & nbsp;& nbsp;& nbsp;& nbsp; 1) The online measurement can be realized without affecting the continuous operation of the sewage treatment plant.
 & nbsp;& nbsp;& nbsp;& nbsp; 2) Portable Doppler ultrasonic flowmeter is small, easy to carry, and easy to install on site. Compared with the traditional laboratory volumetric verification, it saves the demolition and transportation of the tested flowmeter. Moreover, some large-diameter tested flowmeters can not be disassembled because the future verification problems are not considered in the construction and installation. Therefore, the on-site verification can save the verification cost.
 & nbsp;& nbsp;& nbsp;& nbsp; 3) On line detection can also verify the influence of on-site pipeline conditions on the flowmeter, which is a problem that cannot be solved by inspection.
 & nbsp;& nbsp;& nbsp;& nbsp; Of course, it inevitably leads to the disadvantages of online detection
 & nbsp;& nbsp;& nbsp;& nbsp; 1) The tested electromagnetic flowmeter must be "degraded for use".
 & nbsp;& nbsp;& nbsp;& nbsp; Under ideal conditions, the laboratory calibration accuracy of external clip portable ultrasonic flowmeter can reach 0.5%. Considering the uncertainty of field pipeline parameters, only on-line flowmeter below grade 1.0 can be verified. According to the principle of value transfer, the electro-magnetic flowmeter with precision of grade 0.5 can not be verified. Our factory adopts the method of "nominal degradation" to solve this problem, That is to say, the accuracy of the flowmeter is changed from 05 to 1.
 & nbsp;& nbsp;& nbsp;& nbsp; 2) Field flow is not easy to control, it is difficult to achieve full range test.
 & nbsp;& nbsp;& nbsp;& nbsp; 3. Problems and Solutions
 & nbsp;& nbsp;& nbsp;& nbsp; In the actual detection, we also encounter the following problems:
 & nbsp;& nbsp;& nbsp;& nbsp; 1) The output of one pump is larger than that of two parallel pumps. The reason may be that there is gas doping in the water flow, and the measured flow rate is the amount of gas water mixture. Gas gathering and exhaust devices should be installed at the highest part of the main pipe in front of the flow meter, and regular discharge should be ensured to reduce the influence of water aeration.
 & nbsp;& nbsp;& nbsp;& nbsp; 2) In the same system, the flow values are different in different pipe sections with different pressures and diameters. When the flowmeter is in normal use, its flow rate should be more than 1m / s. for dirty sewage, its flow rate should be more than 2m / s.
 & nbsp;& nbsp;& nbsp;& nbsp; 3) In the same pipe, the pipe diameter is the same, and the flow values of different pipe sections are different. When measuring in the vertical pipe section, the indication value is high and unstable, while the flow number measured in the horizontal pipe section is stable and low. When considering the installation of gas gathering and exhaust devices, the flowmeter should be installed at the lower part of the horizontal pipe section as far as possible, so as to check the zero point of the flowmeter in static state.
 & nbsp;& nbsp;& nbsp;& nbsp; 4 Conclusion
 & nbsp;& nbsp;& nbsp;& nbsp;& nbsp; It is a feasible method to use the portable ultrasonic flowmeter with external clamp to test the large diameter flowmeter. Although the development of ultrasonic flowmeter has become more and more mature, it has its scope of application, and there will be problems in the specific detection. This requires us to:
 & nbsp;& nbsp;& nbsp;& nbsp;& nbsp; 1) When selecting the instrument, we must be familiar with both the instrument and the measured object, and consider other factors, so that the measurement can be accurate;
 & nbsp;& nbsp;& nbsp;& nbsp; 2) Combined with the actual situation, the design and installation stage of the equipment should be improved as far as possible.
 & nbsp;& nbsp;& nbsp;& nbsp; reference:
 & nbsp;& nbsp;& nbsp;& nbsp; [ 1] Liang Guowei. Measurement technology and instrument. Beijing: China Machine Press. 2002 < br > & nbsp& nbsp;& nbsp;& nbsp;& nbsp; [ 2] Li Xiaoguang. Basic principle and application of Doppler ultrasonic flowmeter. Application in instrument industry. 2005.3 < br > & nbsp& nbsp;& nbsp;& nbsp;& nbsp; [ 3] Li h, Xue Z. analysis of abnormal phenomena in Doppler ultrasonic flowmeter measurement. China instrumentation. 2004.9