Technical Analysis of Sewage Flow Monitoring Data of Open Channel Flow Meter

Abstract: The technical analysis information of the sewage flow monitoring data of the open channel flow meter is provided by the excellent flow meter and flow meter manufacturers. With the increasingly serious problem of environmental pollution, my country has put forward higher requirements for establishing an environmental management system and increasing environmental monitoring. At present, key sewage enterprises in China have installed automatic sewage monitoring systems to ensure that the data of sewage discharge can be uploaded in real time. For more flowmeter manufacturers to select models and price quotations, you are welcome to inquire. The following is the technical analysis article details of the sewage flow monitoring data of open channel flowmeters. With the increasingly serious problem of environmental pollution, my country has put forward higher requirements for establishing an environmental management system and increasing environmental monitoring. At present, key sewage enterprises in China have installed automatic sewage monitoring systems to ensure that sewage discharge data can be uploaded to the municipal and provincial environmental monitoring departments and national environmental protection authorities in real time. In order to ensure the validity of the uploaded data, the Ministry of Environmental Protection has issued HJ/T355-2007 'Technical Specifications for the Operation and Assessment of Water Pollution Source Online Monitoring System' and HJ/T356-2007 'Water Pollution Source Online Monitoring Data Validity Discrimination Technical Specifications' and other specifications sex document. These documents specify the test items and data requirements for monitoring data related to water quality (such as chemical oxygen demand CODcr, ammonia nitrogen NH3-N, total phosphorus TP, pH value), etc. , but did not ask for it. With the requirement of monitoring the total emission reduction of major pollutants, this situation has been increasingly discovered and paid attention to. At the end of 2013, some people from the environmental monitoring department have proposed technologies and methods for quality supervision and inspection of sewage flow monitoring. However, from the professional point of view of flow measurement, the method proposed by the environmental supervision department only simplifies and reorganizes the relevant measurement verification procedures, without in-depth measurement technology analysis, and cannot guarantee the accuracy of on-site flow data. In this paper, based on a large number of sample data for on-site verification of open channel sewage flowmeters, certain research and analysis have been made, which has laid a certain foundation for further accurate online monitoring of sewage flow. 1 Sewage flow measurement 1.1 Principle and composition Due to the standardization of sewage outlets and the needs of video monitoring, the automatic monitoring system of sewage flow takes the weir and groove open channel flowmeter as the main method of flow measurement. The principle of the weir and groove type open channel flowmeter is to set standard water weir grooves in the open channel (such as Parshall water grooves and rectangular thin-walled weirs, etc.) Single value relationship, according to the corresponding flow formula or empirical relationship, the measured water level value is converted into a flow value. The flow meter consists of a standard weir tank and a liquid level flow conversion instrument (secondary instrument). As shown in Figure 1. Figure 1 Schematic diagram of the weir and groove open channel flowmeter system According to statistics, there are a total of 338 weir and groove open channel flowmeters in the automatic monitoring system of sewage flow in this test. Among them, the Parshall trough type accounts for 68.2%, the rectangular thin-walled weir accounts for 14.5%, and other weir and trough types account for 17.3%. This paper takes the standard Parshall trough open channel flowmeter as an example for analysis. 1.2 Mathematical model for calculation of open channel flow rate in standard Parshall tank The standard Parshall tank flow rate Q is calculated as follows: (1) In the formula: C is the flow coefficient; b is the throat width, m; ha is the measured liquid level, m . According to the basic principle and flow calculation formula of standard Parshall tank open channel flowmeter, the accuracy of flow measurement directly depends on the accuracy of flow coefficient C, the accuracy of throat width measurement and the accuracy of measured liquid level. Among them, the accuracy evaluation of the measured liquid level includes the influence of three factors: the measurement accuracy of the liquid level gauge, the measurement accuracy of the instrument measuring the zero point of the liquid level gauge and the disturbance of the liquid level. 2 Influencing factors of on-site flow measurement accuracy Through the above analysis, we can conclude that in the sewage online monitoring system based on open channel flowmeters, the main factors affecting the flow measurement accuracy are as follows: 1) The type of weir and groove, the main Determine the measurement accuracy of the flow coefficient; 2) The accuracy of the measured liquid level mainly depends on the measurement accuracy of the liquid level gauge and the measurement accuracy of the instrument measuring the zero point of the liquid level gauge; 3) The repeatability of the liquid level measurement is determined by It is determined by the disturbance or stability of the liquid level, which mainly affects the measurement accuracy of the average value of multiple liquid level measurement readings; 4) The geometric size of the weir and groove mainly depends on the throat width (or the size of the plates and grooves in other weirs and grooves). ) the accuracy of the measuring instrument; 5) the water level-flow conversion error, which is generated by the software of the secondary instrument of the flowmeter. Since the flow calculation formulas of various weir and groove open channel flowmeters involve multiple parameters, when the flowmeter manufacturer performs conversion in the software system, each parameter is stored in the form of a database function according to different design size ranges, due to the limitation of storage space. The parameters cannot be set accurately enough,This causes errors in the displayed value of the flowmeter. 2.1 Types of weirs and grooves When choosing open channel flowmeters for weirs and grooves, factors such as flow range and design and construction difficulty are mainly considered on site. According to the national metrological verification regulations JJG771-1990 'Trial Verification Regulations of Weir and Slot Flowmeters for Open Channels' and relevant ISO international standards for various types of weirs and slots, the measurement errors introduced by the flow ranges and flow coefficients of several commonly used weirs and slots are shown in Table 1. Show.

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