0 Preface
With the growing problem of environmental pollution, China has put forward higher requirements for establishing an environmental management system and increasing environmental monitoring. At present, the domestic key sewage companies have installed automatic sewage monitoring systems to ensure that the data of sewage discharge can be uploaded in real time to the municipal, provincial environmental monitoring agencies and national environmental protection authorities. In order to ensure the validity of the uploaded data, the Ministry of Environmental Protection issued the HJ/T355-2007 "Technical Specifications for the Operation and Assessment of Water Pollution Source Online Monitoring Systems" and HJ/T356-2007 "Technical Specifications for the Validity of Water Pollution Source Online Monitoring Data" and other specifications. Sex file. These documents specify the test items and data requirements for the relevant water quality monitoring data (such as chemical oxygen demand CODcr, ammonia nitrogen NH3-N, total phosphorus TP, pH value, etc.), but are accurate on how to ensure the flow monitoring data of the water quantity. However, no request was made.
With the introduction of monitoring requirements for emission reductions of major pollutants, this situation has been increasingly discovered and valued. At the end of 2013, the environmental monitoring department had already proposed techniques and methods for quality supervision and inspection of sewage flow monitoring. However, from the point of view of flow measurement expertise, the method proposed by the environmental supervision department only simplifies and reorganizes the relevant metrological verification regulations, and does not perform in-depth metrological analysis, which does not guarantee the accuracy of on-site flow data. In this paper, according to the large number of sample data of the on-site verification of the open channel sewage flow meter, a certain amount of research and analysis was made, which laid a certain foundation for the further accurate realization of the on-line monitoring of sewage flow.
1 Sewage flow measurement
1.1 Principle and composition
Due to the standardization of sewage outlets and the need for video surveillance, the automatic monitoring system for sewage flow uses the gutter-type open channel flowmeter as the main method for flow measurement. The principle of the gutter-type open channel flowmeter is to set standard water gutters (such as a Parshall gutter and a rectangular thin wall grate) in the open channel and measure the water level according to the specified position. The flow rate and water level through the gutter are presented. In the single-valued relationship, the measured water level value is converted into a flow value according to the corresponding flow formula or empirical relationship. The flow meter consists of a standard volume gutter and a level flow conversion instrument (secondary instrument). As shown in Figure 1.
Fig. 1 Schematic diagram of the guttering open channel flowmeter system
According to statistics, the total flow rate of this test was 338 sets of open channel flow meters. Among them, the Parshall trough type accounted for 68.2%, the rectangular thin-walled nevus accounted for 14.5%, and the other gully type accounted for 17.3%. This article takes the standard Parshall open channel flowmeter as an example for analysis.
1.2 Mathematical model for the calculation of flow in a standard Parshall trough open channel
The standard Parshall trough flow Q is calculated as follows:
(1)
Where: C is the flow coefficient; b is the throat width, m; ha is the measured level, m.
According to the basic principle and flow calculation formula of the standard Parshall open channel flowmeter, the accuracy of the flow measurement directly depends on the accuracy of the flow coefficient C, the accuracy of the throat width measurement and the accuracy of the measured liquid level. Among them, the accuracy evaluation of the measured liquid level includes the influence of three factors: the accuracy of the level gauge measurement, the measurement accuracy of the instrument to measure the zero point of the liquid level gauge, and the disturbance of the liquid surface.
2 Influencing factors of on-site flow measurement accuracy
Through the above analysis, we can conclude that the main factors affecting the accuracy of flow measurement in the online sewage monitoring system based on open channel flowmeters are the following:
1) The type of the gutter mainly determines the measurement accuracy of the flow coefficient;
2) The accuracy of the measured liquid level mainly depends on the accuracy of the level gauge measurement and the measurement accuracy of the instrument measuring the zero level of the liquid level gauge;
3) The repeatability of the liquid level measurement, which is determined by the level of disturbance or smoothness of the liquid level, mainly affects the measurement accuracy of the average of multiple liquid level measurement readings;
4) The geometry of the gutter mainly depends on the accuracy of the measuring instrument of the throat width (or other gutters in the plate, slot size);
5) The water level-flow conversion error is generated by the flowmeter's secondary instrument software. Because the flow calculation formulas of various open channel flowmeters involve multiple parameters, when the flowmeter manufacturer converts the software system, each parameter is stored as a database function according to different design size ranges, due to the limitation of storage space. It is not possible to set the parameters with sufficient accuracy, thus causing the error of the flow meter display value.
2.1 Types of gutters
When the open channel flow meter selects the gutter, the site mainly considers the flow range, design and construction difficulty and other factors. According to the national metrological verification regulations JJG771-1990 "Test procedures for open channel gutter flowmeters" and relevant ISO international standards for various types of gutters, the measurement errors introduced by the flow ranges and flow coefficients of several common gutters are shown in Table 1. Show.
Table 1 Parameters of several commonly used gutters
It can be seen from Table 1 that the measurement errors introduced by the flow coefficients of several kinds of gutters are not much different, and there are no high technical points in various gutter types. After comprehensive consideration of the flow application scope and the difficulty of site design and construction, Parshall trough and thin-walled trout have certain advantages. Compared with the two, thin-walled crucible has the advantages of convenient manufacture and simple installation. The disadvantage is that the head loss is relatively large. Sediment and suspended matter in the sewage are easily deposited before the crucible to increase the water level and affect the measurement accuracy. The Parshall trough has the advantages of wide measurement range, small head loss, and difficulty in sediment deposition in the sewage, but the processing and installation are more complicated.
2.2 Accuracy of Level Measurement
The actual level measurement in an open channel flowmeter involves two aspects of the zero point of the level gauge (ie, the liquid level zero point) and the accuracy of the level gauge. The early float level gauge has been eliminated due to low measurement accuracy. The position meter is also less used due to its susceptibility to effluent corrosion and sediment coverage. Ultrasonic level gauges are now mainly used. Ultrasonic measurement of liquid level is non-contact measurement, which can be affected by the nature of the liquid, but it will also be affected when there are more liquid surface froth and solid floating objects.
The ultrasonic liquid level gauge used in the open channel flowmeter of this experiment has a nominal measurement accuracy of no more than 3mm, which meets the requirements of the environmental protection industry standard HJ/T15-2007 "Technical Requirements for Environmental Protection Products Ultrasonic Open Channel Sewage Flowmeter" Bit measurement error requirements. However, actual measurement results are quite different. The specific data is shown in Figure 2.
Figure 2 level measurement results
Tests have found that there are many problems with ultrasonic level gauges used in the field. The reason for the inaccurate zero is mainly because the zero point is not set or calibrated when the level gauge is installed. Part of the reason for the water level error is that the level meter has not been calibrated after installation, and the other part is that the sensor probe is aging during the use of the level gauge, and the measurement accuracy is not up to the standard.
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