Since sewage is divided into domestic sewage and industrial sewage, the electromagnetic flowmeter with rubber lining and 316 electrodes can meet the needs of domestic sewage. However, industrial sewage generally has the characteristics of many impurities, strong conductivity and strong corrosiveness, so electromagnetic flowmeter is the first choice for measuring sewage. This paper mainly analyzes the selection of sewage flowmeters from five aspects: the measured medium object, flowmeter performance, installation conditions, operating environment, and economic cost, and provides a reference for the reasonable selection of flowmeters in the process of sewage flow monitoring.
1. Identify the object to be tested:
The measured object is industrial wastewater or domestic sewage. Industrial wastewater contains a large amount of impurities and has certain conductivity and corrosiveness. Electromagnetic flowmeters are the most suitable; electromagnetic flowmeters are suitable for measuring the flow of conductive liquids (conductivity ≥ 5μS/cm). , it only needs to be reasonably selected according to the physical properties of the measured medium and the pipe material: electrode material, lining material, grounding method, and protection level.
2. Flowmeter performance
When selecting a flowmeter, the accuracy, repeatability, linearity, turndown ratio, pressure loss, output signal characteristics, response time, unmeasurability and other factors of the flowmeter should be comprehensively considered first. The sensor structure of the electromagnetic flowmeter is simple, and there is no throttling component in the measuring tube that hinders the flow of the medium, so it will not cause any additional pressure loss. It is one of the flow meters with the lowest energy consumption in the flowmeter. The electromagnetic flowmeter has a very wide range, and is not affected by the temperature, viscosity, and electrical conductivity of the measured medium during the measurement process, and has a sensitive response. Satisfaction, the selection of electromagnetic aperture should ensure that the flow velocity of the measured medium is not less than 0.5m/s under the minimum flow condition.
3. Installation conditions
There are problems with the flowmeter during operation, usually due to problems with the installation. The electromagnetic flowmeter can be installed horizontally, vertically (bottom in and top out), or inclined installation (bottom in and top out), but it is required to have straight pipe sections at the front and back of the flowmeter. The general requirement is that the front straight pipe section is greater than or equal to 5D. The rear straight pipe section is greater than or equal to 3D. The electromagnetic flowmeter should be installed to ensure full pipe flow and cannot be installed at the highest point of the pipe. If the flowmeter is installed in the well, the protection level of the electromagnetic flowmeter sensor should be IP68.
4. Environmental aspects
The environmental aspect mainly considers the influence of ambient temperature, humidity, safety factors, signal conditioning and transmission, pressure, atmospheric and electronic interference on the performance stability of the flowmeter. When installing, try to avoid large motors, large transformers, etc. to prevent the introduction of electromagnetic interference.
The ambient temperature of the electromagnetic flowmeter is generally -30 ~ 80 ℃ (up to 150 ℃), the relative humidity requirements are between 5% ~ 95%, and the atmospheric pressure is 86kpa ~ 106kPa.
5. Economic cost
The choice of flowmeter should be considered comprehensively between initial investment and long-term reliable operation, such as purchase cost, installation cost, operating cost, maintenance cost, calibration cost, flowmeter life, spare parts and consumer goods, reliability and other factors. It is necessary to carry out the optimal economical selection under the premise of effectively satisfying the reliable operation of industrial production.