As China’s industrialization accelerates, the problem of environmental pollution is becoming increasingly serious and society’s call for sustainable development is becoming louder and louder. The concept of harmonious coexistence between man and nature is accepted by the people, and the treatment of all kinds of pollution is more urgent.
Because of the need to measure and control the discharge of effluent, additives, sludge, etc. in wastewater treatment processes, flow measurement becomes very weighty. Today we will talk about sewage treatment, the most commonly used several flow meters.
1. Electromagnetic Flowmeters
Electromagnetic flowmeter whose principle of operation is based on Faraday’s law of electromagnetic induction, that is, the measured medium perpendicular to the direction of magnetic lines of force flow, and therefore in the direction of the flow and magnetic lines of force are perpendicular to the medium to generate the induced electric potential.
The relationship between the induced electric potential and the measured medium flow is proportional, using the flow rate and the cross-sectional area of the pipe to derive the instantaneous flow rate. The conductivity of the water is one of the prerequisites for the electromagnetic flowmeter to work properly.
Sewage contains a large number of impurities and salts, with a certain conductivity, so the electromagnetic flowmeters can be used for sewage measurement. For pure water, on the other hand, cannot be measured using an electromagnetic flowmeter. The advantages of the use of electromagnetic flowmeter are high accuracy, stable measurement, a wide range of applications, according to the media selection of electrodes and liners, in the harsh media can also be used to complete the measurement work; the disadvantage is vulnerable to electromagnetic wave interference, the larger the pipe diameter, the more expensive, and large pipe diameter electromagnetic flowmeter disassembly and installation are not easy, difficult to overhaul.
Because the original sewage is containing particles, suspended matter, solid-liquid mixture, corrosive media, and electromagnetic flowmeter big feature is able to measure the flow of acid, alkali, salt solution, and containing solid particles (such as mud) or fiber liquid.
2. Ultrasonic Flowmeters
The working principle of the ultrasonic flowmeter is that, with the help of the time difference measurement principle, through a pair of ultrasonic probes emitting signals through the intermediate medium to reach the other side of the pipe wall, to be received by another probe, the other side of the probe similarly emitting signals to be received by the first probe.
In this process, there is a certain time difference because the signal delivery time is affected by the medium flow rate. Based on this, the ultrasonic flowmeter calculates the corresponding flow value with the help of the relevant formula.
In general, ultrasonic flow meters can be divided into 2 types: external clamping type and insertion type. With the continuous updating and upgrading of technology, ultrasonic flowmeter from a single sound path to the development of multiple sound paths, at present, commonly used is 8 sound path ultrasonic flowmeter, its accuracy can reach the degree of electromagnetic flowmeter, can overcome the problem of poor installation conditions.
If the length of the straight pipe section before the flowmeter can not reach the installation requirements can be installed a multi-sound path ultrasonic flowmeter. The ultrasonic flowmeter has the advantages of automatic data storage, high measurement accuracy, full Chinese display, flexible operation, ease to carry, etc. Compared with electromagnetic flowmeter, the large diameter of the installation of ultrasonic flowmeter (especially in the 1 m above the large diameter) has obvious advantages, and the construction is not difficult, relatively low cost, easy maintenance.
However, the disadvantage of the ultrasonic flowmeter is that the installation of insertion ultrasonic transducer will be affected by the media composition, the scope of application is limited; transducer in the process of use will be aging, affecting the measurement signal strength, reducing the measurement accuracy; when the measured medium in impurities or bubbles, there will also be unable to measure the situation.
In actual use, the park has 2 manufacturers installed ultrasonic flowmeters. Restricted by the installation conditions, the flowmeter is installed in the pump outlet 2 m less than the place, the pump starts, the signal strength of the flowmeter down to 0, when the pump stops, the signal strength of 100. It can be seen that the ultrasonic flowmeter by the medium in the bubble is very influential, bubbles more than can not be used normally.
External clip type ultrasonic flowmeter is easy to carry and install, measurement and testing units are often used as standard measuring instruments (generally imported products) in the field for flow comparison work, but by the degree of temperature resistance of the coupling material between the transducer and the pipeline, the original data of the measured fluid transmission speed at high temperature is incomplete, as well as the impact of the transducer power strength, easily lead to the flowmeter can not work properly.
If long-term installation and use in the field, the probability of failure is greater, therefore, the flowmeter coupling agent needs to be replaced regularly, and tighten the outer wall of the tube on the transducer. This external clip-type ultrasonic flowmeter our company has not considered the use.
3.Open Channel Ultrasonic Flowmeter
Most of our sewage treatment plant sewage discharge way is natural flow down, that is, the flow of sewage in the pipeline or channel is a non-full tube. To this non-full tube state flow of the waterway, that is, with a free surface of the waterway, called “open channel”. Even in the so-called “culvert” waterways, as the liquid is naturally flowing down, so it is also open channel flow. In such waterways, conventional full pipe flowmeters with pressure cannot be used when measuring flow.
Measured sewage contains a variety of impurities and other organic chemical components, although the sewage discharge has been made before the purification process, but, if the use of direct contact with the sewage medium flowmeter, after a long time, the internal flowmeter will inevitably appear scaling or be corroded, thus affecting the smooth measurement work.
Sewage discharge, especially sewage treatment plant sewage discharge measurement instrument, is one of the key monitoring data of the environmental protection department, it must have the following characteristics:
① measurement instrument must be the non-contact measurement of the instrument. Most of the sewage contains a variety of dirt and corrosive substances, only non-contact measurement, in order to avoid the impact of the physical and chemical properties of the medium, so as to ensure the accuracy of measurement and even inertia.
②The detection parts must be clamped on the outside of the pipeline, can be moved to any location at will to measure, without truncating the pipeline, the measurement instrument range than wide, good linearity, moderate price. ③Measuring instrumentation low power consumption, in line with today’s trend of miniaturization of instrumentation.
Ultrasonic open channel flowmeter is a non-contact instrument with the above characteristics. The advantages of the open channel flowmeter are the measurement range is large, flow measurement is not affected by the tributary surface backwater, floating sediment in the water, bubbles and water level changes significantly, and the flow sensor will produce resistance to water flow, simple structure, small volume, easy to install. It Will be used in the PRD region, the standard channel does not need to be modified, can be installed directly, and the installation and construction costs are low.
Marshall trough is a kind of open channel water weir trough, generally, by the contraction section, thick throat channel section, throat channel under the section, divided into large, small, standard type trough, with solid material in the water does not precipitate, the water level raised fall small characteristics, suitable for use in the drainage ditch for the measurement of flow. The open channel flow meter also relies on the ultrasonic level meter to measure the flow in the actual operation process. In addition, the device can be used in harsher environments due to its ability to perform non-contact measurements during actual operation.
Overall, the complex shape of the marshall tank is more expensive than a weir, and the dimensions of the part of the measuring tank are required to be accurate in order to improve accuracy. In addition, when installing an open channel flowmeter such as a marshall tank in the Pearl River Delta, it is important to avoid the effects of high and low tides on the flowmeter and to ensure that the flowmeter has sufficient fall to ensure accurate measurement.
Its advantages are mainly small water level loss (about 1/4 of the weir), almost no precipitation in the water even if there is a solid material, small impact on the approach flow rate, relatively small impact on the water level downstream side, etc. Therefore, it is suitable for measuring the flow of sewage discharged from sewage treatment plants.
Sewage flow metering instrument selection recommendations
In order to ensure that the sewage flow meter in the actual operation process can give full play to its own benefits and promote the development of related work, the operator to strengthen the scientific selection of sewage flow measurement instrumentation.
In the company to determine the sewage flow measurement instrument selection before, according to the actual situation on-site and the comparison of different flow meters, the company’s sewage flowmeter selection related to the summary:
① electromagnetic flowmeter measurement stability, a wide range of applications, can measure a variety of media, but easy to be electromagnetic interference. With the increase in pipe diameter, its price will also become more and more expensive.
②Ultrasonic flowmeter cost is relatively low, high measurement accuracy, stable operation, easy installation, maintenance, the price will not increase with the increase in pipe diameter and more and more expensive, but it will be more and more expensive because of the increase in the sound path.
③ open channel flowmeter measurement range is wide, flow measurement is not affected by changes in the medium, but it is subject to the constraints of the size of the measuring tank.
Generally speaking, in the case of meeting the open channel installation conditions, the flow data is large and can be conveyed through the open channel sewage flow operations, using weir type flowmeter and trough type flowmeter for related operations. In the actual measurement process, this type of equipment can meet the requirements of metering for accuracy and can save energy.
It uses the self-flow method of conveying sewage. In contrast, the electromagnetic flowmeter still has the absolute advantage, regardless of the matter from the principle, or measurement accuracy, are the most stable and excellent.
In addition, in the pipeline transport of sewage flow measurement operations, the operator needs to increase the use of an electromagnetic flowmeter (ultrasonic flowmeter). In the process of using this equipment, the cleaning of the transducer, electrodes, and other components can contribute to the improvement of measurement accuracy.
In addition, in order to save measurement costs, when the diameter of the pipe is less than 500 mm, electromagnetic flowmeter operation can be used; when the diameter of the pipe is greater than 500 mm, it is necessary to rely on ultrasonic flowmeter operation. To improve the accuracy of the flowmeter, a multi-sound flowmeter can be used (there are 4-sound, 8-sound, etc.).