Sewage flowmeter are widely used in the environmental protection industry

Application of flow meters in the environmental chemical industry

The environmental protection industry has very strict control over sewage flow, and flow meters must have high reliability and accuracy. Sewage flow is a dynamic quantity. There is great friction inside the flowing liquid, and complex secondary flows and vortices may occur. In the process of measuring sewage flow with a flow meter, the shape, diameter, fluid form, installation level, etc. of the flow meter will all have an impact on the measurement.

1 Electromagnetic flowmeter

The main components of the electromagnetic flowmeter include a sensor and a signal converter. The sensor is in direct contact with the pipeline medium, and the signal converter is located at the upper end of the equipment. The flow meter is mainly manufactured based on Faraday's law of electromagnetic induction and is used to measure the flow of conductive liquids with conductivity greater than 5 μs/cm. It is an instrument for measuring the flow of conductive media. In addition to measuring the flow rate of general conductive liquids, it can also be used to measure highly corrosive liquids such as strong acids and alkalis, and liquids containing uniform liquid-solid two-phase suspensions, such as mud, mineral pulp, paper pulp, etc. Electromagnetic flowmeters are instruments with high precision and long service life, so all aspects of product design, processing, production, installation, and debugging need to be carefully considered.

Working principle of electromagnetic flowmeter: Based on Faraday's principle of electromagnetic induction, the conductive medium in the electromagnetic flowmeter acts as a conductive metal rod, and a constant magnetic field is generated at its upper and lower ends. When a conductive medium passes through a magnetic field, an induced voltage is generated. Set two electrodes at both ends of the measuring tube to measure the induced voltage generated. Regarding the isolation between the measuring electrode and the fluid in the measuring tube, it is mainly achieved by the lining composed of rubber, Teflon and other materials to ensure measurement safety.

Measurement principle of electromagnetic flowmeter: According to Faraday's law of electromagnetic induction, when a conductor cuts magnetic field lines in a magnetic field, an electric potential will be generated at both ends. The direction of the electric potential is determined according to the right-hand rule. The magnitude of the electric potential is proportional to the length of the conductor in the magnetic field, the intensity of the magnetic induction, and the speed of the conductor in the magnetic field. The measurement tube of the flowmeter is equivalent to a uniform magnetic field. A non-magnetic pipe is set inside the side tube for sewage discharge. When the conductive liquid flows through the side tube, it is actually cutting the magnetic induction lines. . However, if the law of magnetic induction in a uniform magnetic field is to be satisfied, the following conditions need to be met:

(1) The magnetic field is a uniform and constant magnetic field;

(2) The liquid flowing through the measuring tube The distribution of flow velocity axis is symmetrical;

(3) The measured liquid has non-magnetic characteristics;

(4) The liquid flowing through the measuring tube has unidirectional conductivity and uniform conductivity.

For the installation of electromagnetic flowmeters, the installation site has the following requirements:

(1) When measuring mixed phase fluids, choose a place that will not cause phase separation; measure double groups When dividing liquids, avoid installing them downstream where the mixing is not uniform; when measuring chemical reaction pipes, install them downstream of the section where the reaction has been fully completed; try to ensure that the front and rear straight pipe sections are not less than 5D and 2D respectively.

(2) Avoid negative pressure inside the measuring tube;

(3) For some small instruments, a location with the least possibility of vibration should be chosen as much as possible;

(4) The flowmeter needs to be installed away from large transformers, motors and other equipment to avoid electromagnetic interference to the current meter during the operation of these equipment;

(5) Avoid direct sunlight exposure of the equipment ;

(6) Choose an installation site where it is easy to ground the sensor independently.

2 Electromagnetic flowmeter error analysis

Assume that the electromagnetic flowmeter environment, installation, and the evaporation loss of the sewage itself are basically the same.

The main reasons for measurement errors include the suspended solids content and oil content of sewage in sewage stations. The oil content and suspended solids content of treated sewage are significantly reduced, and the measurement accuracy of electromagnetic flowmeters has also been improved. The specific reasons for the measurement error are as follows: the measurement principle of the electromagnetic flowmeter is Faraday's law of electromagnetic induction. The flow of liquid in the measuring tube is equivalent to the movement of a conductor cutting magnetic field lines in a magnetic field, which generates an electric potential at both ends. The amplification process of the electric potential is used to calculate the flow rate of the liquid flowing through the measuring tube. When there is a large amount of suspended solids and oil in the sewage, the conductivity of the sewage decreases, which affects the accuracy of electromagnetic flow measurement.

3 Application of electromagnetic flowmeter in wastewater flow monitoring in environmentally friendly chemical industry

In the environmentally friendly chemical industry, the excitation method of electromagnetic flowmeter mainly uses DC excitation. It uses a DC magnetic field or directly uses a magnetic magnet to generate a relatively constant and uniform magnetic field. It adopts DC excitation method to generate a uniform magnetic field, which has strong anti-electromagnetic interference management and can basically ignore the self-induction phenomenon of liquid in the measuring tube. However, since the DC excitation method will produce electrolysis in the liquid and affect the measurement accuracy, this method is generally used in non-electrolyte liquid flow measurement.

Sewage treatment in the environmentally friendly chemical industry uses electromagnetic flowmeters to detect sewage return, inlet and outlet flow rates. After the relevant requirements and standard installation of the flow meter are completed, the measurement can be used to judge the sewage feed volume and sediment treatment volume based on the flow display. In addition, electromagnetic flowmeter maintenance is relatively simple and cost-effective, which is in line with the development strategy of the environmentally friendly chemical industry.

In the measurement of some chemical production wastewater, phenomena such as liquid precipitation are often foreseen, causing the measurement electrode to be covered and affecting the measurement accuracy. The main reason for this phenomenon in actual application is that the rationality of the design of the flowmeter is open to question. The measuring pipe and the electrode are not at the same horizontal position. The electrode arm of the electromagnetic flowmeter has a low horizontal position in the measuring pipe. When the liquid flows through the measuring pipe, , precipitation problems are prone to occur, the electrodes are covered, and the accuracy of the measurement cannot be guaranteed. The purpose of reducing the diameter of the electromagnetic flowmeter sensor is to make it inconsistent with the diameter of the pipeline transporting the medium. The purpose is to speed up the speed of the medium passing through the electromagnetic flowmeter sensor and reduce the precipitation of the medium at the sensor. However, this design encounters another problem, which is the acceleration of the flow rate.Fast, the noise generated by the flow increases, and the increase in noise has a certain impact on the measurement accuracy. In order to solve this problem, they changed the lining of the electromagnetic flowmeter sensor to a rubber lining, and at the same time increased the thickness of the lining, effectively The noise is reduced, which solves the problem of inaccurate measurement of electromagnetic flowmeter.