Application of On-Line Instruments-Electromagnetic Flowmeter in Water Plant Construction

Abstract: With the continuous development of automation technology, the application scope of automatic control systems and online instruments is becoming more and more extensive, and they have been effectively used in various fields in our country. PLC-based control systems are now mainstream. This paper introduces the main online instruments in the distributed control system with PLC as the basic control unit based on the example of the water plant automation system built on the basis of the new generation of water treatment process in Beijing in recent years (focusing on Shijingshan Water Plant as an example). The type, principle, selection and debugging situation. The system sets up a PLC field control station in the control room of each process structure. All monitoring signals in each PLC field control station are uploaded to the monitoring terminal in the central control room in real time through industrial Ethernet (1000MbpS industrial Ethernet). The control equipment and online instruments that make up this system are based on the fundamental premise of information security, and also need to select mainstream products that are easy to use, simple to install and maintain, and highly reliable according to the different objective factors such as the water quality and environment of the process section. The expansion space is reserved on the above to maintain the scalability of the system.

Common online instruments in water plants

The automatic monitoring system of the water plant needs a large number of online instruments as the foundation to build, and it is very important to select and equip it reasonably. In order to better cope with the daily technical management and maintenance work, it is also necessary to master the basic principles of the instrument.

Pressure transmitter

Pressure transmitter (usually also used as liquid level transmitter) is one of the commonly used process parameter sensors in water plants. Its physical principle is to convert the pressure signal of gas, liquid and other media through the conversion function characteristics of the transmitter and cooperate with the peripheral circuit to transform it into an electrical signal proportional to the original measured physical signal and transmit it to the automatic control system.

The usual types of pressure transmitters are: piezoelectric, piezoresistive, capacitive and so on.

Piezoelectric pressure transmitter is a specific application of the piezoelectric effect. It uses the characteristic that certain crystals will generate charges on their surfaces when they are subjected to external pressure. The number of charges has a fixed proportional relationship with the external pressure.

However, since the charge generated when the pressure is constant or changing slowly is easy to leak through the external circuit, the piezoelectric transmitter cannot be used for static pressure measurement but is suitable for dynamic pressure measurement. Capacitive pressure (differential pressure) transmitters generally use a circular metal film or a metal-plated film as an electrode of the capacitor. When the film is deformed by pressure, the capacitance formed between the film and the fixed electrode changes. It can output an electrical signal that has a certain relationship with it.

Flow Meter

The flow meter in the automatic control system of the water plant is to accumulate the amount of water processed and grasp the instantaneous change of the amount of water in operation. In water plants, flow meters are usually installed at the water inlet before treatment and the discharge outlet after treatment. The two most widely used flowmeters in modern water plants are electromagnetic flowmeters and ultrasonic flowmeters.

Water (mud) quality analysis instrument

Under normal circumstances, the analytical instruments in the automatic control system of water plants mainly include pH meters, turbidity meters, residual chlorine analyzers and thermometers. Among them, the residual chlorine meter is used in the effluent water and the factory water after the filter, mainly to measure the concentration of free residual chlorine in the water; the pH meter is used in the raw water inlet and outgoing water, mainly to measure the pH of the water body; the turbidity meter is used in the raw water inlet. The concentration of suspended solids in the water plant is detected at the plant and the water outlet after sedimentation in the sludge discharge room of the machine addition tank; the thermometer is used to monitor the temperature of the water body in real time in special seasons to prevent the water body from freezing and causing unfavorable water supply.

Liquid level meter

The liquid level instrument is also a commonly used detection instrument in the automatic control system of the water plant. Its function is to control the opening of the inlet and outlet gate valves in real time according to the real-time situation of the liquid level in the pool and the needs of the process. Among them, the hydrostatic level gauge and the ultrasonic level gauge are the most widely used in modern water plants.