Abstract: Industrial boilers play a pivotal role in industrial production. Starting from the purpose of saving energy and reducing consumption and improving work efficiency, this article systematically studies the problems of loos boiler continuous blowdown equipment in industrial production. This article summarizes the existing problems of continuous blowdown of boilers, studies and constructs a retrofit system from three aspects of boiler continuous blowdown operation principles, design ideas, and implementation schemes, and proposes a continuous boiler blowdown control system based on instrument control. Maintaining the position of the liquid level in the continuous blowdown expansion vessel within a relatively stable range improves the stability of secondary steam generation and prevents the loss of deaeration tank steam and secondary flash steam. At the same time, on the basis of ensuring the effect of continuous sewage discharge, the temperature of sewage discharge is effectively controlled to realize the effective recovery and utilization of waste heat.
introductionAs a necessity of industry and society, industrial boiler occupies an important position in national economy and social development. Take our boiler system as an example. Our boiler system mainly provides production steam. In the working process of the boiler, the water in the boiler is gradually concentrated during the continuous evaporation process, so that a large amount of floating oil, various suspended solids and concentrated salts remain on the surface of the boiler water, and it is easy to produce deposited sludge at the bottom of the boiler. , Mud and other substances. In order to ensure the working efficiency of the boiler, maintain high-quality steam operation of the boiler, prevent boiler deposits from corroding the boiler wall, and ensure the safe operation of the boiler, the boiler has a special device for continuous and regular blowdown.1 Current problems with continuous boiler blowdownTaking the continuous blowdown equipment of our boiler as an example, there are currently two main problems.1.1 The output of secondary steam is unstable and the heat loss is largeIn the case of a large continuous blowdown of the boiler, part of the secondary steam produced by flashing will be discharged from the bottom together with the waste water, resulting in a waste of secondary steam. Moreover, because the water flow rate of the continuous blowdown boiler from the boiler is different in different time periods, and the flow rate change is sometimes rapid, the regularity is not obvious, the direct blowdown method will also cause the expansion of the vessel to expand and the separation effect is unstable, and the secondary steam The amount of production is also unstable. Generally, the normal boiler blowdown rate is 5%~10%, that is, the ratio of continuous blowdown sewage to the overall boiler operating water. There are three main factors affecting the blowdown rate, namely the alkalinity of water, the hardness of water and the control value of alkalinity of boiler water. Generally, when the salt concentration of boiler water is relatively low, the blowdown rate is usually low, that is, 4%~5%. We use a pollutant discharge rate of 5%, a pollutant flow rate of 0.5 t/h, a working pressure of 0.8 MPa, and a saturation temperature of 175 ℃. The heat loss per hour is nearly 4×10 5 million kJ, and the calorific value of burning 32 kg of raw coal. Quite, it is not difficult to see that the heat loss of sewage is quite large.1.2 Loss of steam in the deaeration tankThe existing deaeration tank uses boiler steam for thermal deaeration. Therefore, the deaeration tank maintains a certain pressure, which is normally about 0.01 MPa. When the boiler is not discharged continuously, the steam in the deaeration tank will be affected by the pressure. It enters the continuous expansion vessel and is discharged by the manual valve at the lower end, which will cause the loss of steam in the deaeration tank.2 Improvement of boiler continuous blowdown control system2.1 Principle of continuous blowdown operation of boilerContinuous blowdown mainly removes the oily dirt, suspended solids and concentrated salt on the surface of the boiler water and in the water body, thereby reducing the alkalinity of the boiler water and improving the quality of the boiler steam. The continuous blowdown system is mainly composed of a blowdown pipeline and a continuous blowdown expansion vessel. The continuous blowdown expansion vessel is also called continuous blowdown expander. It is directly connected to the continuous blowdown port of the boiler. Its main function is to decompress and expand the continuous blowdown boiler water of the boiler. The blowdown boiler water is decompressed and adiabatic and expanded in the continuous blowdown expander. It is secondary steam and waste water. The secondary steam is led out by a special pipe and enters the deaeration tank for use, while the waste water is directly discharged from the expansion vessel through the manual valve at the lower end.2.2 Improved design ideasWe modified the continuous discharge expansion vessel. Because the lower discharge port of the expansion vessel adopts direct discharge, the flash steam and wastewater will be discharged together under the condition of large flow. In the case of no flow, the deaeration tank steam will be in the deaeration tank. The internal steam enters the expansion vessel and is lost under the pressure. In order to improve this situation, it is necessary to add a set of liquid level control system to keep a certain liquid level in the expansion vessel, which can not only ensure the stability of the pressure in the expansion vessel, ensure the stable generation of flash steam, but also prevent flash steam and removal. Loss of steam from the oxygen tank, save steam as much as possible. This requires the system to have high requirements for liquid level control, requiring stable control, capable of coping with the influence of different continuous blowdown flows on the liquid level, and ensuring the safe and stable operation of the expansion vessel. According to the above conditions, this set of control system is designed independently, which is composed of measuring mechanism, control mechanism and execution mechanism. The core is to upload the liquid level information to the PID controller through the sensor, use the internal PID calculation of the PID controller to realize the liquid level data processing, and feed it back to the electric control valve. By adjusting the valve degree of the control valve, the liquid level of the expansion tank is maintained at a stable level. In the range.2.3 Boiler continuous blowdown implementation plan2.3.1 Selection of liquid level sensor for measuring mechanismThe sensor mainly considers two types of commonly used magnetic flap level gauges and differential pressure level gauges. When the boiler blows into the continuous expansion vessel, it will cause pressure fluctuations in the expansion vessel, which will affect the measurement accuracy of the differential pressure level gauge. The magnetic flap level gauge uses liquid level buoyancy, which is not affected by pressure fluctuations, and is convenient for visual observation of the liquid level on site. So we finally choose the magnetic flap level gauge as the source of our level signal.2.3.2 Field Controller SelectionIf the budget of the field controller is sufficient, consider the common Siemens PLC controller on the market, which has the advantages of powerful computing function and high real-time performance. If the XSC5 series PID instrument controller can be selected from the perspective of control budget, it can effectively adapt to the high temperature environment on site, and the cost is low, which can meet our design requirements. It can receive the 4 mA-20 mA liquid level signal from the magnetic flap level gauge, and control the on-site Siemens electric control valve to ensure the liquid level is stable after calculation by the internal PID algorithm.2.3.3 Field PID parameter tuningThe field control method generally adopts PID control, which is controlled according to the proportion (Proportional), Integral (Integral) and derivative (Derivative) of deviation (PID control for short) is the most mature and widely used control technology in continuous system control theory. . Due to its advantages such as simple algorithm and high reliability, the most commonly used control law of controllers in control systems is PID control. The principle block diagram is shown in Figure 1.PID controller parameter tuning is the core content of control system design. There are many methods for PID controller parameter tuning, which can be summarized into two categories: theoretical calculation tuning method and engineering tuning method. Generally speaking, engineering tuning method is mostly used. Through field test and analysis, the PID parameters are adjusted according to the changes of the liquid level in multiple tests, and the PID parameters should be revised in time according to the changes in the continuous sewage discharge.2.3.4 On-site electric control valve selectionThe field regulating valve is required to be able to withstand high temperature, high reliability, and be able to receive control signals. Generally, 4 mA~20 mA or 0 V~10 V are the main ones, and can be fed back through signals. Based on this situation, Siemens SKD62 series electric control valves are generally used.3 Improvement effect of boiler continuous blowdown control systemThe modified continuous sewage system has been put into operation in the actual factory, and the operation effect is good. After the transformation is completed, through long-term operation observation, when the continuous sewage flow is unstable, the PID controller controls the opening of the regulating valve, responds to changes in flow in a timely manner, and adjusts the liquid level in the expansion vessel effectively. The violent fluctuation of the liquid level is avoided, the liquid level in the expansion vessel is stable, the pressure in the expansion vessel is constant, and the lower liquid level can effectively ensure that the steam enters the deaeration tank, preventing the steam in the deaeration tank from being lost through the expansion vessel.The optimization and transformation of the boiler continuous exhaust system can not only improve the stability of the secondary steam generated by flash evaporation in the expansion vessel, but also avoid the loss of steam and secondary steam in the deaeration tank in some cases, and save energy. At the same time, you can also consider uploading the operating data such as the liquid level in the controller to the central control room in the later stage, so that the operating personnel can remotely find the problem, and can always grasp the boiler blowdown data to better ensure the operation of the boiler system.
Post time: 21-09-21