Optimization scheme of feedwater control for 1000MW ultra-supercritical unit

Abstract: 1000MW ultra-supercritical unit feedwater control optimization plan information is provided by excellent flowmeter and flowmeter manufacturers and quotation manufacturers. The significant difference between the supercritical unit and the subcritical unit is that the boiler adopts once-through furnace. The remarkable feature of once-through boiler is that there is no steam drum. The once-through boiler is a multi-input and multi-output control object, in order to meet the characteristics of fast dynamic response and small inertia of the once-through boiler. For more flowmeter manufacturers to select models and prices, you are welcome to inquire. The following is the details of the article on the optimization of feedwater control for 1000MW ultra-supercritical units. The significant difference between the supercritical unit and the subcritical unit is that the boiler adopts once-through furnace. The remarkable feature of once-through boiler is that there is no steam drum. The once-through boiler is a multi-input and multi-output control object. In order to meet the characteristics of fast dynamic response and small inertia of the once-through boiler, the control strategy of parallel feedforward and small deviation adjustment is adopted for the control of the boiler side. That is, the output of the main control of the boiler is sent to each sub-regulation system of fuel, air volume and water supply in parallel, and the deviation adjustment is carried out on this basis to ensure that the boiler can be adjusted without deviation in a steady state. Feedwater control is the main control difficulty of ultra-supercritical boilers, which is very different from subcritical. The control task of the feedwater control system is to keep the feedwater flow not lower than the minimum required feedwater flow of the boiler at low load, and to maintain an appropriate fuel-water ratio when the boiler enters the once-through operation mode. The following takes two 1000Mw ultra-supercritical coal-fired turbo-generator units in the fourth phase of Huadian International Zouxian Power Plant as an example to introduce the ultra-supercritical water supply control system. 1. Feed water control object The boiler feed water system is equipped with 2 sets of 50% capacity steam-driven variable-speed feed pumps, and 1 set of 25% BMCR (boiler maximum continuous evaporation) capacity variable-speed electric feed pumps as backup. The steam-driven feed pump is designed with two steam sources of high and low pressure, which can be switched automatically. The high-pressure steam source is cold reheat steam, the low-pressure steam source is four-stage extraction steam, the auxiliary steam used in the factory is used as the starting and debugging steam source, and the small machine exhausts steam. to the main condenser. The control of the feed pump is controlled by DCS and Siemens WOODWARD505 controller. The 505 controller receives the remote control speed signal sent by DCS, controls the opening of the high and low pressure door, adjusts the speed of the small machine, and meets the water supply requirements of the system. The closed-loop control of the water flow is realized in the DCS, and the WOODWARD505 controller realizes the closed-loop control of the pump speed. The WOODWARD505 controller adopts the single 505 operation mode, and the operation functions of some operators of the 505 operation panel are made special operation screens in the DCS to realize remote operation. The signal transmission adopts two methods of hard wiring and communication. 505 outputs 2 valves for controlling high and low pressure steam sources. During normal operation of the unit, the four-extraction incoming steam is used. When the low-pressure regulating valve is fully opened and the four-extraction steam source cannot meet the operation needs of the small unit, the high-pressure regulating valve is opened to introduce cold and reheated steam. The high pressure regulating valve is located in front of the main valve of the small engine in the system layout. The high and low pressure air sources are mixed in front of the main valve and then enter the turbine through the low pressure regulating valve. The electric feed pump changes the performance curve of the pump by adjusting the position of the spoon tube of the hydraulic coupling to change the speed of the feed pump, so that the working point is moved, so as to achieve the purpose of adjusting the flow rate of the pump. The speed regulating mechanism of the spoon tube of the rotational speed control unit adopts an electro-hydraulic servo mechanism. The spoon tube acts according to the control signal, and drives the fan-shaped gear shaft to rotate through the crank and connecting rod. The amount of oil flushing can realize the stepless adjustment of the output speed. The operation mode of the spoon tube adopts electro-hydraulic servo mechanism, and the electro-hydraulic servo system consists of an electromagnetic actuator, a double-acting hydraulic cylinder and a position detector. The electromagnetic actuator receives the control signal of 4-2OmA, and the position of the actuator is controlled by this signal. The position of the electro-hydraulic servo system is controlled by an internal positioner solenoid valve. The signal triggers the action of the magnetic controller. The electromagnetic force is controlled by controlling the piston of the multi-directional hydraulic valve. The position detector detects the position difference and feeds the signal back to the positioner, enabling the system to operate precisely and quickly. This allows for a 'soft start' of the coupling. 2. Control scheme Modern large-scale unit units all use variable speed pumps to control the flow of feed water. Zou Power Phase IV ultra-supercritical 1000MW units use 2 steam-driven variable speed pumps as the main feed pumps, 1 electric feed pump as the start-up feed pump, and Used as a backup pump for the system. Various types of variable speed pumps have their own safe working area. In order to prevent cavitation of the pump and improve the working efficiency of the pump, when controlling the variable speed pump, it is necessary to ensure that the pump works in a safe working area by changing the speed, pressure and flow. Inside. The safe working area of ​​the pump is related to the pressure. When the pressure is high, the safe area is wider, and when the pressure is low, the safe area narrows. In order to ensure that the pump works in the working area, measures are taken as follows: ①At low load, when the flow rate of the pump is lower than the minimum flow rate, the recirculation door is automatically opened to increase the flow rate in the pump body,Therefore, the working point of the feed pump in the low load stage is also within the upper limit characteristic curve. ② When the flow rate is higher than a certain value, the recirculation door will automatically close. When the amount is large, if the safe working area is narrow, the working point may be outside the lower limit characteristic area. In order to prevent the occurrence of this phenomenon, the method used is to increase the resistance of the upper water pipeline, that is, to close the flow control valve at the outlet of the small pump to increase the outlet pressure of the pump and move the working point back into the safe area.

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