Introduction to Deaerator Level Control

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Introduction to the Deaerator Level Control in XX Power Plant CWBAOBAO 2003-08-01 At present, XX Power Plant has four 600MW supercritical pressure units in operation, and the deaerator level control in this plant is part of the plant’s automatic control system. Its feature is that, due to the thermal system and operating characteristics of the unit, the deaerator water level control can automatically select between single-impulse or three-impulse control under different operating conditions. A brief introduction is provided below for our joint learning*. I. Process flow for deaerator water level control. The process flow is shown in Figure (1); the output of each condensate pump and each steam-driven feedwater pump is 50% of the MCR. The electric feedwater pump operates at variable speed via a hydraulic coupling, with an output of 30% MCR. The normal water level in the deaerator tank is 2875 mm, with a water capacity of 425 T. The unit operates in slip mode at dry conditions (i.e., within the 160MW–600MW range). Under normal conditions, the high-pressure heater drain flows by gravity stage by stage to the deaerator tank. #The drain water from the 2nd to 4th low-pressure heaters flows by gravity stage by stage to the low-pressure heater drain tank, and then is pumped into the inlet on the water side of #3 low-pressure heater via the low-pressure heater drain pump; the drain water from #1 low-pressure heater flows directly to the condenser expansion tank. The water level control of the deaerator is achieved by throttling the deaerator water level control valve at the outlet of the shaft seal heater, thereby changing the flow rate of condensate water entering the deaerator. II. Deaerator water level regulation and control section: The schematic diagram of the deaerator water level control is shown in Figure (II). The system employs both three-pulse cascade control and single-pulse control to meet the requirements of different operating conditions. Measurement element: a) LT: The operating parameters of the deaerator tank are relatively low (rated: p=0.97 MPa, t=176°C); therefore, there is no correction for measurement errors in the water level measurement section, as is the case with bubble level measurement. However, to improve system reliability, three level transmitters were used, and the average value of these three was taken as the deaerator level feedback signal. b) LS: The water level switch is used to detect low water level 1, low water level 2, high water level 1, high water level 2, and high water level 3, thereby triggering alarms or activating relevant protection mechanisms. c) FT1: The feedwater flow measurement signal is derived from the feedwater flow feedback in the boiler coordination control; a orifice plate flow meter is used, and the average value of the three flow transmitters is taken as the feedwater flow, with adjustments made for feedwater temperature. d) FT2: The flow measurement point for the condensed feedwater entering the deaerator is installed at the outlet of #4 low-temperature heater. It is also a throttle orifice flow meter, but the intermediate value of the three flow transmitters is used as feedback for the condensate flowing into the deaerator, without any temperature correction. Actuator: The actuator is a pneumatic flow control valve, equipped with feedback devices for indicating the valve position as well as for detecting when the valve is fully open or fully closed. Control section: a) As shown in Figure (II), the set water level for the deaerator is 2875 mm. b) The selection between the three impulse and single impulse control modes is made by using switch T1, depending on the size of the feed water flow rate. c) When the water level switch LS is at a high level of 3, in addition to shutting off all steam and drain flow to the deaerator, it also acts on the water level control circuit of the deaerator: the selector switch T2 sets the valve position command to zero, and a digital signal is sent to activate the solenoid valve in order to quickly close the deaerator’s water level control valve. d) The water level control of the deaerator can be adjusted by means of an automatic/manual switch, as well as selector switch T3 to change the control mode (the tracking portion of the schematic is omitted). e) When the difference between the set value and the feedback value of the valve position of the deaerator’s water level control valve exceeds ±15%, an abnormality alarm for the valve is issued after a 10-second delay, and the control mode is automatically switched to manual control. f) Single impulse control: Single impulse control is used in operating conditions where the water supply flow rate is less than 450 T. The output obtained after the deaerator level deviation is processed by the proportional-integral regulator 3 represents the desired valve position for the deaerator’s level control valve. Based on the dynamic characteristics of the object, the proportional coefficient k of the proportional-integral regulator is set to 0.10, with a time constant of 1800 seconds. Since this regulator operates only under conditions of low feedwater flow, it also has an upper limit, which is 50% of the output. g) Three-pulse control: When the feedwater flow rate is greater than 500 T, the water level control in the deaerator is a typical cascade three-pulse control system. The main controller (e.g., integral regulator 1) ensures regulation of the water level without any static deviation; its proportional coefficient k is 23.0, the time constant is 1500 seconds, and there are upper and lower limits of ±300. The output of the main controller, together with the feedwater flow rate and the condensate flow rate, will serve as inputs for the secondary regulator (e.g., integral regulator 2). The proportional coefficient k of the secondary regulator is 0.02, the time constant is set at 7000 seconds, and the upper and lower limits are 0 and 95 respectively; its output represents the desired valve position for the water level control valve of the deaerator. The function converter 1 is essentially determined based on the impact of the corresponding feedwater heater drain flow rates at different feedwater flow rates on the deaerator level. The function converter 2, the multiplier, and the secondary regulator together form a proportional-integral element whose proportional coefficient can be automatically adjusted according to different load demand values (MWD) or boiler input values (BID); as a result, the entire deaerator water level control system possesses a certain degree of adaptability in the three-shot mode. This enables **improved control quality of the deaerator water level across the entire load range from low load to high load (210~600MW)**. h) Reasons for using two control methods: The reason why different control methods are employed to regulate the deaerator water level of this unit at various feedwater flow rates is determined by the characteristics of the steam-water system and the properties of supercritical units operating in sliding pressure mode. The steam-water system of the boiler’s economizer, water wall, and steam-water separator is shown in Figure (3). At the beginning of operation of a once-through boiler, it is necessary for the water wall tubes to maintain a minimum flow rate to ensure the safety of these heating surfaces. When the boiler’s steam production is below this minimum safe flow rate, the excess water returns to the condenser via the steam-water separator and its water tank. The minimum safe flow rate for this boiler is 425 T. Therefore, when the feedwater flow rate is less than 450 T, the boiler’s feedwater volume remains essentially constant, and the change in the deaerator level due to variations in feedwater flow is also minimal. Therefore, using a single impulse system meets the requirements for control quality while also reducing the need to tune the parameters for the entire water supply control process. After the boiler enters dry operation, the feedwater flow rate becomes a proportional function of the boiler’s input demand value (BID); therefore, the feedwater flow rate changes continuously with BID, with more significant variations when the load changes. To improve the control quality of the deaerator water level, it is necessary to use a more complex three-pulse cascade control system. Based on the characteristics of the unit, it can be seen that in the range where the boiler operates in dry or wet mode (130–160 MW), the boiler’s feedwater flow rate is approximately 500 T, which corresponds to the transition point between single-impulse and three-impulse control methods. During three-impulse control, in order to ensure good control quality of the deaerator water level under different dry-load conditions, the secondary regulator can have different parameter settings for various loads, granting the control system a certain degree of adaptability. III. Parameter curves related to deaerator water level control during actual operation IV. Conclusion The design of this deaerator water level control system takes into full account the operating characteristics of the unit as well as the features of the system; practical operation has shown that it is able to maintain the deaerator water level within the normal range, whether under normal operating conditions or under significant internal and external disturbances. This control system can switch between different control modes based on certain conditions, and it can also automatically adjust the characteristic parameters of the regulator according to varying loads during normal operation. These have played a significant role in improving the control quality of the entire water level regulation system. It is worth drawing parallels from to learn from.