The normal operation of deaerator provides water for steam generator. The stability of deaerator liquid level is of great significance to ensure core cooling. This paper introduces the principle of three-impulse control of deaerator level controller, and analyses the system response and response measures under abnormal input parameters of deaerator level controller.
The main function of deaerator is to remove oxygen and other non-condensable gases in feed water, ensure the quality of feed water, reduce corrosion and improve heat transfer efficiency. At the same time, deaerator itself is a mixed heater, which plays the role of heating feed water and increasing the temperature of feed water. The capacity of deaerator water tank in our plant is 338 m3, which can supply the water requirement of power plant for 5 minutes under full load operation. Deaerator liquid level is an important control parameter for unit operation, because too low deaerator liquid level may cause feed pump cavitation and trigger linear power reduction of reactor, while too high deaerator liquid level will submerge deaerator head, which not only affects the effect of deaeration, but also may cause feed water to flow back to steam turbine through steam extraction pipeline, resulting in water hammer accident and loss. Bad steam turbine. The liquid level control of Deaerator in our factory adopts three-impulse PID regulation, which realizes the automatic control of the liquid level of deaerator better. However, the abnormal input parameters of the controller itself will lead to the fluctuation of the deaerator level, and improper handling will lead to the transient of the unit. This paper introduces the fault treatment ideas of the deaerator level when the input parameters of the controller fluctuate due to the abnormal input parameters of the controller. The liquid level control of Deaerator in our factory adopts three-impulse internal cascade plus feedforward control mode. The control of deaerator level is influenced by many variables, such as condensate flow rate, feed flow rate and deaerator level. These three variables can be measured are so-called “impulse”. Input 1 (IN1) is condensate water flow, input 3 (IN3) is feed water flow, input 5 (IN5) is deaerator liquid level, deaerator liquid level controller set LOOP1 is the main loop, LOOP2 is the secondary loop. Internal cascade: refers to the main and secondary loops are connected in series, and the output of the main loop acts as the setting point of the secondary loop. There are two loops (LOOP1/LOOP2) in each controller. LOOP1 is the main loop for level control. The liquid level measurement value of deaerator is compared with the liquid level setting value. After PID calculation, the calculated output is added with the feed water flow signal as the output of LOOP1. The LOOP1 output is used as the remote setting point (RSP) of the secondary loop LOOP2. LOOP2 is a secondary loop for condensate water flow control. The set value of condensate water flow is compared with the actual value of condensate water flow. After PID calculation, as the output of LOOP2 controller, it is used to control the opening of liquid level control valve of field deaerator, and then control the condensate water flow. The feed water flow signal of evaporator is used as feed-forward signal, which is combined with the output of main circuit and serves as the setting point of LOOP2 of secondary circuit. The level control of Deaerator in our factory is composed of main level control and auxiliary level control.
The main level control is to control the level of deaerator by the main level control valve when the main condensate pump is running. Auxiliary level control is that when the IV power supply is lost, the auxiliary condensate pump supplies water to the deaerator through the auxiliary level control valve, and maintains the required level of the deaerator during the period of discharging the reactor decay heat. Controller operation mode: The operating handle 64321-HS4410A has three locations “LT4410A, LT4410B, LT4410C”, which are used to select the master and slave positions of three level controllers. When a position is selected, two controllers are put into operation: one at AUTO and one at STANDBY. The level controller at the AUTO position is used to regulate two level control valves selected by the control handle 64321-HS4410C at the AUTO position, and the level controller at the STANDBY position controls the remaining one at the STANDBY position. Control valve operation mode: 64321-HS4410C on PL10 has three locations, namely “LCV4207A, B”, “LCV4207A, C” and “LCV4207B, C”. The two valves selected by 64321-HS4410C are controlled by the controller selected by 64321-HS4410A, while the other valve is controlled by the standby controller when the deoxidizer level is below 3000mm. Generally speaking, under normal operation, one of the three level controllers controls the two deaerator upper valve, automatically maintains the level of the deaerator at 3380 mm, and the other controller is in standby state. When the level of the deaerator is lower than 3000mm, the third level control valve controlled by the standby controller is put into control, and the three levels are controlled. The control valves work together to ensure that the water level of the deaerator is maintained at 3380 mm. Event: In 2006, the main control system of a domestic power station appeared the alarm of “low flow rate of main condensate pump” and “open of recirculation valve of condensate pump”. Inspection immediately found that the flow of water on deaerator decreased significantly, deaerator liquid level decreased, emergency manual power reduction. After the power of the reactor is reduced to 60% FP, the water flow on the deaerator gradually returns to normal, and finally returns to the normal value. After confirming the stability of the unit, the instrumentation and control personnel checked and found that the input failure of the main feed water flow signal sent to the deaerator level controller was caused. The main feed-water flow signal acts as feed-forward and plays the role of advanced regulation. When the feed-water signal is lost, the remote setting value (RSP) of the controller decreases rapidly at full power, which makes the final output of the controller drop to about 10% instantaneously, and the level control valve of the deaerator is almost completely closed. At the same time, because the actual feed water flow remains unchanged, the load balance of the system is destroyed, and the deaerator water level begins to fall, which leads to the increase of RSP in the main circuit.
Through the PI function of the secondary circuit, the output of the controller starts to increase to track RSP, and the corresponding deaerator level control valve opens to increase, then the deaerator level begins to rise slowly. How does the system respond when multiple feed-water flow signals fail? The main feed-water flow is controlled and calculated as a feed-forward term. The controller compares the measured liquid level signal with the liquid level setting point to produce a percentage output, which is added to the percentage value of the main feed-water flow signal as the condensate flow. The setting point of the loop, the flow setting point and the condensate flow rate are compared, and then the signal is sent to the valve for control by PID operation. So without a feed water signal, thermostatic element the setting point of condensate flow circuit will decrease, but the effect is not significant, because when the setting point of condensate flow decreases, the valve will close down, the liquid level of deaerator will drop, and the setting point of condensate flow will rise again after the liquid level of deaerator decreases. The principle of losing more feed water flow signals is the same, but the disturbance of liquid level will be greater. Special Operation Procedure (SOP) of power plant instructs operators to stabilize the liquid level of deaerator by reducing the feed water flow of evaporator when the liquid level of deaerator is lower than a certain value. When the total feed water flow signal fails, there will be an indication of “FF input failure” on the interface of the controller. Although the simulator verifies that the deaerator level starts to rise when it drops to about 3000mm, the operator can still respond by reducing the power appropriately, but considering the particularity of the control system of the nuclear power plant (mainly due to the particularity of the control system). Xenon poisoning), the single power reduction should not be too large to prevent Xenon poisoning, each power reduction to observe whether the deaerator level can be adjusted back. In this case, the individual does not recommend that the controller be manually operated to increase the upstream flow, because the manual control of deaerator level under high power is very risky. If the disturbance is introduced into the system due to improper manual operation, the consequences are often more serious, so it is not recommended to use it as a last resort.
Event: In November 2012, a domestic power plant instrument control personnel carried out the maintenance work of deaerator level transmitter. After obtaining the consent of the main control, the fuse was replaced. When the old fuse was dismantled, the operator notified that the water flow in the condensation water was lost. Then it stopped working immediately. After ten seconds, the water flow on the condensate water recovered automatically, and the liquid level of the deaerator decreased slightly. The reason is that the terminal of the replaced fuse is close to the terminal of the failure signal fuse.
From the structure of the terminal, the contact is between the upper and lower copper sheets clamping the metal sheet in the middle. Influences: The signal of water flow in condensation water is sent to the level controller of deaerator. Due to the signal failure, the controller automatically enters the failure safety (FAILSAFE) mode. The current FAILSAFE parameter setting is that the output is automatically reduced to 4 mA. Therefore, the deaerator level control valve automatically closes, the condensate pump outlet flow rapidly drops to 0, and the recirculation flow control valve begins to open. The level of deaerator drops rapidly. After 18 seconds, the signal is restored from IRR, and the controller is transferred to normal control mode. The liquid level control valve opens rapidly, the flow rate of condensate pump outlet is restored immediately, and the liquid level begins to rise. The response of the process is normal. If this condition does not recover in time, operators should first confirm that the controller interface appears “PV input failure”, press the page turning key to turn to the ALARM page, and if X5 indicates the loss of condensate flow signal, then the controller should be switched to manual, manually increase the upflow, deaerator liquid level abnormal fluctuation treatment standard. In Quasi-Operation process, manual triggering of linear power reduction is required when the liquid level of deaerator is lower than the prescribed value due to unknown reasons, but the cause of the accident has been confirmed. Normally, the liquid level of deaerator is abnormally closed by the control valve of the liquid level of deaerator. When the auxiliary condensation pump is involved in the control, the liquid level of deaerator decreases from the set value to the automatic trigger value of linear power reduction. It takes about two minutes, so the operator has enough time to manually increase the output of the controller to restore the flow of water on the deaerator to normal value. At the same time, the changing trend of deaerator liquid level needs to be closely monitored. If the trend of liquid level decline has not been alleviated, the deaerator power can be reduced to respond appropriately. Under this condition, the state of the unit varies greatly, and it is difficult for the operator to control the upper water flow manually to maintain the deaerator level, which will be a great challenge to the operator’s psychology and skills. Deoxidizer liquid level measurement signal drift (high drift) will lead to deoxidizer liquid level reduction.
If the operator can find the deoxidizer liquid level drop in the first time, it can be further confirmed by the deoxidizer liquid level controller display. At this time, the operator can switch the controller to manual control the deoxidizer liquid level, and wait for the liquid level to stabilize in the manual control. Consider switching controllers. If the deaerator level measurement fails, the word “PV input failure” will appear on the controller, then press the page-turning key and turn to the ALARM page. If X1 is prompted to indicate the loss of liquid level signal, the controller will enter the failure safety mode, and the deaerator level control valve will automatically close. At this time, the principle of handling the accident and the signal of condensate flow will be lost. The effect is the same. Deaerator level controller is the core of deaerator level control.
With the increase of unit operation time, there is a risk of aging. When the input parameters of deaerator level controller are abnormal, it will bring great disturbance to deaerator level, which may affect the evaporator level directly to the heat output of the loop. As operators, they should recognize the impact of various failure modes and take reasonable intervention measures to avoid transient guarantees of unit safety.