Abstract: The reliability of feed water pumps has an important impact on the safe and stable operation of nuclear power units. Considering that there is currently no standard or specification in China for temperature protection of bearings in nuclear power plant feed water pumps; At the same time, based on the experience feedback of in-service nuclear power plants, it is necessary to optimize the bearing temperature measurement points to further improve the reliability of the feed water pump. This article analyzes the temperature protection scheme of feed water pump bearings, especially the reasons for misoperation of measurement points, setting of measurement points, and logical processing of measurement points. Combined with data from the water pump industry and nuclear power plants, an optimization scheme suitable for temperature protection of feed water pump bearings in nuclear power plants is proposed. The aim is to provide guidance for temperature protection schemes of feed water pump bearings in subsequent projects and improve the reliability of feed water pumps.
Keywords: feed water pump; Temperature measurement points; bearing
Electric feed water pumps play an important role in the safe operation of nuclear power plants. This article focuses on the study of the logic scheme for bearing temperature protection, analyzing the logic scheme from both qualitative and quantitative perspectives, and providing a suitable logic scheme for bearing temperature protection of electric feed water pumps in nuclear power plants.
1 Introduction to Electric Feed Pump System
1.1 System functions
The main function of the nuclear power plant feed water system is to provide satisfactory feed water to the steam generator under different operating conditions.
The electric feed water pump provides power for the feed water to meet the pressure requirements of the nuclear island steam generator. The function of this system is to cooperate with the feed water flow regulation system, extract and pressurize the water in the deaerator, and send it to the steam generator through a high-pressure feed water heater.
1.2 System composition
A typical electric feed water pump in a nuclear power plant consists of a booster pump, an electric motor, a hydraulic coupling or gearbox, and a pressure stage pump.
2 Overview of protection logic scheme for electric feed water pump
According to the composition of electric feed water pumps, combined with the original design schemes of mainstream feed water pump manufacturers, the protection of electric feed water pumps generally includes the following categories:
① Protection of the water pump body;
② Protection of the motor body;
③ Protection of hydraulic couplers or gearboxes;
④ Protection from other signals.
From their own perspective, water pump manufacturers should minimize the setting of temperature measurement points as much as possible to reduce costs; On the other hand, temperature measurement points are mostly set as single point pump trip signals. According to the statistics of the under construction CPR1000 unit, the single point pump trip caused by the bearing temperature signal of the feed water pump accounts for about 70% of the total pump trip signals of the feed water pump.
In addition, based on experience feedback, the original protection logic scheme of the feed water pump is not entirely reasonable, mainly manifested in: ① the original bearing temperature protection logic scheme has too many single point pump tripping signals, increasing the risk of false temperature signals leading to the misoperation of the feed water pump; ② For important positions, such as thrust bearings, the number of measurement points is too small.
3 Analysis of Logic Scheme for Temperature Protection of Water Supply Pump Bearings
As an important part of thermal automation, temperature protection is installed on the bearings of electric feed water pumps. However, in the actual process, a sudden increase in the measured temperature may be caused by a malfunction of the temperature measuring element. For a protection logic scheme where a single temperature measurement point triggers a pump trip signal, a sudden increase in temperature can cause the feed water pump to trip incorrectly, which may affect the stable operation of the unit.
3.1 Qualitative analysis of temperature protection logic scheme
Thermal resistance is made by utilizing the principle that the resistance of a metal conductor varies with temperature. The change in temperature rise and fall is a continuous and small process.
When the thermistor is disconnected and the terminal contact is poor, its contact resistance will increase, and for protection, its temperature will suddenly rise, leading to protection action. Based on past engineering experience, there are often false and unrealistic signals generated in engineering.
Therefore, from a qualitative analysis perspective, the generation of false signals at a single temperature measurement point is more advantageous for the equipment itself, but may have an impact on the overall availability of the factory. Therefore, the temperature protection logic scheme needs to be determined by comprehensively considering factors such as equipment safety, power plant availability, and investment.
To avoid the generation of false signals as much as possible, there are two main processing methods in engineering: ① changing the Z-terminal orientation of the temperature signal, that is, modifying the temperature signal to only be used for alarm and display, without relevant logical control; ② Processing the signals generated by temperature measurement points involves processing the temperature signals in a certain way to avoid false signals as much as possible. The processing methods mainly include adding delay links, checking for delay and bad points, increasing rate thresholds, adopting redundant configurations, and performing logical operations.
Based on engineering experience, it is recommended to prioritize the use of changing temperature signal orientation and redundant configuration with logical operations to avoid the generation of false signals.
The core issue for changing the direction of temperature signals is to determine the range within which this method can be adopted. Through communication with manufacturers, comparing specific engineering schemes of nuclear power projects at home and abroad, and combining with the operating experience of nuclear power units, it is recommended to adopt a change in temperature signal direction for the radial bearings of the feed water pump. That is, the temperature signal of the radial bearings is only used for alarm and display, and not as a pump trip signal for the feed water pump.
In addition, for thrust bearings, it is recommended to use redundant configuration and logical operations for processing. The following text will analyze this method from a quantitative perspective.
3.2 Quantitative analysis of redundant measurement point settings
The thrust bearing of a feed water pump generally consists of several fan-shaped tiles, distributed in a dispersed manner. According to feedback from the feed water pump manufacturer, the temperature measurement points installed on one side (inner or outer) of the thrust bearing of the feed water pump should not exceed three. Further analysis will be conducted on the different logical operations of temperature measurement points within 3.
There are two types of logic system failure modes, refusal to operate fault and misoperation fault. The former refers to the presence of refusal to operate faults in certain input channels of the logic, which may cause logic refusal to operate faults. This is a dangerous fault that will result in the inability to start protection actions normally; The latter refers to the occurrence of false signals in certain input channels, which can cause logical malfunctions and result in protection malfunctions, reducing the availability of the power plant.
Through logical operations, it is known that two out of three logical operations have a relatively low probability of rejection and misoperation.
4 Recommended scheme for temperature protection of bearings in feed water pumps
4.1 Recommended solution description
In summary, for the radial bearings of the feed water pump, temperature measurement points are only used for alarm and not for logical control. Considering the need for online maintenance, one measurement point is set and double thermal resistors are configured; For the thrust bearing of the feed water pump, three measuring points are set on the inner and outer sides of the thrust bearing, and a two out of three logic operation is performed to obtain the output signal of the Z-terminal.
4.2 Feasibility analysis of recommended solutions
4.2.1 Analysis of radial bearing scheme
For radial bearings, high temperature of the radial bearings in the feed water pump usually occurs in the absence of lubricating oil. As the feed water pump is equipped with a dedicated lubricating oil system, the lubricating oil system has a main oil pump and an auxiliary oil pump. In addition, the lubricating oil system is also equipped with protection for low lubricating oil pressure, which can ensure good lubrication of the radial bearings during the operation of the feed water pump. The temperature of the radial bearings is relatively controllable. Even if there is poor lubrication, the temperature rise rate of radial bearings is relatively small. Modify the protection logic to an alarm, appropriately reduce the alarm temperature, and after the alarm signal is issued, the operator can take necessary measures in a timely manner.
Regarding the setting of temperature measurement points for radial bearings, due to the limited number of measurement points, current radial bearings are relatively easy to arrange and alarm functions are also easily implemented in DCS. Therefore, it is technically feasible to modify the radial bearing to an alarm mode.
4.2.2 Analysis of thrust bearing scheme
For thrust bearings, due to the perpendicularity error between the rotor thrust disc and the rotor axis, some of the bearing shells of the thrust bearing will be subjected to large forces; If the flatness of the bearing is not good and the deviation exceeds the thickness of the Z small oil film, it will damage the oil film between the bearing and the thrust disc; If the relative height difference of the bearing pad installation is large, it will also cause some thrust pads to be subjected to excessive force. Once the thrust bearing is worn out, the temperature rises rapidly. If automatic pump shutdown is not set, it may damage the relevant equipment of the feed water pump.
Meanwhile, in DCS, there are many systems that apply two out of three logical operations. Therefore, it is also feasible for thrust bearings to perform two out of three calculations simultaneously. In addition, the relevant standards for boiler feed water pumps in thermal power plants also recommend using a two out of three redundant configuration for bearing temperature measurement points.
5 Conclusion
For the temperature protection logic scheme of electric feed water pump bearings in nuclear power plants, it is recommended to set a single temperature measurement point for radial bearings and equip them with dual thermal resistors; It is recommended to set three measuring points on each side of the thrust bearing and use a two out of three logic operation. From a technical perspective, this solution can ensure the safety of the feed water pump equipment and improve the reliability of the system and even the unit; From an economic perspective, compared to the original plan of the water pump manufacturer, only a few additional thermal resistors were added, and the investment cost did not significantly increase.
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