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Regulating Method and Pump Selection of Main Water Supply System of PWR Nuclear Power Station
Abstract: By analyzing the control requirements of the thermal parameters of the secondary circuit of PWR nuclear power station and the operation characteristics of the electric constant speed feed pump, electric speed control feed pump and steam speed feed pump, from the point of view of construction, maintenance and operation economy, This paper discusses the ways to adjust the main water supply system of PWR nuclear power plant and the options of feed water pump and puts forward the suggestion of selecting the priority of feed water pump configuration scheme of PWR nuclear power plant. Key words: pressurized water reactor nuclear power station configuration of feed pumps China's in-service and under construction in the nuclear power station has 6, 11 units, with a total installed capacity of 8 700 MW. Among the five pressurized water reactor nuclear power plants, the main feed pumps have different types and configurations. Daya Bay and Ling'ao Nuclear Power Station adopted two sets of 50% (referring to the rated water flow, the same below) with a 50% standby electric pump steam pump, Qinshan a 3 50% electric fixed-speed pump, Qinshan Phase II 3 sets of 50% electric speed pump, Tianwan nuclear power station with 5 25% electric constant speed pump. Foreign PWR nuclear power station main feed pump selection configuration is not the same way, all countries have the style and habits, but the trend is more and more inclined to the electric feed pump. Although the main feed water pump systems of different types and configurations can meet the safety and functional requirements of nuclear power plants, the economics of investment and operation and maintenance as the most important auxiliary system of a conventional island nuclear power plant are quite different. China's nuclear power development in the face of unprecedented opportunities (in 2020 the installed capacity to reach 36 ~ 40 GW), but also faced with national requirements to achieve independent design, independent manufacture, independent construction, independent operation and reduce the project cost and tariff, Involved in the huge market competition challenges. Therefore, in accordance with the PWR primary water supply system operating requirements analysis to the operating characteristics of different types of pumps, determine the best PWR main feedwater pump type and configuration program on realization of China's self-designed pressurized water reactor nuclear power plants, nuclear power plants is to reduce the cost very necessary. 1, the main water supply system regulation The main function of the main water supply system of PWR nuclear power station is to send the water supply of temperature, pressure and water quality to the steam generator, and use the water supply system regulation function to maintain the water level of the steam generator within a given range, It is an important thermal system to ensure the safe operation of nuclear island and the quality of soft drinks. As with conventional power plants, there are also two main types of PWR systems, that is, constant speed feed water supply control system and speed control feed water supply system. Two kinds of feed water control system are under higher load (more than 15% ~ 20 %) Three impulse adjustment control principle. Under low load, due to the low steam parameters and small load changes, the pseudo-water level of the steam generator is not too serious. The requirement of maintaining a given water level is not too high, and the steam flow rate and the feed water flow rate are too small to measure accurately. Under the load using single impulse bypass adjustment control method. 1.1, constant-speed water supply pump control system Constant-pressure water supply control system is the easiest water supply control mode. Its essence is to maintain the same pump characteristics curve, by adjusting the water supply valve opening to change the water flow resistance loss, that is, to change the water supply pipe curve to change the feed pump operating point. As shown in Figure 1, the water supply Control Valve is closed, and the pipe resistance characteristic curve is changed from R1 to R2. The working point of the feed pump is changed from 1 to 2, so that the feed water flow rate decreases from Q1 to Q2, so as to adjust the feed water flow rate and steam occurrence The purpose of the water level. Case of Figure 1 a constant speed to the pump characteristic curves 1.2, the shift feed pump feedwater control system of the transmission to the pump feedwater control system is water pipe resistance curve given (or substantially constant) to change to the pump characteristic curve by altering the pump speed , To achieve regulation of water flow, steam generator water level control purposes. As shown in Figure 2, the feedwater pump speed adjustment mechanism will feedwater pump speed is adjusted from n1 to n2, feed pump characteristic curve will be changed from Q-H1 to Q-H2, in a given water supply pipe resistance curve (here Constant value), the operating point of the variable speed feed pump characteristic curve in Figure 2 transitions from 1 point to 2 points, reducing the feed water flow rate from Q1 to Q2 to adjust the feed water flow rate and the steam generator water level. Variable speed feed pump prime mover mainly small turbine and electric motor in two ways. Steam driven water pump driven by a small steam turbine, steam turbine to receive water flow control system or speed demand signal, and through the steam turbine into the flow control mechanism to adjust the feed pump speed. In normal operation, the steam turbine enters the steam from the main steam turbine and the steam from the reactor under low load conditions. When the load of the main steam turbine increases, the extraction pressure for the steam turbine will also increase accordingly. The output of the steam pump will increase automatically when the opening of the adjustable door is not changed. Therefore, the control of the feed water flow of the steam-driven feed pump has certain self-balancing ability. Electric Speed ​​Control Feed Pump Adjusts the speed of the feed pump through a fluid coupling installed between the motor and the feed pump. The hydraulic coupler receives the flow or speed demand signal of the water supply regulating system and adjusts the speed of the feed water pump by changing the filling amount of the hydraulic coupler through the spoon pipe control mechanism. Because the hydraulic coupling can only decelerate, it is necessary to increase the speed to the upper limit beforehand by using the gear-up gear. Since the 1980s, advanced industrial countries, especially the United States, have gradually used frequency-variable speed control motors for large-scale power station variable-speed feed pumps (feed pump power is 4-20 MW). However, China is still in its infancy in this respect. 1.3, PWR steam generator pressure control requirements Conventional power plant boiler heating surface heat transfer temperature up to several hundred degrees, the boiler design is not subject to strict size constraints, but no special requirements of nuclear safety, thermal design freedom, so It can be designed according to the constant pressure (the main steam pressure is not inconvenient to maintain) and the operation mode (Figure 3). The operation mode (Figure 4) can also be designed according to the sliding pressure (the main steam pressure increases as the load increases). Figure 3 regular boilers constant pressure operating curve Figure 4 conventional boiler pressure running curve PWR nuclear power station is not the case, the pressure of the first loop water temperature (which can be seen as conventional boiler hearth temperature) is low, the second loop steam is always in a non-overheated state , One or two loop heat transfer temperature difference is small, the average logarithmic average temperature is only about 50 ℃, and arranged in a limited space cement containment. To meet the stringent requirements of nuclear safety, thermal design freedom is very limited. In the thermal shutdown state, the reactor output power is close to zero, the reactor basically no temperature difference between the two circuits, reactor core import and export no temperature difference, that is, the average temperature of the primary circuit and the secondary circuit steam temperature is basically the same. When the load on the secondary circuit increases, in order to ensure the normal conduction of the heat transfer surface of the steam generator, the average temperature of the primary circuit must be increased or the secondary circuit steam pressure must be reduced. If the second loop steam pressure (temperature) remains unchanged (BE lines in Figure 5), the average temperature of the primary circuit must be kept large enough to ensure that the appropriate logarithmic average temperature difference between the primary and secondary circuits, at this time to the pump outlet pressure diagram 5 AF line. However, the excessive increase of the average temperature of the primary circuit will bring a series of problems to the safety design of the entire nuclear island. First, the average temperature of a loop increases, the volume of the regulator must be increased. The increase of the volume of the regulator will directly lead to the increase of the water charge of the primary circuit, that is, the increase of the total latent heat of vaporization of the coolant of the next circuit in the primary circuit of a large circuit. In order to ensure the high safety of PWR nuclear power plants under the first-break large-scale accident, its containment volume must be correspondingly increased, which is not technically and economically feasible. In order to avoid the above problems, the average temperature change range of primary loop is limited in mature PWR design (the average temperature of M310 reactor does not change more than 20 ℃). In this way, under the premise of ensuring nuclear safety requirements, in order to maintain the normal heat power transfer between primary and secondary circuits, the pressure or temperature of the secondary circuit steam must be reduced as the load increases as required (line BC or BGC in Figure 5) At this point to pump outlet pressure to maintain a basic constant (AD line in Figure 5). 2, a variety of economic analysis of water pump configuration 2.1, electric speed control pump and steam speed feed pump capacity at the same time, the electric speed control pump and steam speed feed pump main difference is that the prime mover in different forms, Different energy input methods and speed control methods. Assuming that both feed pumps meet the feed water regulation requirements in terms of speed range, the difference in feedwater economy between the two speed regulators is primarily in terms of investment, operation, and maintenance. 2.1.1. Economic Analysis of Operation It is assumed that the starting point of energy transfer is to the steam turbine