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: PWR nuclear power station feed pump configuration scheme
There are 6 nuclear power stations under construction and under construction in mainland China and 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. While nuclear power development in China is facing unprecedented opportunities (with installed capacity of 36-40 GW in 2020), it is also facing the problems of independent design, self-made manufacturing, self-construction and autonomous operation required by the state and reduction of construction costs and on-grid electricity prices. Involved in the huge market competition challenges. Therefore, according to the operation requirements of the main feedwater system of PWR nuclear power plant, the operating characteristics of different types of feedwater pumps are analyzed to determine the optimal type and configuration scheme of main feedwater pumps for PWR nuclear power plants. In order to realize the independent design of PWR nuclear power plants in China and reduce the cost of nuclear power plants very necessary.
1, the main water supply system regulation
The main function of PWR main feedwater system is to send temperature, pressure and qualified water to the steam generator and to maintain the water level of the steam generator within a given range by using the regulation function of the water supply system. It is to ensure the safe operation of the nuclear island And soft drinks important thermal system. 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 regulation system are under higher load (more than 15% -20 %) Three impulse adjustment control principle. Under low load, due to low steam parameters and small load changes, the pseudo-water level of the steam generator is not so serious. The requirement of maintaining a given water level is not too high. In addition, the steam flow rate and the feed water flow rate are very small and it is difficult to measure accurately. Under the load using single impulse bypass adjustment control method.
1.1, constant speed water supply pump control system
Constant speed feed water supply control system is the simplest 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.
Figure 1 constant speed feed pump characteristic curve
1.2, variable speed feed water pump regulation system
Variable speed feed water supply control system is to change the feed pump characteristic curve by changing the feed pump speed given (or almost constant) resistance characteristic curve of the feed pipe to achieve the purpose of regulating feed water flow and controlling the water level of the steam generator. As shown in Figure 2, the feed water pump speed adjustment mechanism will feed water pump speed is adjusted from n1 to n2, feed water pump characteristic curve will be changed from Q? H1 to Q? H2, in a given water supply pipeline resistance characteristic curve Constant value) of the case,
Figure 2 variable speed feed pump characteristic curve
Working point transition from 1:00 to 2:00, so that water flow from Q1 reduced to Q2, to achieve water flow and steam generator water level adjustment.
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 inlet flow control mechanism to adjust the 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 difference up to several hundred degrees, the boiler design is not subject to strict size limits, but no special requirements of nuclear safety, thermal design freedom, so you can press constant pressure (main steam pressure to maintain the basic inconvenience) Operation mode (Figure 3) design, but also by sliding pressure (main steam pressure increases with the load increases) mode of operation (Figure 4) design.
Figure 3 conventional boiler constant pressure operating curve
Figure 4 conventional boiler slip operating curve
The PWR nuclear power station is not the case, the pressure of the first circuit water temperature (which can be seen as conventional boiler hearth temperature) is low, the second loop steam is always in the non-overheated state, the first and second loop heat transfer temperature difference is small, the logarithmic mean temperature difference Only more than 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 (Figure 5, BE line), 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, this time to the pump outlet pressure diagram 5 AF line.
Figure 5 steam generator outlet steam pressure and load relationship
However, an excessive increase in the average temperature of the primary circuit can cause a series of problems for 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 thermal 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 water supply pump configuration of the economic analysis
2.1, electric speed control pump and steam speed feed pump
When the capacity is the same, the main difference between the electric speed control water supply pump and the steam speed control feed water pump is that the prime mover has different forms, the energy input modes are different, and the speed regulation modes are different. 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, Operational Economic Analysis
The starting point for energy transfer is assumed to be the main steam turbine extraction point for supplying steam to the steam turbine (see Figure 6). Then the steam-driven variable speed feed pump energy transfer conversion from the extraction point to the small steam turbine inlet steam pipe, steam turbine through the flow part of the heat - machine conversion process and the pre-pump deceleration mechanism. Electric speed control pump to the energy transfer of the main transfer point after the host pumping point of the flow through the heat part of the machine conversion process, generator machine - conversion process, the plant with the transformer, switch cables, motor electromechanical conversion process, hydraulic coupling And its speed-increasing mechanism or motor frequency conversion system. As small steam turbine pipelines and switch cables loss is relatively small, not included in the analysis of the comparative factors here.
When the total power of steam turbine generator N is the same, the use of steam speed control feed pump compared with the use of electric speed feed pump makes the unit power increase â–³ Nn positive, indicating that the use of steam speed feed pump is economical in operation , On the contrary it shows that the use of electric speed control water pump is economical in operation. The degree of benefit can be derived as follows:
The use of steam speed feed pump when the unit output net power N1:
Figure 6 two drive mode energy transfer diagram
N1 = N- â–³ N
△ N = DexHηiηe
Where:
Dex: Turbine intake;
H: adiabatic enthalpy drop downstream of the main steam turbine extraction point;
ηi: relative internal efficiency of the downstream of the main steam turbine extraction point;
ηe: main turbine mechanical efficiency.
When the electric speed control feed pump is adopted, the unit output net power N2 is the difference between the total power N of the unit and the power NM of the electric speed control feed pump:
N2 = N-NM
NM = NTηd = NTηgηtηeηmηwηf
Where:
NT is the turbine shaft power;
NT = DexHηipηep = NMηd
ηg: generator efficiency;
ηt: step-down transformer factory efficiency;
ηm: motor efficiency;
ηw: speed gear mechanism efficiency;
ηf: hydraulic coupler efficiency;
ηip: small turbine relative internal efficiency;
ηep: small turbine mechanical efficiency.
The use of steam speed feed pump compared with the use of electric speed control pump configuration benefits:
ΔNn = N1 - N2 = NTηd - ΔN or
△ NnN = NTN (1ηd- △ NNT)
= NTN (1ηd-ηiηeηipηep)
Thus, as long as 1ηd> ηiηeηipηep can benefit.
Take a million-kilowatt PWR nuclear power unit as an example:
Turbine power NT = 2 × 7.961 MW;
The relative internal efficiency ηi of the downstream of the main steam turbine extraction point is 83.3%.
Small turbine relative internal efficiency ηip = 70.7%;
Generator efficiency ηg = 98.79%;
Factory use step-down transformer efficiency ηt = 99.3%;
Motor efficiency ηm = 96.9%;
The main steam turbine mechanical efficiency ηe = 99.6%;
Turbine mechanical efficiency ηep = 99%;
Speed ​​gear mechanism efficiency ηw = 96.5%;
Hydraulic coupler efficiency ηf = 92.6%;
Substitute in the above formula:
ΔNnN = 2 × 7.9611000 × (10.846-0.833 × 0.9960.707 × 0.99)
= -0.0053% or
â–³ Nn = -0.053 MW
Considering the mechanical loss of 0.102 MW of the pre-pump gear reduction mechanism of the steam speed governor, the net gain / loss of the steam speed governor feed pump is 0.155 MW compared with that of the electro-hydraulic coupler speed governor pump. In other words, the steam speed control pump configuration and electro-hydraulic coupler speed pump compared to the operation is not economical.
We know that if the use of FM motor, hydraulic coupling can be completely eliminated the growth of gear mechanical loss, only about 3% of the loss of conversion (that is, the speed gear mechanism and the hydraulic coupling efficiency of the two merged into 97% Of inverter efficiency). at this time
ΔNnN = 2 × 7.9611000 × (10.918-0.833 × 0.9960.707 × 0.99)
= -0.153% or
△ Nn = -1.53 ​​MW
In other words, compared with variable-speed motor feed pump, steam-driven feed pump net loss of 1.53 MW.
Due to the limitation of the long blade length and the length of the shaft, the early high capacity full speed steam turbine diverts a part of the low pressure steam for the feed water pump turbine to some extent to relieve the pressure of the exhaust area of ​​the main engine and reduce the exhaust residual speed loss. In addition, due to the immature technologies of early generation of hydraulic coupler, conventional thermal power units of 200 MW and above are generally equipped with steam-driven variable-speed feed pumps. Especially in the northern areas where the cooling water temperature is relatively low, the discharge volume of the low- Large, the use of steam-driven pump configuration is more obvious. One of the main reasons why the four million-kilowatt-class nuclear power plants in Daya Bay and Lingao Phase I selected two steam plus one power feed pumps was limited by the capacity of the generator at that time. These factors are not major issues with the progress of power generation equipment manufacturing industry. Especially for large-capacity half-speed turbo-generator units, the length of the blades or the area of ​​exhaust steam have not constituted the main factor that must select the configuration of the steam-driven feed pump.
2.1.2, investment economy analysis
In the feed pump itself, based on the electric pump to increase investment costs are mainly motor, hydraulic coupler (including gear growth mechanism) or inverter system, motor lubrication system, switchgear and cables. The main investment items for steam-driven feed pumps include small steam turbines and related valves, extraction pipelines, regulating lubricating oil systems, pre-pump speed reduction mechanisms, steam turbine steam sealing systems, hydrophobic steam releasing systems, and steam turbine exhaust steam pipes.
After the first quarter of Ling Ao Nuclear Power Plant nuclear power plant steam speed control feed pump with hydraulic coupler electric speed feed water pump system equipment and materials price cumulative analysis found that the steam speed feed pump investment Costs higher than the electric speed control pump investment costs 70%. If you consider the unit and its associated system of steam feed pump covers an area of ​​about 2 times the electric water pump, the space occupying space is at least 3 times the electric feed pump factor, then the steam speed feed pump investment than the electric tune Speed ​​to pump investment costs may be higher than 100%.
Although I did not get the cost information about the frequency conversion system, according to the report, even in the United States to consider the cost of frequency conversion system, the investment cost of steam speed feed pump is generally more than 100% higher than the cost of electric speed feed pump. This is enough to prove that the steam-driven feed pump is much less economical to use than the electric feed pump.
2.1.3, maintenance economy analysis
Take the first phase of a nuclear power plant at Ling Ao Nuclear Power Station as an example. The electric water supply pump, in addition to the pump itself, is mainly equipped with a motor and a fluid coupling, plus a very simple lubricating oil cooling system (no fixed lubricating oil Purification system). The steam-driven water pump in addition to the pump itself equipment, there are small turbines, front pump deceleration mechanism, lubricating oil system, steam turbine steam release system, a small steam seal system, and even the car system (after cancellation) Like a small power station. Especially the hydrophobic system and oil cooling filter system is quite large, and equipped with the corresponding rotating equipment, compared with the main steam sparrows are small and complete, compared with the electric water pump will undoubtedly greatly increase the maintenance workload, spare parts and spare parts The number of consumables. Another small turbine also need regular (about 4 years) to open the cylinder to check the flow through the state. It is reported that the annual maintenance costs of variable speed motor drive mechanisms in Europe and America are only about 25% of the annual maintenance costs of steam turbines or gas turbines of the same capacity.
2.2, electric speed control water pump and electric constant speed water pump
2.2.1, Operational Economic Analysis
It can be seen from Fig. 1, Fig. 3 and Fig. 4 that, for large-scale conventional boilers, whether constant pressure operation or sliding pressure operation, if the constant speed water supply adjustment mode is adopted, large throttling losses are caused at different degrees under low load Amount â–³ Np:
ΔNp = ÏQ2 (H2-H3) ηp
Where:
Ï is the water density;
Q2 for the operating point 2 of the water flow;
H2 for the operating point 2 pump lift;
H3 for the operating point 2 deduct flow control valve throttling loss of feed pump head;
ηp for the pump efficiency.
In addition, the pump at low flow away from the best working point, the pump efficiency will be greatly reduced. In other words, for large-scale thermal power units, the use of fixed speed water supply control mode of operation is not incontrovertible fact. However, this is not the case for nuclear power plants. Due to the special requirements of the main steam parameters of the pressurized water reactor nuclear power plant described above, the pressure before the water supply regulating valve generally changes little with the load. In this way, as long as the feed pump characteristic curve is kept flat within the load range, the flow rate of the constant-rate feed pump can be adjusted without substantial throttling.
The above objectives can be achieved through the operation of multiple low-capacity feed pumps in parallel. According to the theory of pump operation, when multiple feed pumps are operated in parallel, the multi-pump operating characteristic curve can be drawn as long as the pump flow rates in parallel operation are added under the same pressure conditions. The P3 line in Fig. 7 is the operation characteristic curve of two equal capacity constant speed feed pumps after being tied. It can be seen from the parallel pump multi-pump characteristic curve becomes more flat. If using 4 to 5 low-capacity feed pumps in parallel operation (Tianwan nuclear power plant feed pump configuration), the curve will be more flat. Because each pump capacity is small, according to the different load units in turn need to put in, so each pump can run at its best efficiency point, that is, within a certain load range of the pump can be in the best efficiency point.
Figure 7 parallel constant speed feed pump operating characteristics
For a 100% capacity or even 2 sets of 50% capacity fixed-speed pump configuration, to achieve the above objectives is very difficult, the main reasons are:
(1) The single pump or double pump configuration can easily cause instability of the water supply system caused by the over-flatness of the characteristic curve of the feed pump under low load, and may cause the flow induced vibration of the pipeline in severe cases. Such problems have been encountered during the first phase of Ling Ao Nuclear Power Station commissioning and have been resolved after doubled capacity expansion (from about 36% to about 72%) of the pump minimum flow recirculation system.
(2) Under the partial load of the unit, the single pump or double pump configuration is easy to make the feed pump to run at partial load, thus reducing the efficiency of the feed pump.
In theory, the smaller the capacity of a single pump, the more the number of parallel pumps, 100% load range of the parallel characteristics of the more flat. However, if the number of parallel pumps is too large, the capacity factor of single equipment will work. In this case, the total investment of multiple fixed-speed feed pumps will increase. There are 5 25% and 4 33.3% of the electric pump configuration schemes for PWR nuclear power plants currently under active construction or supplier's proposal for new construction with constant speed water supply regulation.
In addition, multi-pump parallel operation also reduces the impact of a single pump trip on steam generator water level regulation, reduces the risk of steam generator shutdown due to low-low water level trips, and increases the safety of reactor operation.
The above analysis shows that if properly selected, the PWR nuclear power plant using multiple electric constant speed water pump parallel operation of the configuration can be basically achieved without throttle loss of water flow control, eliminating the governor, improve the operation Reliability, avoiding about 8% hydraulic coupler energy loss or about 3% frequency conversion system energy loss, so it can be said that the PWR nuclear power unit is a more ideal and more economical configuration of the water pump.
2.2.2, investment
As the constant speed feed pump is not equipped with a motorized speed control feed pump such as a fluid coupling or a frequency converter, the equipment cost can be further reduced. Take a case of a 1 MW nuclear power plant at Ling Ao Nuclear Power Station as an example, canceling the hydraulic coupling can reduce the cost of the electric water pump by about 10%.
2.2.3, maintenance economy analysis
If using fixed-speed electric pump configuration program, the system is more simple, more flexible layout, maintenance costs can be further reduced.
2.2.4, localization
According to the current experience and status quo of domestic nuclear power equipment manufacturers, there is still a certain amount of procurement for small turbines, feed pumps, hydraulic couplers, 10 MW capacity motors, and large-capacity inverter units in the 1 000 MW class nuclear power unit feed water pump set Difficulty. If multiple fixed-speed electric water pump configuration scheme, feed pump and drive motor production are in the domestic power plant equipment manufacturing capacity range, it can be easily achieved nuclear power plant main feed pump equipment localization, to further reduce equipment investment costs.
3, conclusion
In summary, according to the characteristics of safe operation of PWR nuclear power plants, analysis from the perspectives of investment, operation and maintenance shows that the order of selection of main pump configuration schemes for PWR nuclear power plants in turn should be: multiple electric constant speed feed pumps, frequency conversion Speed ​​electric water pump (3 sets of 50%), with hydraulic coupler electric speed feed pump (3 50%), steam speed feed pump. Taking into account the VVVF electric pump configuration has never been used in large nuclear power plants, so for some time in our country, the main water supply regulating system of pressurized water reactor nuclear power station should choose more than one electric constant speed pump configuration scheme or with hydraulic coupler Electric speed control pump (3 50%) configuration scheme is appropriate.
Sildenafil Intermediates,Sildenafil Intermediate Cas 139756-02-8,Sildenafil Amino Cas No.139756-02-8,Sildenafil Amino 139756-02-8
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