These pipes supply the steam from the separator drums to the turbine and to plant internal plant needs. This includes steam for the pressure safety system of the MCC, and through a reducer to the plant steam system. Table 5.10 lists the design steam parameters.
The piping is schematically presented in Fig. 5.13. From the separator drum (1) the steam is carried along 16 pipes (325 x 19) mm, to two steam headers (2) (630 x 25) mm. From there the steam of one MCC loop is collected by four steam pipes (3) (630 x 25) mm, and fed to the turbine (5). Each separator drum is connected by the pipe line (3) to two turbogenerators.
To ensure a uniform longitudinal sink from the separator drum, steam pipes (3) are connected mid-way to the steam headers (2). Each of the steam pipes (3) has a parallel Steam Discharge Valve (SDV-C) (7) to direct the steam to the condensers of the turbines. The pressure of the steam is also controlled, and peaks of pressure are eliminated by the high pressure steam loop (8) The two high pressure steam loops, one for each MCC loop, consists of pipes and discharge valves, and is connected by four pipes to the steam zones of the two separator drums. The Steam Discharge Valve (SDV-A) (10) and six Main Safety Valves (MSVs) (9) discharge the steam to the pressure suppression pool of ACS tower. When the pressure decreases bellow 5.89 MPa (60 kgf/cm2), two valves (which control steam flow rate to one turbine) close and two similar valves of the other turbine are closed when the pressure falls below 5.39 MPa (55 kgf/cm2) (this is absolute pressure).
Fig. 5.12 A schematic representation of the GDH check valve
1 - disk, 2 - protective cross, 3 - suspender
5.4.1 Protection of the MCC from Pressure Surges
The MCC is protected from over-pressures by a steam discharge system employing pressure relief valves. One loop of this system consists of:
The SDV-A and MSVs are located in the high pressure steam loop (8) joining the four steam headers of the separator drums of one loop. The steam discharged through these valves goes into the ACS tower. The parameters of the steam discharge system are shown in Table 5.11 [43].
Table 5.10 Operation parameters of the steam
|
Absolute pressure in the separator drums, MPa |
6.47-6.96 |
|
Absolute pressure of the turbine supply inlet, MPa |
6.18- 6.67 |
|
Inlet temperature of the turbine, oC |
279.5 |
|
Water content in the steam flow (mass fraction) at the turbine inlet, % |
< 0.5 |
|
Maximum flow rates for two turbines at 4200 MW (th), kg/s |
2055-2125 |
Table 5.11 Parameters of the protective steam discharge valves [43]
|
Trademark |
Number per reactor |
Activation pressure (off/on), MPa (kgf/cm2)* |
Capacity per valve, kg/s (t/h) |
|
SDV-C |
8 |
6.96/6.77 (71/69) |
152.8 (550) |
|
SDV-A |
2 |
7.06/6.77 (72/69) |
97.2 (350) |
|
MSV group I |
2 |
7.36/7.06 (75/72) |
97.2 (350) |
|
MSV group II |
4 |
7.45/7.16 (76/73) |
97.2 (350) |
|
MSV group III |
6 |
7.55/7.26 (77/74) |
97.2 (350) |
* excess pressure
Fig. 5.13 Steam piping
1 - separator drum, 2 - header, 3 - main steam pipes, 4 - SDV-D, 5 - turbogenerators, 6 - condensers, 7 - SDV-C, 8 - high pressure steam loop, 9 - MSV, 10 - SDV-A
5.4.1.1 Fast-Acting Steam Discharge Valve SDV-C
Steam pipes leading to the condensers of all turbines are provided with fast-acting steam discharge valves. To ensure a uniform drain from the separator drum, two steam pipes are connected to each SDV-C valve. This keeps the piping hot even when the reactor is shut down. If condensation occurs in the lines, the inclined inlet-outlet pipes facilitate free flow of the condensate. Each SDV-C contains a throttle to decrease the energy of the steam flow.
The SDV-C valves admit a portion of the steam from the steam piping, control it in the throttle valves and supply it to the condensers of the turbines. These valves are activated under the following conditions :
An individual SDV-C is limited to a 152.8 kg/s flow rate. This adds up to 1222 kg/s for all eight SDV-C in the steam pipelines or 50% of the maximum 2444 kg/s steam generated by the reactor at 4800 MW (th). The valve is activated in about 10 s after the limiting 7.06 MPa pressure is reached. The fast-acting steam discharge valve is an (on/off) isolating throttle described in Table5.12 [39].
Electric drives of the SDV-C are operated in the following three modes by:
Table 5.12 Fast-acting steam discharge valve to turbine condenser [39] (type - 1034-300/300-7, manufacturer - Tchechovskoy Plant of Energy Engineering, Russia)
|
Number per reactor |
8 |
||
|
Capacity, kg/s |
152.8 |
||
|
Overpressure protection (excess), MPa |
6.96 |
||
|
Opening/closure time, s |
< 10 |
||
|
Operating temperature, oC |
280 |
||
|
Pressure at outlet, MPa * |
1.0 |
||
|
Temperature at outlet, oC * |
183 |
||
|
Nominal diameter at inlet and outlet, mm |
300/300 |
||
|
- average lifetime, years |
30 |
||
|
- average lifetime between major servicing, years |
5 |
||
|
- average lifetime between major servicing, cycles |
750 |
||
|
- probability of safe operation, % |
92 |
* Design values
Each valve is connected to an automatic controller and a position indicator on the MCR. In the first operating mode, the SDV-C is activated by a high pressure signal from the separator drum. In the second mode, the computer returns either a "close" or a "keep closed" command under a combined action of the following conditions: increased pressure in the turbine condenser (pressure in condenser is more than 0.023 MPa), an excessive level of the condensate, temperature downstream of steam discharge valve is more than 100 oC, or failure of the electric currant. Finally, remote manual control (third case) is used in transient operation during start-up or cool-down of the reactor, or in the case of failure of the other two operating modes.
5.4.1.2 High Pressure Steam Loop
The steam piping consists of two identical high pressure steam loops. Each of these loops is connected by four pipes (630 x 25) mm to the steam zones of the separator drums of the respective closed loop of the MCC, as shown in Fig. 5.10. A high pressure steam loop contains six MSVs and one SDV-A. The SDV-A valve is connected to the high pressure loop by two pipes (325 x 16) mm. The MSV of the loop and the SDV-A discharge the steam through their individual pipes (630 x 12) mm to the fifth pool of the ACS tower. Part of the steam also escapes through closed MSV and SDV-A valves. This steam is collected and directed through a (630 x 8) mm pipe to the condenser in the machine hall. The inclined supply and discharge pipes facilitate free flow of condensate.
The SDV-A obtains steam from the steam piping, reduces its energy by throttling and directs it to the fifth condensation pool of the ACS tower. It is activated when turbine load decreases substantially doe to:
SDV-A is also available to cool the reactor in case of an accident. It will be activated in those conditions when the opening of the SDV-C valve is insufficient to control the rise in pressure or when the SDV-C valve fails to function.
The activation pressure of SDV-A is set at 7.06 MPa, nominal steam flow rate after activation is 97.2 kg/s. The fast-acting steam discharge valve is an (on/off) isolating throttle, as shown in Table 5.13 [39].
Table 5.13 Fast-acting steam discharge valve to fifth pool of the ACS tower SDV-A [39] (type - 1034-300/300-7, manufacturer - Tchechovskoy Plant of Energy Engineering, Russia)
|
Number per reactor |
2 |
||
|
Capacity, kg/s |
97.2 |
||
|
Overpressure protection (excess), MPa |
7.06 |
||
|
Opening/closure time, s |
< 10 |
||
|
Pressure at outlet, MPa* |
1.0 |
||
|
Operating temperature, oC* |
280 |
||
|
Temperature at outlet, oC* |
183 |
||
|
Nominal diameter at inlet and outlet, mm |
300/300 |
||
|
- average lifetime , years |
30 |
||
|
- average lifetime between major servicing, years |
5 |
||
|
- average lifetime between major servicing, cycles |
750 |
||
|
- probability of safe operation, % |
92 |
* Design values
Two types of fast-acting steam discharge valves are operated by electric drives. They differ only in their activation pressure, which is 7.06 MPa for the SDV-A and 6.96 MPa (excess pressure) for the SDV-C valve. The computer control issues either "close" or "do not open" directions at 6.77 MPa operation pressure. Remote manual operation from the MCR is available.
The MSV are intended for emergency protection of the piping and other primary system components from pressure rises for which the activation of the SDV-A, and SDV-C safety valves become insufficient.
The activation pressure for separate groups of MSVs are given in Table 5.11. The opening/closing time for MSVs is less that 0.5 s, the time delay for opening is less that 4 s and for closing about 10 s. All of the original 24 Main safety valves (12 valves per unit) were replaced by new more reliable French-design valves in 1996. Safety significant characteristics of these valves, such as actuation pressure (on/off) and capacity of these new MSVs remain the same as for old ones.