Normally, the steam flows through the turbine control valves to the high pressure turbines. These valves are isolated on a number of plant upset conditions. Steam, however, continues to be generated in the core. To control pressure in the MCC, this steam is relieved either through the steam discharge valves, or through the main safety valves. The SDVs are power-operated and operate through an active control system to maintain pressure. Eight SDV-C valves provide a steam
exhaust to path directly to turbine condensers. These valves are used for pressure control during a normal plant shutdown, as well as operational upset conditions. Two additional SDV-A valves relieve steam to the suppression pool of ACS system. There are twelve MSVs which are set to operate in three banks. When the MCC pressure reaches the specified setpoints for the first bank, valves of the first bank open and discharge steam to the suppression pool. If pressure continues to rise, the second and third bank valves may also open. Table 3.1 gives a list of setpoints, reset parameters and other characteristics of SDVs and MSVs.
Table 3.1 Setpoints and parameters of steam discharge and main safety valves
|
Valve |
Number of valves |
Capacity per valve, kg/s |
Opening pressure, MPa |
Reset pressure, MPa |
Delay for opening/ closing, s |
Opening/ closing time, s |
|
SDV-C |
8 |
152.78 |
7.06 |
6.77 |
- |
10/10 |
|
SDV-A |
2 |
97.22 |
7.16 |
6.87 |
- |
10/10 |
|
MSV, Bank 1 |
2 |
97.22 |
7.46 |
7.16 |
4/10 |
0.5/0.5 |
|
MSV, Bank 2 |
4 |
97.22 |
7.55 |
7.26 |
4/10 |
0.5/0.5 |
|
MSV, Bank 3 |
6 |
97.22 |
7.65 |
7.36 |
4/10 |
0.5/0.5 |