SBCS (Steam Bypass Control System) Flashcards
What is the valve group/ordering for SBCS?
1001: 0-25%
1004: 25-50%
1003&1006: 50-66.7%
1002&1005: 66.7-83.3%
1007&1008: 83.3-100%
Where do the SBCS valves direct their steam?
3 - 2 -1, A-B-C
A has more due to its size
How do the SBCS valves fail? Loss of power/loss of air
- Air (80#) used to open & close the valves (fail closed / spring pressure)
- NNN-D11 and NNN-D12. Upon a loss of either
supply the following effect on operation occurs:
Steam Bypass Control System loses Logic Power, Steam Bypass Control Valves fail CLOSED
and cannot be operated in manual or auto.
(NKNM45 or NKND42) - all of the valves to lose power to the solenoids and** fail closed.**
What pressure does the SBCS condenser interlock occur? What does it do?
Main Condenser vacuum interlock
* 2 PTs per condenser shell
* 1 PT for interlock #1 (2 of 3 shell logic. Used for the quick open circuit.)
* 1 PT for interlock #2 (2 of 3 shell logic. Used for the permissive circuit.)
* Setpoint ~ 5.5” HgA
o** Interlock seals in and must be manually reset **when < 5” HgA (B06 or SBCS panel).
** Does NOT defeat 1007/1008 signals*
* Requires loss of vacuum in more than one condenser shell to actuate.
How large of a load rejection can the plant withstand on SBCS alone? With RPCB?
Calculations have determined that the plant can withstand a ** 56% turbine load rejection with the aid of the SBCS (no RPCS initiation)** and without enabling the SBCS interlock (Condenser shell
pressure limit is not exceeded) during worst case conditions (hot and humid summer periods).
Partial load rejections above 56% can be mitigated with the aid of both SBCS and RPCS, without a reactor trip, assuming that the RPCB is initiated when the load rejection exceeds 56%.
What are the steam header pressure transmitters for SBCS?
PT-1024 Modulate
PT-1027 Permissive
Describe the plant response to a failure of PT-1024
PT-1024 Steam Pressure failure
Fails High (1300 psia):
* Full modulation demand (100%).
* Valves open if permissive met.
* Runback signal is non-functional (above runback region).
Fails low (900 psia):
* No modulation demand (0%).
* No alarm
* SBCS runback is always enabled (within runback region)(continuous runback)
Describe the plant response to a failure of PT-1027:
PT-1027 Steam Pressure failure
Fails High (1300 psia):
* Generates a permissive signal
Fails Low (900 psia):
* Blocks auto permissive for modulating circuit
* No alarms
When will SBCS generate an AWP?
AWP = SBCS Trip alarm
Permissive & Modulate signal generated OR Quick Open (X1&X2 or Y1&Y2)
Assuming that SBCS is connected and not in Emergency Off, an Automatic Withdrawal Prohibit
(AWP) signal is sent to CEDMCS from the SBCS when ANY of the following conditions are met:
- Modulation demand and a permissive
OR - Quick Open X1 and X2
OR - Quick Open Y1 and Y2
This signal is generated to prevent automatic CEA withdrawal when in an already overpower condition (i.e., primary power greater than secondary power).
What will cause a Quick Open Block?
Quick Open Blocks
* Loss of a MFP
* LOFP signal comes from RPCB, Blocks X and Y signals
Reactor Trip
o Trip signal from DFWCS (sensed by CEDMS UV coils 1 & 2 or 3 & 4 logic)
o Blocks Y signal
Reactor Trip with Low Tavg ** (573.5°F**)
o Tavg from RRS
o Blocks X signal
Explain pressurizer pressure biasing effects on SBCS:
PZR pressure biases the setpoint program when
above 2250 psia. This bias results in a decrease
in the demand setpoint at the rate of -0.2 psia
steam header pressure for every increase of 1
psia of PZR pressure. This results in the bypass
valves opening earlier when operating at higher
PZR pressures. The impact that a failure (HI or
LO) has depends on where the plant is presently
operating.
How does a loss of power affect the SBCS master controller?
SBCS is OOS, when restored in Manual at 0% output
* Loss of power (> 2 seconds) – transfers to manual and **may be in “Emergency Off” **when restored
How long does the PT-1027 permissive signal last?
When the output reaches the bistable trip setpoint, the permissive bistable will trip. Tripping of the bistable generates the auto permissive signal to energize the permissive solenoid on all eight bypass valves for at least** 15 seconds**. This 15 second ensures permissive signal is present after the demand has decayed away to prevent a quick close of the valves.
How is SBCS availability calculated?
- Steam Bypass Availability
o Determine if bypass valve(s) is functional. If** all **are met:
Both Condenser Interlocks CLEAR (not applicable for 1007 / 1008)
Master Controller in REMOTE-AUTO
Individual Controller in AUTO
Valve Mode Selector Switch in AUTO or MANUAL (Permissive) (Not in OFF)
o Summer adds up the available valves and multiplies it by ~7%. (~ 56%)
Describe AMI Threshold
- AMI Threshold
o Determined by comparing turbine (TLI) and bypass valves capability to AMI Manually Selected Setpoint.
o Manually selected setpoint is determined by the position of the thumbwheel on AMI SETPOINT controller (B04).
o Normally set at ~ 50%. (changes over core life, refer to 40OP-9ZZ05)
o Black needle shows setpoint.
o AMI threshold is the LOWER of the two (GREEN needle).
