AOP-6 Loss of Condenser Vacuum Flashcards
What is the purpose of AOP-6?
To stabilize Turbine Backpressure at less than 6 inches Hg.
What are the AOP-6 entry conditions?
- EK-0111, “VACUUM LO” (24” Hg)
- EK-0110, “VACUUM PRETRIP” (21.5” Hg)
- EK-3522, “CLG TWR PUMP P-39A TRIP”
- EK-3523, “CLG TWR PUMP P-39B TRIP”
- PT-0763 Turbine Backpressure (6” Hg)
What are the automatic actions of AOP-6?
Turbine trips at 18-21 inches Hg in Main Condenser vacuum.
What is the reactor and turbine trip criteria for AOP-6?
Reactor Power greater than or equal to 15%
- Main Condenser vacuum lowers to less than 22” Hg
- Loss of both cooling tower pumps
Turbine Trip
- Reactor Power less than 15%
- Main Condenser vacuum lowers to less than 22” Hg
- Loss of both cooling tower pumps
What are the Immediate Actions of AOP-6?
None
When does a loss of condenser vacuum occur?
Event occurs when Main Condenser cooling is reduced and/or air removal capacity is lowered/challenged.
Why is the target for vacuum at 24 inches Hg?
The real limit for Turbine operation is a backpressure of 6” Hg.
This roughly corresponds to a 23.2” Hg vacuum.
The 24 inches Hg target ensures the 6 inches Hg backpressure limit is met and provides a margin from the automatic Turbine/Reactor trip when above 15% power.
Where do you see EK-0111, “VACUUM LO” (24” Hg) and EK-0110, “VACUUM PRETRIP” (21.5” Hg)?
C-11 (left), second column, 4th and 5th rows.
Where do you see EK-3522, “CLG TWR PUMP P-39A TRIP” and EK-3523, “CLG TWR PUMP P-39B TRIP”?
C-126. 1st and second columns, 4th row.
Where can you read turbine backpressure?
C-01. Above Bleeder Trip Valve indicating lights.
How does the Vacuum Trip work?
It is a Balance Beam Trip.
Vacuum trip is part a “balance beam” mechanism on the Main Turbine Front Standard that holds a dump valve (a different one from the overspeed trip) closed against spring pressure, as long as adequate bearing oil and condenser vacuum are present.
When this dump valve opens (ie, on loss of vacuum, bearing oil, or solenoid trip), the auto stop oil drains back to the lube oil reservoir, auto stop oil pressure drops quickly allowing the interface valve to open. This dumps EHC pressure which allows the turbine steam valves to go closed.
If both cooling tower pumps trip, why can’t you still operate at reduced power with only the Dilution Water Pumps?
If neither cooling tower pump is in service the Main Condenser water boxes will not remain full enough to condense the steam before the steam-air mixture reaches the air removal section of the tube bundle.
Therefore, even though the dilution water pumps can support significant Main Condenser heat removal, the air ejectors will not be able to remove the air
and the Plant will still need to be tripped.
What benefit is there to lower power reduction rates?
Power reduction at lower rates allows the use of boron and may prevent entry into
Technical Specifications LCO 3.1.6 (PDIL).
When do you reduce power for air in-leakage?
It is expected that on a condition where air in-leakage is greater than air removal capability it may be necessary to reduce Turbine load and therefore reduce the heat load on the Main Condenser to improve vacuum slightly and to allow time to identify the source of air in-leakage or to improve air removal capability.
In this case the rate of power reduction will be commensurate with the rate of vacuum loss.
Why might reducing power be the wrong choice when air-leakage is identified as the reason for lowering condenser vacuum?
The words “as necessary” are important since reducing power for air in-leakage/air removal problem events makes it difficult to determine when the air in-leakage has been successfully isolated.
