Steam Bypass Control Systems Flashcards
Describe SBCS Design Features.
- SBCS Controls RCS Temperature and S/G pressure, and minimizes the undesirable lifting of the PZR Code Safeties and Main Steam Safety Relief valves during certain Reactor/Steam plant transients
- SBCS also reduces the chance of any potential uncontrolled release to the atmosphere through the Main Steam Safety Valves
- Accommodate load reductions of 57% w/o tripping the Rx or Turbine and w/o opening the PZR safeties or Main Steam Safety Valves
- Maintain the RCS at hot zero power conditions
- Provides a means for manual control of the RCS temperature during heatup or cooldown
- Provides a Condenser Interlock which will block steam bypass flow when unit condenser pressure exceeds a preset limit
Describe SBCS General Information.
SBCS is designed to remove excess heat from the NSSS during:
a) Load reductions
b) Reactor trips
c) When high secondary system header pressure exists
* Total Capacity – 57%, Bypass Valves are Air Operated
* SBCS Valves (5): Are direct acting valve with independent 60 gal air volume tank (needed to achieve the < 1 second quick opening time)
* Failed closed on loss power and loss of air (< 55psig) via the air volume tank
* Side-mounted handwheel can be used to operate each of the valves. Manual operation requires engagement of the manual override shaft to the actuator stem by inserting a pin into the slot on the manual override shaft and bypass air valves are installed to equalize the pressure in the top and bottom of the actuator
* Each valve positioner has a Quick Trak controller key pad in the field which gives valve status information. Certain “Trip Close” malfunctions may need to be reset locally prior to regaining control of the valve.
* The SBCS has no safety design bases and is classified as non-seismic.
* Overpressure protection of the Main Steam System is provided by ASME Code safety valves.
Describe the components of the SBCS.
- 5 Valves that dump steam to the Condenser (PCV 8801-8805), Reverse Acting
- Overlap valve modulation - Open sequentially but with overlap in each group for better flow control with:
a) 8801 opening first, (~5% valve), and when 90% open demand signal, then:
b) 8802 starts to open (~13% valve), and when 8802 is 90% open demand signal, then:
c) 8803, 8804 and 8805 start to open (each are ~13% valves). All valves are ganged together. - 1 Master controller (PIC 8010 / PCV-8805) – RTGB 102 [202]
- 4 Manual / Automatic Controllers (HIC-8801 / 8802 / 8803 / 8804),
- Permissive Mode Selector Switch, RTGB 102 [202]
Describe the SBCS Power Supplies.
- The Power Supply A (PSA, normal) is powered by 120 VAC Vital Bus #1, Ckt #22, and the Power Supply B (PSB, redundant) is powered by 125 VDC Bus #1B, Ckt #17.
- DC power to solenoids: 125 VDC, Bus 1B, Ckt #17.
- PSA and PSB supply the Eycon controllers and PT-8010 and 8020.
- [2A and 2B Vital AC (SUPS Inverter) – primary power to SBCS DCS components]
- [2A Battery (PP-218) and 2B Battery (PP-219) – back up power to SBCS DCS components]
- [SAS SUPS (PP-276) – Power to “A” train flat panel display]
- [SAS SUPS (PP-277) – Power to “B” train flat panel display]
Describe the SBCS Inputs.
SBCS Inputs: -
1) Validated Main Steam Pressure (Averages P-8010 & P-8020 vs. Arbiter input P-0802)
2) Validated Steam Flow (Averages F-8011 & F-8021 vs. Arbiter input P2236A)
3) Validated RCS Tavg (Averages Rx Reg 1 & Rx Reg 2 Tavg vs. Arbiter 1° & 2° inputs from QSPDS)
4) Validated Condenser Vacuum (2/3 logic from ‘A’ or ‘B’ Condensers)
5) Actual Load Demand (DEH rate of change)
6) Reactor Trip undervoltage ( U/V relays from Rx Trip swgr)
* Main Steam Header Pressure (Pressure Setpoint 1 & Pressure Setpoint 2)
* 5 inputs from RTGB M/A stations
* Emergency off PB on SBCS control cabinets
* Test signals from SBCS control cabinets
Describe the SBCS Permissive Switch.
- Located on the RTGB, near the Steam Dump controllers. Has the following positions:
OFF: Operation of the SBCS valves blocked, permissive solenoid valves de-energized.
AUTO: (normal mode): permissive relays energized by any demand signal from the permissive circuitry.
MANUAL: Makes up the valve operation permissive by energizing the permissive solenoids. This allows valve operation if NO low vacuum and NO emergency off interlocks are present.
Placed in “Manual” when performing a plant cooldown – so when steam pressure drops below reduction < 806 psia, all the SBCS valves do not go closed. Being greater than 806 PSIA is usually the last permissive remaining that allows auto operation ( < 806 psia - loss of Quick Open-2 permissive)
Describe the SBCS Emergency Off Pushbutton.
- Located on the SBCS overview on DCS,
- Pushing the button will close Bypass Valves.
- To regain control of the valves depress the “Emergency Off Reset” PB
Describe the SBCS Quick Open Signals.
Used initially on a Unit Trip to assist in control of Tave and Stm Pressure
Limit Tave and Stm Pressure rise following large Load Reduction transients while unit operating.
Two different signals make up logic for actuation of the quick opening solenoid valve:
Quick Open 1 signal:
- Rate of ∆ for Steam flow is > {16} [19] % per min
OR
- RX TRIP WITH Tavg is >555°F AND < 3°F below post Rx trip Tavg
Quick Open 2 signal
- Rate of ∆ for DEH load ∆ is > {16} [19] % per min
OR
- RX TRIP AND Steam pressure is >806 psia
- Quick Open signals are sent to the SBCS valves by way of a Quick Opening solenoid valve which shifts to port a 2” air line directly to the SBCS valve operating piston . (Takes ~ 1 second to open the valve fully)
Each individual PCV-8801 thru 8805 Quick Opening solenoids will energize if:
a) That particular HIC-8801 thru 8805 controller is in “Auto”
AND
b) “Quick Open 1” AND “Quick Open 2” signals are present
Describe SBCS Valve Modulation Signal.
Positioner controls air to SBCS valves, (Takes ~3 – 9 seconds to open the valve fully)
Primary means of controlling temperature following RX trip once Quick Open signal goes away
Two modes of valve modulation:
Pressure Modulate
Tavg Modulate
Describe SBCS Pressure Modulation Program.
If PIC-8010 is in AUTO:
* Controls HIC-8801 at all times
* Controls HIC-8802, 8803 & 8804 only if the Rx is not tripped
* Compares Actual Steam Pressure vs. setpoint of 900 psia
* If pressure is greater than the setpoint, PCV’s will start to slowly open, starting with PCV-8801
* Auto - (reactor critical) - sends sequential valve control signals to all SBCS valves with HICs in Auto
Based on maintaining a 900 psia steam header pressure setpoint and allows PCV-8805 to open on QO signal
If PIC-8010 is in Manual:
* Sends sequential valve control signals to all SBCS valves with HICs in Auto, and
* Manually operates PCV-8805 (starting at a PIC-8010 Master STA output DMD of ~32%)
If a Rx Trip occurs:
* The setpoint is driven to 940 psia for 4 minutes, followed by a 5 minute ramp back to 900 psia
* This will only affect the position of PCV-8801.
* PCV-8802 thru 8805 position will be controlled by the Tavg Modulation Mode.
Describe SBCS Tave Modulation Program.
- Entered only if Master controller PIC-8010 is in AUTO and Rx Trip has occurred
- PCV 8802-8805 are no longer controlled by PIC-8010, and instead become controlled by the Tave Program
- Tave programmed input ranges from 580F to 541F.
- As Tave is being reduced to 541°F, PCV’s 8802-8805 slowly modulate closed,
- Further cooldown from 541F to 532F is performed by PCV-8801 in the pressure Modulate Mode
Describe the Steam Bypass Master Controller (PIC 8010/PCV-8805) - RTGB.
- Setpoint is remotely derived by the SBCS Calculator (900 psia). Following a Rx trip, the setpoint is driven to 940 psia for 4 minutes, followed by a 5 minute ramp back to 900 psia.
- Tracks the Quick Open signal to provide a bumpless transfer back to the Modulate mode
- While in MANUAL, it directly controls PCV-8805 and any other bypass valve which controller is in auto, provided the valve permissive and interlocks are satisfied.
- Quick Open & Tave Control circuits bypass the Master controller if in AUTO on Rx trip
- Auto transfers into the MANUAL mode if:
a) Loss of BOTH Steam Pressure signals (BAD)
b) Bad arbiter along with a Deviation
Describe the SBCS Eycon controller Man/Auto (M/A) Stations.
Eycon controller HICs-8801 through 8804 can individually modulate their respective valves when in manual provided the valve Permissive and Condenser Vacuum Interlocks are met.
Describe Manual Operation Of Steam Bypass Valves.
- 8801-8804 controlled with its own HIC,
- 8805 is controlled by the Master Controller in Manual
- Valves can also be operated locally
Describe SBCS Condenser Vacuum Interlock.
- > 12” Hg Abs blocks both modulate and QO paths, prevent damage to Turbine & Condenser
- When vacuum is regained the valves will re-open if M/A stations are in AUTO
- If any of the M/A Stations are in MANUAL, When Vacuum is regained the valves will not re-open
- Prevents inadvertent Cooldown if vacuum is restored and controller is in Manual
- Must RESET by pressing “Vacuum Reset” PB located on the DCS SBCS Overview
