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Balance of Plant (BOP)

The balance-of-plant systems convert main-steam energy to electrical output and return feedwater to the steam generators. From an EOP perspective the BOP is relevant as the normal heat sink (turbine + condenser + main feedwater) whose loss drives the plant onto AFW and on to the secondary-side heat-removal recovery procedures (E-0, FR-H.1).

Function

In normal operation: main-steam from the four SGs drives the HP and LP turbines; exhaust steam condenses in the condenser against circulating water; condensate is pumped through low-pressure heaters, the deaerator, and the main feedwater pumps back to the SGs at ~440 °F through high-pressure heaters.

In transient response: a turbine trip closes the turbine stop valves and dumps steam to the condenser via the dump valves (if condenser available); main feedwater isolates on SI; AFW takes over SG feed. Loss of condenser vacuum forces atmospheric dumping via ARVs.

Components

flowchart LR
    MS["Main Steam<br/>(from 4 SGs)"]
    TSV["Turbine Stop Valves<br/>«TSV-POS»"]
    HP_T["HP Turbine"]
    MSR["Moisture Separator<br/>Reheaters"]
    LP_T["LP Turbines<br/>(3 stages)"]
    GEN[("Main Generator<br/>«GEN-MW»")]
    COND["Condenser<br/>«COND-VAC»<br/>«COND-LVL»"]
    DUMP["Condenser Dump Valves<br/>«COND-DUMP-VLV»"]
    CW["Circulating Water<br/>(cooling tower or<br/>once-through)"]
    HOTWELL["Hotwell"]
    CP_A(("«COND-PUMP-A»"))
    CP_B(("«COND-PUMP-B»"))
    LP_HTR["LP Feedwater Heaters<br/>(4 stages)"]
    DEAER["Deaerator"]
    MFP_A(("«MFP-A»"))
    MFP_B(("«MFP-B»"))
    HP_HTR["HP Feedwater Heaters<br/>(2 stages, ~440 °F)"]
    FWI["MFW Isolation<br/>«MFW-ISOL»"]
    SG["To SGs"]

    MS --> TSV --> HP_T --> MSR --> LP_T --> GEN
    LP_T --> COND
    MS -. "turbine bypass" .-> DUMP --> COND
    COND --> HOTWELL
    CW -. cooling .-> COND
    HOTWELL --> CP_A & CP_B --> LP_HTR --> DEAER --> MFP_A & MFP_B
    MFP_A & MFP_B --> HP_HTR --> FWI --> SG

    classDef turbine stroke:#856404,stroke-width:2px
    classDef pump stroke:#1f618d,stroke-width:2px
    classDef vessel stroke:#6c3483,stroke-width:2px
    class HP_T,LP_T,GEN turbine
    class CP_A,CP_B,MFP_A,MFP_B pump
    class COND,HOTWELL,DEAER vessel
  • Main turbine — tandem-compound HP + 3 LP stages; ~1180 MWe gross at Vogtle; moisture-separator-reheaters between HP and LP for saturated-steam cycle.
  • Main generator — direct-coupled to turbine shaft; hydrogen-cooled stator; output to switchyard via main transformer.
  • Condenser — multi-shell surface condenser; saturated steam condenses at ~3 in Hg abs (about 100 °F); circulating water cools the shell side.
  • Condenser steam dump valves — bypass steam from main steam header directly to condenser; capacity sized for 100% load rejection at Vogtle (≈40% steam-flow dump).
  • Condensate pumps — typically two, vertical multistage; pump hotwell condensate up through the LP feedwater heaters.
  • Deaerator — strips dissolved oxygen from condensate; provides NPSH for main feedwater pumps; storage volume for transient flow mismatch.
  • Main feedwater pumps — typically turbine-driven, two pumps; full flow on both; ~16000 gpm each at full load.
  • HP feedwater heaters — final feed temperature ~440 °F; isolated on FWI signal (string check valves) to prevent heater-source inventory from flooding the SG.
  • Main feedwater isolation valves — close on SI / FWI; AFW takes over SG feed via separate nozzles.

Instrumentation

  • Generator output: «GEN-MW»
  • Turbine stop valve position: «TSV-POS»
  • Condenser vacuum: «COND-VAC»
  • Hotwell level: «COND-LVL»
  • Condenser dump valve position: «COND-DUMP-VLV»
  • Condensate pump statuses: «COND-PUMP-A», «COND-PUMP-B»
  • MFW pump statuses: «MFP-A», «MFP-B»
  • MFW isolation valve position: «MFW-ISOL»
  • MFW flow per SG: «MFW-FLOW-SG-A»…D

Setpoints

Parameter Value Source
Rated thermal power 3625 MWt Vogtle UFSAR §1.3
Gross electrical output ~1180 MWe Vogtle UFSAR §1.3
Condenser design vacuum ~3 in Hg abs (100 °F) Vogtle UFSAR §10.4.1
Condenser steam dump capacity ~40% MS flow Vogtle UFSAR §10.4.4
MFW final feed temperature ~440 °F Vogtle UFSAR §10.4.7
MFW isolation on SI automatic Vogtle UFSAR §10.4.7
Loss-of-condenser-vacuum trip «COND-VAC» < 22 in Hg Vogtle Tech Spec 3.3.1

Normal alignment

  • Turbine on line at rated load; generator synced to grid
  • Both condensate pumps and both MFW pumps in service
  • Condenser dump valves closed; available on demand
  • Deaerator at LCO storage level
  • MFW isolation valves open; AFW pumps standby

Failure modes

  • Loss of condenser vacuum — sealing water failure, air in- leakage, CW loss. Turbine trips at «COND-VAC» < setpoint; steam dump unavailable; ARVs become the only steam-relief path. Cross- links to FR-H.1 (heat sink threats).
  • MFW pump trip / both — feed flow lost; AFW automatic actuation on low-low SG level. Response embedded in E-0 / ES-0.1.
  • Loss of circulating water — cooling tower or intake failure; condenser pressure climbs; turbine trip via vacuum interlock.
  • Generator load rejection — sudden grid disconnect; turbine overspeed protection trips; steam dump must handle full load rejection or SG safeties lift.
  • Feedwater heater string isolation — HP heaters trip; feed temp drops, increasing reactor power demand; runback or trip may follow.

Relationship to EOPs

BOP loss is the dominant initiator of secondary-side EOP entry: turbine trip + MFW loss drives the plant to AFW heat removal. Most recovery flows (FR-H.1 / FR-H.2 / FR-H.4) treat BOP restoration as a long-term recovery path; the immediate EOP focus is on AFW and steam-relief integrity rather than restoring the turbine cycle.

References

  • Vogtle UFSAR §10.1 (Summary description — steam and power conversion)
  • Vogtle UFSAR §10.2 (Turbine generator)
  • Vogtle UFSAR §10.4.1 (Main condenser)
  • Vogtle UFSAR §10.4.4 (Turbine bypass steam dump)
  • Vogtle UFSAR §10.4.7 (Condensate and feedwater)
  • Vogtle Tech Spec 3.3.1 (Reactor protection trip — vacuum)