Module 3.4 — Extraction steam heating, drain cascading, and why heater tube leaks are a distinct failure mode from condenser leaks with their own chemistry signature.
Click any component to see its leak risk and chemistry implications. LP heaters sit under vacuum before the DA; HP heaters are pressurized after the DA — that pressure difference changes which direction a leak flows.
Feedwater heaters progressively raise feedwater temperature on its way to the boiler using steam extracted from various turbine stages — extraction steam that's already done partial work in the turbine gets a second useful job heating feedwater, improving overall cycle efficiency. Low-pressure (LP) heaters sit between the condenser and the deaerator, typically operating under vacuum similar to the condenser itself; high-pressure (HP) heaters sit between the deaerator and the economizer, operating at pressurized conditions closer to boiler feed pressure.
Like the condenser (Module 3.1), feedwater heaters are tube-and-shell heat exchangers, and like condensers, their tubes can fail. But the chemistry signature and risk profile differ meaningfully from a condenser leak. Where a condenser leak introduces raw, untreated cooling water, a feedwater heater leak introduces extraction steam/condensate that's already been through at least some of the treatment cycle — meaning the contamination risk is generally lower in degree, but not zero, since extraction steam can carry its own impurities depending on where in the turbine it was pulled.
As extraction steam condenses inside a feedwater heater giving up its heat, the resulting drains (condensed extraction steam) are typically cascaded backward — drained to the next lower-pressure heater, and eventually back to the condenser or deaerator — rather than being pumped forward with the main feedwater flow. This design choice means a heater drain system carries its own water quality considerations distinct from the main feedwater path, and drain cooling/flash tank equipment along the cascade path is itself a location where problems (fouling, level control issues) can develop somewhat independently of main feedwater chemistry.
Historically, many feedwater heaters — particularly LP heaters — used copper alloy tubes for the same heat transfer advantages that made copper attractive in condensers (Module 3.1). This is one of the more common reasons a feedwater train ends up needing AVT(R) rather than AVT(O) (Module 2.4): even without any condenser copper alloy tubes, copper alloy feedwater heaters alone can be enough to require oxygen-free reducing conditions to protect that metallurgy. Module 3.5 covers copper alloy chemistry in more depth.