A deaerating feed tank (DFT), often found in steam plants that propel ships, is located after the main condensate pump and before the main feed booster pump.
The deaerating feed tank's surge volume allows the ship to change "bells" (steam turbine power output) and change the ship's speed without running the feed pump dry or flooding the turbines with liquid water.
This draws more water from the condenser, potentially to the point of going dry and starving the boiler or steam generator resulting in a loss of propulsion.
When the bell is decreased, turbine power output is reduced and the feed rate drops.
The feed tank serves as a surge volume to take this excess condensate and avoid losing vacuum.