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. It has these three purposes:
Based on the relevant theoretical Rankine cycle diagram, there are four main processes, or steps:
In the practical implementation of a Rankine cycle, it is common to break the single pump (process 1 to 2) into three pumps: (in water flow order: condensate pump, feed booster pump and then feedwater pump).
A surge volume allows the plant to change bells (power output level) without running the feed pump dry or flooding the turbines. Consider the plant running in a steady state condition.
The bell is increased, more power output demanded, the rate of feed is increased. This draws more water from the condenser, perhaps to the point of being dry and starving the boiler resulting in a loss of propulsion. This is until the water, converted to steam, provides its energy to the turbine and then is condensed in the condenser.
The bell is decreased, less power output demanded, the rate of feed is decreased. Since less water is drawn from the condenser the condensate level rises, covering more condenser tubes, reducing the ability of the condenser to maintain vacuum and, if the level is allowed to go high enough, vacuum could be lost and/or water could impinge (and damage) the turbine blades as the turbine normally sits directly above the condenser.