4.2.4 Low pressure feedwater heaters
Low pressure (LP) feedwater heaters are basically simple straightforward 'tube and shell' heat exchangers, with the condensate passing through the tubes and the bled-steam being admitted to the shell. This type of heater, arranged vertically, has been used for many decades, but two advances worthy of note have been made in recent years and are now treated as modern practice.
With the turbine exhaust being connected to its underslung condenser by a large, deep, transition piece, a significant volume of unused space exists between the exhausts of a double-flow cylinder and above the condenser tubenests (see Fig 1.68). This space can be effectively utilised by inserting one or two LP feedwater heaters in a horizontal attitude.
A number of advantages accrue from such an installation. The bled steam piping is short in length, giving a cost saving and a lower pressure drop, the lower drop resulting in a slightly higher temperature for heating the condensate, with consequential improvement in cycle efficiency. Bled-steam valves are omitted — both the traditional isolating valve and the non-return valve. It has been established by experiment and experience that steam reflux on turbine trip, when the stage pressure collapses to condenser pressure, is negligible and hence there is no significant contribution to rotor overspeed.
With the heaters being almost completely within the condenser 'neck', there is economy in turbine hall area and in the elimination of supporting steelwork, access ladders and platforms.
The omission of an isolating steam valve prompted the development of improved methods of preventing heater flooding. Neck heaters are equipped with valveless primary drains and a valveless secondary drain, the latter being arranged physically a little higher than the primary drain outlet.
External LP heaters, although still of the traditional form, are now arranged in a horizontal attitude, elevated so that the bled-steam piping from the turbine falls to the heaters, thus assuring drainage. This minimises the risk of water entering the cylinder and the possibility of damage by impact on rotating components.
The heaters are 'stacked', so that the ultimate LP heater can drain its bled-steam condensate into the next heater, and that heater can drain its condensate into the condenser above the working level of condensate in its base.