13.2 Water extraction devices
Since the steam enters the turbines wet and becomes progressively wetter as it expands through the HP blading, arrangements are usually made for internal moisture extraction at each stage. Where appropriate, tip seals fitted to stationary components are slotted to permit extraction of water collecting on the outer flow boundary.
This water is either removed with steam bled from extraction points between the fixed blade carriers or, in stages which have no bled-steam tapping, drained through the casing via pipes, drillings and orifices to a stage at lower pressure.
Water extraction grooves may also be incorporated where necessary to assist the feedheating belts in extracting additional moisture. These consist of an annular groove in the cylinder wall at the trailing edge of the moving blades connected to a series of nozzles which discharge water and a small quantity of steam into the space between blade rings and outer cylinder.
The turbine cylinders should be self-draining; the HP cylinder into bled-steam pipes and into the cold reheat pipes, which are drained at all times to the drains receiver; the LP cylinders into bled-steam pipes and the condenser. If pockets at each end of the HP cylinder are formed in the casting between the cold reheat branches, these are drained continuously into the cold reheat pipes.
Interstage drainage introduces considerable quantities of water to the feedheater system and the provision of extra large drain arrangements in the design of this system requires careful consideration. In common with conventional machines, every effort is normally made to ensure that there is no possibility of water refluxing to the turbine. Present specifications require individual routing of drains to the turbine drains receiver and prohibit the commoning of drains from sources at different potentials.