Turbine Drainage

Steam enters the HP turbine dry with superheated. As it passes through the stages the degree of superheat falls to a point in the final stages of the LP turbine they dryness factor is less than one and water droplets are entrained. When the water droplets form they are very small and travel at the same velocity as the steam. As the stream passes through further stages the water droplets fail to keep in the steam stream with the changes in direction and velocity. The droplet size increases and is removed from the steam by centrifugal action and by contact with the blades.
These droplets may impact the leading face of the rotating blades abd lead to erosion and cause a retarding effect. The damage is proportional to the swirl velocity and therefore is 4 times worse at the tip than the root.
The water droplets tend to flow to the tips of the blade and from there passes to the casing or pass on to further stages. The erosion causes pitting, perforations and blade failure.

Damage to the blades may be reduced by brazing or electron beam welding on a stellite strip
However these can be undermined by erosion and be thrown off causing considerable damage.
One method of reducing this problem is by reheating the HP exhaust system by passing through the boiler. This has the added effect of increaseig plant efficiency but at increase cost of pipe run. Reducing blade height and therefore speed can lessen effects as can taper twisting the final stages (see seperate page)