2.3.3. Tip leakage
This leakage, as its name implies, arises from steam passing through the small clearance required between the moving blade tip and the casing, or between the end of the fixed blades and the rotating shaft. The problem is overcome to a certain extent through the use of inter-stage seals.
At the blade tips there is a shroud band which extends around the entire circumference of the moving blades, joining the tips. The shroud is sealed against the casing by several knife edges.
The extent of the leakage depends on whether the turbine is of an impulse or reaction type. Reaction turbines suffer a pressure drop across the moving blades, so encouraging flow through the clearances. Consequently, it is more important for a reaction turbine to have good tip sealing than it is for an impulse turbine (Fig 1.15).