5.8.1 Erosion mechanism
Turbine blade erosion is initiated by a relatively small proportion of the water droplets in the steam separating out and collecting on the concave pressure face of the diaphragm blades, where a film is formed which is drawn towards the trailing edge by the drag of the steam.
Here the film grows and passes around the trailing edge on to the convex face where, usually in a region of strong secondary flow, droplets move back and forth increasing in size until they are torn away by the shearing action of the main steam flow. Relatively large drops (50-200 /mi in diameter) are produced and have to be accelerated from rest by the steam. These large drops arrive at the inlet plane of the moving blade row at only a fraction of the absolute velocity of the steam and are subsequently struck by the convex surfaces of the moving blade row. This causes the abrasion on the leading edge of the blade known as erosion Fig 1.91.