How does impulse blading work in a steam turbine?

In a steam turbine, impulse blading functions by converting the thermal energy of steam into kinetic energy before it reaches the moving blades. This process occurs in two distinct steps:

• Expansion: Steam passes through stationary nozzles where it expands to a lower pressure, significantly increasing its velocity and kinetic energy.1
• Energy Extraction: The high-velocity steam is directed onto moving blades (or buckets). In an ideal impulse stage, there is no further pressure drop or acceleration as the steam passes through these blades; instead, the blades change the direction of the steam flow, extracting its kinetic energy to generate rotational work.1

Because there is no pressure drop across the moving blades, the cross-sectional area of the flow passage remains constant, resulting in a characteristic symmetrical blade shape that is thick in the middle and sharp at the ends. This design avoids the axial thrust loads associated with reaction blading, as pressure forces on the rotor play no role in the impulse process. For maximum efficiency in a symmetrical impulse stage, the rotor velocity is typically designed to be approximately one-half of the entering steam velocity.12