When the steam pressure drop occurs in only the stationary members of a turbine, how is the turbine described?

When steam pressure drop occurs only in the stationary members of a turbine, it is described as an impulse unit. In an impulse turbine, the steam passes through stationary blades (nozzles) where it expands and accelerates to high velocities before striking the moving blades. The drop in pressure happens as the steam exits the nozzles and enters the rotating blades, which solely convert the kinetic energy of the fluid into mechanical work.

The defining characteristic of an impulse unit is that it operates based on the momentum of the steam jets created by the divergent flow through the nozzles. In this configuration, the energy drop is realized by converting the steam's potential energy into kinetic energy entirely at the nozzle stage rather than through expansion in both stationary and rotating parts.

This design contrasts with a reaction unit, where the steam undergoes pressure drop in both the fixed and moving blades, allowing for work extraction through a combination of kinetic and pressure energy changes. Understanding this fundamental difference between impulse and reaction units is crucial for differentiating turbine types and their operational principles.