Why are large steam turbines split into parallel low-pressure flow sections?

Large steam turbines are split into parallel low-pressure flow sections to manage the massive increase in steam volume that occurs as it expands from high-pressure, high-temperature conditions down to condenser pressures.

As steam expands, its volume can increase more than a thousandfold. To maintain a constant steam velocity through the turbine, the annular flow area would theoretically need to increase by a similar magnitude. Without splitting the flow, this would necessitate the use of excessively large diameter casings and turbine blades of impractical lengths near the exit. By dividing the low-pressure stream into multiple parallel flow sections, engineers can keep the turbine dimensions and blade lengths within manageable limits while maintaining efficient mechanical energy conversion.1

This design approach is a key feature of multiflow and compound arrangements, which help mitigate the physical constraints imposed by the significant expansion of steam during the power generation process.1