Explain the significance of 'thrust collars' in steam turbine operation.

Thrust collars in a steam turbine are critical components designed to manage axial thrust. Axial thrust is a force that acts along the axis of the turbine rotor, pushing it in one direction. This force is created due to the pressure differences across the turbine blades and the geometry of the steam flow path. Without proper management, axial thrust can cause the rotor to shift its position, leading to rubbing between rotating and stationary parts, damage to seals, and ultimately, turbine failure. Thrust collars are typically attached to the turbine rotor and interact with a thrust bearing assembly. This assembly is designed to absorb the axial thrust and maintain the rotor in its correct axial position. The thrust collars are usually made of hardened steel and have a smooth, polished surface to minimize friction. The thrust bearing is designed to handle both the normal operating thrust and any transient thrust loads that might occur during start-up, shutdown, or load changes. The design and maintenance of the thrust bearing and collars are crucial for ensuring the reliable and safe operation of the steam turbine.