Thermal expansion difference refers to the relative expansion of the shaft compared to the cylinder. If the cylinder's expansion is significantly less than the shaft's expansion at this point, the thermal expansion difference may not necessarily shift toward positive values. If unit parameters remain unchanged and load is stable, the thermal expansion difference and axial displacement will not change. During unit startup/shutdown and when steam parameters change, the expansion difference will vary. Axial displacement will also change due to load variations. Changes in axial displacement during operation inevitably cause changes in the expansion difference.
The expansion difference where the turbine rotor expands more than the cylinder is called positive expansion difference. When the cylinder expands more than the rotor, the expansion difference is called negative expansion difference.
Expansion differential values are critical operating parameters. If the differential exceeds limits, thermal protection activates to trip the main unit, preventing collision damage between rotating and stationary components.
During startup, heating devices typically control cylinder expansion, while rotor expansion is primarily regulated by turbine inlet steam temperature and flow, along with shaft seal steam temperature and flow. Expansion differential generally increases during startup. During turbine shutdown, as load and speed decrease, the rotor cools faster than the cylinder, causing expansion differential to generally develop in the negative direction. This effect is particularly pronounced during slip parameter shutdown. In such cases, steam heating devices must be employed to inject cooling steam into the cylinder jacket and flange passages to prevent expansion differential protection from tripping.
In steam turbine generators, a pressure drop occurs between the steam pressure upstream and downstream of the moving blades due to steam performing work within the blades and steam leakage through the partition steam seal gaps. This pressure drop generates an axial thrust on the turbine rotor in the direction of steam flow, resulting in axial displacement. If the axial displacement exceeds the minimum clearance between the moving and stationary parts of the turbine, the stationary and rotating parts will collide and become damaged. Increased axial displacement causes the thrust bearing temperature to rise excessively, leading to carbon burnout and severe vibration in the unit. Therefore, emergency shutdown is mandatory; otherwise, serious consequences will ensue.
