Define blade tip clearance and explain its importance in overhaul acceptance criteria.

• A. The distance between turbine casing and bearing housing; determines alignment.
• B. The gap between blade tip and casing; affects efficiency, stability, and risk of rubs; must be within OEM-specified limits after reassembly and run-in.
• C. The distance between combustor and turbine stage 1.
• D. The gap between stator and rotor.

Answer: (B)

Blade tip clearance is the radial gap between the tips of the rotating turbine blades and the surrounding casing or shroud. This small space is crucial because it controls tip leakage—gas flowing over the blade tips instead of through the designed aerodynamic path. If the gap is too small, the blades can rub the casing during operation due to thermal growth and rotor bow, leading to wear or damage. If the gap is too large, a significant amount of work is lost to leakage, reducing efficiency, affecting stability, and potentially causing aerodynamic instabilities.

In overhaul acceptance criteria, the clearance must be within the OEM-specified limits after reassembly and run-in because parts change with mounting, installation tolerances, cooling, and thermal expansion once the engine heats up. The run-in allows the clearance to settle to its real operating value, so measurements reflect what the engine will actually experience in service. Verifying this parameter helps ensure there’s no rubbing, maintains efficiency, and supports safe, reliable operation.

Others describe different gaps (such as casing-to-bearing alignment, combustor-to-turbine spacing, or stator-to-rotor clearance), which are not blade tip clearance and do not directly pertain to the tips’ gap to the casing.