In view of the large number of blades in any turbine machine, the existence of unavoidable sources of vibration excitation and the serious consequences of the failure of just one blade, an intimate knowledge and understanding of the vibration characteristics of the blades in their operating environment is essential.
Vibration excitation can arise from a variety of sources but principally involves the following categories:
1. Non-uniform pressures, velocity or changes in the angle of steam flow resulting in a periodic fluctuating force on the rotating blades. This may be caused by:
- Steam entering the rotating row over only a portion of its circular path (partial admission).
- This may exist in the control stage of some HP turbines but not in LP stages.
- A change in the direction of flow, particularly from axial to radial at exit from moving blades.
- Good aerodynamic design should minimize the magnitude of this non-uniformity and ensure that multiple harmonics of the synchronous speed are not produced.
- Flow distortions produced by the presence of steam extraction passages for feedheater tappings.
2. Periodic effects due to manufacturing constraints or structural features. These can include:
- Inexact matching of stationary blade geometry at horizontal joints. Blade-pitching at diaphragm horizontal joints may not be uniform and this can give rise to excitation at even multiples of rotational frequency.
- Leakage through gaps in stationary blade shrouds and diaphragm discs at horizontal joints. The current arrangement of diaphragm support precludes this effect.
- Eccentricity of diaphragms and other station any elements with respect to the rotating blade assembly. Again, current design practice should minimize this.
- Ellipticity of stationary parts, such as end walls, seals, etc.
- Non-uniformity in the gauge or thickness of stationary blades. Modern quality control and manufacturing normally obviates this effect
- Moisture removal slots.
All the above sources give rise to excitation frequencies at the rotational frequency or low multiples (harmonics) of that frequency.
3. Nozzle wake excitation caused by the aerodynamic force-fluctuations are seen by the rotating blade as it passes each stationary blade or traverses each stationary blade pitch. This is seen by the rotating blades as excitation at the nozzle passing frequency (rotational frequency multiply on number of stationary blades) and its multiples. A number of sources can also give rise to excitation having no direct relationship to rotational speed. In the stationary flow passages, these can include:
- Acoustic resonances in inlet passages, extraction lines or other cavities, excited by the flow past them.
- Vortex-shedding from stay bars, etc.
- Unsteady flow separation from stationary blades, etc.
- Unsteady shocks in choked stationary blade pass¬ ages.
- Surface pressure fluctuations, due to impingement of turbulent flow onto rotating blade shrouds, discs, etc.