The foundations need to combine rigidity, to avoid excessive movement, and flexibility, to prevent excessive forces being present at the bearings. Dangerous resonances at all speeds must be avoided and the natural frequencies of the foundation as a whole must not be close to running speed.
Two methods of avoiding resonant conditions and unacceptable vibrations are possible:
- Overtuned or high-tuned, the natural frequency of vibration of the foundation structure is above the running speed of the machine and away from any harmonics of this speed. This is referred to as a stiff foundation.
- Undertuned or low-tuned, the natural frequency of vibration of all or some of the foundation structure is below the operating speed of the machine. This is sometimes referred to as a flexible foundation.
In designing turbine-generator foundations it is therefore necessary to consider 'tuning' the foundations and there are two aspects to this:
- Analysis during the design process and before construction to establish design values of natural frequency (and harmonics of that frequency) well removed from running frequency and its harmonics.
- Incorporation within the support structure of a means for adjusting the natural frequency after erection and commissioning to remove or control unwanted resonances and forced vibrations.
Analysis methods used at the design stage model the plant and its foundations mathematically as a series of masses, beams and springs as shown in Fig 2.47. Because of the complexity of the model and the number of possible modes and frequencies of vibration, computer programs are used to assess the forces and deflections in the structure in response to a variety of static, transient and oscillatory forces. The design studies also consider the response of the foundation to the effects of shaft misalignment, out-of-balance forces, and load and electrical fault torques.
The subsoil elasticity is important at some sites but the effects vary with subsoil type and foundation type. These can sometimes be ignored.
Rigidity is achieved by establishing the mass of the support structure and sub-foundation at about ten times the mass of the rotating shafts. With flexible support structures, the foundation mass can sometimes be reduced as the elasticity of support reduces the dynamic forces transmitted to the sub-foundation and subsoil.
In calculating the loads on the structure, account must be taken of pipework loads (both hot and cold) and the effects of condenser weight, condenser water weight and vacuum forces within the condenser and LP turbine. When underslung, the condenser itself is usually spring-supported, so only a proportion of the weight is taken by the support structure. All the condenser weight is taken by the sub-foundation.
In this way, design variations can be assessed to reach the best compromises of foundation size, weight, ease of construction and erection, and the balance between deflection amplitudes and transfer of forces.
For high-tuned foundations, natural frequencies above 60 Hz would be expected for vibration in a vertical direction. For low-tuned steel foundations, natural frequencies of 12-18 Hz in the vertical direction and 1.5-3 Hz for transverse vibrations are possible. With spring foundations, the vertical natural frequency can be reduced even further.
If, despite careful analysis at the design stage, resonance or excessive movement is experienced in the foundation, it becomes necessary to tune the structure on-site. Two basic methods are available for this:
- Adding mass to columns or panels at selected locations to reduce vibration amplitude.
- Adding (or removing) structural strength to (from) the foundation structure to change its stiffness and move the natural frequency away from a plant excitation frequency.
Both methods have been used, separately or in combination, on occasions, although post-commissioning tuning is generally not necessary.