5 Turbine foundations
The turbine-generator foundations consist of the support structure, the sub-foundation and the subsoil; they perform the following functions:
- Support the static load of the turbine-generator and associated pipe loads, and transmit these forces to the subsoil
- Restrain the plant from undue movement due to dynamic forces resulting from the load torque,
unbalance forces, electrical faults, etc., by trans mitting and absorbing the associated energy
- Maintain the alignment of the plant under all operating conditions
Accommodate the thermal expansion of the structure and static parts of the plant under all operating conditions.
Raise the plant above the turbine house floor to provide access for electrical connections and pipework. Bottom connection of the main steam pipework is particularly desirable to avoid dismantling pipework during maintenance and to prevent the pipework draining into the turbine. The height of the turbine above the basement floor level is partly determined by the need to accommodate the condenser and condenser neck, when an underslung condenser is used, and partly by consideration of condensate drainage requirements. The bled-steam piping should drain away from the turbine and the drains from LP heaters must be returned to the condenser. For this to be achieved with gravity drainage requires a certain minimum height difference between turbine and condenser hotwell.
For main turbine-generator plant, these requirements have been satisfied by either reinforced concrete or steel support structures, which are described in more detail in later sections.
It is also necessary to monitor the movement of the foundations throughout the life of the station. Settlement of the whole foundation in service is not very serious, provided that it is not large enough to affect electrical and steam connections, but differential movement between bearing supports must be avoided if the alignment of the plant is to be maintained within reasonable limits. Multi-limb mano-metric level measuring systems are therefore installed on the bearing supports to monitor relative level changes. A schematic diagram of such a system is shown in Fig 2.46.
'Slave' units are fitted to each bearing support, with a master reference unit at one point and the whole system is filled with water. A uniform level is thus established by the water surface and movement of the bearing support relative to the fixed water level can be detected by suitable sensors fitted on each slave unit. Sensors using micrometer adjustment of an electrical contact probe, and floats with LVDT position measurement have been used in the past, but both systems have disadvantages. Present day systems use an ultrasonic sensor fitted in the base of each slave unit which measures the position of the water surface by sound reflection technique. Accuracy of measurement is typically ±0.05 mm, with a minimum measurement range of ±2.5 mm about nominal level.
Cooling water is circulated around jackets on each slave unit to eliminate errors due to manometer water temperature variations. Air pressure variations are nullified by connecting the air spaces of each slave unit together in an 'air balance box' (not shown in figure). The balance box is then vented to atmosphere at a single point.
To allow any relationship between bearing support level changes and foundation structure temperatures to be studied, temperature sensors are buried in the foundations during construction or attached during erection.