planning is critical for this item. Advanced methods such as pumped concrete should be used for construction, as they provide a better quality structure.
6.5. INSTALLATION OF CONDENSER
Traditionally, the condenser has been brought into the building in multiple pieces. Openings are left in the turbine building wall directly in line with the final location of each piece. Each piece is located in the pit, and then slid into position. All the pieces are then connected into one piece.
In recent plants, advantage has been taken of the VHL cranes now available and used in the open top method of construction. The condenser has been lifted as one piece, lowered into the turbine building, and then slid into position as one piece. This approach has eliminated openings in the turbine building wall, and has reduced the schedule, both for full installation of the condenser, as well as for turbine building construction completion.
6.6. CASTING OF TURBOGENERATOR PEDESTAL
The turbo-generator pedestal consists of the bottom raft, vertical columns and top slab. The turbo-generator pedestal is designed as per the non–safety system codal requirements but, as a general convention, is checked for safe shutdown earthquake conditions for a collapse. It is, however, the designer’s choice to consider various loading combinations, including dead load; live load; erection load; vertical and horizontal dynamic loads; seismic loads; short-circuit forces on generator supports; and various combinations of behaviour of equipment during normal and abnormal conditions. This also includes the failure of turbine blades.
The turbo-generator pedestals remain within the turbine building, but are kept structurally isolated from the rest of the building by providing an isolation gap of approximately 50 mm. This prevents vibrations from the turbo-generator pedestal from being transferred to the rest of the building structure. Nearly all plant developers use this approach as a part of modularization and working out the economics during construction.
Normal N-20/40 concrete is conventionally used for casting the turbo-generator pedestals. As turbo-generator pedestals are heavily reinforced, conventional methods of casting them require large mobilization of human resources as well as plant and machinery to achieve the desired quality. This is due to the fact that turbo-generator bottom rafts, columns and top slabs must be cast in a single pour individually, as a codes and standards requirement.
In the casting of turbo-generator pedestals, temperature-controlled concrete is now being used as an improvement on normal concrete. The use of temperature-controlled concrete facilitates large single pours of mass concrete, while maintaining the peak temperature within the concrete due to heat of hydration within the specified limits, and reducing the total time required to pour the large mass of concrete. The use of concrete pumps and mechanized concrete batching plants of appropriate size with ice flaking facilities in tropical countries; and the mobilization of concrete transit mixture, further reduce the total time required and improve the quality of concreting. However, continuous concreting also requires the mobilization of requisite plant and machinery, with sufficient backup systems and human resources for levelling of concrete, even for a shorter duration. Figure 82 shows the turbo-generator bottom raft being cast.
It should be noted that the turbo-generator pedestal bottom raft is cast on a sound rock foundation, which may need slightly deeper excavation. However, in the case of deep alluvial soil, suitable pile foundations may be used, depending on the site conditions.
The concrete pedestals, which are normally 15–20 m high, can be cast in a maximum of two pours as per the code requirements. Use of temperature controlled concrete and self-vibrating shutters (form vibrators) ensures proper compaction of the concrete as well as casting of the columns in a single pour. The top sections of the columns are given a haunch shape to support the top deck and proper distribution of the stresses. Figure 83 shows turbo-generator pedestal columns cast in a single pour.
The turbo-generator top deck is also cast in a single pour in the same manner in which the bottom raft is cast. Figure 84 shows the turbo-generator top deck ready for casting.
In the construction of turbo-generator pedestals, vibration isolators are now being used below the turbogenerator top deck, as an advancement. The vibration control through these isolators suppresses unwanted vibrations, eliminates transmission of vibrations along with the possible resonance effect, and limits vibration on
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FIG. 82. Construction of turbogenerator pedestal at Tarapur, India (credit: NPCIL).
FIG. 83. Casting of turbo-generator pedestal columns (credit: NPCIL).
FIG. 84. Turbo-generator top deck (credit: NPCIL).
the foundation and equipment during operations. Use of these isolators further compensates for settlement and misalignment of the foundation of equipment. Various varieties of vibration isolators are available on the market. It is, however, recommended that only reliable and time-tested vibration isolators with an efficiency as high as 95% are used in the turbo-generator pedestals. The vibration isolators are pre-numbered and designated with respect to
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FIG. 85. Seismic isolators (credit: NPCIL).
their position in a pre-stressed condition. These vibration isolators are placed over a resilient adhesive pad, which in turn is placed over the prepared smooth concrete surface on the concrete structure column. The relative position with respect to the design position of each isolator is checked before continuing. The top deck is cast with in-built machined embedded parts correlated to each vibration isolator. Figure 85 shows vibration isolators being erected in a turbo-generator pedestal.
While casting the top deck, the specified design gap is kept between the bottom of the machined embedded part and the top of the isolator. This gap takes care of the settlement after concreting, if any, before the load is transferred to the isolators.
The turbo-generator pedestal can thus be constructed in parallel with the rest of the turbine building. This parallel way of working, coupled with open top construction, can appreciably reduce the total time needed to erect the turbine building and associated systems.
6.7. ERECTION OF STRUCTURAL STEEL
Because of the span of the turbine generator hall and the size of the turbine building crane, the turbine building structural steel consists of large heavy sections or even built-up sections. The roof trusses, in particular, are very heavy.
6.7.1.Conventional method
The conventional method used to erect the turbine building structural steel has been to first erect the columns. These columns are quite large, heavy and self-supporting. Temporary braces are connected to hold the columns after aligning the structural columns. The roof trusses are then placed one at a time and connected to the steel column. The permanent bracing, both vertical and horizontal, is connected to the columns and the beams. In some cases, the permanent vertical bracing is connected to the steel columns before the roof trusses are erected.
The heavy turbine building crane, normally with a lift capacity of 200–300 t, is then installed. Advantage:
—Can be performed in all countries, as expertise and equipment are readily available. Disadvantage:
—Requires considerable time.
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6.7.2.Advanced methods
The advanced methods that are used in the erection of the turbine building structural steel are the open top method of construction, and modularization.
In these methods of construction, the structural steel columns may be erected as in the conventional method. After temporary bracing of the columns, the roof trusses and the roof horizontal bracing are set-up in modules, and placed in position using the VHL crane.
After the placement of the roof trusses, the vertical bracing is connected to the columns. The turbine building crane is placed in position using the VHL crane through a bay in which the horizontal roof bracing has not been erected.
Advantages:
—Very short erection time;
—Better quality final structure;
—Provides very good safety for workers. Disadvantage:
—More expensive to implement.
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