Page 357 - Steam Turbines Design, Applications, and Rerating

Rerates, Upgrades, and Modifications 331

TABLE 16.1 Reasons and Modifications for Turbine Redesign
Reasons for modification Possibly due to Candidates for modification
Performance and efficiency New technologies Buckets/blades
upgrade New materials Rotor
Reliability upgrade Improved Control system
Life extension manufacturing tools Thrust bearing
Modification and reapplication Improved Journal bearing
rather than buying new manufacturing Brush seals
turbine processes Improved leakage control
Process change Nozzle/diaphragm
Maintenance Lube and hydraulic system
Problem solving Auxiliaries
Casing modification
Exhaust end
Condensing bucket (L-0)
Valves—bar lift
Valves—steam lift
Tip seals
Coatings

16.1 Performance and Efficiency Upgrade
The basic power and/or speed requirements of a steam turbine may
change after commissioning, for various reasons. The most common
reason is an increase (or decrease) in the power required by the driven
machine due to a plant expansion or de-bottlenecking. Other reasons
include a search for increased efficiency, a change in the plant steam
balance, or a change in steam pressure or temperature. Because steam
turbines are periodically refurbished, an opportunity exists to update
the design for the current operating environment.
In many cases, the desired objective of a rerate is an increase in out-
put power. An increase in power usually requires more steam flow area
inside the turbine, which may or may not be possible within the physical
limits of the existing casing. A decrease in power is nearly always possi-
ble simply by blocking off some flow area, but maintaining efficiency
usually requires a more sophisticated solution. Sometimes increased
flow is accomplished by removing the second stage of the turbine. It may
seem counterintuitive that removing this stage results in additional
power, but this modification increases the flow capacity of the stages
downstream of the control stage. Increased flow capacity in the down-
stream stages allows the turbine to pass more flow as a whole (the
control-stage nozzle may also need to be modified). More flow translates
into more power, even though the turbine efficiency will be reduced.
For example,consider a turbine with a nominal efficiency of 80 percent.
If the flow capacity is increased by 10 percent,the power will be increased