General Electric began offering the MS7001FA around the turn of the 21st Century.  The most obvious distinguishing feature of the 7F technology was the “cold-end-drive” design.  The generator is connected to the compressor end of the turbine rotor, a departure from GE design concept for over 50 years.  Also, unlike the MS7001EA design, the 7F turbine rotor spans only two bearings.

These “F-Technology” units are ISO rated at approximately 150 MW.  The greater power output rating was largely due to higher turbine firing temperatures.  They are equipped with the latest “high tech” parts and components.  For instance, the turbine section features single-crystal, first-stage buckets that are internally cooled by air from the gas turbine’s own axial-flow compressor, similar to the 7EA designs.  Fig. 20-1 shows the end view of a first-stage turbine bucket for an F-machine.  Cooling holes and passageways are designed within the buckets.  Notice that even the pressure side of the bucket has holes along the perimeter.  Compressor inlet filtration is critical to keep the cooling air clean, so as to not clog these tiny cooling holes in the buckets, precipitating likely failures.       Air flows through the inner passageways of the turbine bucket and out through the blade tips. The air inside is approximately 500F, while the outer surface of the bucket is at approximately 2400 F at base load.


Fig. 20-1- First-stage 7FA turbine buckets (air cooled)

Fig. 20-2 shows interlocking, second-stage turbine buckets used on 7FA turbines.  This stage is also cooled with air extracted from the turbine’s own compressor.  The temperature gradient allows the bucket material to cool and operate at higher surface temperatures.  Buckets also have thermal barrier coatings (TBC) on the surfaces to protect the metal, as was explained in blog Chapter 19.


Fig. 20-2- Interlocking, air-cooled F-technology 2nd stage turbine buckets (shroud end view)

The 7FA first-stage turbine nozzle segments are also air-cooled and very sophisticated in design.  The nozzle partitions have cooling air holes both on the leading and trailing edge of the partitions.  Also, the inner and outer sidewalls have cooling holes and show a TBC shown in white. See Fig. 20-3 and 20-4.


Fig. 20-3- First-stage air-cooled turbine nozzle segments showing internal cooling holes

Air-cooled nozzle partitions are another way that gas turbines can accommodate higher firing temperatures, which increases power output and thermal efficiency.  Cooling air is extracted from the axial-flow compressor presents a slight loss in overall efficiency but the benefit is there.


Fig. 20-4- First-stage turbine nozzle segments showing air-cooled partitions

Turbine fuel nozzles have become far more sophisticated with the need for improved exhaust emissions and nitrous oxide (NOx) reduction in exhaust gases.  See Fig. 20-5.  Fuel nozzles of this design cost over $100,000 each, with a full set costing over one million dollars.


Fig. 20-5- Gas Turbine Fuel Nozzles for DLN-1 Operation

Diaphragms are an integral part of the second-stage nozzle.  They contain radial seals adjacent to the turbine rotor.  Extraction air from the axial-flow compressor is blown into the seal area. The “angel wings” on each end seal the rotor near the roots of the turbine buckets.


Fig. 20-6- Nozzle Diaphragm with labyrinth (high-low) radial seals

One might ask, therefore, what are the firing limits on gas turbines?  It seems that certain “thresholds” are being approached and perhaps even touched upon with modern gas turbines.  For simple-cycle (SC) applications, the limit for thermal efficiency seems to be about 40 percent.  Single-crystal turbine buckets, internal cooling, ceramic coatings of some components are approaching limits.  Combined-cycle (CC) applications appear to be “cresting” at 60 percent total plant thermal efficiency for gas and steam turbines in these configurations.  Even if suppliers like GE, Siemens-Westinghouse and Mitsubishi Heavy Industries claim to have exceeded these levels, the operators doubt that operation will allow such diverse applications as plant cycling, daily starts and shutdowns or peaking power production.  Operational flexibility may be more of a limiting factor than reaching or surpassing the mythical limits of 40 percent (SC) and 60 percent (CC).

Looking to the future, the H-class turbines may be seeking other means of cooling.  Under consideration is “steam cooling” to replace compressor extraction air from the axial-flow compressor in combined-cycle applications.  However, the ability to monitor and control water “chemistry” will be a strong consideration in the quality of the steam being used.

In conclusion, just when we think that gas turbines have reached their limits on firing temperature and efficiency, manufacturers seem to come up with other ideas for metals (buckets and nozzles), surface coatings and internal cooling methods.  In other words, as we enter the second decade of the 21st century, I have only this advice: STAY TUNED!