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The HP steam for injection into a turbocharged STIG is raised at a pressure in the range of three to ten times the gas turbine combustor pressure. After expansion in the back pressure steam turbine, the steam mixes with the air compressed in the topping compressor. Then both steam and air flow into the gas turbine combustor where the air and steam are raised to the expander inlet temperature by combustion of fuel. For optimum efficiency additional L.P.steam is also injected into the combustor. Modifications required to a gas turbine to adapt it to a turbocharged system include: injection ports for the steam: changes to prevent reversal of cooling air because the pressure at the inlet to the expander exceeds the pressure at the outlet of the main compressor: strengthening of couplings and shafts because of increased torques: adjustment of the thrust balance and strengthening of casings because of the approximately two atmospheres higher pressure: incorporation of ports for extraction and return of the compressed air. A recuperated gas turbine will not require the last modification. Because of the higher power of STIG, more powerful reduction gear and electric generator will be required than the equivalent machine without steam injection. The power of a gas turbine is increased approximately 100% by Turbo-STIG conversion. As most power of an expander goes to drive the compressor, which is not increased, the power of the expander is increased by only 33-50%. The major stress in expander blading arises from centrifugal force which is not increased by conversion and the increase applies only to the blade bending stress which is normally 1/3 of the total blade stress. Turbo-STIG conversion increases expander blade stress by only about 15%. The increased pressure at the inlet to the expander of the STIG passes the increased flow at the normal velocity. At the exhaust of the expander, the pressure is not increased and velocities will be raised with the standard design. This will increase leaving losses. To minimize additional losses it is prudent to select engines for STIG applications which have generous exhaust area. Failing this, a modification to increase the back end area for a Turbo STIG is less serious than opening up the expander all the way through as required for a normal STIG.Theflowpathofanexpandercanbe increased at minor cost by changing the angle at which the nozzles and blades are inserted. The flow areas of uncooled rows can additionally be increased by cropping the trailing edges of nozzles and blades. If high exhaust flow is a problem, it may be limited in the Turbo-STIG by closure of the variable compressor inlet guide vanes or stators thus restricting the airflow of the engine. Power is reduced by this procedure but efficiency is affected little. The full air flow can then be restored by reopening the compressor vanes or stators if full steam injection is not required or in hot weather when flow is naturally reduced. This works for the Turbo-STIG but not for non turbo STIG because the reduced guide vane setting reduces surge pressure ratio as well as flow. Commercially successful gas turbines are without exception multiple products. The cost of design and tooling for sophisticated machinery is so high that the cost of "one off" production or other than simple modifications to a standard design is prohibitive. Modification of static components for STIG applications may be acceptable but changes to blade path and rotating components must be minor to be affordable. This is accomplished by Turbo- STIG . Other STIG systems require extensive end-to-end modification of either the compressor or the expander for all the steam produced by recovered heat to be accommodated. Cogeneration With the turbo system, steam can be exported for cogeneration at the pressure prevailing before, or after, the steam turbine. Steam which is exported does not increase the flow through the expander and does not increase its inlet pressure. Steam extracted after the steam turbine will have produced power in the steam turbine which drives the topping compressor. However the flow through the gas turbine expander is not increased by the steam which is extracted. In this case, the pressure rise in the topping compressor resulting from the extracted steam reduces the pressure rise and the power absorbed by the main compressor. The power not required by the main compressor then becomes output and improves overall efficiency. If required the boiler can be fired so the extraction of steam for cogeneration does not reduce the injected steam. Water consumption of a STIG cycle Higher firing temperatures, which produce more NOx, and more stringent air pollution regulations, have required water or steam injection into most gas turbines for NOx control. Thus most gas turbines now require a demineralization system for the injected water or steam which for NOx control is about 27. of airflow. Steam injection for power augmentation does not require a demineralization plant to be added-only enlarged. The water flow must be increased from about 2% of airflow to 7-1S/. of airflow. The water in the exhaust of a STIG can be recovered by cooling the exhaust gas below the dew point to condense the moisture and capture the resulting liquid water drops in chevron water separators. Many "condensing" economizers are in commercial service on gas fired boilers. A cycle diagram for a "Dry" LM 5000 S T I G as conceived in the study reported in reference 8) is shown on figure 2. The exhaust of a STIG is substantially cooled in its heat recovery boiler. For full water recovery it must be further cooled to about 100 F by heat rejection to an extraneous heat sink such as ambient air or water unsuitable for injection. Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1989/79160/V004T09A002/4456981/v004t09a002-89-gt-100.pdf by guest on 21 October 2020PDF Image | The Turbocharged Steam Injected Gas Turbine Cycle
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