3.2.4.1General
The three-stage turbine section is the area in which energy in the form of high temperature pressurized gas, produced by the compressor and combustion sections, is converted to mechanical energy. Gas turbine hardware includes the turbine rotor, turbine casing, exhaust frame, exhaust diffuser, nozzles and shrouds.
3.2.4.2 Main Components of the Turbines
1 .Turbine Base and Support
The base that supports the gas turbine is structural steel fabrication of welded steel beams. And plate its prime function is to provide a support up on which to mount the gas turbine is mounted to its base by vertical supports at three location. The forward support at the lower half-vertical flange of the forward compressor casing and the two on either side of the turbine exhaust frame.
2. Turbine Rotor
The turbine rotor assembly consists of the forward and turbine wheel shafts and the first, second and third stage turbine wheel assemblies with spacers and turbine buckets. Concentricity control is achieved with mating rabbets on the turbine wheels, wheel shafts, and spacers. The wheels are held together with through bolts mating up with bolting flanges on the wheel shafts and spacers.
Selective positioning of rotor members is performed to minimize balance corrections as shown in Fig. (3.34) and Fig. (3.35).
Wheel Shafts
The turbine rotor distance piece extends from the first-stage turbine wheel to the flange of the compressor rotor assembly. The turbine rotor shaft includes the No. 2 bearing journal
Wheel Assemblies
Spacers between the first and second, and between the second and third -stage turbine wheels determine the axial position of the individual wheels. These spacers carry the diaphragm sealing lands. The 1-2 spacer forward and aft faces include radial slots for cooling air passages. Turbine buckets are assembled in the wheels with fir-tree-shaped dovetails that fit into matching cut-outs in the turbine wheel rims. All three turbine stages have precision investment-cast, long shank buckets. The long-shank bucket design effectively shields the wheel rims and bucket root
fastenings from the high temperatures in the hot gas path while providing mechanical damping of bucket vibrations.
As a further aid in vibration damping, the stage-two and stage-three buckets have interlocking shrouds at the bucket tips. These shrouds also increase the turbine efficiency. By minimizing tip leakage. Radial teeth on the bucket shrouds combine with stepped surfaces on the stator to provide a labyrinth seal against gas leakage past the bucket tips. Figure (3.33) shows typical first-, second-, and third-stage turbine buckets for the MS9001FA. The increase in the size of the buckets from the first to the third stage is necessitated by the pressure Reduction resulting from energy conversion in each stage, requiring an increased annulus area to accommodate the gas flow.
3. Turbine Stator
The turbine casing and the exhaust frame constitute the major portion of the turbine gas turbine stator structure. The turbine nozzles, shrouds, and turbine exhaust diffuser are internally supported from these components.
4. Turbine Casing
The turbine casing controls the axial and radial positions of the shrouds and nozzles. It determines turbine clearances and the relative positions of the nozzles to the turbine buckets. This positioning is critical to gas turbine performance. Hot gases contained by the turbine casing are a source of heat flow into the casing. To control the casing diameter, it is important to reduce the heat flow into the casing and to limit its temperature.
Heat flow limitations incorporate insulation, cooling, and multi-layered structures. 13th stage extraction air is piped into the turbine casing annular spaces around the 2nd and 3rd stage nozzles. From there the air is ported through the nozzle partitions and into the wheel spaces. Structurally, the turbine casing forward flange is bolted to the bulkhead flange at the aft end of the compressor discharge casing. The turbine casing aft flange is bolted to the forward flange of the exhaust frame.
Figure (3.36) shows typical first-, second, and third-stage turbine Element. The increase in the size of the buckets from the first to the third stage is necessitated by the pressure reduction resulting from Energy conversion in each stage, requiring an increased annulus area to accommodate the gas flow.
5. Nozzles
In the turbine section there are three stages of stationary nozzles which direct the high-velocity flow of the expanded hot combustion gas against the turbine buckets causing the turbine rotor to rotate. Because of the high pressure drop across these nozzles, there are seals at both the inside and the outside diameters to prevent loss of system energy by leakage. Since these Nozzles operate in the hot combustion gas flow; they are subjected to thermal stresses in addition to gas pressure loadings as shown in Fig.(3.37)
First-Stage Nozzle
The first-stage nozzle receives the hot combustion gases from the combustion system via the transition pieces. The transition pieces are sealed to both the outer and inner sidewalls on the entrance side of the nozzle; this minimizes leakage of compressor discharge air into the nozzles. The gas turbine first-stage nozzle contains a forward and aft cavity in the vane and is cooled by a combination of film, impingement and convection techniques in both the vane and sidewall regions. The nozzle segments, each with two partitions or airfoils, are contained by a horizontally split retaining ring which is centerline supported to the turbine casing on lugs at the sides and guided by pins at the top and bottom vertical centerlines. This permits radial growth of the retaining ring, resulting from changes in temperature, while the ring remains centered in the casing.
The aft outer diameter of the retaining ring is loaded against the forward face of the first-stage turbine shroud and acts as the air seal to prevent leakage of compressor discharge air between the nozzle and turbine casing. On the inner sidewall, the nozzle is sealed by a flange cast on the inner diameter of the sidewall that rests against a mating face on the first-stage nozzle support ring. Circumferential rotation of the segment inner sidewall is prevented by an eccentric bushing and a locating dowel that engages lug on the inner sidewall. The nozzle is prevented from moving forward by the lugs welded to the aft outside diameter of the retaining ring at 45 degrees from vertical and horizontal centerlines. These lugs fit in groove machined in the turbine shell just forward of the first-stage shroud T hook. By moving the horizontal joint support block and the bottom centerline guide pin and then removing the inner sidewall locating dowels, the lower half of the nozzle can be rolled out with the turbine rotor in place.
Second-Stage Nozzle
Combustion air exiting from the first stage buckets is again expanded and redirected against the second- stage turbine buckets by the second-stage nozzle. This nozzle is made of cast segments, each with two partitions or airfoils. The male hooks on the entrance and exit sides of the outer sidewall fit into female grooves on the aft side of the first-stage shrouds and on the forward side of the second-stage shrouds to maintain the nozzle concentric with the turbine shell and rotor. This close fitting tongue-and-groove fit between nozzle and shrouds acts as an outside diameter air seal. The nozzle segments are held in a circumferential position by radial pins from the shell into axial slots in the nozzle outer sidewall as shown in Fig. (3.38)
Third-Stage Nozzle
The third-stage nozzle receives the hot gas as it leaves the second stage buckets, ithird-stage buckets. The nozzle consists of cast segments, each with three partitions or airfoils. It is held at the outer sidewall forward and aft sides in grooves in the turbine shrouds in a manner similar to that used on the second stage nozzle. The third-stage nozzle is circumferentially positioned by radial pins from the shell.13th stage extraction air flows through the nozzle partitions for nozzle convection cooling and for augmenting wheel space cooling air flow as shown in Fig. (3.38).
Diaphragm
Attached to the inside diameters of both the second and third-stage nozzle segments are the nozzle diaphragms. These diaphragms prevent air leakage past the inner sidewall of the nozzles and the turbine rotor. The high/low, labyrinth seal teeth are machined into the inside diameter of the diaphragm. They mate with opposing sealing lands on the turbine rotor. Minimal radial clearance between stationary parts (diaphragm and nozzles) and the moving rotor are essential for maintaining low inter stage leakage; this results in higher turbine efficiency.
6. Shrouds
Unlike the compressor blading, the turbine bucket tips do not run directly against an integral machined surface of the casing but against annular curved segments called turbine shrouds. The shrouds’ primary function is to provide a cylindrical surface for minimizing bucket tip clearance leakage. The turbine shrouds’ secondary function is to provide a high thermal resistance between the hot gases and the comparatively cool turbine casing. By accomplishing this function, the turbine casing cooling load is drastically reduced, the turbine casing diameter is controlled, the turbine casing roundness is maintained, and important turbine clearances are assured.
The first and second-stage stationary shroud segments are in two pieces; the gas-side inner shroud is separated from the supporting outer shroud to allow for expansion and contraction, and thereby improve low-cycle fatigue life. The first-stage shroud is cooled by impingement, film, and convection. The shroud segments are maintained in the circumferential position by radial pins from the turbine casing. Joints between shroud segments are sealed by interconnecting tongues and grooves.
7. Exhaust Frame
The exhaust frame is bolted to the aft flange of the turbine casing. Structurally, the frame consists of an outer cylinder and an inner cylinder interconnected by the radial struts. The No. 2 bearing is supported from the inner cylinder. The exhaust diffuser located at the aft end of the turbine is bolted to the exhaust frame. Gases exhausted from the third turbine stage enter the diffuser where velocity is reduced by diffusion and pressure is recovered. At the exit of the diffuser, the gases are directed into the exhaust plenum. Exhaust frame radial struts cross the exhaust gas stream. These struts position the inner cylinder relation to the outer casing of the gas turbine.
The struts must be maintained at a constant temperature in order to control the center position of the rotor in relation to the stator. This temperature stabilization is accomplished by protecting the struts fro fairing that forms an air space around each strut and provides a rotated, combined airfoil shape. Off through the space between the struts and the wrapper to maintain uniform temperature of the struts. This air is then directed to the third wheel space. Trunnions on the sides of the exhaust frame are used with similar trunnions on the forward compressor casing to lift the gas turbine when it is separated from its base. Figure (3.39) shows the exhaust duct which included silencers and a flange of the turbine casing.
8. Bleed valve
During starting the surge phenomenon must be avoided to prevent axial play to the rotor which destroy the engine. To protect the engine, a bleed air valve is used to bleed a part of air from the compressor delivery to the exhaust during starting to control the air flow rate or pressure ratio at specified value for each speed In GE there are four bleed valves, two from the stage 13 from the compressor and two from the stage 9 from the compressor as shown in Fig. (3.40)
9. Silencers
The disclosed silencer has noise gas duct in parallel with but with as to define gas passage means and noise duct. The noise-absorbing space means is closed by noise at inlet and outlet ends of the duct. A plurality of gas chambers aligned along the line of gas flow through the duct are formed in said noise absorbing space means by disposing noise therein as shown in Figure (3.41) Silencers noise -
absorbing partitions disposed in a spacing from outer walls of the duct, so noise -absorbing space means in the noising noise-shielding sectional walls Figure (3.41) shielding plates noise-shielding.
The three-stage turbine section is the area in which energy in the form of high temperature pressurized gas, produced by the compressor and combustion sections, is converted to mechanical energy. Gas turbine hardware includes the turbine rotor, turbine casing, exhaust frame, exhaust diffuser, nozzles and shrouds.
3.2.4.2 Main Components of the Turbines
1 .Turbine Base and Support
The base that supports the gas turbine is structural steel fabrication of welded steel beams. And plate its prime function is to provide a support up on which to mount the gas turbine is mounted to its base by vertical supports at three location. The forward support at the lower half-vertical flange of the forward compressor casing and the two on either side of the turbine exhaust frame.
2. Turbine Rotor
The turbine rotor assembly consists of the forward and turbine wheel shafts and the first, second and third stage turbine wheel assemblies with spacers and turbine buckets. Concentricity control is achieved with mating rabbets on the turbine wheels, wheel shafts, and spacers. The wheels are held together with through bolts mating up with bolting flanges on the wheel shafts and spacers.
Selective positioning of rotor members is performed to minimize balance corrections as shown in Fig. (3.34) and Fig. (3.35).
Wheel Shafts
The turbine rotor distance piece extends from the first-stage turbine wheel to the flange of the compressor rotor assembly. The turbine rotor shaft includes the No. 2 bearing journal
Wheel Assemblies
Spacers between the first and second, and between the second and third -stage turbine wheels determine the axial position of the individual wheels. These spacers carry the diaphragm sealing lands. The 1-2 spacer forward and aft faces include radial slots for cooling air passages. Turbine buckets are assembled in the wheels with fir-tree-shaped dovetails that fit into matching cut-outs in the turbine wheel rims. All three turbine stages have precision investment-cast, long shank buckets. The long-shank bucket design effectively shields the wheel rims and bucket root
fastenings from the high temperatures in the hot gas path while providing mechanical damping of bucket vibrations.
As a further aid in vibration damping, the stage-two and stage-three buckets have interlocking shrouds at the bucket tips. These shrouds also increase the turbine efficiency. By minimizing tip leakage. Radial teeth on the bucket shrouds combine with stepped surfaces on the stator to provide a labyrinth seal against gas leakage past the bucket tips. Figure (3.33) shows typical first-, second-, and third-stage turbine buckets for the MS9001FA. The increase in the size of the buckets from the first to the third stage is necessitated by the pressure Reduction resulting from energy conversion in each stage, requiring an increased annulus area to accommodate the gas flow.
3. Turbine Stator
The turbine casing and the exhaust frame constitute the major portion of the turbine gas turbine stator structure. The turbine nozzles, shrouds, and turbine exhaust diffuser are internally supported from these components.
4. Turbine Casing
The turbine casing controls the axial and radial positions of the shrouds and nozzles. It determines turbine clearances and the relative positions of the nozzles to the turbine buckets. This positioning is critical to gas turbine performance. Hot gases contained by the turbine casing are a source of heat flow into the casing. To control the casing diameter, it is important to reduce the heat flow into the casing and to limit its temperature.
Heat flow limitations incorporate insulation, cooling, and multi-layered structures. 13th stage extraction air is piped into the turbine casing annular spaces around the 2nd and 3rd stage nozzles. From there the air is ported through the nozzle partitions and into the wheel spaces. Structurally, the turbine casing forward flange is bolted to the bulkhead flange at the aft end of the compressor discharge casing. The turbine casing aft flange is bolted to the forward flange of the exhaust frame.
Figure (3.36) shows typical first-, second, and third-stage turbine Element. The increase in the size of the buckets from the first to the third stage is necessitated by the pressure reduction resulting from Energy conversion in each stage, requiring an increased annulus area to accommodate the gas flow.
5. Nozzles
In the turbine section there are three stages of stationary nozzles which direct the high-velocity flow of the expanded hot combustion gas against the turbine buckets causing the turbine rotor to rotate. Because of the high pressure drop across these nozzles, there are seals at both the inside and the outside diameters to prevent loss of system energy by leakage. Since these Nozzles operate in the hot combustion gas flow; they are subjected to thermal stresses in addition to gas pressure loadings as shown in Fig.(3.37)
First-Stage Nozzle
The first-stage nozzle receives the hot combustion gases from the combustion system via the transition pieces. The transition pieces are sealed to both the outer and inner sidewalls on the entrance side of the nozzle; this minimizes leakage of compressor discharge air into the nozzles. The gas turbine first-stage nozzle contains a forward and aft cavity in the vane and is cooled by a combination of film, impingement and convection techniques in both the vane and sidewall regions. The nozzle segments, each with two partitions or airfoils, are contained by a horizontally split retaining ring which is centerline supported to the turbine casing on lugs at the sides and guided by pins at the top and bottom vertical centerlines. This permits radial growth of the retaining ring, resulting from changes in temperature, while the ring remains centered in the casing.
The aft outer diameter of the retaining ring is loaded against the forward face of the first-stage turbine shroud and acts as the air seal to prevent leakage of compressor discharge air between the nozzle and turbine casing. On the inner sidewall, the nozzle is sealed by a flange cast on the inner diameter of the sidewall that rests against a mating face on the first-stage nozzle support ring. Circumferential rotation of the segment inner sidewall is prevented by an eccentric bushing and a locating dowel that engages lug on the inner sidewall. The nozzle is prevented from moving forward by the lugs welded to the aft outside diameter of the retaining ring at 45 degrees from vertical and horizontal centerlines. These lugs fit in groove machined in the turbine shell just forward of the first-stage shroud T hook. By moving the horizontal joint support block and the bottom centerline guide pin and then removing the inner sidewall locating dowels, the lower half of the nozzle can be rolled out with the turbine rotor in place.
Second-Stage Nozzle
Combustion air exiting from the first stage buckets is again expanded and redirected against the second- stage turbine buckets by the second-stage nozzle. This nozzle is made of cast segments, each with two partitions or airfoils. The male hooks on the entrance and exit sides of the outer sidewall fit into female grooves on the aft side of the first-stage shrouds and on the forward side of the second-stage shrouds to maintain the nozzle concentric with the turbine shell and rotor. This close fitting tongue-and-groove fit between nozzle and shrouds acts as an outside diameter air seal. The nozzle segments are held in a circumferential position by radial pins from the shell into axial slots in the nozzle outer sidewall as shown in Fig. (3.38)
Third-Stage Nozzle
The third-stage nozzle receives the hot gas as it leaves the second stage buckets, ithird-stage buckets. The nozzle consists of cast segments, each with three partitions or airfoils. It is held at the outer sidewall forward and aft sides in grooves in the turbine shrouds in a manner similar to that used on the second stage nozzle. The third-stage nozzle is circumferentially positioned by radial pins from the shell.13th stage extraction air flows through the nozzle partitions for nozzle convection cooling and for augmenting wheel space cooling air flow as shown in Fig. (3.38).
Diaphragm
Attached to the inside diameters of both the second and third-stage nozzle segments are the nozzle diaphragms. These diaphragms prevent air leakage past the inner sidewall of the nozzles and the turbine rotor. The high/low, labyrinth seal teeth are machined into the inside diameter of the diaphragm. They mate with opposing sealing lands on the turbine rotor. Minimal radial clearance between stationary parts (diaphragm and nozzles) and the moving rotor are essential for maintaining low inter stage leakage; this results in higher turbine efficiency.
6. Shrouds
Unlike the compressor blading, the turbine bucket tips do not run directly against an integral machined surface of the casing but against annular curved segments called turbine shrouds. The shrouds’ primary function is to provide a cylindrical surface for minimizing bucket tip clearance leakage. The turbine shrouds’ secondary function is to provide a high thermal resistance between the hot gases and the comparatively cool turbine casing. By accomplishing this function, the turbine casing cooling load is drastically reduced, the turbine casing diameter is controlled, the turbine casing roundness is maintained, and important turbine clearances are assured.
The first and second-stage stationary shroud segments are in two pieces; the gas-side inner shroud is separated from the supporting outer shroud to allow for expansion and contraction, and thereby improve low-cycle fatigue life. The first-stage shroud is cooled by impingement, film, and convection. The shroud segments are maintained in the circumferential position by radial pins from the turbine casing. Joints between shroud segments are sealed by interconnecting tongues and grooves.
7. Exhaust Frame
The exhaust frame is bolted to the aft flange of the turbine casing. Structurally, the frame consists of an outer cylinder and an inner cylinder interconnected by the radial struts. The No. 2 bearing is supported from the inner cylinder. The exhaust diffuser located at the aft end of the turbine is bolted to the exhaust frame. Gases exhausted from the third turbine stage enter the diffuser where velocity is reduced by diffusion and pressure is recovered. At the exit of the diffuser, the gases are directed into the exhaust plenum. Exhaust frame radial struts cross the exhaust gas stream. These struts position the inner cylinder relation to the outer casing of the gas turbine.
The struts must be maintained at a constant temperature in order to control the center position of the rotor in relation to the stator. This temperature stabilization is accomplished by protecting the struts fro fairing that forms an air space around each strut and provides a rotated, combined airfoil shape. Off through the space between the struts and the wrapper to maintain uniform temperature of the struts. This air is then directed to the third wheel space. Trunnions on the sides of the exhaust frame are used with similar trunnions on the forward compressor casing to lift the gas turbine when it is separated from its base. Figure (3.39) shows the exhaust duct which included silencers and a flange of the turbine casing.
8. Bleed valve
During starting the surge phenomenon must be avoided to prevent axial play to the rotor which destroy the engine. To protect the engine, a bleed air valve is used to bleed a part of air from the compressor delivery to the exhaust during starting to control the air flow rate or pressure ratio at specified value for each speed In GE there are four bleed valves, two from the stage 13 from the compressor and two from the stage 9 from the compressor as shown in Fig. (3.40)
9. Silencers
The disclosed silencer has noise gas duct in parallel with but with as to define gas passage means and noise duct. The noise-absorbing space means is closed by noise at inlet and outlet ends of the duct. A plurality of gas chambers aligned along the line of gas flow through the duct are formed in said noise absorbing space means by disposing noise therein as shown in Figure (3.41) Silencers noise -
absorbing partitions disposed in a spacing from outer walls of the duct, so noise -absorbing space means in the noising noise-shielding sectional walls Figure (3.41) shielding plates noise-shielding.
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