STEAM TURBINE HIGH PRESSURE VENT AND SEAL SYSTEM

Abstract: A steam turbine having a chamber in the inlet end thereof adjacent the thrust balance piston and dummy ring formed so that the temperature is substantially reduced while the pressure is only slightly reduced to optimize the pressure temperature affect on the balance piston and rotor utilizing labyrinth seals and a static seal together with vent ports in fluid communication with the chamber and an area downstream of the first row of rotatable blades to provide a more efficient and reliable turbine.

Description: This invention relates to steam turbines and more particularly to a system for sealing and venting the high pressure end thereof. For efficiency reasons, the utility industry generally requires multivalve variable inlet nozzles steam flow area control for turbine generators. As a result, individual valves provide steam flow to individual inlet nozzle chambers and their associated nozzle vanes. Inlet nozzle vanes and the first rotating blade row are combined to form a control stage. Nozzle exit steam in the axial space between the nozzle vanes and the rotating blades can flow along a leakage path between nozzle seal strips and the rotor at both the base of the nozzle and the outer diameter of the blade shroud. The quantity of leakage has an effect on efficiency since no work is contributed by the steam that bypasses the rotating blade row. The leakage steam temperature for conventional fossile turbo generators is in the vicinity of 520°C at rated power and decreases with load. However, even at half rated load, this leakage steam temperature is above 480°C. In high pressure turbines this high temperature nozzle exit leakage steam can flow between the rotor and the nozzle chamber assembly to the rotor thrust dummy balance piston. For a given geometry, the rotor material strength, with respect to maximum creep tangential stress, decreases with increasing temperature. Therefore, it is desirable to reduce this leakage temperature. One method of reducing the steam temperature the rotor is exposed to is shown in United States Patent 3,206,166. In this patent the control stage flow direction is opposed to the direc­ tion of the following row of high pressure turbine blades and a venting and sealing system isolates the nozzle exit leakage steam from direct contact with the rotor. Single flow high pressure turbines and combined high pressure intermediate pressure turbines require a rotor thrust balancing dummy piston that has a diameter approximately equivalent to the average mean diameter of the blade path. The high pressure rotor thrust dummy piston is exposed to exit steam from the control stage rotating blade which over the load range is 25 to 55°C cooler than nozzle exit steam. However, the opposed flow control stage has the disadvan­ tage of being less efficient in delivering the steam to the following row of blades because of the loss associated with turning the flow 180° around the nozzle chamber to the following rows of blades. It is desirable to utilize the efficiency of the straight through flow control stage and at the same time bathe the rotor in steam significantly cooler than inlet nozzle exit steam. U.S. Patent 4,150,917 shows rotor cooling for single and double axial flow steam turbines which utilizes motive steam taken from the motive steam flow path before and after the control stage or first row of rotating blades. It is the principal object of the present inven­ tion to provide the efficiency advantage of straight through flow control stage and a significantly cooler steam supply bathing the rotor in the critical area of the nozzle chamber and thrust dummy balance piston and to reduce the nozzle exit leakage steam quantity at the base seal for further increase of the control stage efficiency. With this object in view, the present invention resides in a steam turbine comprising an outer cylinder; an inner cylinder disposed within the outer cylinder; a blade ring disposed partially within the inner cylinder and partially within the outer cylinder; a nozzle chamber assembly disposed within the inner cylinder for introducing motive steam to the turbine rotor blades and having nozzle chamber and nozzle block portions; a rotor having a plural­ ity of circular arrays of blades and a thrust balance piston formed thereon; a dummy ring disposed within one end of the inner cylinder adjacent the balance piston; and labyrinth sealing means disposed between the dummy ring and the balance piston and forming a limited leakage rotating seal therebetween; characterized in that a stationary sealing is means is disposed between the nozzle block and the blade ring; that labyrinth sealing means are disposed between the nozzle chamber assembly and the rotor so as to form a seal therebetween, such that the inner cylinder, nozzle chamber, nozzle block, blade ring, dummy ring and rotor and said sealing means cooperating to form an en­ closed seal chamber which confines the steam acting on the balance piston; and that a port is disposed in the blade ring so as to provide fluid communication with the sealed chamber and located down stream of the first circular array of rotor blades. Stationary seals which provide metal to metal contact are disposed between the nozzle block and the blade ring and labyrinth seals are disposed between the I.D. of the nozzle chamber assembly and the rotor. The inner cylinder, nozzle chamber, nozzle block, blade ring, dummy ring and rotor and the seals cooperate to form an enclosed sealed chamber which confines the steam which acts on the dummy piston and a number of ports are disposed circumfer­ entially in the blade ring with fluid communication to the sealed chamber and the ports are located downstream of the first circular array of rotating blades to provide cooled steam to the thrust dummy piston chamber. The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, in which: Figure 1 is an elevational view partially in section of a steam turbine designed in accordance with this invention; and Figure 2 is an enlarged sectional view of a portion of the turbine shown in Figure 1. Referring now to the drawings in detail there is shown a steam turbine comprising an outer casing or cylin­ der 3, an inner casing or cylinder 5 disposed within the outer cylinder 3, a blade ring 7 disposed partially within the inner cylinder 5 and partially within the outer cylin­ der 3. A nozzle chamber assembly 9 is disposed within the inner cylinder 5 and has nozzle chamber and nozzle block portions 9 and 13, respectively. A rotor 15 is rotatably disposed in the turbine and carries a plurality of circular array of blades or blade rows 17 disposed in series. Interdigitated with the array of rotatable blades 17 are circular arrays of stationary or nozzle blade rows 19 mounted within the blade ring 7. Disposed on one end of the inner cylinder 5 is a dummy ring 21. A thrust balance piston 23 is disposed on the rotor 15 adjacent the dummy ring 21. A labyrinth seal 25 is disposed between the dummy ring 21 and the thrust balance piston 23 and comprises a plurality of circumferential rings serially disposed on the balance piston and a plurality of fins extending radially inwardly from the dummy ring. The fins interdigitate with the circumferential rings and are also disposed radially adjacent the center portion of the circumferential rotor rings and cooperate therewith to form a high pressure running seal between the dummy ring and the trust balance piston. As shown more clearly in Fig. 2, a similar labyrinth seal 27 is disposed between the nozzle chamber 9 and rotor 15 and comprises a plurality of circumferential rings 31 serially disposed on the outer periphery of the rotor 15 and a plurality of fins 33 disposed to extend radially inwardly from the radially inner surface of the nozzle chamber 9. The fins 33 interdigitate with the rings 31 and are disposed radially adjacent the center portion of the rings 31 cooperating therewith to form a high pressure running seal between the nozzle chamber 9 and the rotor 15. A labyrinth seal 35 is also disposed between the nozzle block 13 and the blade disc 37 on the rotor 15 adjacent the first blade row or first circular array of rotating blades to form a running pressure seal between the nozzle block 13 and the blade disc 37. A labyrinth seal 39 is also disposed between the blade ring 7 and a shroud ring 40 disposed on the outer periphery of the first row or circular array of rotating blades to restrict the flow of motive steam from bypassing the first row of blades. A pressure tight stationary seal 41 is disposed between the nozzle block 13 and the blade ring 7 to prevent steam from leaking therebetween. A series of ports 43 are disposed circumferen­ tially in the blade ring 7 immediately downstream the first row of rotatable blades allowing steam which has passed through the first row of rotatable blades to pass into and fill a chamber 45 bounded by the inner cylinder 5, the blade ring 7, the nozzle chamber assembly 9, the dummy ring 21, the thrust balance piston 23 and the rotor 15 resulting in a pressure zone wherein the temperature is substantially reduced in chamber 45 therein providing the efficiency advantage of a straight through flow control stage and a significantly cooler steam supply to the rotor thrust balance piston 23 without substantially reducing the pressure on the thrust balance piston and reducing the amount of leakage steam bypassing the first row of rotating blades increasing control stage efficiency.

Claims: 1. A steam turbine comprising an outer cylinder (3); an inner cylinder (5) disposed within the outer cylinder (3); a blade ring (7) disposed partially within the inner cylinder (5) and partially within the outer cylinder (3); a nozzle chamber assembly (9) disposed within the inner cylinder (5) for introducing motive steam to the turbine rotor blades and having nozzle chamber and nozzle block portions (9, 13); a rotor (15) having a plurality of circular arrays of blades (17) and a thrust balance piston (23) formed thereon; a dummy ring (21) disposed within one end of the inner cylinder adjacent the balance piston (23); and labyrinth sealing means disposed between the dummy ring (21) and the balance piston (23) and forming a limited leakage rotating seal therebetween; characterized in that a stationary sealing means (41) is disposed between the nozzle block (13) and the blade ring (7); that labyrinth sealing means (27) are disposed between the nozzle chamber assembly (3) and the rotor (15) so as to form a seal therebetween, such that the inner cylinder (5), nozzle chamber (9), nozzle block (13), blade ring (7), dummy ring (21) and rotor (15) and said sealing means cooperate to form an enclosed seal chamber (45) which confines the steam acting on the balance piston (23); and that a port (43) is disposed in the blade ring (7) so as to provide fluid communication with the sealed chamber (45) and located down stream of the first circular array of rotor blades (17). 2. A steam turbine as claimed in claim 1, characterized in that the labyrinth sealing means (35) between the nozzle chamber assembly (13) and the rotor (15) comprises a first labyrinth seal between the nozzle block (13) and the rotor (15) disposed adjacent the upstream side of the first circular array of rotor blades and a second labyrinth seal 27) disposed between the nozzle chamber assembly (9) and the rotor (15). 3. A steam turbine as claimed in claim 2, characterized in that the second labyrinth seal between the nozzle chamber assembly (9) and the rotor (15) comprises a plurality of circumferential rings (31) serially disposed on the rotor cooperatively associated with a plurality of circular fins (33) which interdigitate with the rings (31) and are disposed radially adjacent the center portions of the rings (31) to form a high pressure low leakage laby­ rinth seal.

Inventors: BROWN, BOYD, GROENENDAAL, JOHN COMBE, JR.
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1986
Main classification: F01D5
IPC codes: F01D3, F01D5